The pyrolysis of peat

261

Jotuna of Ana!,ricol and Applied Pwz+s& 5 (1983) 261-332 Elscvicrscience PublishersB.V_ Amsterdam - Printed in The Netherlands

Review THE PYROLYSIS

OF PEAT

A COiMPREHENSIVE REVIEW OF THE LITERATURE CHRISTIAN ROY * and ESTEBAN CHORNET Departmenr of Chtvnicai Engineering. Liricersit&de Sherbrooke. Sherbrooke, Q&MC. JIK ZRI

(Canada)

CHARLES

H. FUCHSMAN

Center for Encironmenta1Studies, Berni& State Unicersi~. Berni&, .WV 56601 tW.SA.)

(Received November 12th.1982:acceptedJune 12th.1983)

Part I: Funhenttds I- Invoduction .......................................................... . Mucu of put p~wl~is ................................................ 11. Patcokeandsani-coke .............................................. 22 Pats .......................................................... 3. ?hcroleofpcatandilsconstituentsonthamaldecomposition ...................... 4. Paramctctsg~~ngthcp~l,uticrc3ctionsofpat .............................. 4.1. Temperature ...................................................... 4 2. Hatpretratmcnt .................................................. 4.3.Contacttitne.. .................................................... 4.4. Hatingrate ...................................................... 45 Presswe ......................................................... 4.6. Panicle size ....................................................... 4.7 !kcondarycracking ................................................. 5 ~ecombined~~l~~ofpeatuilhothersubstanccr. ............................ 5.1.qTolyisinthepresulceofu~wr ........................................ 5S Pyro~ysisintheprtscnaofpetrohtnruidues .............................. 53. Pyrol@s in the presatcc of methane ..................................... 54. fitoI>ti in the p-a of ammonia and other tliv taining compounds 6. Kinetics ............................................................. 7. ThermograimcuicanddiffmntidrhamalanalSresofpeats ....................... 8. Catalytic effects ....................................................... 8.X. Alkaline salts ...................................................... 82 Ironoxides .......................................................

8.3. Lwiszuids

.......................................................

8.4.Strwgoxidizingacidr ................................................ gJ.Imprcgn21tedmetalions .............................................. 9. Ractionmbchaniuns ...................................................

0165~2370/83/503.00 E 1983ElsevicrSciencePublishersB-V.

262

.......

265 266 272 276 276 280 281 283 284 287 287 291 291 293 293 294 295 298 299 299

300 301 301 302 302

Pan II: Induwial applicuiions Commcrci31 prod;lcu of put pyoIysis ~~~.~.~l~~~~.~..~~~~_----------ll--..~~.... IflLScmicokc _________________._._._._._._-.._-_._._..-_..._..___._. I~~cokc._._..._.___........~.....~..................-....~.~...... 103. Aaisaticadwn ____ ._. _. _. _._._. _. -. -. . . . . . I. -. . . . . . _. . . . ______. IO_-t Pclt~.............._....................................._._._ Gtn~1~irrrr~ufor~tfcdCocohng~rions............... -RU p>r+iS

p roccLt

I~I.Thedr)ingphw

_

. . . . . . . . . . . . .

.~_~~.~......~.~...~.----~~.................~~..... _._._________._

I7- -

Scrucrurdchanga(110-160’C)

IL-L

Dccamposition

125. Coke fomatiua

(690~9CWC)

__. _.

309 313

._.___..___._._._._._...~.~........

313

_. _ _ _. . . . _ . _. _ _ _ _ _. _ _ _. _. _ _. . _. _ _ _. _ _. _. 123. The hrcakdoun of carbohydrates and formation of bednate ( 160400°C) - . e_. . I . _. of humic acids and formation of semi-coke (300400’C)

305 30s 306 307

. _. . . _. . . . .

_ . _ . _ . _ . __ _ . . _ . _ _ _ . __ _ . _ . _ _ _ _ . _. _ . _ . _ Icdusxrid pear cd&g practice . _ . _ . _ _ _ _ _ . - . - - - . - . - . . - . - . - . - . - . - _ - 1 I . I I I . - . 13.X. Chan;~~tieso.Cindus~ri~ipcjtp~rrlI~~ir ._ _ ._...._._ _..__ _._._ ._._.. ___ 1::’ _-- L~~~~fr~mindrlrrri~1tx~~cn.r _. ..___.._._._ _._ ..____ _._._.___.._.._

313 31:

115 316 316 316

323

PART I: FCSDAMEST.M_S

1. ISTRODCC-I-IOS

Peat is a dark-coiored organic soil consisting mainly of u-ater. Water contents of 85-905 are common. The balance is largely organic material comprising the residues of higher plants and of the microorganisms of decayThe process of decomposition of plant residues to form stable organic substances relativeIy free of visible plant structures is termed humification. Pent with a low degree of humification contains considerable amounts of fibrous material and exhibits a cellular structure under the microscope. Highly humified peat retains only little fibrous or cellular matter. and appears as a dark amorphous easily dispersed mass. Inorganic matter as measured by ash content of peat (calculated on a water-free basis) varies greatly with location and geological history- High-moor peats usually have ash contents below 10%. while low-moor peats have more inorganic matter. The term “peat” is usuall_v not applied. in North America or Europe. to soils u boss inorganic content exceeds 25 or 30% In some countries (e.g.. China). however_ organic solids with 50% ash are often identified as peat. Peat is r~garckd geoIogicaII_v as an intermediate stage betu-een plant substance and coal. Cons=_uentiy the peat literature employs the terminologies of plant chemistry. soil chemistry. and coal chemistry. often in confusing and inconsistent ways. The following terms which are used to characterize the chemical components of peat. are not mutually exclusive- Their quantitacit.-e values in any sample depend on the analy.icaI procedures employed.

263

Bilumens. The organic matter in peat which dissolves in hot organic solvents_The solvents chosen are usually hydrocarbons, ethers or alcohols, or some mixture thereof. P’robizumens. Bitumens recovered from the condensed products or pyrolysis. ZVaxes. The portion of the bitumen fraction which is soluble in hot alcohol but insoluble in cold alcohol. Resins. The portion of the bitumen fraction which is soluble in cold alcohol. Asphahenes. The portion of the bitumen fraction which is insoluble in hot alcohol. Water-soluble and easi& hy&o&zabblesubstances (WS + EH). The fraction of debituminized peat which dissolves on boiling with 6% hydrochloric acid in 2 h. HemicelZuZase. The portion of the WS + EH fraction which is attributed to carbohydrates. It is usually computed by measuring the amount of reducing substance in 6% hydrochloric acid hydrolysis filtrate. Po&peptide (or protein). The portion of the WS + EH fraction which is attributed to amino acid chains in peat. It is computed from the nitrogen content of 6% hydrochloric acid hydrolysis filtrate, after correction for the ammonium ion content. Humic acids. The fraction of peat which is soluble in dilute aqueous alkali at moderate temperatures, and acid-insoluble. The humic acid determination is usually made on the residue after the 6% hydrochloric acid treatment. If the determination is made on whole peat, or on debituminized peat which has not been treated with aqueous hydrochloric acid, the humic acid fraction will include elements of the waxes and hemicellulose, respectively. Hymaomelnnic acid. The portion of the humic acids which is alkali-soluble. Fzd~icacid The organic matter remaining in solution after humic acids are precipitated by acidification of alkali humate solutions. Ce#&.se. The fibrous carbohydrate component of peat. It is sohrble in cold 72% sulfuric acid and hydrolyzable to ghxcose by boiling in 5% sulfuric acid. L&in. A fraction which may or may not be similar to the lignin of higher plants. It is the portion of the residue, after humic acid removal, which is soluble in hot concentrated aqueous sodium hydroxide. Humins. Broadly defined, the organicfraction which is not soluble in diiute sodium hydroxide at moderate temperatures. More narrowly defined, it is the high molecular-weight organic fraction which remains after removing bitumens, carbohydrates (hemicellulose plus cellulose), humic acids, and lignins. Ash. The residue of peat after combustion. It consists of mineral inclusions in the peat (e.g., sand) and the oxides and carbonates resulting from

combustion of mctaI salts of organic acids and other metal-organic complexes originaMy present in peat. Tht pyrolysis of peat appears to ha\-e been first described by Patin [l] in 1663. Later K=
scribed by Gordon [3]. is the book by F.G. Wieck (The Little Peat Book. or Properties. Production. and Use of Peat). published in Chemnitz (now KarI-S!arx-Stadr). Germany in 1837. Wieck described the coking of peat in retorts. ovens. and coaI-burning piles. KeppeIer [4]_ in his studies on dewatering peat. cites the book by Vignoles (Statements Respecting the Method and Cost of Producing Coke from Turf). published in London in 1850. Vignoles was one of the earliest writers to rrddrtis the difficult question of removing water from peat. prior to coking. According to Vignolzs. peat must be heated to 150-16OOC. to permit satisfactoc removal of water by filtration. 1vith the rapid development of coal-based industry in the mid-nineteenth ccntuq. work on peat. and especially on its carbonization. became peripheral to more basic studies on coal coking. Industrial developments in the high-temperature treatment of peat are described in other sections of this Pap-This article is intended to describe the thermal decomposition of peat in both chemical and technological terms. and to indicate the nature of the changes which occur in the peat fractions considered individually and coHectivel_v. Since the data on thermal changes in peat come in part from industrial sources much of the information is empirical in character. The wet carbonization of peat is not treated in the text. The objective of wet carbonization is removal of water with the concomitant destruction of the hydrophilic nature of crude peat. This reaction yields solid products w-host structural properties are similar to peat chars obtained on low-temperature conventional carbonization. Several wet carbonization processes have bssn devczloped and their dacription can be found eIsewhere [5.6].

Z PRODLX-TS

OF P&4T

PI-ROLI-SIS

The products of peat pyro!ysis are (1) a solid carbonaceous residue, usustll~-termed coke or semi-coke depending on the degree of carbonization; (2) a Iiquid distiltate. consisting of a viscous organic phase. called peat tar, and an aqueous phase which usuaISy contains acetic acid , methanol and other low-molecular weight polar organic substances: and (3) a non-condensable pyroiysis gas. These pyrolysis products are discussed in separate

sections of this chapter.

265

2-i. Peat coke and semi-coke

The carbonized solid product obtained by heating peat to about 550°C is generally known as peat semi-coke. The term char, which is less specific, is sometimes used interchangeably with semi-coke The use of the term peat coke to characterize the pyrolysis product obtained from peat at about 9OOOCis not entirely satisfactory, although prolonged usage has made it impractical to eliminate the term. The terminological difficulty arises from the implied similarity to coal-derived coke. Although peat and coal-derived cokes share certain characteristics, chiefly high carbon content, they differ structurally in ways which can be significant in the study of their physical properties and chemical reactions. Coal coke forms by thermal decomposition of solids fuels which undergo a plastic stage during carbonization, resulting in a hard anisotropic product with graphitoidal crystallites dispersed in an amorphous matrix. Although all coals can be pyrolyzed to yield high-carbon content products, only specific coals (high-volatile bituminous mainly) yield the peculiar crystallineamorphous aggregate commercially recognized as coke. Peat does not undergo any plastic stage during pyrolysis. The resulting peat coke is almost entirely amorphous and isotropic. The carbon structure of peat coke forms along the structural lines of the cellulosic fibers and other solid organic substances initially present. With the loss of water, and especially of oxygen, these quasi-crystalline structuresare unstable and break down. However, the breakdown products lack the mobility of molecules of the plastic phase in coking coal, and so the peat product is relatively incapable of growing large crystallites from the graphitoidal nuclei which begin to appear at high temperatures. Meunier [7] considered semi-coke to be a carbonaceous residue with a relatively high Volatile Matter content (V-M. Index > 4-5). Carbonaceous residues which were not cokes but whose Index was lower than that were termed pseudo-cokes. Kalkreuth and Chomet [8] showed that the anisotropic properties of coke were absent from peat coke, It would thus appear that, by Meunier’s criterion, the peat product should be called pseudo-coke. In this article however, the familiar term “peat coke” is used. It is a well understood term, and little confusion should result as long as its crystal structure is not identified with that of coal-derived coke. In a review of literature on the low-temperature decomposition of peat, Christiansson[9Jreported that peat semi-coke derived from carbonization on sn industrial scale generally has an elementary composition based on organic matter of 84-88% C, 2-3.5% H, 5-9% 0 and 2-4% N. The total sulfur content amounts to O-2-0.48, whereas the content of volatile sulfur is only 0.05-0.1% The phosphorus content is 0.05-0.1%. The ash content varies considerably but lies as a rule within the limits of S-208 of the dry matter_ The calorific value can be given as 7300-8200 kcal kg-* of organic matter_

Industrial peat coke. produced in Germany and Finland. contains about SO-938 fixed carbon (usually 90-91%), l-5-3% volatile matter, 1.5% H. O-l-1.4% N, and 3.5-10% ash [lo]. Earlier data derived from Soviet peat coke studies are given by Naucke [ 111. 2.2 Pear rar

The term peat tar is normaIIy used in the literature to refer to the tota pyroIysis condensate. Sometimes however. it is intended to refer only to the heavy water-immiscible fraction of the distillate. Unless otherwise specified. the term peat tar as used in this paper. refers oniy to the organic phase. Peat tar is a compIex Iiquid. useful as a fuel and as a source of many organic compounds. Its analysis provides some insight into the nature of the peat from which it is derived. and into the course of the pyroIytic process. The physical and chemica1 properties of peat tar vary with the kind of peat and the conditions of carbonizaticn. The tar is a dark-brown to almost black. strong-smciling product which at room temperature is a semi-solid or a viscous Iiquid. It has a calorific value of 8800-9500 kcal kg-’ [9J. Softening points are reported to be between 28OC [12] and 36*C [13]. The viscosity of tars from low- and high-temperature coking of high-moor peats is about 37 mm’ s-!. at 50°C. Tar collected from gas generators using peat as a raw materia! is more viscous. typicaII,v 90-250 mm’ s-r. at 50°C 1131. Moisture-free peat tar usually has a density of 0.92-0.98 [9,14]. Howwer. some laboratory studies by Keppeler and Hoffmann [15] yielded peat tars v.ith densities in the range l-O-l.1 and. in one case when pyrolysis was conducted at 800°C. a density of 1.254 was recorded. Kaucke [16] has reported densities of 1AM-l.1 for peat tars from gas-generator operations using low-moor peat. Densities higher than 1.0 can be explained by the prcssnce of organic compounds with condensed aromatic rings. Eg.. the density of anthraczne is 1.25. It is of course possible that in some cases uc.Iusion of carbonaceous solids may also have contributed to higher dtnsitie5. GencraII_v 60-70% of peat tar distills within the temperature range ISO-35O’C. A slight decomposition often commences. hou.ever. at 300-325=c [9]. Literature values for the elementary composition of peat tar at the industrial or pitot IeveI have been reviewed 19.14) and are summarized in Table 1. Such literature data refer to tar that does not contain any appreciable amount of water-soluble matter. By including the water-soluble fraction to peat tar for the elemental analyses. Christiansson [9] later on obtained the follow-ingresults at the laboratory scale: 72.0% C. 9.4% H. 3.8% N and 14.8% 0. The c!ementary co.mposition of peat tars obtained after vacuum pyrolysis was found to be substantiahy different to that previously reported 19.171. This variation u-i11be discussed elsevr-herein the paper.

267 TABLE 1 Elcmmtaq

composition

of peat tars at the industrial or pilot

Ehlent

56weigh

C

78 -80 8 -10 O-S- 2 (25-3) 8 -105 O-l- 01

H N 0 S

plantlevel lg.141

*

* Nitrogen content of tars from low-moorpeat.Ochervaluespresumably represent principallytarpfromhigh-moorpeats.

The main components of peat tar are different kinds of hydrocarbons,

largely of an aliphatic nature. aliphatic esters, heterocyclic oxygen compounds, aliphatic acids, and phenols. Ketones, alcohols, bases, and neutral nitrogen compounds are also present. Results reported by several investigators and analyzed by Christiansson [9] indicate that tar derived from reIatively highly decomposed peat generally contains 40-558 neutral oil, mainly composed of hydrocarbons and oxygen compounds, 5-15% solid paraffin hydrocarbons, lo-20% phenols, 5-10% acids, less than 5% bases, and lo-20% other matter, mostly of an asphaltic nature. Similar analyses have been reported in the Russian literature [18,19] and are in agreement with the data cited by Christiansson. Much earlier Chemozemov [20] had reported that the paraffins and asphaltenes appear mainly in the heavier higher boiling fractions (> 2OOOC)of peat tar; asphaltenes being present only at low concentrations and the paraffins being absent in the lighter low-boiling fraction ( < 200°C). A somewhat different picture was presented by Boisselet [21] who characterized the hydrocarbon composition of lowtemperature peat tar as 40-60% paraffins and 3-10% aromatics, with the balance being olefins. Fractionation of peat tars by differential solubility in organic solvents has been used as an analytical tool for many years. Thus Rigot [22] in his literature review of peat utilization cites extensively from the fractionation studies of V.E. Rakovskii in the 1930’s and 1940’s. By vacuum pyroIysis of tar from moderately decomposed low-moor peat, Kravtsova and co-workers [18,23] obtained the distribution of distillate fractions presented in Table 2. Losses were less than 0.1%. The light fraction boiling between 65 and 15OOCobtained from the coking plant at Redkinsk was lower according to Grigor’eva and Karavaev [24] since it represented 4% of the total tar. Boisselet [21] characterized peat tar distillates in terms suggestive of their potential uses (Table 3). Detailed knowledge of the composition of peat tar is mainly limited to relatively low-boiling components and other groups of substances which can be easily isolated. Thus the composition of n-alkanes

TABLE 2 DistiIIstc fractions of peat tar aaording

to Kravtsva

and et+--orkcrs [18.23)

Boiling rang (‘Cl

Yield <% of car. moisture-a&-free basis)

cl70 170-200

157

X0-230 EO-50 2X-Xi0 300-330 s 330

6.0 17.6 16.4 17-7 8.9 17.6

separated from low-temperature peat tar was studied via a gas-liquid chromatographic (GLC) technique by Rakovskii et al. [U]. Prior to GLC anal_vsis the saturated hydrocarbons were first separated into compounds with strai&t and branched chains by urea complexing- Solid and liquid normal hydrocarbons c\ere obtained. The n-C,O_zl hydrocarbons represented 2.05 of the original tar: th,o CIs_r9 hydrocarbons predominated. It was also concluded that hydrocarbons isolated from the initial tar and its distillates {boiling below 4OOT) u-ere identical in chemical composition. Low-boihng fractions of neutral oil derived from peat tar are characterized by a high content of oxygen compounds. Christiansson [9] studied the neutral oil fraction boiling at 20-200°C_ Furan derivatives. aliphatic ketones. thiophenes. and saturated and unsaturated aliphatic and aromatic hydrocarbons could bc identified. Kravtsova et aI. [18] anaIyzd a fraction of neutral oils with a boiling point beIow 135OC. The composition of the neutral oils was determined by liquid chromatography with a subsequent study of the fractions by chemical analysis and IR and UV spectroscopy_ The three major goup components wcrc: paraffin-naphthenic hydrocarbons containins 6-33 carbon atoms. aromatic hy-drocarbons such as alkylated mono-. bx-. and polycyclics. and oxygen-containing compounds. which proportion u-as 17.3. 37-3 and 30.6 5. rspcctively_ In a comparative study of the structural group composition of aromatic hydrocarbons in undistilled low~smpcrature peat tar and a neutral oil fraction. Kashirina and Rakovskii [26] TABLE 3 DistIIIa~c:fI;IcCms of pe3t LX according to J3oisscIct[Zi] T3r frasrion

5 weight

269

noted however that the content of bicycIic compounds was higher in the neutral oil than in the untreated tar. The numbers of aromatic and naphthenic rings per molecule were determined in both cases. The ?leutral oxygen compounds in the 200-270°C peat tar fraction were studied by Budyak and Karavaev [27j after their separation by dilution with petroleum ether and filtration through non-activated silica geI. According to the authors the chemical group fraction principally consisted of cyclic compounds connected by aliphatic chaius with 0, S, and N in the chains. A neutral oil fraction from peat tar boiIing between 150 and 350°C was examined by Kashirina and Rakovskii [28]. The distillate contained 22.5% and 7.8% saturated and unsaturated hydrocarbons, respectively, and 32.8% aromatic hydrocarbons (S in reference to the amount of tar). The saturated hydrocarbons were 78.4% normal C r4_*, paraffins and 21.6% isoparaffins. The unsaturated hydrocarbons comprised normal olefins (27.9%) and branched-chain olefins (67.0%). Several adytical studies on the aliphatic acids in peat tar have been reviewed and reported by Christiansson [9]. Investigators have isoIated saturated and unsaturated ahphatic acids with 5 to 10 carbon atoms while others claim that they have found high-molecular weight fatty acids with 8 to 26 carbon atoms. Much has been written about phenols in peat tar. The phenolic fraction consists of substituted and unsubstituted phenols. Quantitative determination of the phenolic constituents in peat tar has been reported by Stramkovskaya et al. [29] and Stipnik and Levin [303. The latter found that the simplest monohydric phenols in peat tar (unsubstituted pheno1 and the cresols) made up to 59.6-72.5% of the combined phenols, while the dihydric phenols (catechol, hydroquinone, resorcinol. and their homologs) amounted to about 8% or a Iittle more. As u-as previously poiuted out, peat tar contains smaII amounts of nitrogenous bases. By carbonizing a Sphagnum peat up to 550°C KeppeIer and Hoffman [31] ascertained the elementary composition of bases obtained to be 69.7% C, 7.4 % H, 11.5% N, 0.7% S, and 10.7% 0. Particularly noticeable is the high content of oxygen, the bonding of which is not known. Pyridine and the nearest higher homologs have been identified by Evdokimova et al. [32] using IR spectroscopy. Their results are included in Table 4 which lists 100 compounds that have been found in the tar fraction or aItemativeIy in the aqueous condensate from the low-temperature pyrolysis of peats. Some of the compounds may be the result of secondary reactions that occur in refining procedures. A few have not been completely characterized and the identity of some has not been established beyond aI.I doubt. Most compounds on the list, however, commonly occur in tar and aqueous phase fractions and no doubt many more that are present have not been identified. It should be borne in mind, however, that tar obtained on high-temperature decomposition of peat, including tar obtained from peat

270 TABLE

4

compounds formed by put c2rbonization Formula Hz0 SH, CH,O CH,O, CH,O Cs, C, H,O, GH,O Cd4602 ‘2-W,

C,H,S C,H,X C,H,O, C,H,S C,H,O C,H,lJ CSH,, CsH*, CsH,, C& C,H,O C,H,O, C6H602

C,H;O, C6H60,

C,H+ C,HiS CbHTS C&H& C6wJ C6HE C&z C&4 C6HI4 CJ-44 CA W-W

CiHIO CiH,O C,H,O, C,H,OI c-I-rap3 T;H,S C,H,N C,H,N C,H,S CiH9S C,HoS

xame Water Ammonia Formaldehyde Formic acid ~fethanol Carbon disulfide Aatic acid Acetone Propionic acid Furan Thiophene gm1c But>tic acid Pyridine 2-~fethylfuran C?;clopcntalX 2-Pentale IsOptnt3IX n-Pcntane BerKcnc Phenol Catechol Hydroquinone Resorcinol PJTOg3IIOl Aniline a-Picolinc &PicoIine y-Picoline 2.5Dimcth~ifuran CycIohexane Meth_vIqcIopentanc n-Hcxanc 2-~fcthyIjxncane 3-Mcthylpcmne Toluene m_CrcsOl 0-CresoI pCreso1 Guaiacol Methykatechol ZEthylpyridine 1EthyIpyridint 4-Ethylpyridine 2ILutidine 24-Lutidille 25Lutidinc 2.6-Lutidine

Ref-

9. 33, 34 35 34 9. 36. 37 3s 9.34.36 9. 37 34 14 39. 40 41.42 34 9.22 32 42 14 18 14 14 14 18.23.39.40 9. 23. 29. 30. 43. 44 30.4445 30 30 45 32 32 32 32 14 18.23.39 18. 23 18.23.39 18. 23 18. 23 18.23.39.40 9. 23. 29. 30. 43 9,23,29.30 9.23,29.30 43.44 44 32 32 32 32 32 32 32

271 TABLE 4 (cominueti) Formula

Name

C,HPN C,HqN

3~Lutidine 3-Methylaniline

C7H*, GH7N GHS who CUHIO G&,9 GH,, GH,oO GH,oO GH,oO GH,oO GH*oO Who0 GP*,O GH,oO GH,oO GP*oO2 _*

CgH,,N CgH,,N C*H,,N CSH,, C9H,.S C9Hs

CgH,N C9H,o CgH*2 C9H,2 C9H,2 C9H,2 C9HlZ C9H*2 C9HIZO C9H,20 GH*20 GH*20 w-I*20 G% C*o% GdW C,OHSO C*lP,2

‘Xb C*oH*4 GOHI, Go %

n-Heptane Indole styrene Ethylbenzese m-Xylene o-XyIa,e p-Xylene m-EchyIphcnol c+EthyIphenoI p_EchyIphcnoI 2.1DimethyIphe;loI 24-DimechyIphenoI 2,5-Dimethylphenol 26Dimethylphenol 3.4-Dimethylphenol 3,5-DimethyIphenoI Creosol 4EthyIphenoIresorcinoI 2,3.5-Trimethylpyridine 23.6-TrimethyI~tidine 2,4,tGTrimethyIp~tidine n-Ofztane Quinoline indcne Dihydroquinoline Indan Cumene HemimeIIitcne MesityIene n-PropyIbenzcne o_EthyItoIuene Pseudocumene o_IsopropyIphenoI 2,3,5-TrimethyIphenoI 2.4,6-Trimeth~IphenoI 3,4,5-TrhnetiyIphenoI 1,MethyI-S+thyIphenoI n-Noaane Naphthakne a-Naphthol &NaphthoI TetraIin Durene Isodurene Prehnitcne I-Methyl-3-propyIbe

Refa-cnces 32 32 l&23,39 41.42 18, 23 1% 23,39.40 18.23.39.40 18.23,39,40 18.23,39.40 9. 29 23 9. 23, 29 23,29.30 23.29 23.29.30 23.30 9. 23 9. 23 44 30 32 32 32 18,23.39 42 18. 23 9 18. 23 18, 23 18. 23 18. 23 18. 23 18, 23 18. 23 30 30 30 30 30 39 15,23,38 30 30 23 18. 23 18, 23 18, 23 18. 23 (Continued on p. 272)

272 TABLE

4(concinucd)

Formula

Name

RefCreIUXS

CdLO

23.5.6-TetramcthyIphcnol Carbvole TetrahydrourbazoIe Anthncenc

30 42 41 15

C,:HkJS C,,H,,S C:aH,,,

* Inadequate terminology. thus CorrnuI~not shown.

gasification. has undergone considerable degradation. The table shows that few higher-molecular-weight compounds ha\-e been identified in peat tar.

.; . THE TIOS

ROLE

The yields

OF PEAT

AXD

r’fs COSSTITUENTS

OS

THERMAL

DECOMPOSI-

tar. water and gas may vary within wide Emits with difkrmt peats and different conditions of carbonization. The latter fact. in particular. makes it difficuIt to cornFare the majority of studies reported in the literature. According to Christiansson [9]. slightly decomposed peat as a ruIe yields at 500°C. 32-35s semi-coke, 10-14s tar, %-3X water. and 17-7’G -.c gas. based on the organic matter originally present in the peat. High& decomposed pest yields 36-428 semi-coke. 1%25% tar. IS-234 water. and 17-229 gas. The yieids of semi-coke and tar gcr,eraIIy increase with the degree of decomposition. whereas the yield of water decreases considerabIy and the yield cf gas remains comparatively constant. The yicid of tar is particuIarIy VariabIe. Variation in the properties of peat semi-cohe with the type of peat used has been reported by Christiansson [9]. He carried out an anaiytical investigation of semi-coke prepared in a Fischer-type retort by carbonizing different types of peat to a final temperature of 535°C. The results are shown in Table 5. Although the variations in the chemical composition of the commercial semi-coke derived from different types of peat are not completely known, certain information may nevertheless be gathered from the Iiterature [9]. The carbon content seems to increase with the degree of decomposition of the peat. u.hiie the hydrogen content decreases. The nitrogen content is generaIIy direct& correlated with the nitrogen content of the peat and is always higher than the latter. The sulphur content of industrial semi-coke. which is always Iow. is generally of the same magnitude as that of peat. Finally. the calorific =paIueof the semi-coke increases with the degree of decomposition of peat. According to Drozhalina and Rakovskii [46] the high-moor peats carbonize of semi-cok

273 TABLE 5 Ekmcntary composition of semi-coke derived from different types of peat [9] chssifxcatim

c ca

H (9)

N CO

0 ce

High-moor pcac slightIy decomposed

845-853

3.4-3.7

1.0-1.8

10.0-10.3

1.8- 3.4

825

3.4

1.5

126

3.0

85.0

3.4

26

9.0

11.8

792

35

26

lA7

125

815-842

3.1-33

l-8-24

10.9-128

4.4-10.2

820-85.0

29-3.4

26-5.3

8.0-10.9

11.8-20.0

High-moor peat,

moderately decomposed High-moor peat. higblydecomposcd Low-moor peat. slightly decinnposed Low-moor peat, moderately dccomposcd Low-moor peat. highly dccompascd

more rapidly

at a given

temperature

than the low-moor

peats,

Ah (RI

which

means

that the percentage of carbon atoms increases more rapidly. One explanation is that the oxygenated compounds in high-moor peat are primarily aliphatic iA nature and thermally unstable and decompose almost completely below 700°C, forming highly unsaturated, carbon-rich fragments of reactive, radical-form solid compounds. The oxygenated components in low-moor peats, which include a large proportion of aromatic organic compounds, are thermally more stable. These compounds continue to decompose up to 900°C. The rate at which polynuclear aromatics form and condense is directly associated with the type and chemical nature of the oxygenated compounds in high-moor and low-moor peats. Baranchikova and Rakovskii [47j concluded that the chemical structure and thermal stability of the oxygen-containing functional groups origix~Uy present in the peat determine the chemical nature of the surface of the coke The number of aromatic rings per carbon atom is greater in coke from Iow-moor peat than in coke from high-moor peat at the same temperature. However, the formation of the polynuclear aromatics, especially above 7OO”C, proceeds more rapidly in coke from high-moor peat. Presumably, above 900°C the degree of aromatization of the coke substance from high-moor and low-moor peats is the same 1461. Mal’ et al. [41] carried out laboratory carbonization experiments for a variety of low-moor and high-moor peats with degrees of decomposition ranging from 5 to 45% For both types of peat, the content of nitrogen atoms in the semi-c&e increased with the content of easily hydrolyzable components (carbohydrates and amino acids) in the peat. Semi-coke retains 58-83% of the nitrogen atoms in uncarbonized high-moor peat. In the case of low-moor peat the retention is only 47-54%. The relationship of the chemical composition of peat tar to the kind of

peat carbonized has been examined by several authors. The ether-soluble neutral substancescontained in peat tar seem to increase with an increase in the degree of decomposition of peat [9]. MaI’ et al. 1411obsenred that tar obtained in the high-temperature pyrolysis of high-moor peats contained 0.35-O.%% N in the form of heterocyclic amines as compared to 0.95-1.30% for tar from Iov+moor peats. The higher ratio of amino acids to carbohydrates in high-moor peats. when compared to low-moor peats, is associated with an increase of some nitrogenous substances in the tar (e-g., amides, pyrroles, indoIes, and tetrahydroccbazoles). According to Kaganovich and Rakovskii [48j. the amounts of fatty acids and pyrolytic water collected after Iow-tempcrature decomposition of peat are greater for low-rank (oxygen-rich) peat. The variation in the composition of peat gas as a function of the composition of the original peat has not been widely studied- DrozhaIina and Rakovskii 1491 reported that in high-moor peats carbon monoxide is obtained from the destruction of thermally unstable oxygen-containing compounds while in low-moor peats pyrolytic carbon monoxide IvouId be formed by the destruction of the thermally stable phenolic compounds. According to the authors. most of methane from high-moor peats comes from bitumens. whereas methane from Iou=moor peats originates partIy from bitumens but mostly from decomposition of hydrolysis residues. Thk carbon dioxide evolved varies directly with the humic acid content of the pear. Thus. the compositions of the gas evoked in pyrolysis of high-moor and Iow-moor peats appear to be different at the same temperature of processing. According to Mal’ et al. 1411more XH, is evolved from low-moor peat than from :?i&-moor peat during high-temperaturepyrolysis. Kashirskii [SO] reported that high-moor peats yielded more hydrocarbon gases than Zovv-moorp~2ats. The thermal dr3compositionof different peat constituents can be studied either by carbonizing a number of peats rvith greatly varying chemical composition or by carbonizing isolated peat fractions. Most studies have heen carried out with isolated fractions. Christiansson [9] extensively revit7t’cd the carlitr literature on the subject. Carbonimtion experiments vcith peat of extremely IOW decomposition. containing as much as 70-805 carbohydrates. afford low yields of coke and tar. The FicId of water is extremely high. This is what one would expect from the carbonization of cellulose and reIated carbohydrates. The behavior of the carboh_\-drate fraction of highly decomposed peat is less clear. It is difficult to characterize the contribution of humic matter to carbonization yields bczausc even extremely highly decomposed peat contains only 50-60s humic matter in addition to small amounts of bitumen. carbohydrates. and other components_ Since the coke yieid is high and the tar yieid is usually low from high& decomposed peats. similar results would be expected from h_\rpotheticaIIy pure humic acids, Finally. the bituminous constituents of peat give ;ivcy high Field of tar_

275

Drozhalina and Rakovskii [49] pyrolyzed different types of peats in the range 300-9OOOC in order to establish the mechanism of formation of the major pyroQt.ic gases and the relation between the quantity and composition of the evolved gases and the chemical composition of the peats. For all peats investigated, the range of intensive decomposition of humic acids and of water-soluble compounds was 300-6OOOC. Thereafter carbon dioxide evolution was negligiblti The sudden increase of hydrogen at 400-450°C was expiained by dehydrogenation associated with the condensation of aromatic compounds. These reactions continue up to 900°C Unsaturated hydrccarbons were obtained from bitumens and unhydrolyzable residues. In the vacuum decomposition of various peats at 150~4OO*C, Bel’kevich and Minkevich [51] concluded that carbon dioxide was predominantly formed by decomposition of humic acids, water-soluble compounds and bitumen while carbon monoxide mainIy resulted from decomposition of humic acids and Iignocellulose. Reilly and O’Sullivan [52] were the fit investigators to study the carbonization of peat by carbonizing isolated peat constituents. Christiansson [9] has reviewed the experimental results based on such procedures, the weak point being related to the difficulty of obtaining large amount_ of peat constituents without affecting their purity. According to prevailing opinion, bitumens yield large amounts of tar while peat humic acids give high yiekls of semi-coke with negligible yields of tar- Kazakov 153,541, however, concluded that the contribution of peat humic acids to the formation of tar varied considerably, according to the amount of humic acids present as humates. Decomposition of the carbohydrate constituent in peat seems to yield pyrolytic water and little tar. Tar derived from humic acids is mainly composed of phenols. According to Kazakov 1541, nitrogen bases from the tar are predominantly formed during the decomposition process of peat humic acids while phenols of relatively low molecular weight are formed chiefly from resinous substances, and only partially from humic acids and Iignin. Tn the formation of neutral oils from peat tar, all of the peat components participate: bitumens, humic acids, l&in, and carbohydrate. Although the components of peat tend to interact during pyrolysis, some components, e.g., the bitumens, appear to behave almost independently. It seems likely that in a peat coking or semi-coking operation, extraction of bitumens prior to pyrolysis would probably not significantly reduce coke or semi-coke yields [44]. The thermal decomposition of different peat constituents can be studied by successive treatment of peat with different reagents and by carbonization of the original peat and the residues re maining after each treatment. The thermal decomposition yields of the extracted fractions can then be calcuIated by difference. Such approach was followed by Christiansson [9], Ruschev and Mikhailov [55] and Bel’kevich and Minkevich [56]. The latter authors defined the participation degree of a peat component as the ratio

276

between the weight loss of the component and the weight loss of the peat, while the degree of thermal decomposition was defined as the ratio between weight loss of ths peat component and the content of this component in peat (H-form). These studies, which were conducted for the range of temperatures 150-4OOOC.provided estimates of the thermal stability of the various peat components. The conclusions of all these authors about yields are in general agreement with those cited previously. 4. PARAMETERS

GOVERSISG

THE PYROLYTK

REACTIOSS

OF PEAT

Several authors have studied the physical parameters presumed to affect the thermal decomposition of peat. The variables studied include temperature. pretreatment of peat, retention time in the reactor, heating rate. pressure. and particle size. The occurrence of secondary cracking reactions has also been studied. These variables are discussed below. 4.1.

Tempermwe

Warren et al. [57J have carried out instructive slow-carbonization experiments at final temperatures of 250-600°C uith a peat whose efementaqcomposition u-as59.2% C, 5.9% H. 2.1% N. 0.2% S. and 32.6% 0. The results of the experiments are shown in Table 6. The yield of tar, which was 3.6% [m.a.f.) * at 25OOC.was increased up to 45OOC.and then decreased. In the conventional slow pyrolysis of peat the liberation of tar begins at about I70-300°C. though the bulk of it is liberated betu-een 250 and 500°C. As indicated by Ito [58J.additives can greatly affect tar formation. This subject wiI1 be discussed further in another section of the article. TABLE 6 YieIds upon carbonization

of peat to mrying final tempentunv

(‘Z u-eight. m.a.f.1 [57J

Product

250°C

3oG”C

350°C

4uoQC

450°C

500°C

55OT

6ooOC

Ssni-coke

88.9 3.6 2.4 3.6

63.9 13.2 S-l 14.3

55.5 13.0 13.8 18.8

51.2 13.6 17.0 17.4

46.3 16.6 18.6 220

43.5 16.2 15.7 23.1

40.2 14.4 18.6 25.4

39.1 14.8 18.9 26.4

T3r

U?ltcr G3S

Similar experimental results were also obtained by Tanner (cited by Christiansson 191)and by Ivanov [36J. who followed the progress of peat pJToIysis and the yields of products t,o llOO°C and SOOOC.respectively. The >-icId of solids constantly decreased vrith an increase of the decomposition

temperature in the range of X0 to 300°C. * maf.

= moisture and ash free.

The rate of decrease in solid residue upon slow peat carbonization (initially containing 54.0% C and 6.0% H) was reported by NekIeevich et aI. [59]. The yields, based on % weight, m.a.f., are reproduced in Table 7. Even at 900°C the coke still undergoes a slight loss of weight which is mainly attributable to the evolution of hydrogen [17]. Similarly, Kokurin et al. [60] reported that in the fast pyrolysis of peat the solids constantly decrease with an increase in the decomposition temperature_ On the other hand Kashirskii and Koptilov [33] found an optimum temperature range (780-83OOC) for producing the maximum yield of coke under flash pyrolysis conditions (lOOO°C see-‘). The range of temperatures investigated was from 720 to 87OOC. The composition of peat semi-coke depends on the type of peat used and on the fmaI temperature to which it is heated. VolatiIe matter content may decrease by about 50% by heating to 600°C instead of 5OOOC.This increase in temperature is accompanied by an increase in fixed carbon and in combustion value in the solid product. In general the semi-coke obtained from high-moor peat had lower voIatiIe content and higher combustion value than the one obtained from low-moor peat [61]. The calorific value of the chars, studied at the laboratory level by NekIeevich et al. [59] in the temperature range of 105 to 360°C, varied between 5019 and 6324 kcal kg -l. According to Novichkova and Rakovskii [62] the final temperature of pyrolysis also affects strength and abrasion-resistance of the semi-coke. Strength increases from 350 to 5OOOCwhereas abradability follows the inverse cute. Warren et al. [571 reported the elementary composition of semi-coke obtained from carbonization of peat at final temperatures varying from 250 to 600°C. Their results are reproduced in Table 8. Smol’yaninov et al. [63], however, reported that the nitrogen content of peat semi-coke decreased with an increase of temperature from 200 to 45OOC. With further increase of temperature the nitrogen content increased sIightIy and again decreased. The dynamics of nitrogen transfer into volatile products may thus depend on the nature of the peat, although the variations may be small enough to reflect minor variations in the experimental procedures. The caIorific values of peat coke samples has been studied by Chistyakov [64] in the temperature range of 500 to 900°C. At 500 and 9OOOCthese were 8082 and 7981 keal kg-‘, respectively, The maximum value (8199 kcaI kg”) was reached at 6OOOC.In these experiments the initial peat had a calorific TABLE 7 Yields of char upon carbonization of peat at varying find tanpcratures Tempemur~(~C) 56 Weight

[S9]

1GS 150 180 190 200 22G 240 260 280 300 320 340 360 100.0 98.2 96.9 95.7 94.2 92.3 89.3 86.8 83.8 77.8 68.6 66.8 605

278 TABLE

8

Ehncncaq

composition of semi-coke obtained

from carbotition

of peat at varying finai

temperatures [571 kkmcnt

Put

2sO~C

3ooT

350%

400°C

450%

5ooT

55ooc

6oo”c

C H 0 K S

592 5.9 326 21 02

635 6.2 27.7 24 02

74.8 5.4 16.7 3.0 0.1

77.2 5.1 14.6 29 02

79.1 4.4 13.4 3.0 0.1

842 3.8 8.7 3.0 03

826 4.1 10.1 3.1 0.1

87.8 3.4 5.7 3.0 0.1

89.9 3.0 4.1 2s 02

reported that the coke attained value of 5562 kcal kg- ‘. This investigator maximum mechanical strength at 800-900°C. The strength also depended on the initial moisture content of peat. According to Naucke (651 the coke from industrial plants had a higher compressive strength than the coke from experimental laboratory tests. The specific electrical resistance of peat particles (7-10 mm size). measured 0%.er the range 550-15OOOC. decreases steadily from > 10’ to - 2 ohm cm. respectively [64.66]. Naucke [65] explained the increase in electrical conductivity of peat coke in terms of increased numbers of electrons in the conductive orbitals in the pyrolyzed material. The density of semi-coke and coke was reported by Naucke [65J over the temperature range 175-26OOOC. It increased non-IinearIy from 1.32 to 1.95 g cm->. Structural changes in the residual solid materials obviously affected both density and strength especially in the range 550-650°C. The temperature range 800-9OOOC is characterized by the formation of larger aggregates of carbonaceous material and thicker surface Iayers. The crushing strength of the coke increased correspondingly. Saucke [65] showed the evolution of the H/C and O/C ratios of peat semi-coke and peat coke while heating at temperatures up to 26OOOC. Elementary compositions of peat cokeobtained by pyrolysis .at temperatures up to 900°C. as reported by Chistyakov [64] are shown in Table 9. The formation of peat tar and its most important components during the TABLE

9

Ekmcnta~

composition of peat coke as a function of find pyrolysis temperature [64J

Elrmcnt

Inidd Deal .

500°C

6OOOC

7OOoc

900°C

C

575 6.0 34.6 1.7 02

89.4 3.3 5.5 1.7 0.1

92.4 2.5 3.3 1.7 0.1

94.8 1.8 22 1.1 0.1

97.0 1.0 1.2 0.7 0.1

H 0 s S

279

carbonization process has been studied by examining solvent-fractionated tar, obtained at different carbonization temperatures. Laboratory experiments [29,60,67,68],pilot plant studies [9,69,70], and industrial experience [9,15,69,70] have indicated that yields and composition of peat tars are function of the carbonizing temperature and the initial quality of the peat. Christiansson [9], for instance, reported that the content of ether-soluble phenols was only 7.7% for tars obtained at 3OOOCbut reached 37.1% in tars obtained at 5OOOC.Such increase was almost solely due to an increased yield of low-molecular weight phenols with boiling points lower than 225OC.At a higher range of temperatures, Stramkovskaya (291reported that the yield of volatile phenols (phenol, cresols, xylenols, and ethy1phenoIs)increased by a factor of 1.5 by going from 600 to 800°C. She also reported that tars from peat pyrolysis at 600-700°C contained twenty times more phenol than tars obtained from coal coking. According to Christiansson [9], the proportion of high-molecular weight paraffin hydrocarbons reaches a maximum at 350-400°C but thereafter decreases rapidly and ceases at about 500°C. Stipnik and Levin [39] showed, by chromatographic techniques. that the yield of benzene, toluene, o-, m-. and p-xylene and ethylbenzene increased monotonically between 400 and 7OOOC.Smol’yaninov and Maslov [71] reported that the yield of pyrobitumens from peat varied substantially in the temperature range 220-36OOC.The yield was also dependent on the kind of peat investigated, an observation made earlier, in pilot plant studies, using solid heat carriers [69]. These authors noted that tar taken at 500°C contained less carboxylic acids and asphaltenes, presumably because of extensive cracking. According to Hering (cited by Ivanov [36]) hygroscopic water is liberated up to 150-160°C- Beyond this temperature, liberated water was considered to be water of decomposition. Alcohol methyl formation occurred at about 160°C and continued up to 300°C. Acetic acid appeared at 2OOOCunder normal pressure conditions and was produced up to 500°C, the largest quantities being produced during the interval 300-400°C. Ammonia began to appear at 3OOOCand increased in quantity up to 7OOOC.According to Kaganovich and Rakovskii [48] the maximum yields of fatty acids and ammonia are obtained at temperature intervals corresponding to the maximum release of water of decomposition. The formation of secondary reaction products is reduced by vacuum pyrolysis (eg, lo-12 mmHg in the work of Ivanov [36]). Van Heek et al. [72] reported that decomposition water from the slow carbonization of peat is formed between 200 and 800°C. Below 2OOOCall uncombined water is removed. Most water of decomposition is formed in the interval 200-600°C. The first maximum in the water formation curve occurred around 300°C A second maximum occurred between 400 and 45OOC. Christiansson [9] indicated that the gas obtained from low-temperature

2SO

peat carbonization at Iow heating rates contains about 45-608 CO,, 12-208 CO, lo-159 CH,, 6-12s H,, 2-4s C2Hs, 2-4% C,H, and 2-4% N2 (vol SE). The proportion of carbon dioxide decreases beyond 600°C as increasing amounts of hydrogen and carbon monoxide are evolved. The cumulative gas yield increases monotonically with increasing temperature. while the solid residue decreases correspondingly, as shown in Table 6. A somewhat different pattern for the evolution of gases was observed, however. by Degtev [73] who heated peat mixed with metal spheres by an induction furnace to SOfJ-700°C. According to the author, the volume of gases increased exponentially with temperature within the experimental in te=aI. The evolution of non-condensable compounds on slow thermal decomposition of peat at low and high temperatures has been the subject of several studies and reviews [9.17.49.72.74-771. During the first stage, at lOO-150°C. onIy smaI1 amounts of carbon dioxide and monoxide are observed. There is no significant loss of organic matter from peat on heating to 170OC. At about I70-18OOC decomposition of organic material is detectable. the rate increasing with further increases in temperature. Water is given off. along with carbon dioxide which evolves at a maximum rate in the 250-3503C range. and with carbon monoxide which reaches its first maximum rate of evolution in the 300-350°C range. and its second between 550 and 800°C depending on the type of peat. The maximum content of methane in the gas mixture varies between 32 and 42% (vol a) for al! types of peats; it falls at 550-6OPC [49]. Hydrogen appears in the gases at about 350-400°C and becomes a significant component above 450°C. The percentages of saturated and unsaturated light hydrocarbons (ethane. ethylene. propane. propene. butane and butene) are at the highest at 425-525OC [17]. Chukhanc.- et al. [78] obsen.ed that in the fast pyrolysis of peat the reiative voIume of carbon dioxide diminishes over the temperature range 2CKL800°C. while hydrocarbons as well as carbon monoxide and hydrogen increased. 4.2. Heai pretreatmen The preliminary heat treatment of peat at low temperatures is defined as the bertinizaticn process. Such treatment eliminates much of oxygen content of peat as water. carbon dioxide and carbon monoxide before proceeding witht he secondary p_yrolysis of the solid residue (the bertinzte) at higher temperatures. The polymeric nature of the peat substrate is thus changed during the treatment. The bertinization process is generally conducted under fast pyroIysis conditions since slow heating makes the bertinization less complete: less oxygen forms pyrolytic water and more goes into tars [79]. Contact times in the range 0.06-O 6 set have been used [79-841. In these experimental studies

281

peat particles were suspended in a gaseous heat transfer media such as superheated steam [80,84] and flue gases with oxygen concentrations of 0.25 to 6% 181-831. Peat berti&ation has been studied in the range 200-450°C, the optimum range being 250-4OOOC[80-83,85,86]. Above 400°C peat pyrolysis becomes more intense, the calorific value of the evolved gases increases substantially, and more tar is produced [79,80&I]. More valuable products are expected from the subsequent pyrolytic treatment of peat bertinate than from peat which has not been beri&ed. The non-condensable pyrolytic gases are characterized by a higher calorific value [86,87J Good quality coke can be obtained at higher yields when peat is subjected to preliminary heat treatment [85,88,89]. Such treatment shotrId be carried out under conditions which exclude the steam-gas mixture from the zone of subsequenthigh-temperaturepyrolysis. Maslov and Smol’yaninov [90] also reported that higher yields of bitumens can be extracted from peat when the latter is preheated to 260-28OOC. 4.3. Conract time The extent of the interaction among solid residues, steam, tarry vapors, and other gaseous products is largely determined by the contact time spent by such products in the pyrolysis reactor. Gas-solid and liquid-sohd interactions as well as secondary reactions in the gaseous phase are important in pyrolysis since they may lead to recondensation or repolymerization of the broken organic macromolecules, secondary thermal cracking, catalytic cracking and oxidative or reductive reactionsTwo significant factors in such secondary reactions are the heating rate of the original material and pressure of gases and vapors in the system- Heating rate is important mainly at the solid-phase level where recondensation of the primary products may occur under slow-heating rate conditions. Total pressure of the system, and especially partial pressures of the released organic vapors which can be diluted by a carrier gas, are of great importance in pyroIysis since the number of gas-phase collisions between the molecules increases proportionally to the square of the pressure. Although heating rate and gas pressure are not wholly independent, it is convenient first to consider them separately. Workers in the field of fast pyrolysis of biomas currently agree that a contact time of the order of 0.1 s distinguishes between so-called fast and sIow pyrolysis 1911.Such short contact time of the reactants and products in the reactor is possible when very high heating rates of the solids typically of the order of 100°C s-’ or higher, are used in the processes. Flash pyrolysis invoIves even higher heath rates. Most work that has been devoted to the study of fast-flash pyrolysis of peats has been reported by Russian investigators beginning in the early

sixties. When compared to slow. conventionaI pyrolysis of peats. fast-heating and short-contact time conditions bring about drastic changes in both quahty and quantity of the products obtained. Gravil’chik et aI. [92] evahxated the effect of contact time under fast heating conditions for the thermal decomposition of peat bertinates. During 0.05 s of pyrolysis at 800°C. gas. water. tar. and low-boiling hydrocarbons tzvolved at 44.6. 8.2. 7.5. and 2.2% yields, respectiveIy (based on the initial organic mass): whereas when the pyrolysis required 240 s. the yield of gas and low-boiling hydrocarbons increased to 54.7 and 3.2%. respectively. and the one of tar and water decreased to 3 and 4%. respectively. These results are apparently contrary to those reported by Korchunov [93] who indicated the possibility of increasing the yield of peat coke under thermal shock conditions. by developing maximum interaction among the pyrolytic products of peat bertinate. By increasing the duration of interaction of the vapor-gas mixture and the solid residues from the destruction of peat bertinate. Korchunov obtained higher yields of coke. An opposite effect is observed when treating raw peat feedstock under similar conditions [60.85]. Optimum conditions of temperature and contact time leading to the maximum yields of peat tar have been reported by Stipnik and Levin [30] for the fast pyroIysis of two kinds of peat in the presence of a steam-gas mixture. ,Maximum yields of tar were reached at 450 and 575°C and 63.5 and 97 h-r space velocity of the steam-gas mixture in case of the high-moor and low-moor peat. respectiveIy. Stonans and Dubava 1941attributed the increased yield of volatile matter on rapid heating of peat not only to the temperature effect but also to the lack of stabilization of the pat structure during heating. Such stabilization usually occurs on slow heating. Some aromatic compounds that might othe~ise condense and carbonize can be swept out of the pyrolysis chamber and recovered as liquid products. if the contact time between solid and vapor at high temperatures is short. Kashirskii et aI. [40] pyroIyzcI peat at 67@-SOO”C under conditions which reduced average contact time to 0.20-0.46 s and obtained a highly aromatic hydrocarbon product. pyrobenzol. consisting essentially of benzene. toluene. ethylbenzene. thiophene. and the three isomers of xylene. The pyrobenzol yield was 2% (dry peat basis). The chemicar composition and properties of the peat pyrolysis residue is influenced by residence time in the reactor. Investigations by Kokurin et al. [60] on fast pyroIysis of peat showed that the contents of H. 0, N. and S in the solid residue decreased when increasing temperature from 500°C to 900°C and time from 0.1 to 180 s. whereas the C content increased. Density of the dry mass of solid residue increased with increasing time and decreased with increasing temperature. The change in total pore volume of the solid residue did not follow a regular pattern with respect to time and temperature. The evolution and quality of various pyrolytic gases with respect to time and temperature was also studied by these authors and by Reprintseva

283

[95,96]. Korchunov [97] has described the increase in the multiple ring aromatic hydrocarbon fraction in the solid pyrolysis products of peat as a function of temperature and time under shock heating conditions. Infrared absorption spectra have been reported by Smutkina et al. [98] for peat and for the solid residues of the rapid peat pyrolysis at 320-750°C at various heating times (0.04-0.4 s)_ During rapid heating. carboxyl, hydroxyl, and simple ether groups appeared to be relatively stable. IR spectroscopic analyses of tars obtained in the fast pyrolysis of peat have also been reported

WI-

Piottukh [99], Shapatina [lOO], and later Kunin et al. [loll reported the operation of pilot plant units based on the principles of fast pyrolysis of peat using sand as a heating medium. In the last case the contact time observed between the vapor-gas mixture and the sand was reported to be 1.5 s. 4.4. Heating rate In fast pyrolytic reactions, it may be assumed that the original material is brought up to the operation temperature almost instantaneously. In slow pyrolysis. the heating rate of the feedstock can be used to calculate the nominal residence time of the solids. In fast pyrolysis of peat, contact time of the residual solids and the vapor-gas mixtures is of importance in determining the extent of gas-solid reaction. At slower heating rates, contact time still remains an important parameter. but the course of the reaction is strongly influenced by even a slight variation of the heating rate_ Relatively low heating rates reduce the rate of generation of volatile matter from the solids, particularly in the temperature range 200-400°C [102], and this in turn simultaneously influences the contact time of the vapor-gas mixtures and the residual solids in the reactive zone- One practical Uustration of the relevance of the heating rate in pyrolytic processes will be found for instance in the manufacture of peat coke briquettes used in the metallurgical industry. Such processes require that the heating rate of the original material be 1.2OCmin-‘, or even lower, in order to prevent breaking of the carbonized structures caused by the escape of the volatile matters from the solids. Sundgren and Ekman [102] reported that when carbonization of some types of peats was carried out at 3OC min-‘, the peat briquettes did not remain intact, and breakages were common. Comparative experimental studies have shown that the chemical reactions which occur under fast-heating conditions differ substantiahy from those conducted under slow heating, withr espect to both the yields and the composition of the pyrolytic products. As compared to conventional slow-heating processes, fast pyrolysis of peats may bring a rise of up to 20% in the amount of Volatile Matter (V-M,) produced [78]. Such an increase in V.M. would be attributable to the amount

of tar only, since the yields of pyrolytic water and of gases are lower at the faster heating conditions [67.78]. A corresponding decrease in the amounts of residual solids should be observed under fast heating conditions. Chukhanov et al. [78] also determined that in the fast pyrolysis of peat the yields of saturated hydrocarbon gases, carbon dioxide, and hydrogen decreased compared to slow pyrolysis. while those of unsaturated hydrocarbons and carbcn monoxide increased. The pyrolytic product composition is also affected by a change in the heating regime. Malashenko et al. [103] found that the unstabIe high-molecular weight tar containing N. 0 and S was obsen.ed in higher amounts when peat was heated rapidIy. Stonans [67] reported that 50% of the peat tar obtained in the fast pyrolysis was insoIubIe in benzene while this percentage was only 4% during the slow-heating process. Yields of phenols are approximately the same although differences were noted in the distillate cuts [104]. Korchunov et al. [105] reported that the rapid heating of peat decreased the carbon content in the solid pyrolysis products by a factor of 1.5 as compared to slow heating to the same temperature. 4.3. Pressure

The totai pressure in the system as we11 as the partial pressures of the released organic vapors in the reactive zone affect thermal decomposition reactions. However. it is difficult to distinguish between contact time effects and those attributabIe to the frequency of gas-phase coIIisions. Since the pioneer work of Reilly and Pyne in 1926 [106] on the thermal decomposition of peat under reduced pressure. there have been few comprehensive studies of the roIe of pressure in peat pyrolysis. These authors reported that vacuum pyrolysis of peat yielded more high-boiIing paraffins and Iess low-boiiing and gaseous products (e.g.. ammoni& acetic acid, and methanol) than are obtained by pyrolysis at atmospheric pressure. Christiansson [9] conducted laboratory and pilot plant studies on the effect of reduced pressure (5 10 mmHg) on the yields and composition of the major products from low-temperature peat pyrolysis. His feedstock was a moderately decomposed low-moor peat containing 55.4% C. 6.6% H. 3.4% X. 0.35 S. and 34.352 0. The calorific value of the peat used was 5560 kcal kg-’ of organic matter. The yields of pyrolysis products are given in Table 10. The pilot unit was an indirectly heated vertical retort with forced internal circulation of the carbonization gas. The batch apparatus with a capacity of 16.2 kg of air-dry peat balls can be considered as a small-scale model of an industrial unit. In the laboratory experiments the heating rate was 16OC min-i while in the larger-scaIe unit it was only 2.3OC min-r. The final temperature reached was 500°C. Laboratoq-scale carbonization under reduced pressure afforded a high yield of tar. while the yields of other products were Iower than those

285

TABLE 10

Yidds (% weight,m.a.f.) of main produns upon carbo.nization of peat under different conditions [9] Carbonization method Laboratory scale, reduced pressure Laboratory scale. atmospheric pressure Pilot. atmospheric pressure

Semi-coke

Tar

Water

Gas

(%)

(4)

(4)

(4)

34.1 36.0 38.8

29.9 22.6 17.9

19.1 23.8 24.6

16.9 17.6 18.7

obtained by carbonization of well decomposed peat under atmospheric pressure. On carbonization under atmospheric pressure the yield of tar decreased considerably and the yields of other products, especially the yield of pyro!ytic water, increased. In the pilot unit, tar yields were lower, while the yields of the other products. especially that of semi-coke, increased. In industrial carbonization, one may expect even Iower yields of tar along with greater yields of semi-coke and slightly greater yields of water and gas. The figures calculated for the elementary composition and the calorific value of the tar and the semi-coke obtained in the different carbonization experiments are presented in Tables 11 and 12. The high oxygen content of the pyrolysis condensate was attributable hugely to water-soluble, oxygen-rich substances. The calorific values were correspondingly lower. There is, however, a marked increase in the carbon and hydrogen content and of the calorific value of condensate when peat is carbonized under more severe conditions. Data are reported by Christiansson [9] on the variation of the yields of peat tar fractions (bases, fatty acids, phenols, paraffm, asphah and oil soluble in ether; water-soluble tar; insohrble tar) as a function of carbonization pressure conditions. The Iow carbon content in the semi-coke at reduced pressure characterizes a carbonization process carried out under mild conditions. The nitrogen content of the semi-coke accounts for 4549% of the nitrogen originally present in the peat. The nitrogen in the semi-coke presumably originated, at TABLE

11

Elementary composition and calorific value of the tar upon carbonization of peat under different conditions [9] Carbonizationmethod Laboratory scale. reduced p.ressure Laborator)- scale. atmosphaic pressure pilot, atmospheric pressure

C

H

N

0

calorific value

(%)

(%)

(%)

(%)

(kdkg-‘)

CaIorific value of original Peat in tar (S)

67.0

9.1

4.1

19.8

7650

41

72.0

9.4

3.8

14.8

8300

34

76.8

10.0

2.4

10.8

9000

30

2S6 TABLE 12 Elementcomposition and calorific value of the semi-coke upon carbonization under different conditions [9] Carbonization method

Laboratory SUIC. rcchIced pressure Laboratory SC&.

ztmosphcricpressure Pilot. atmospheric P-m

of peat

:)

N (a)

0 (W

Calorifac value (kcal kg-‘)

calolific vaIue of original peat in semi-coke (S)

81.6

3.5

4.5

10.4

7570

46

83.6

3.8

45

8.1

7870

51

S3S

3.6

4.3

8.3

7840

55

C (5)

least in part. from proteinaceous materials in the peat. The carbon content was substantiahy higher under atmospheric pressure. probably because of decomposition of certain parts of the primary tar. particularly low-mokcular weight organic compounds. The calorific value of the semi-coke produced at reduced pressure was caIcuIated to be 7570 kcaI kg- ‘_ Only 46% of the calorific vatue of the peat remained in the semi-coke. Semi-coke produced under atmospheric pressure had a caIorific value of 7840-7870 kcaI kg-‘. corresponding to about 51% (Iaboratoq testing) and 55% (pilot) of -he calorific va.Iue of the original peat. The volume composition and the calorific value of the gas from the different carbonization experiments arc shown in Table 13. In a11carbonization experiments. carbon dioxide was the main component and made up to 70-808 by weight of the gas. Thus. upon Iaboratory-scale carbonization at both reduced and atmospheric pressure, 5% of the calorific value of tht peat is contained in the gas. while upon carbonization in the pilot unit the corresponding vahte increased to about 6%. A further increase is likely to occur upon industrial carbonization.

TABLE 13 Composition (~019) and ca!orific x~luc of the gas upon carbonization of peat under different conditions [9] Carbonizationmethod Laboratory scaie. reduced pressure Laboncory scale. atmospheric pressure PiIot atmospheric pressure

Hz

CH,

CO

CO,

C,H,

N,

calorific G&le (kcal Nm”)

IS.3

6.8

162

515

3.2

4.0

2000

10s

14.5

11-l

57.7

26

4.0

2210

16.0

124

128

49-6

3.6

5.6

2370

287

4-6. Particle size Few studies have been conducted on the effect of particle size on the pyrolysis of peat. Skele and Vajars [84] reported that the yield of volatiles produced during the bertinization of peat increased with the decrease of the particle size from 148 pm to 74 pm. Gavril’chik et al. [107] pyrolyzed peat bertinate particles with an average size between 0.05 and 2.0 mm. The particles were rapidly heated to 800°C and maintained at this temperature for 26 s. The yields of tar, pyrolytic water and gas changed from 2.7,4.0. and 54.8 to 7.8. 7.8, and 43.3% , respectively, with increasing size of the parUesThe yield of hydrogen and carbon monoxide decreased by factors of 2.0 and 1.5, respectiveIy. The IR spectra of the tars showed increasing aromatization with increasing particle size. The yield of coke increased by 4% with an increase in average particle size from 0.05 mm to 2.0 mm, although a minimum was observed for a par-We size of appro_ximately O-3 mm. According to Korchunov [SSJ a sharp increase of coke yield is observed at the transition from the 0.1-0.2 mm size class to the O-2-0.4 mm size cIass while the further increase in coke yield is small for the particle size range O-4-1.3 mm. The significant increase of coke yield for the 0.2-mm peat particles was explained by the interaction of the organic vapor-gas mixtures and the solid surface of the wide-pore system inside the fuel particles. 4.7. Seccmdaq- cracking Low-temperature carbonization (< 6OOOC) yields up to 30% of tar depending upon the method of carbonization. This tar is rich in phenol and tar acids. As the temperature of decomposition is increased, the yields of tar and phenols decrease. High-temperature pyrolysis (> 600-7OOOC) exposes the primary volatile products to more intensive cracking conditions so that phenols form polynuclear aromatics and ahphatics are aromatized and the light oil becomes more aromatic. Laboratory investigations into the thermal cracking of peat-derived materials. as distinct from the conventional carbonization of peat. have provided information on three types of products: (a) volatiles released from peat and cracked before condensation; (b) carbonization by-products; and (c) gasification by-products. Table 14 summarizes the yields of gas and tar that have been obtained by carbonization (or gasification) followed by thermal cracking of peat-derived materials_ Where available, the product yields are quoted in % wt. of original peat; otherwise results are presented in % wt. of feedstock. However, it must be stressed that feedstocks such as neutral oil, phenols. etc. represent only a small percentage of the parent peat. A major objective in the vapor-phase thermal cracking of peat-derived material is the production of a high-calorific value gas with a high content of

01

0 C

-

-.

. , ..

Gasification byproducts

Carbonization by-products

the low=

Neutral oil from

0vcrall: 35 * Cnlorific value: 7980 kcal/n?

Condwuxl peat 730 tar

_.L..._

.

.

.

on p, 290)

112

Overall: 47 * Calorific value: 9600 k&/n? Unrat. HC: 418, v/v Sat. HC: 14.6-28.68, v/v

Paraffin fraction 730 (180-36OOC) of peat tar from w Prod=

(Continueb

122

112

111

108

I15

110

&wall: 37-57 l Unsat. MC: hi& %

.~._... ...._ . ... .

Gwolinc (b,p. s 220°C): 25 * Gasoline (b.p. s 22OOC): 25-46,8 *

Substancw insoluble in hot bcnzcne: 42.S 56,7% of dry tar fwclion

Yiuld: 5.6-12.7

High-rnolccular 685 weightparaffin and wax fraction of peat tar from glu.pKxhlccrb

Unsut, HC: 418, v/v Sat, IIC: 14.6-28,6%, v/v

Overall: 35-45 *, 600 d/ton * Calorific value: 3700-l 1000 kcul/m’ Unsat, HC: 26,0-33.096,v/v Sat, HC: 26.5-33.5%,v/v

nS/ton * Cnlorificvuluc: 6000800 kcal/n+ Overall: 800-1000

Condcnaedpeat 685-730 tar

tcmpcraturc carbonization of peat

..“.._

.

650-800

Condcnscdpent 425,15,5tar 18 ulm 600-650

Volatila from the lowtcmpcraturc, fast cnrbdzntion of peat

E

290

e.j

v

R

o

,... " ~ .~ ~ -.-.- :=

o

..~

° i

~_. -r., P

V

?.

i e~

11

e. aW~

m

.w

~g m

.:...

°.

291

hydrocarbons, particularly unsaturated hydrocarbons. The latter may be used for the production of lubricants and liquid fuel [108]. Heat transfer is improved by the presence of solid heat carriers (e.g., sand, ceramic packing) or by the introduction of steam into the cracker. It was generally found [109-1121 that steam (either added extemahy or originating from the moisture content in the original peat) substantially decreases the residual tar yield and, conversely, greatly increases the gas yield. According to Rebrov [112], peat tar from gas producers and particuIarIy its paraffm and neutral fractions can be substituted for petroleum raw materials in the production of high-calorific fuel gas. Alternatively, the cracking process would be weII suited for the production of aromatics [108,113,114]and gasoline [llS]. During the pyrolysis of peat, the volatiles released undergo further reactions on the hot surfaces within the bed as they pass through it. These effects may be enhanced when the primary volatiles are recirculated through the bed of solids. In these reactions the peat coke particles serve as catalytic sites for cracking the mcirculating tar, thereby increasing yields of gas with a high content of methane, aromatic and unsaturated hydrocarbons [116,1171.Peat coke was used by Ma&al et al. 11181for the catalytic cracking of petroleum products. It was found that high-moor peat coke packing resulted in a higher degree of aromatization and lower yields of pyrolyzates than a low%roor peat coke packing having smaller pores. Aerding to Kashirskii and Lobacheva [123] recycIing of the gas product stripped off gasoline together with peat feedstock during the flash pyrolysis of peat between 800 and 1000°C resulted in an increase of the aromatized gasoline fraction of the liquid products. The higher the temperature, the higher the contents in benzene and toluene and the lower the proportion of unsaturated hydrocarbons. An elevated temperature was also favorable to higher gas yields, especially methane, carbon monoxide and hydrogen_

5. THE COAMBINED PYROLYSIS OF PEAT WITH OTHER SUBSTANCES

The addition of various substances to pyrolyzing peat can affect the course of the decomposition. Additives may be used to improve the yield or quality of peat pyrolysis products by direct participation in the pyrolysis reactions. They are thus not simply cataIysts (cataIysts are treated in a separate section in this paper). The additives considered in this section are water, petroleum residues, methane, and various nitrogen-containing substances, including ammonia 5. I_Pyro&sisin the presence

of water

SeveraI workers have studied the effect of steam and superheated steam on the thermal decomposition of peat. The chief effect is probably mechani-

292

ral. the steam helping to convey the pyrolysis products from the carbonizing zone quickly- In addition it probably exerts some favorable influence on temperature control- Steam probably also causes hxdroljsis, but this effect has not been clearly demonstrated. Steam may originate from the moisture initialIy present in the feedstock material or it may be added during the pyrolysis process- T&se two modes of addition of water to peat may cause different reaction patterns. In the production of a metallurgical coke from peat sods or briquettes. large quantities of steam originating from the moisture in peat and the gaseous substances generated by decomposition tend to break up the peat lumps. Severa investigators [102124.125] reported that the initial moisture in peat has a great effect on the strength of the final briquette. Amagaeva et al. [llO] reported that less tar seas formed by wet peat (20-35s moisture) than by dry peat during rapid carbonization at 500-55OOC followed by a secondary cracking at 650-800°C. The analytical characteristics of tar obtained from both processed feedstocks were nearly the same. On slow pyrolysis up to 5OOT. Murav’eva [126J found that the yields of gas were 17.8% and 19.7% w/w for dehydrated and natural peat. respectively. The gas obtained from dehydrated peat contained less carbon dioxide and its calorific value w-as 2770 kcal mole” as compared to 2448 kcal mole-’ for gas obtained from a natural peat. Several studies have been conducted on the effect of steam externally added during the thermal decomposition process of peat. Steam is generally used in fluidized bed reactors in order to enhance the heat transfer to the peat particles. However. the role of steam during the pyrolytic reactions of peats varies with temperature. According to AMazinaand co-workers [127.128]. Steam participates in pyrolytic reactions even at low temperatures_ In the range X0-3OOOC. the presence of steam leads to a more complete elimination of oxygen from the solid residue in the form of oxygen-containing gases. The residue is correspondingly richer in carbon. SimuItaneously there is an increase in the yield of tar. including phenols. especially dihydric phenols. and in the amount of hydrogen in the gas [129]. Enhanced yields of tar [127.128]. u-axes [130]. fatty acids. phenols. and ammonia [48] have been observed when steam and superheated steam were used in peat pyrolysis beIow 500OC. Cnder such conditions the yield of semi-coke decreased while the yield of gases increased. The yield and calorific value of the latter increase with the peat-to-steam ratio within the range 1%l-12 [131]. Above 550°C the introduction of steam during the pyrolysis of peat increases the yield and the calorific value of the gas. partly because of steam cracking of the tar to form hydrocarbon gw [132]. Under such conditions tar yields are reduced. in contrast with the situation occurring under milder temperature conditions previously noted. Kashirskii et al. 11331 showed that the volume and relative proportion of hydrogen rapidly increased as a function of the temperature in the range 700-1000°C. The yields of carbon

293

monoxide and carbon dioxide also increased. Evidence is given that above 7OOT carbon from the coke particles is rapidly gasified by steam with formation of hydrogen and carbon monoxide. Such oxidation of peat particles by steam is favoured by the highly developed contact surface area, and also by the fact that, as a result of chemical degradation of the molecules of organic matter and the rupture of chemical bonds, the reformed carbon structure is highly disordered and therefore highIy reactive [133]. X2_ P’ro&r~s in the presence of petroleum residues Particularly promising are processes permitting several valuable products to be obtained simultaneously, such as gas or liquid products and an important solid fuel or coke. One of the processes in which the production of energy-rich gas and strong coke is possible is the pyrolysis of heavy petroleum residues on peat [129,134,135].In such a process, the initially formed peat coke acts as a pyrolysis catalyst for the coking reactions of the petroleum residue. The carbon deposited on the peat coke strengthens the latter up to 15%. The by-product gas from this process may have either a high content of hydrocarbons and a corresponding high calorific value (6000-7000 kcal NmD3) or considerabIe amounts of hydrogen (- 508, v/v), depending on process conditions [129J. Rakovskii et al. [134] reported that the liquid by-products contained 50-808 polycyclic aromatic compounds. Factors affecting coke formation during combined pyrolysis are the temperature, which varied in the range X0-900°C, and the residence time of the vapors in the pyrolysis zone [129,134,136].By changing these parameters it is possible to direct the process towards more unsaturatedhydrocarbons or towards more hydrogen in the gas phase. The yieid and composition of the gas formed also depends on the feedstock and the peat coke characteristics [137]. Similarly, the specific surface area and porosity of the peat cokes formed by such processes are affected by the type of petroleum fraction [138]_ Ma&a and Rakovskii [129] reported that peat tar can completely or partially replace the heavy petroleum products in such processes. 5.3. Pyrolysis in the presence of methane The rapid pyrolysis of pulverized peat in a medium of natural gas has several advantages according to Kashirskii and Atoyan [139]. As is known, the organic matter of all types of peat is characterized by a high oxygen content. The water vapor and carbon dioxide formed in the thermal decomposition of peat can react with the methane or the products of methane cracking. The non-catalytic conversion of methane by steam and carbon dioxide occurs in two stages. The first process is the thermal cracking of methane (methane)

according

Co the equation

CH,-,C+2H, In the second stage of the process the carbon formed is gasified according the equations

to

c+Co~-,2co C-+H~o4CO+H~

considcrabIe part of the hygroscopic and pyrolytic water is used up in the oxidation of methane. and the total yield of unreacted excess water is thereby decreased. Expenditure on reprocessing and purifyzing powdered moss peat in a natural gas medium at 800~850°C Atoyan (1411 produced a highly aromatic gaseous stream. The liquid products included resins. pyrobentene and an aqueous condensate. A

5.4. P_wo&-sisin he presettce of atttttntttiaattd other nirrogett-cottzainingcotnpur4t:ci.T

MaI ;Ind bfaksimenok [143.144] studied the low-temperature decomposition of peat in a stream of anhydrous ammonia gas. They found that the thermoS_vsis with ammonia of high-moor peat. which had a high carbohydrate content. the yield of tar was 30-505 higher than without it, whiIe the content of nitrogen in the tar increased by a factor of 4 to 5, reaching 7-8s. The content of heterocyclic amine nitrogen increased by a factor of 4 to 5

295

and volatile phenols by a factor of about 2, w;;ile the yield of water-soluble fatty acids decreased [143]_ The nitrogen bases were mainly quinohnes and pyridines. In the thermal decomposition of low-moor peat with ammonia, the tar yield did not increase, but the nitrogen in it was raised by a factor of 1.5 to 2 and the yield of heterocyclic amine nitrogen was increased by 30 to 40% [143]. Optimum operating conditions for the production of tar, phenols, heterocyclic bases (e-g, pyridines, quinolines, pyrroles, indoles, carbazoles) and other nitrogen-containing compounds during the carbonization of slightly decomposed high-moor peats in presence of ammonia were reported [42]. Hypothetical mechanisms for the interaction of ammonia and peat have appeared in the literature [42,48,145]. Rakovskii and Volkov [146] reported that a hardened, nitrogen-rich coke can be obtained from the pyrolysis of nitrogen-containing organic compounds over peat coke. In these experiments, pyridine and quinoline were chosen as starting substances because of their high thermal stability. It was shown by X-ray diffraction that at 775OC the coke film shows signs of graphite structure, at 800 and 850°C the process of graphite formation becomes more evident, and at 950°C the lattice is very close to that of graphite.

6. KIrSEllCS

The objective of kinetic studies on thermal decomposition is to develop qu;rntitative expressions which describe the extent of the pyrolytic reactions. Such a description is both of practical and mechanistic importance. AIthough industrial pyrolysis of peat is an old art. the development of kinetic models began only with the initial general pyrolytic approach of Van Krevelcn et al. 11471. Until then, numerous observations on the evolution of different components (gases, tar and residual solids) from the pyrolysis of peat have been systematically made by various authors. Christiansson [9j summarized these observations which did not lead to quantitative kinetic descriptions of the pyrolytic reactions. A discussion of the modern work on pyrolytic models and their applicability to peat has been developed by Roy PIKinetic models developed for peat have been essentially pheno,menological aiming at simple expressions describing either the conversion (weight changes) or the evolution of the different volatile compounds. These phenomenological models do have sound physicochemical bases but fail to provide a detailed mechanistic vision of the sequence of elementary steps involved in the decomposition process. Table 15 presents a summary of the most important contributions to the kinetic analysis of peat pyroIysis. Initial models centered the analysis on the evolution of total volatile matter yielding simple expressions through first

296

TABLE 15 Sunmary of significant contributions Pat

to the kinetic analysisduring thermal decomposition

-Kinetic model

Rt!suIts

Year(s)

Referencts

dv/dr = k (I - Y)” V= frxtion of wIatiIes a: time t k = x-, cspl(- E/RT)

Study focussed on effects of heating rate.

1960

148

dLr/dt=k(vO-Y) L’ = volatile matter at time I 1;. total ~-0latiics released k - k,, cxp( - E/RT)

E = 8-S kuI/moIe k, = 0.9-10’ min-’

1961

149

Tried 1s; and 2nd order expressions bctwcn 350750°C for the rapid p>-re I_vsi!sof peat

1st order fit found satisfactc c at 350450°C. 2nd order fit betwzen 550750°c.

1965. 1966

150.151

Aut~calvtic model / = I-e-hr f = conversion k and n - chaficteristic parameters x- =x-, exp(- E/RT) Studies conducted between loo-&WC in vacua.

Little +ffect found a-hat conducting p,vrolysis in absence of either bitumen H,O solubles. usiIy hydroI>zabIc compounds. humic materials or unhydroI>zable residue. 10.9 c E (13.6 kcal/mole

1968-1973

152-157

FoIlo~s model developed by Juntgcn ad Van Heck (IZS] for non-isothcrmaI pyroI>-sis_ Studies fctcused on individuzd anaIysis of CO1. CO. H,O and Hz.

Determination of entropy v&aes through preexponential factors alrowed mechanistic

1974-1980

159-163

Sequential model as foIIou-s: k: gases pwt % tar< &, coke

It is concluded that totaI tar yield is a function of relative values of k,. k, and ci,. Quantification is not showa

1977

164

Study conducted in the range 190465°C for the slow p~~oIyis of peat.

SpCCUkltiOns-

of

297

TABLE 15 (continued) Kinetic model

RCSlllU

year(s)

Completestatistical anaQsis prcsaltedfor H,. CHa, CO. CO,. C,H,. C,H,. C,H,, C,H,, C,H,

1981

Refefcncts

overall kinetic cxprcs-

sionsfollow

f = I--exp(kr) where f = conersion First order kinetics for individual gas evolution based on model of Weher and Ngan[165] which assumes a Gaussian distribution of energ;cs of activation with fucd pre!cxpoIlmtial factora

17

and V-&o 40.4 ag E g 712 kcal/molc

and second order models, as well as autocatalytic expressions. These initial models (essentially two- or three-parameter models) provided energies -of activation well in the range in which diffusional limitations occur Thus, values between 8 and 14 kcal mole- ’ have been reported by Agroskin and Miringof [149] and Bel’kevich and co-workers [152-1541. More recent work by Kravtsova and Smol’yaninov [164] suggested that a combination of reactions in series through the formation of stable intermediates (i.e., tar) could also describe the pyrolytic process. A novel approach put forward by Roy [17j suggested that the activation energies were not fued and distinct parameters for each of the individual gases but should rather reflect the heterogeneity of the peat material. Thus, Roy proposed a distributed activation energy model approximated by Gaussian functions. Such an approach extended to peat the formalism developed by Weimer and Ngan [165] based originalIy on the work of Hanbaba et al. [166]_ From the kinetic literature two distinct options seem clear: (1) The total approach in which overall conversion is expressed as a function of time; this option quantifies total reactivity, but does not provide any insight into the evolution of the individual products. (2) The selective approach which focusses on the kinetics of individual products, thus providing insight into the selectivity of the overall pyrolytic process. This review of previous work suggests that there is still place for a combined approach (reactivity-selectivity) which at the same time would try to provide a quantitative description of tar and water formation as functions of time. Such a kinetic approach will eventually have to incorporate the decomposition kinetics of the different poIymeric structures present in peat. Basic data is thus needed on the decomposition of the individual polymers or groups of polymers.

7. ‘IHJZRMOGRAVIMETRIC AXD DIFFEFEWIAL

THERMAL

ANALYSES

OF PEATS

The thermogravimetric (TG) and the differential thermogravimetric (DTG) analyses of peats have been used as tools to investigate chemical reactions of the major constituents of peat under pyrolytic conditions. Paulik and Weltner [167] conducted TG and DTG investigations of the pyrolysis of Hungarian peats up to a temperature of 1000°C in a stream of nitrogen (15 1 hr-I). at 3°C min-*. starting with 1.0-g samples. The curves indicate three main phases of thermal decomposition. The first phase. occurring up to 2oOOC. is characterized by moisture removal. Tbe loss of free moisture occurs slowly even at room temperature: adsorbed water is lost at about llO°C. and colloidally bound water at llO-200°C. Xext is the main phase. 200-6OOOC. in u-hich organic compounds of different chemical structures decompose in specific tzmperaturc ranges. The last phase in the temperature interval 600-1000°C is dominated by coking reactions. The main pyrolytic phase in the temperature range 200-600°C is characterized by three major DTG peaks which appear in the temperature intervals 210-33OOC. 300-460°C and 450-6SOOC. The first of these three regions represents the extensive thermal decomposition of cellulose and hemicellulose fractions and of the sugar-like by-products resulting from hydrolytic reactions. There is only a moderate decomposition. which starts at about lSO°C. of the humic acids. the lignin and the bitumen constituents in this temperature internal. These constituents. however. decompose at their ma.ximal speed in the second temperature interval and the lignin and the humic acids in general peak at 350°C and 350-4UO°C. respectively. The last peat DTG interval (450-6SOOC) is characterized by the presence of condensed ring structures as well as low 0. high C and high H levels. due to a series of condensation and polymerization reactions. It was postulated by the authors that many chemical reactions (e.g.. h_vdrolysis. decarboxylation. isomerization. free radicals chain reactions. condensation. etc.) occur in sequence or in parallel during the whole interval of thermal decomposition. Similar studies along this line were also reported by Russian investigators [157]. Lozbin and SmoI’yaninov [168] found a correlation between defined temperature ranges and the thermal decomposition of distinct peat constituents. Such an approach seems valid to characterize different types of peat [159.169]. Differential thermal anaIysis (DTA) is a method of determining the temperature at which thermal reactions occur in a material undergoing continuous heating. It also involves a determination of the exothermicity or cndothermicity of such reactions. Reprintseva [1703 reported that the maximum exothermic effect obsen-ed in the DTA of peats between 100°C and 700°C was 140-148 kcal kg-’ of dry peat. In general it is found that the decomposition of peats. which is endothermic at first up to about 270°C, becomes exothermic at about 260-500°C. with the strongest effects depend-

299

ing on the kinds of peat investigated. In some cases, however, endothermic effects appear at about 400°C [S]. Strong exothermic effects were noted by FaIyushin and Marina [171] in reed peat at 850-875OC as compared to medium or sphagnum peats. The heat evolution was accompanied by a large yield of gas, especially carbon dioxide and hydrogen. The role of some peat components (e-g., humic acids, fulvic acids, easily hydrolyzable compounds, etc.) has been investigated by Filimonov and Rakovskii [172]. The intensity and the peak temperatures of the thermal reactions appear to be highly dependent on the type of peat under study [9,171,172].

8. CATALYTIC

MCI-S

CataIysts may be included in peat pyrolysis to improve the product-mix or the product characteristics (e.g., the porosity or strength of semi-coke). Since peat contains substantial amounts of mineral matter certain authors have suggested in-situ catalytic effects due to some of the minerals in peat. Christiansson [9] indicated this possibility although the low temperatures used in carbonization are not favorable for the activity of such minerals. The works of Falyushin et al. [173] and Pankratov et al. [174] indicated that some of the minerals themselves undergo changes during the pyrolytic processes. By and large, catalytic effects are induced by external addition of catalysts in rather massive amounts reaching up to 35% by weight of dry peat. The major groups of catalysts explored in the pyrolysis of peat can be categorized as follows: alkaline salts, oxides of iron, Lewis acids, strong oxidizing acids. and impregnated metal ions. 8.I. Alkaline salts Alkaline salts as additives have been thoroughly studied. KeppeIer and EdIer [175] revievled the Iiterature on the subject- In Trutzer’s Na-caI process (1751the addition of a mixture of sodium carbonate and calcium hydroxide yielded 30% ells out of which 70% were distillable below 325OC.Pascal [176] and Michot-DuPont [177] reported that upon addition of 2% calcium acetate, 0.1% iron oxide and 2-4s sodium carbonate the pyrolysis of peat at 500°C yielded considerable quantities of non-phenolic oils. The increase of tar yields with alkali addition was also documented experimentally by Keppeler and EdIer [175] in tests conducted using Fischer retorts. The claims of Michot-DuPont were not confirmed by tests in other Iaboratories [175,178]. Reactor configurations and mixing may be rcsponsibIe for discrepancies among authors. Brat (cited by Keppeler and Edler [175]), added 20% calcium oxide to peat and autoclaved the mixture at 170°C. He obtained a tar which was rich in low-boiling neutral oils.

300

In a systematic study. Ito [58] investigated the effect of alkaline substances (sodium hydroxide. potassium hydroxide. sodium carbonate. calcium oxide, and calcium carbonate) at concentrations of lo-30% by weight of dry peat. In al1 cases gas yields were higher than with untreated peat. Sodium carbonate and calcium oxide favored the production of semi-coke whereas potassium hydroxide increased the tar yields. The latter result was also confirmed by Drozhalina et al. [179]. fto [58] suggested a complexion of groups with added alkaIi. Keppeler and EdIer [175] Iignin or -OCH, reported optimum yields of tar (Fischer assay) when peat containing sodium hydroxide. calcium carbonate. sodium carbonate. potassium hydroxide or calcium o.xide (5-35 3 by weight of dry p eat) was heated in the rrzsence of superheated steam (250-3OOOC). The results of Ito [58]. Drozhalina et al. [179]. and Keppeler and EdIer [175] are in sharp contrast with the work of Khidasheli [180]. who reported no improvement in tar production with additions of sodium hydroxide and potassium hydro.xidc (up to 4.~G by weight). This author pointed out the importance of uniform heating conditions throughout the retort. Even when sodium chIoride acts as homogenizer of the heat flux. peat is more uniformly heated and higher tar yields are observed. A mechanistic explznation of the role of alkaline materials during thermal decomposition of peat has been proposed by Drozhaiina et a1. [179]. According to these authors alkaline additions are responsible for intra- and intermolecular redistribution of oxygen. leading to dismutation of aldehydes with formation of aIcohoIs and carboxylic acids. Oxygen can be easily removed .JF subsequent decarboxyiation reactions. Alkalis also promote redox activity leading to carbon dioxide and tar formation. probably via condensation reactions of primary decomposition products. The addition of water vapor during thermal decomposition leads to the formation of a Iiquid phase u-hich readiIy dissohes hydroaromatic structures. The latter can be further hydrogenated by intramolecular reactions or by reactions with hydrogen from external sources 11751. 8.2. Iron

o.eies

Iron oxides constitute a second important group of added catalysts to peat. ?he practical intertrt lies in the reduction of iron ores by peat semi-coke produced in the early stages of pyrolysis. Smol’yaninov and co-workers [121.181.182] have elaborated a complex reaction sequence invohing carbon monoxide reduction of iron oxides. along with shift reactions. The authors concluded that the catalytic effect of the iron oxides is probably responsible for an increase in both the oxygen-containing compounds in the tar and the low-molecular u-eight fractions. These observations are consistent with the work of Stramkovskaya and co-workers [183,184]. who showed an increase of the low-molecular-weight phenolic compounds in the tar.

301

Kravtsova and co-workers [23,185] noted? however, that in the semi-coking process of peat-ore materials containing iron, the distillate fraction boiling below 300°C was denser than the corresponding fraction obtained from crude peat. 8.3. Len+s acids The presence of Lewis acids strongly influences the thermal decomposition of peat. In the presence of such acids Drozhalina and co-workers [179,186] and Luk’yanova et al. [187] obtained reduced gas yields, and a finely microporous char. Maxina et al. (1881postulated that oxygen-containing compounds in peat formed thermally unstable organometallic complex with zinc chloride. These complexes decomposed at lower temperatures, yielding larger amounts of water and oxygen-containing acids than was the case in control experiments. Bel’kevich and co-workers [189.190] reported that hydrogen-form peat (peat which has been treated with cold dilute mineral acid to remove exchangeable cations), impregnated with aluminum chloride and iron chloride, respectively, yields more char and less condensable liquid. Infrared studies indicate that trivalent cations inhibit elimination of functional groups in the temperature range 150-3OOOC.thereby promoting condensation reactions. Electron paramagnetic resonance (EPR) signal increases confirm these interpretations. 8.4. Strong oxidizing acids Strong oxidizing acids and, in particular, phosphoric acid are known to develop a fine micropore structure in peat leading to high surface area active carbons. A structural study conducted by Mazina et al. [191] has shown that cotton-grass peat can yield very uniform pores with a mean diameter of lo-12 A. DTA studies reported by Makeeva et al. [192] suggest that the reaction of phosphoric acid with peat organic matter is responsible for evolution of volatiles at 100°C lower temperatures than in the case of untreated peat. Drozhalina et al. [179] postuIated that the addition of phosphoric acid to peat promotes the dehydration of organic compounds leading to the formation of stable oxygen-links among the solid products of low-temperature carbonization. The presence of phosphoric acid increases char formation through polymerization reactions. At higher temperatures, phosphorus compounds catalyze the degradation of quinone groups and promote the cleavage of ether bonds present in the aromatized carbonaceous material.

302

8.5. Impregnmed

metal ioxs

The effects of impregnated metaI salts in minute quantities have been recently studied by Bel’kevich and c-workers [193.194]. Oxidized peats were impregnated with CL?‘. Co’? and I%” and pyrolyzed under vacua. Nickel and cobalt enhanced carbon dioxide and carbon monoxide formation. AI1 mttals hindered dehydration reactions. IR spectra showed that the metal ligand bonds to function31 groups were destroyed at 450°C or below. More recent@;. Roy and Chornet [I951 have documented statisticahy the effect of impregnated nickel. cobalt. iron. and potassium from their nitrate salts. When compared nith untreated peat. these samples showed no significant diffenznce in the solids or tar yields obtained at dosages from IO.7 to 20.4 mg of metal per 8 dry peat. A significant catalytic effect was noticeable for the formation of carbon monoxide. carbon dioxide. and acetylene. The authors stigzested that the presence of silica in the mineral matter might have pm-cnted any significant catalytic action in the tar formation. In the p>.rolysis of alkaline earth salts of o.xidized cotton-grass peat the yields of liquid products and carbon monoxide decreased and the yields of solid residues and carbon dioxide increased in the order magnesium. calcium. strontium. and barium [196.1,97]. Addition of zinc acetate and cobalt acetate was reported 10 increase the pyrolysis residue. probably through complexation of the zinc and cobalt [198.199]. In summary. cataIytic effects have been reported to occur during thermal decomposition of peat. Conflictin_e results may stem from lack of statistical rigor and from influences of reactor configuration. mixing. and experimental procedures. It seems clear however that aIkaIis are active in tar formation through redox mechanisms. whereas Lewis acids and strong oxidizing agents Iikc phosphoric acid favor polymerization leading to enhanced residual solid production.

9. REACTIOS

MECHAXISMS

Investigations of the mechanisms of peat pyrolysis follow two different paths: la) one concerned with volatile matter formation, typical of low-temperature carbonization and (b) another concerned with coke formation resulting from structural changes (repolymerization) of the pyrolysis residue. A discussion on the decomposition of peat has been recently presented by Fuchsman [200]. who indicated that the pyroIytic reactions up to 300°C are easily idealized in terms of conventional low-temperature reaction mechanisms. e.g. loss of carbon dio.xide by decarboxylation of acid groups, dehydration by splitting off water from hydroxylated aliphatic structures and generation of Iow-molecuiar weight aIcohoIs by simple rearrangement of esters. At higher temperature it is not possible to use the low-temperature

303

reactions common to organic chemistry to describe the course of the pyrolyric process of peat. The aromatic structures break down at 300-400°C. Bitumens and other organic residues reportedly decompose in the range of 340-500°C. As early as 1959 Rakovskii [201] and Kaganovich and Rakovskii [48] suggested that thermal dissociation of a c id- ba se complexes (e.g.. pyridinium phenoxides) and reactions of water with unsaturated sulfur- and nitrogencontaining compounds did occur to an appreciable extent during the thermal decomposition of peat. The authors suggested an autogeneous hydrogen atom transfer leading to the formation of aromatic and saturated hydrocarbons from unsaturated cyclic compounds. Studies of physical parameters influencing the pyrolysis of peat were carried out by Russian workers who focussed their attention on transport properties (diffusion) and heat and mass transfer (degree of packing) upon pyrolysis. Filimonov and Rakovskii [202] showed that no diffusional differences existed in the treatment of fine (0.5 ~rn) and coarse (15 cm) peat particles under typical coke-oven conditions. Chukhanov et al. [78] showed, however, that high hearing rates had an effect on the pyrolysis process, presumably favoring fast release of volatiles initially formed, thus inhibiting their polymer~7~tion to non-volatile residues. BeFkevich and co-workers [56,156,203,204] studied the behavior of the individual peat constituents during pyrolysis. Water-soluble components were decomposed at low temperatures, their destruction reaching 37~ at 150°C. whereas at 250°C they were 84~ destroyed. Other carbohydrate-type components were almost entirely destroyed at 250°C, hurnic materials and residual lignin were more stable. Fig. 1 shows the temperature ranges of thermal decomposition for the individual components. Once the low-temperature decomposition of peat is complete a further •~

"~

.'~3

;~3

3~_

3~o

-:.C

-=~C'

~,"

_'2f~~.

~"-C

C~C

7"C

~,.- r * . , q c K . , " ~ : c r e s : l n d

;.-.'~:~V ~.'~.~

......

. *~['2. . . . . . . . . . . .

_~e-~

N

~-.: . c , - ~ ~. . . . . . . . .

.-

-

:._ _..~. ~ .-" - - .

Fig. 1. Range of thermal decomposition of different peat constituents (°C).

!

304

increase in temperature brings about structural rearrangements which produce the polymeric materials that constitute the final semi-coke residue. Naucke [65.205] showed. by electrical resistance measurements and infrared studies, that up to 55O*C only small aggregates of polycondensed aromatic compounds exist At higher temperatures (> 65OOC) thin-plate structures are formed and graphite-like structures appear as early as 725°C. Electron diffraction patterns showed only few diffuse rings, corresponding to some graphite reflections. These observations indicated that numerous single aggregates of hexagonal (non-graphitic) lamellae occurred. These lamellae had scarcely any azimuthal orientation and were known as turbostratic carbons- only by very prolonged heating at very high temperatures (up to 26GOOC)could graphite structures be formed. Rakovskii and Novichkova [206] studied the softening of peat and its semi-coke formation. They indicated that due to its great porosity coke structures cannot be formed without the application of pressure. The authors also suggested that the bitumen present in peat. along with some water-soluble compounds and lignin, acts as a molten phase. The whole peat material under these conditions gains plasticity with further condensation of aromatic rings and cementation of the polymers into semi-coke. Elsewhere. the authors presented schemes for the aromatization processes leading to semicoke formation. It was suggested that arylation reactions are essential to impart strength to the coke [207.208]. Novichkova and Rakovskii [62] indicated that the amount of lignin in the original structures greatly conditions the stabrlity and the resistance to abrasion of the semi-coke.

PART

11sIXDCSTRIAL

10. COMMERCIAL

APPLICATIOSS

PRODUCI-S

OF PEAT PYROLYSIS

The major types of commercial solid products of peat pyrolysis are semi-coke. coke. and activated carbon. Semi-coking is the basic process. It requires heating peat to 550-6OO*C_ Continuing the semi-coking process to higher temperatures yields coke. Controlled oxidation of semi-coke yields activated carbon. Peat semi-coke is produced in several countries, particularly in Eastern and Xorthem Europe. Peat coke is produced only in West Germany (at Elisabethfehn) and in Finland (at Pertieingjoki). Activated charcoal is produced from peat in The Netherlands and in IrelandAlthough the products and by-products of peat pyrolysis are chemically and physically distinguishable from those of the pyrolysis of wood, coal, and oil. these materials of diverse origins are to some extent interchangeable and often compete on a price basis. Products of superior quality can, of course, command higher prices- Since price and availability of alternative raw

305

materials are influenced not only by geographic location, but also by political factors, interest in peat varies from country to country, and from time to time. The design of peat coking operations reflects experience with the carbonization of other substances; but the properties of peat, particularly its moisture content and fibrous character, impose special requirements. In the carbonization of peat, combustible, non-condensable gases are produced along with a water- immiscible condensate (peat tar) and an aqueous condensate (tar water) which cxmtains water-soluble organic substances. In some cases the value of the by-products from tar and tar water have been sufficient to cover the entire cost of the coking process. Such was the experience of the Ziegler process plants at Oldenburg. germany in 1901 [209]. More recently the value of such by-products has been quite small. Naucke [210] estimated that peat tar accounted for only 3-4s of coking plant income_ In modern plants a large part of the non-condensable pyrolysis gas is used to meet the heat requirements of the process. Excess gas may be burned to generate electricity. 10-I. Semi-coke

Peat semi-coke produced at temperatures as low as 250-300°C has been considered in the past as a heating domestic fuel, a fuel for power generation, and as an agricultural fertilizer [211]. Semi-coke is used as a replace-’ ment for charcoal in forging; as a fuel in cupola furnaces and in railroad locomotives; as a carburizin g agent [212]; as a carbon source for gas generators [213]; and as a reagent in calcium carbide production [214J. It can replace wood charcoal in copper and zinc metallurgy [214]. Semi-coke has also been used in the manufacture of anodes for the electrochemical industry. Peat semi-coke can be used to produce coke, which can find application as a metallurgical reducing agent, particularly in ferro-silicon manufacture if its phosphorus and sulfur contents are low [214]. Coke can also be used as a carbon source in the synthesis of carbon disulfide It can be used as a substitute for ueakly coking coals in coking blends [2X]. When the semi-coke product is obtained as dust it can be employed in the production of porous insulating brick [214]. Either semi-coke or coke can be partially oxidized to produce activated carbon, useful as 2n adsorbent and decolorizing agent [194,216]. The semi-coke residue is also suitable as a filler in the production of phenol-formaldehyde resin moldings [154]. 10.2. Coke Peat coke has ben used for a variety of applications in the metallurgical, ceramic and chemical industries, being favored on account of its specific structural features. Fuchsman [2171 recently reviewed the potential industrial

306

applications of peat coke, the major areas of which are indicated below. (1) Peat coke may be used as fuel, particularly in special burning applications. (2) Peat coke generally may me as chemical reducing agent, especiaIly in high-temperature metallurgy. (3) Carbon can be used as an adsorbent and as a cata.Iyst support. (4) Carbon can be used to form anodes in electrochemical c&s. (5) Carbon can be used as a bonding agent in rubber, as a pigment in paints, or as a lubricant in the metal-working industry. (6) Carbon from peat can be used in the manufacture of carbon disulfide. At present the major non-fuel uses of carbonized peat are as peat coke in the metallurgical field, u.here it is used mainly in the manufacture of ferrochrome, ferrosilicon and pure silicon, and as activated carbon in the adsorbent field. Peat coke also finds use in carbon electrodes and as a carburizing agent. Applications in the other areas mentioned are not, at present. economically significant [217,218]. 10.3. Acricated carbon Activated carbon of commercial quality cannot be produced by simple pyrolysis of peat, although active adsorbent surfaces are created by conventional methods of carbonization. To be useful industrially, the active surfaces must be large for a giwn weight of carbon and they must te accessible to the

fIuid from which impurities are to be removed. These requirements for maximum useful surface are achieved in highly porous structures. The pore sizes needed for effective penetration of liquids are greater than those required in gas purification. Particles of semi-coke are generally quite porous. but the pore sizes are too small to allow effective use of the interior surfaces for adsorption. Treatment with steam at temperatures between 900 and 1000°C enlarges the pores by consuming carbon in the water-gas reaction: C + H,O + CO + Hz. At temperatures much below 900°C the reaction rate is uneconomically slou-. Above llOO°C the reaction rate at the outer surface of the carbon

particle becomes unacceptably rapid. and too much carbon is consumed overaI1 for a given degree of pore enlargement [219]. Semi-coke is usually preferred to high-temperature coke as the feed to activated carbon furnaces because it is cheaper and because its surfaces, which lack a distinctiy graphitic structure. are more easily oxidized with steam. Peat coke can also be used to make activated carbon. However, the hardness and density which are valued in peat coke are disadvantages in the feed to a process for making activated carbon. Therefore. coke from indirectly heated retorts is less suitable for this purpose than are the products of directly heated rotary kilns or shaft furnaces [ll]. But the reactivity of coke .aith steam may be adverseIy affected by a variety of factors. Thus. the reactivity of Finnish coke produced in a rotary kiln was less than other types

307

of commercial and experimentaI cokes 12201.The available data suggest that the final cooling of peat coke with a water spray may somehow contribute to this loss of reactivity. A variety of carbon sources (e.g., peat coke, peat semi-coke, wood charcoal, etc) can be used to make activated carbon. The choice of raw material is largely determined by cost and availability of the respective feedstocks. Activated carbon is produced from peat by Norit at Klazienaveen, The Netherlands at approximately 25,000 tons/year, and by Irish Ceca, Allenwood, County Kildare, Ireland at about 1,500 tons/year [219]. In Finland, pilot plant studies in recent years have been directed towards the conversion of fine peat coke from the Per%ein8joki plant to activated carbon. Coke, about l-3 mm in diameter would be treated with steam at about 900°C, then sieved and ground and, if necessary, would be treated with a binder, extruded, and recarbonized before steam activation [221]_ It is estimated that activated carbon can be obtained in 50% yield from peat coke. Rotary kilns have been preferred for steam activation of coke [220] but for small-s&e operations fluidized beds are likely to be more economical. Although commercial Finnish peat coke reacted more slowly with steam than did other cokes, the activated carbon ultimately obtained -was of superior quality, having a surface area of 675-1000 m2 g-r [by the Brunauer-Emmett-Teller (BEI) method] and a micropore volume of O-38-0.43 ml g-i [221]. It is possible to produce activated carbon by treatment of pyrolyzed peat with substancesother than steam, e-g, by impregnation with zinc chloride, phosphoric acid, sulfuric acid or potassium sulfide [219]. At activation temperatures which are generally lower than those used with steam, the impregnated materials react with the carbon surfaces, enlarging the pores. It is usuahy necessary to leach out residues of the chemical additive from the furnace product and dry the carbon powder before the product is suitable for industrial use.

The amount of peat tar produced is of some importance Tar production generally represents a potential loss of coke yield, but sometimes tars may be attractive per se as chemical feedstocks. Naucke [210] reports that actual commercial yields of peat tar, presumably at Elisabethfehn, are about 5%, but that laboratory experimental values from peat similar to that used in the coking process, are closer to 10% He also notes that Christiansson in Sweden had reported a tar yieId of 21.4% from well decomposed high-moor peat_ The large difference may reflect differences in operation. If the heating rate is low and if the particles are large, or if there is a large bed through which the tars must pass, the tars can recombine with or condense onto the

308

-lids. They thus have further opportunity to pyrolyze and char. Where heating rates are rapid. or where tar removal is facilitated by small particle size, reduced pressure or rapid gas flow, the tars are swept out of the pyrolysis chamber before having a chance to undergo further degradation. Fries (cited by Naucke. [210]) showed that faster pyrolysis increases tar yield and decreases semi-coke yield. Although the condensation of peat tars onto pyrolyzing peat particles may increase yield, the condensation of such tars onto the walls of gas ducts may cause serious corrosion problems and sufficient buildup to obstruct the gas flow [II]. The problem can be partially met by maintaining temperatures high enough to reduce condensation in sensitive parts of the equipment. but periodic cleaning and repair of the gas flow system appears to be desirable. Systems proposed for the commercial production of peat coke or semi-coke utilize the combustible gases produced in the semi-coking stage as fuel for the process. The removal of tar and water from the gases is desirable for satisfactory burner operation. Peat iar can be used independently as a commercial fuel. It has a solidification temperature of 15-40°C and a heating value of 80004000 kcal kg-‘. Its value as a chemical intermediate appears to vary from one country to another. In Germany the redistillation of peat tars to generate commercially marketable products was considered uneconomic [210]. In the Soviet Union pesticides and wood preservatives are apparently being produced from peat tar [219]. The amounts of material redistillable from peat tars below 230°C has been variously reported at 3.617% of the total recoverable peat tar. Not all of this material was originally formed at such high temperatures, however. Some of these relatively low-boiling fractions may be breakdown fragments from redistillation. Numerous studies have been conducted in Germany and in the Soviet Union on the composition. fractionation. and utilization of peat tars [16.189.222].The peat tar produced at the Wielandt furnaces in Elisabethfehn (West Germany) reportedly contains 31.2% paraffins. 0.8% free bases. 13.1Gcphenols. 28R neutral oils. and 25% pitch-forming substances. The paraffins reportedly contain 15% esters and 5-105 ketones. Peat tars can be treated with aliphatic hydrocarbon solvents to yield small amounts of peat w-a... In the Soviet Union the tars are more commonly fractionated by distillation. yielding low-boiling cresolic fractions useful in insecticide preparations. higher-boiling fractions used in the production of wood preservatives and emulsifiers for heavy oils. and yielding pitch. DistilIation of peat tar is conducted up to 32SOC.at which temperature thermal cracking begins. Water-condexable by-products appear to offer little incentive for recovery. Naucke as reported by Fuchsman [37] indicated yields of bases (calculated as ammonia) of O-1-2.2%, fatty acids (calculated as acetic acid) of O-5-2.58. and phenols of 0.9-2.2%. In the experimental plant at Boksitogorsk in the Soviet Union an attempt was made to convert the acetic acid from a wet carbonization process into calcium acetate, whxch can be

309

thermally decomposed to yield acetone and recyclable calcium carbonate [219]. The process appears to have been abandoned. A large number of industrial applications have been reported in the Iiterature for the tar formed when converting peat in either semi-coke or coke. The two principal methods for producing chemical products from peat tar arc distillation and extraction. These techniques have been reviewed by Fuchsman [223]. It has been suggested that the tar recovered from such processes can serve as a source of chemicals such as phenols, fatty acids, organic bases, hydrocarbons, solid paraffins, and other similar substances[34,223-2261.Pyroligneous Iiquors produced by the chemical processing of peat constitute a valuable source of raw material for the production of acetic acid, methanol, and ammonia although the solution is sometimes too dilute to permit economical recovery of the solutes. On the other hand, it cannot be discharged without prior treatment because the concentration levels are high enough to generate serious water-pollution problems [34,223]. Refining processes also permit the use of peat tar for the production of such various chemicai materials as soIve.nts,Iiquid fuels, asphalts, lubricants, and pitch. Soviet sources list these uses, emphasizing the antiseptic and antimicrobial values of the phenols, and pointing out that the pitch (melting range SO-55OC) can be used as a binding agent for foundry forms, and as a water repellent in road construction [22,223,227]. In G ermany virtuaIIy all the peat tar is converted to heating oil [223]. The tar can be cracked or hydrogenated to yield motor fuel of high octane number and antiknock value and aviation fuel. pitch, and high quality gas [22,223.228-2301. Peat tar has been ahemativcly used for the manufacture of insecticides [223], surface-active agents [231], adhesives and candles [223]. According to Zubko [232], the pyrolysis of peat gives tars which after distillation leave a residual pitch which can be converted to electrode-grade coke. Volgin [233] reported on the injection of low-temperature peat tar into gasifiers containing hot peat semi-coke. The use of tar d ecreased the consumption of peat and was found to be economicahy justified. Under normal conditions, 225% of the tar evoIved as a vapor and 7?.5% was converted into useful industrial gases-

11. GESERAL

REQUIRESf~

FOR PEAT FED TO COKING OPERATIONS

Since removaI of water from peat is an essential precondition to carbonization, the water content of peat fed to the retorts greatly affects yields and costs. The peat as harvested contains typically 80-958 water. Although peat, dried for fuel use in electric-power generating stations in Ireland and

310

Fiid may contain about 50% water, it must be dried further to be used in coking. Acceptable vaIues are 3040% in Finland and the Soviet Union [212,234]. German coke operations use peat containing 20-28% water [ll]. The peat used for coking is mainIy of the high-moor (ombrotrophic) type [235]. Low-moor (minerotrophic) peat generally contains too much inorganic material to meet the low-ash requirements of peat cokes. When low-moor peat is used to produce semi-coke, a somewhat inferior but presumably usable product is obtained 12141. Although higher ash content may be tolerable for some activated carbon products, the low density of recent high-moor peats confers a particular advantage over low-moor peats even in this application. Thu. high-moor peats are generally preferred for aI1 peat pyroIysis operations. Relatively undecomposed light-colored upper peats (German: Weisstorf) are selected for activated carbon processes while more highIy decomposed darker. older peats (German: Schwarztorf) are preferred where dense cokes and semi-cokes are to be produced. Peat is harvested as sods and field-dried before coking. Harvesting peat by miIIing is unsuitable because of the fine particle size of the milled crumbs. Peat coke is preferred as reIativeIy coarse lumps. In the 1930s considerable research was conducted in the Soviet Union on the utilization of milled peat for semi-coke production. Although techniques were developed for briquetting milled peat and for p_yrolyzing the resultant briquettes, the process offered no economic or quality advantage over the use of sod peat [212]. Hand-cut sods are both more expensive and less suitabIe for producing hard dense coke than are machine-molded sods [235). The machines increase the density of the peat. presumably by partially breaking down the fibrous structure of the peat. The sod molding process generahy involves passage of the fresh peat through a spiral mixer-conveyor. Up to a point. repeated passage through the mixer increases the abrasion resistance of the coke derived from the resultant peat sod. However. it is possible to overwork the peat. In a study by Keppeler and Wiese (reported by Naucke, 12101) more than four passes through the spiral results in a sharp reduction in abrasion resistance Ekman (2361 reports a similar phenomenon; the strength of peat coke samples increases with sod molding time up to a maximum of 10 min. More prolonged moIding diminishes coke strength. It thus appears likely that coke hardness requires the preservation of some structural elements originahy present in the peat. Peat sods used for pyrolysis in Germany are characteristically about 40 x 40 X 160 mm in size [210]. They dry slowly under fieId conditions. ?he shrinking sods retain their original shapes, although with some distortion. The reduction in volume is Iargely attributable to water loss. However, because of structural constraints resulting from the presence of organic fibers and other solid particulate matter. cracks and air-filled voids develop even in sods still containing 30-40s moisture. Sods dried to about 50% moisture

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and thus suitable for fuel use usually have apparent densities of 0.7 to 0.9. Using some average values from the paper of Ekman and Pikku-PyhW5 [234], one can estimate the magnitude of sod cracking Sods with an average initial moisture content of 80% were dried to about 35% moisture. In so doing they shrank 62% in voiume. Assuming the peat solids to have an average density of 1.5 g cm e-3[235] the shrinkage should have been 74% if no air spaces or cracks had formed. For the example given, therefore, about 32% of the air-dried sods consisted of air-filled cracks and voids. The molding process removes entrapped air, breaks some fibers and other weak structures, and thereby reduces resistance to uniform shrinkage. Shrinkage for multiply molded sods in the exampIes given by Ekman and Pikku-Pymta was 63-678, while in the case of unmolded sods it was 54-594; for equivalent drying The unmolded sods had more or larger cracks. Schiiring (cited by Naucke [llj) showed that mechanical treatment particularly at moisture levels below 78%, increases the apparent density of peat Mechanical molding ahers the surface of the sods. A skin apparently forms which resists rapid water penetration during heavy rains while still permitting acceptable water evaporation rates in more favorable weather. Sods stored in piles in the open field require some aeration to promote drying. This aeration, however, favors the activity of microorganisms of decay_ This microbial activity in turn generates heat. If there is insufficient surface area to dissipate the heat at moderate drying temperatures, the temperature may rise to the point where self-ignition occurs. Severe fire losses have thus been sustained. Sods should have more or less uniform moisture content before being fed to the carbonization plant. Naucke [ll] recommends stochpiIing sod peat for one or two years to achieve this result. The sods should be individualIy homogeneous in water content, to minimize cracking during subsequent processing. The variation in moisture content among sods should be small to facilitate temperature control in the furnace operations. The drying of sods can be accelerated by placing them in a stream of heated air at temperatures up to 15OOC.In a test reported by Naucke [ll], the average moisture content of the peat declined from initial values of 28-36% to 8-16% in 3.0-3.5 h. The moisture removal was, however, nonuniform, the center of the sod remaining almost unchanged in moisture content. Under such conditions one would expect severe cracking. The outer layers tend to shrink peripherally, but the sods can shrink radialIy only as the interior portions also lose water. Naucke cautions against the use of gas temperatures above 180°C in artificial drying of peat, and against use of excessive rates of temperature increase in the 180-250°C range (drying phase) of coke oven operations. In both cases the risks are those of mechanical disintegration of the sods from the stresses generated by rapid evolution of steam.

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The need to have appropriately sized particIes of semi-coke or coke, and the inevitable production of fine particles by abrasion and cracking of sods and other larger lumps of peat have drawn attention to peat briquetting as a route to mechanically strong carbonaceous products. Small particles of peat obtained by milling or produced in drying and coking are suitable for briquetting. although some attention must be given to the water content [237$ Pat to bc briquetted should contain 10-14s moisture [238]_ This conforms with the older literature (e.g.. Carter [2393) which notes 10-15s as the requirement. According to Naucke 12403 the acceptable range is 12-22% water. MasIov and Smol’yaninov [90] prepared briquettes from peat containing 11.11% water_ Rudolph et al [216] report working with Irish briquettes containing 14.0% water and Finnish briquettes containing 22.8% water. Briquettes are formed by the application of pressure to peat-filled metai molds of appropriate size. Finnish briquettes prepared by Outokumpu Oy technology are 180 X 70 x 35-40 mm [238]. They are formed without additionaI binder under pressures of 130 kN m-’ (= 1300 kg cm-‘). Naucke [240] reports pressures of 700-1200 kg cm-’ and temperatures of 35-50°C. Maslov and Smol’yaninov [90] produced thenno-briquettes at a pressure of 300 kg cm-’ and at 16OOC. Binders (especiaIIy sodium carbonate or potassium carbonate) may be added to briquettes to increase strength of the p_yroIyzed product Peat briquettes can be pyrolyzed to yield semi-coke or coke. WhiIe it is important that the briquettes be we11compacted to avoid mechanical disintegration on heating. they must be porous enough to pe.rmit water and pyrolysis gases to escape during heating. The briquettes described by Hanni [238] have a packed buik density of 1.0 g cmB3 and a specific gravity of 1.2 g crnw3_ Voids comprise about one-sixth of the apparent volume of the briquettes. Sundgren and Ekman [102] studied the effects of peat briquetting techniques on the yield and quality of peat coke produced by subsequent pyrolysis. We11 decomposed Sphagnum peat (H6 on the Von Post scale) briquettcd at 190-210°C gives the strongest briquettes and the greatest yield of coke after pyrolysis at compressive strength of about 180 kp cm-‘_ Such briquettes exhibit maximum gas evolution at about 400°C. The gas evolution rate and the total amount of evolved gas were markedly reduced by reducing the rate of heating during coking Of three rates of heating studied, 1.7, 3.3, and 7_2OC min-r. the lowest heating rate produced the best results. The effect of heating rate is significant only up to 500°C For final coking bctwecn 500 and 1000°C more rapid heating rates have little effect on yield or strength. Yields of coke up to 44.2% were obtained from the thermally briquetted peat.

12 THE PYROLYSIS PROCESS 12.1. Xhe a3yingphase

In reviewing reports of temperature effects on peat pyroIysis one should distinguish between laboratory observations and industxiaIexperience. Many laboratory studies involve smaII amounts of material which are rapidIy heated to some preassigned temperature, where they can be retained for varying periods of time. The method is convenient for quasi-isothermal rate studies. In industrial processes large amounts of material must be brought from a low initial temperature to an elevated temperature under conditions which cannot be isothermal either with respect to time or to the geometry of the particIes. It is therefore important to distinguish between the temperatures of the equipment and the gas on one hand, and the temperatures of the peat particles on the other_ The difference between the two sets of temperatures provides the driving force for the pyrolysis; the temperatures of the peat particles largely determine the nature of the pyrolytic reactions at a given time- The commercial temperatures associated with the drying phase of peat pyroIysis are usuaIIy given at 160-18OOCto about 25OOC[11], but these should probably be regarded as gas temperatures, with solid temperatures variable but considerably lower. According to PauIik and Weltner’s differential thermogravimetric studies on heated peat samples, adsorbed water is lost most rapidly at 90°C, while the rate of loss of coIIoidaIIy bonded water is the greatest at 17OOC. Even at the lowest temperatures of the industrial heating of peat, the pre-drying and drying phases, one encounters not only loss of free and adsorbed water but also chemical changes of a Iess reversible nature. The conventional temperature range usuaIIy assigned to simple drying is 20-120°C [235,238], or 1%llO°C [241]. Water is evaporated and gases are removed from fibers. Nevcrthekss, some physical changes may occur due to the flow of waxes (which melt at about 81°C [219]), resins and other Iow-melting relatively non-volatile organic substances. ‘The simple flow of waxes over surfaces from which water has been evaporated may contribute to the loss of hydrophilic properties, a phenomenon frequently observed in dry peat sampIes. During the temperatures at which the solid is stiII below 110°C coIIoid-chemicaI changes occur in peat, along with a slight decomposition of humic acids observable at temperatures as Iow as 105OC[Xl]. 12.2.SzmczzuaZchanges (110-f6OOC) With the peat soIids at 110-160°C, more significant changes occur. Steam continues to evoIve but some of it comes from c0IIoida.I structures in which water molecuIes formed part of the hydrogen-bonding structure. The change

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in peat coI1oid.sis reflected in the wet carbonization process which normally occurs in this temperature range [214]. Peat heated, even in the presence of water, undergoes a structuraI rearrangement so that water which previously could not be removed mechanically can now be readily separated by filtration. In addition to these structuraI changes, which result in the release of bound water. chemical decomposition occurs generating small amounts of furfuraI, substituted furfurals, methanol, formic acid, and acetic acid. These organic products are derived principahy from simple carbohydrates, especiahy pentoses and pentosans. Carbon dioxide evolution is also observable at this stage. probably from the decarboxylation of compounds such as galacturonic or ghxcuronicacid. According to Maucke [210], heating in the range NO-200°C is characterized by a rapid loss of aliphatic carbon and a corresponding increase of hydrogen-bonded aromatic carbon (i.e., of simple aromatic structures) in the solid residue. The probability that more radicaI pyroIytic changes have also occurred is supported by the work of Maslov and Smol’yaninov [90]. In briquetting studies on Iow-moor peat, they showed that by simply heating peat to 160°C and then rapidly cooling the product. one obtains a marked increase in benzene-ethanol extractabIe materiaIs. particularly of asphahenes. If the sample is retained at 160°C for a longer period. as it would be in thermal briquetting. the quantrty of benzene-ethanol extract is again reduced. presumably by polymerization and other condensation reactions. It is thus Iikely that the water Iosses below 16OOC.resuIting from the Ioss of neighboring hydrogen and hydroxyl groups from carbohydrate structures. generate relatively non-polar compounds which are soluble in benzene-ethanol mixtures. Tbun et aI. [241] note that polymerization of waxes and resins starts at llO°C. restthing in hardening and embrittlement of peat particIes. If heating is rapid the initial decomposition products may be volatilized before they have a chance to undergo further condensation reactions. The products of such condensation reactions char at high temperatures. Thus. one can rationahi the common observation that slow heating increases the pyrolysis Field from pear. Since slow heating aIso increased the equipment and energy costs for a given throughput. one must balance increased operating costs against increased yield of solid product and decreased yield of volatile by-products. 12.3. The breakdown of carbohydrates and formation of bertinate (I60-300°C) Weight losses of peat. especially above 16OOC.are common.Iy reported on a dry. ash-free basis, i.e., on the basis of the organic matter originally present in peat. According to Abel 12421these losses amount to 6.4-12.48 in the 160-220°C range, and an additional 5.6-8.3% in the 220~250°C range.

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These losses appear to be directly correlated with pentosan content. Abel’s studics show that water-soluble organic substancesmake their main contribution to the volatilized decomposition products of peat pyrolysis at temperatures below 19OOC However since water-soluble organic substances are present only in low concentrations in peat, the yield of organic volatile products at temperatures below 190°C is small, although fme peat dust and other non-volatile matter may be entrained in the steam evolved. Largescale degradation of peat carbohydrates occurs in the range ZOO-320°C [241]. The water recovered in the pyrolysis products is almost entirely derived from these decomposition processes. The evolution of water and carbon dioxide greatly reduces the oxygen content of the solid product, Since the hydroxyl groups which arc lost in this process are largely responsible for the hydrophilic character of the solids, the latter become noticeably hydrophobic. Abel [242] observed a maximum rate of weight loss at about 230°C in continuously heated peat. This temperature corresponded to the middle of the range of pentosan decomposition. Paulik and Weltner [167] associated the wide temperature range over which carbohydrates decomposed, with the wide range of molecular weights. Simple sugars dccomposcd most rapidly at about 22OOC while cellulose reached a maximum rate of decomposition at 300°C. Such chemical changes together with an apparent increase in solid surface area are probably responsible for the effectiveness of oiI adsorption of peat bertinate [241]. (Bertinate is a low-temperature semicoke obtained at about 50% yield based on the initial organic content of the Peat-1 The volatile decomposition products up to about 3OOOCconsist largely of water and carbon dioxide and only smal1 amounts of tars and carbon monoxide [235]. There is an advantage in separating the off-gas stream from the low-temperature stages of pyrolysis from the gases generated at higher temperatures+ in order to minim& the carbon dioxide content of the high-value hydrocarbon-carbon monoxide-hydrogen mixture evolved above 300°C- Water vapor is easily removed from effluent gas streams by cooling and condensation. 12.4. Decomparition of humic acids and formation of semi-coke (300-600°C) At temperatures above 300°C non-carbohydrate components of peat become the principal contributors to the tars, to the organic components of the tar water, and to the non-condensible gases. According to Naucke [210], using Van Krevelen’s method for interpreting elemental analyses of pyrolysis residues, condensed aromatic structures are formed rapidly in the temperature range 200-500°C In the range 400-55OOC this condensed structure becomes more crystalline and can be detected by X-ray diffraction analysis [210]. Such crystalline patterns mark the early stages of graphitization. Aliphatic structures have virtually disappcarcd from the solid residue at 500°c

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The bitumens make their maximum contribution to weight loss at about 375OC according to Abel [242]. or at 450°C according to PauIik and Weltner [167]. Humic acids decompose in the temperature range 3GG-4GG°C [242] mainly between 350 and 4OOOC[167J. Lignins present as plant residues in the peat contribute to peat weight loss principally at 310°C [242] or 340-35GT [1673. Lignin degradation appears to involve mainly demethylation of me:hoxyl groups. The radical decomposition of aI1 these components continues at a diminishing rate as the temperature rises to 550-600°C, the normal termination point of the semi-coking process. The decomposition in the semi-coking range yields considerable amounts of tar, methane. carbon monoxide, and hydrogen. The larger fragments of the decomposing lignins and humic acids. probably free radical in character. recondense to form thermahy stable benzenoid and other aromatic structures. Some of these aromatic structures are volatilized and appear in the peat tar as phenols and as condensed polycyclic hydrocarbons. According to Stramkovska_va et aI. [243] tar from a peat semi-coking process consists of 82.7% neutral oil, 14.1% phenol. 1.1% acid. and 2.1% base. 12.5. Coke fornation (&IO- 900° C) Above 500°C and particularly above 6OOOC. ahcyclic carbon structures disappear while aromatic structures account for nearly all the carbon present [210]. At 725OC X-ray diffraction data corresponds to distinctly graphitoidal forms [210]. The high-temperature reactions include recrystahization and continuous loss of hydrogen. In experimenta cokes the increase in temperature between 500° and 800°C is accompanied by a large increase in the strength of the coke. These changes are attributed to the deposition of carbon. by pyroIysis of volatile carbon compounds. on the surface of the solid coke [213]. The same changes in mechanical properties are not observed in commercial coke from the Wielandt coke ovens.

13. ISDL’STRIAL

PEAT COKISG

PRACTICE

13.1. Characteristicsof industrialpeat p_yro&sis Interest in the chemistry of peat pyrolysis has been generated primarily by the industrial problems of peat coking practice. Pyrolysis residues and condensates were commercial products before they became serious subjects of laboratory study. Problems of the energetics of heat transfer during coking were solved empirically by oven designers before the sophisticated methods of the physicist and chemical engineer were avaiiable. Indeed the present conceptual framework of the peat pyrolysis literature, particularly in

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the schemes of chemical analysis and in the identification of important variables, bears the firm imprint of early peat scientists who were eminently practical men. This history has led to the view, among peat coke technologists, that idealized closely controlled laboratory experimentation on pyrolysis is not adequate to explain the dependence of yield aqd chemical composition of pyrolysates on industrially controllable variables_ This view underlies the large role of pilot plants and semi-commercial operations in the development of peat technology. Areas of scientific concern include the following. (I) Large masses of pyrolyzing peat in industrial ovens are thermally non-uniform and chemically heterogeneous. By comparison, small laboratory batches usually consisting of fine particles tend to be thermally more uniform, and chemically more homogeneous throughout the heating cycleSince secondary reactions of volatile pyrolysis products with each other and with solid surfaces are likely to be affected by concentration and tempemture gradients and by contact time, the role of those reactions tends to be underestimated in laboratory experiments. (2) In some industrial systems part of the peat mass is used as fuel to furnish energy for the endothermal reactions of pyrolysis. Even in ovens which do not intentionally burn peat it is impractical to exclude air totally from the pyrolysis chambers_ It is therefore desirable to regard the peat coking system as a combination of imperfectly separated regions of combustion and of pyrolysis, and to consider the interactions of products generated by both processes. By reviewing briefly the development of peat coking ovens one can sense how basic problems of pyrolysis chemistry are related to industrial practice. Such a review may encourage the development of improved theoretical models and of laboratory approaches better suited to solving those problems. Peat coking operations can be divided conveniently into ~0 main types: direct heating, in which the peat to be pyrolyzed is heated by contact with hot combustion gases, and indirect heating, in which the peat is separated from the hot combustion gases by gas-impermeable heat-exchange walls. A simple form of direct-heating pyrolysis of peat was common in Germany in the 19th century [209] and in Russia during the early part of the present century [212]. Within a brick-lined chamber, about 150-450 m3 of peat was piled, the surface ignited, and the chamber closed. Controlled air access, provided through holes in the oven wall, sustained limi:ed combustion. Heat from the surface, conducted into the core, produced the desired charring. Pyrolysis gases, mingled with the combustion products, were vented. The unburned liquid fraction of the pyrolysate accumulated at the base or on the walls, and could be collected and used as a commercial tar or auxiliary fuel. Yields of solid product ranged from 25 to 32% of the weight of the air-dried peat, depending on the moisture content and quality of the peat. The operation, from loading to unloading, including initial heat-up and final

31s

Fig. 2. Tmditional pc~~~~king by controlIed burning of the surface of the peat pile- ( - ) Heat flow (3) conductive ad radiant heat loss. (b) conwctive heat loss, (c) heat consumed in pyoiysis. ( 4 ) Material flow: (1) combustion gases. (2) volatile pyrolysatc, (3) air.

cool-down required 14 days for smal1 batches. and up to 55 days for large batches (see Fig 2). In the late-nineteenth century increasing value attached to the waterimmiscible tars and oils forxned by peat pyrolysis prompted modification of oven design. Efficient collection of the pyrolysis condensate was achieved in the cylindrical Hahnemann and conical Wagemann ovens [209]. The peat was iaited. and the top of the oven closed. A drain pipe at the bottom collected the pyrolysis tars as the heat from the top was conducted down towards the bottom (Fig. 3).

Fig. 3. Peat coking (schematic) in the Hahnanann oven. ( - ) Heat fJow: (a) conductive and radiant heat 1o.s~.(b) convective heat loss. (c) heat consumed in pyrolysis. ( - ) Material flow: (1) combustion products and gseous pyrolysaatc (2) liquid condensate from pyroQsis, (3) air.

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These direct heating systems all had the inherent defect of burning part of the valuable peat in order to pyrolyze the rest of it. The Lottmann oven [209] conserved high quality peat by heating indirectly, using cheaper and poorer fuels to supply the required heat (Fig 4). Hot combustion gases were conveyed through pipes imbedded in the mass of peat_ Reduction in batch size and &se spacing of heat exchange pipes reduced the operational cycle to about 5-7 days. The yield of solid product probably ranged from about 40 to 50R, depending on the degree of carbonization achieved. Typically, the yields of tar amounted to 4-5% of the dry weight of the peat. The increase in yield of semi-coke in the indirect process was accomplished in part, by the use of an external fuel. However, some increase in yield must be due to the increased polymerization of low-molecular weight pyrolysis fragments, and their subsequent carbonization. In directly heated ovens, the increased gas flow would have removed some of this initially volatile fraction before it had the chance to condense and char. The indirect heating system posed metallurgical problems which were an immediate consequence of the peculiar chemistry of the system. The iron heat exchange pipes carried hot combustion gases generally containing excess air. The outer surface of the pipes were exposed to the strongly reducing system of carbonizing peat. As a result the metal was subject to severe oxidation on one side, and carbon embrittIement on the other. Mechanical failure of heat exchange pipes therefore constituted a major problem in such systems, at least until the advent of suitable alloys. Between 1894 and 1905 a profound change occurred in peat coke oven design, marking the beginning of recognizably modern engineering conceptions. These changes which aRered both the economics and the chemistry of the process. are associated with the names of Martin Ziegler and Wilhelm

Fig. A Peat coking (schematic) in the Lottmann aveIl. ( - ) Heat flow: (a) conductive and radiant heat less, (b) wnwctive heat loss in escaping pyolysis gases, (c) heat wnsumcd in pyrolysis ( - ) Material flow: (1) combustiongases from furnace, (2) gaseous Pyrolysate, (3) liquid condensate from pyrol@s.

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Wielandt- ZiegIer built plants in Germany and Russia [20,212,244,245]. WieIandt operated only in Germany, but his original plant, with many modifications, is the 0nIy one of its type to survive until our time [219]. The basic innovations introduced by ZiegIer and Wielandt into peat coking were: continuous operation instead of batch operation; counter-current movement of energy and matter; and a recycIe loop whereby pyrolysis gases of high fuel vaIue were piped to a separate furnace and burned to furnish the heat for the process (Fig. 5) The major implications of these changes were: (1) The operation. by ahowing the peat to enter continuously at one point and Ieave at another, repIaced the time-temperature regime by position-temperature control. which allowed closer regulation of the process. i.e.. temperature contro1 could be used to favor desirable reactions and minimize undesirable ones. By raising the temperature in the hottest part of the furnace to 1000°C one could produce peat coke, Lower temperatures were used when semi-coke was to be produced in the same oven. (2) Continuous operation, by eliminating the thermal inertia associated with the heating and cooling of massive brick equipment, required Iess time for completion of peat pyroIysis. The reaction time was shortened to periods measurable in hours instead of days or weeks, permitting high peat coke production rates. IndividuaI ZiegIer ovens with about 300 n? capacity could handle about 80 tons/day of air-dried peat. (3) Localized temperature control limited the region of high-temperature oxidation-reduction stress to only the hottest parts of the cast iron heat exchange walIs. reducing maintenance expense. When, as in the case of the ZiegIer furnaces. heat resistant alloy parts became available, the equipment maintenance problem became even less important [219]. (4) The movement of solids in a direction opposite to the movement of

Fig. 5. Put coking (schematic) for continuous indircctl_v hcatcdovens.utilizingpyo~yis gas fkm: (1) peat. (2) coke. (3) pyrolysis as fuel (Zitglcr and Wi&ndt types)_ ( p+ ) Material voLaiIcs. (4) p~-rol_vsis condensate. (5) non-condcnsible gaseous product of pyrolysis. (6) combustion gases

321

hot gases on the other side of the metal walls promoted high fuel efficiency, since even the heat content of relatively cool gases could be used to warm cold incoming peat. This counter-current system also facilitated a relatively even temperature gradient, simplifying temperature control, The waste gases, still possessing some sensible heat, were often used to preheat air to the combustion chamber, and to dry by-products such as calcium acetate, produced from acetic acid present in the aqueous fraction of the condensate [209]. (5) The utilization of pyrolysis gases as process fuel made the coking process more nearly self-sufficient, without serious loss of the valuable peat solids. But it made the analysis and quantification of the non-condensable fraction of the volatile product an essential element in process design. In the Ziegler ovens it was possible to generate a gas with a combustion value of 2877 cal rnw3,and a composition (56, v/v, dry basis) of: CO, 27.4; 0,2.2; N, 22.5; CO 8.6; CH4 14.8; C,,H, 1.0; and H, 23.6. Since this gas represented about one-half the dry weight of the peat fed to the furnace, it provided a fuel which actually exceeded process needs. With increased electrification of industry, such excess fuel could be used to generate power to operate lights and motors. One chemical consequence of this development was an increase in the importance of gas analysis in the control of operations and the design of peat coking plants. The improvements in the indirect-heating system. exemplified by the Ziegler and Wielandt ovens, were also applicable to the direct-heating process for coking peat. That is, it was possible to have continuous, countercurrent direct-heating systems, in which the pyrolysis gases, burned as fuel, were recycled into the carbonizing peat mass (Fig 6). This system was developed by the J. Pintsch organization in the 1920s and early 1930s for installations in Germany and the Soviet Union [2X2,235,245,246].By feeding the hot combustion gases directly into the peat, heat transfer is improved, diminishing the size of the oven and eliminating the metal corrosion and embrittlement of heat exchange materials. However, direct heating markedly decreases yield, due in part to the gasification reactions of the carbonized peat with steam and with carbon dioxide (C + Hz0 + CO + H,; C + CO, 4 2 CO), and in part by the sweeping action of the gas, which removes moderately volatile materials before they have a chance to condense and char. Some furnaces, conceptually related to the Pintsch ovens. were modified in operation to maximize the production of gases of high fuel value [247,248] mainly by passing the pyrolysis gases through glowing coke. This alteration increased the hydrogen content and decreased the methane content of the gases. More energy-efficient modifications of the principles of the Pintsch design were embodied in ovens built and operated in Germany [249] and in the Soviet Union [212,235]. However, no peat coke furnaces of the Pintsch type now remain in operation.

Fig. 6. Put coking (schematic) for continuous directly heated ovens. utilizing pyrol_usis 8;ts a.s fuel (es Pintsch type). ( 0 ) Material flow: (1) peat. (2) coke. (3) pyolysis volatilcs plus combustion gases. (4) pyrolysis condensate. (5) non-condcnsible fuel gases. (6) excess gases vented.

In the 1970s.a new type of direct-heating system designed by Outokumpu Oy has been put into operation in Finland. It differs from previously designated plants in several important respects_ By using a rotary kiln, a slightly incbned horizontal rotating cylinder, somewhat like a cement kiln, it further improves heat exchange between solids and hot gases. But the most interesting concept which the Finnish unit employs is that of shielding the carbonizing particles with their omn escaping pyrolysis gases. Oxidative attack by hot combustion gases is thus greatly reduced (Fig 7). The process requires close control of temperature and heating rates.

03-

Fig. 7. Pyrolyzing peat particle in a stream of hot combustion gas. showing gas shield proIccting solids from oxidation. ( - ) Main direction of radiant heat flow into pyrolyzing put particle ( - ) nxtterial fkvs: (1) shickling gas zone. non-turbulent. consisting of pyrdysis gasp (2) outer mixing zone of pyrolysis gases with hot combustion gases. (3) main fIoa_of hot combustion gas. (4) pyrolyzing put partick

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13.2. Lessons from industtiaiexperience

From the complex history of industrial pyrolysis, one can identify some physical relationships of cominuing interest to the laboratory experimenter and analyst. (1) Both the solid and gaseous products of peat pyrolysis vary in yield and composition with the mass of the product being pyrolyzed, the rate of temperature change, the duration of the test, the nature of the circumambient atmosphere.,and the rate of gas movement. (2) Variations in spatial inhomogeneity in continuous pyrolysis and of temporal inhomogeneity in batch processes produce corresponding differences in product anaIysis, particularly with respect to condensable organic fractions (peat tars and oils). (3) The iron-carburizing potential of high-temperature pyrolysis gases may significantly affect coke oven design in indirectly heated systems. (4) Rate of increase in temperature and particle geometry affects the rate of gas evolution from pyrolyzing particles, and therefore affects the efficacy of the gas shieId protecting the particle from oxidation by hot combustion g-(5) The quantity and combustion value of non-condensable pyrolysis gases produced per unit mass of peat are important analytical data in the design and a-aluation of modern energy self-sufficient peat-coking systems.

ACK?SOWLEDGEMENTS

The authors thank Ms. Shari Chapman of Bemidji State University, Bemidji, lMinnesota,and DipI.-Chem. Renate Schneider of Hannover, ERG., for assistancein obtaining copies of many difficult-t*Iocate articles. This work was partially supported by the Natural Science and Engineering Research Council (Canada); the Federal Ministry of Environment, Canadian Sen
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143 SS- Mal’ and G-1. Maksimatok, Vestsi Akad. Navuk B. SSR Ser. Khim. Navuk, No. 3 (1966) 76; C A, 66 (1967) 30869v. 144 S.S_Mal’ and G-1. Maksimenok,Percrab. Ispol’z Torfa Saprcpelci,(1971) Uo, C. A., 77 (1972) 142176~ 145 S.S. Mal’ and G-1. Maksimcnok,Vestsi Akad. Navuk B. SSR Ser. Kbim. Navuk. No. 4 (1970) 90; C. A 74 (1971) 23796t 146 V-E Rakovskii and VZ Volkov, Torf. Promst. 39 (1962) 21; C A., 57 (1962) 10119c. 147 D.W. van Krevelcn.C van Hecrdcn and FJ. Huntjas, FucI, 30 (1951) 253. 148 YrrN. Korchunov and RS. Tyul’panov, Inzh. Frz. Zh, Akad. Nauk B. SSR 3 (1960) 102; c. k. 54 (1960) 23272f. 149 AA. Agroskin and N.S. Miringof. Nauchn. Tr_ Vsu Nauchno Issled Iust Ptxlzann. Gazife Ugld. No. 4 (l%l) 3; C. A., 58 (1963) 3836 150 SM. Rcprinti~ Gotie Tvrrd Topl.. Tr. Vscs. Kant. 2nd (1965) 240; C_ A 73 (1970) 79253y. 151 S-M_ Rcprintwxq Tcplo i Masopwn os, 4 (1966) 308; C A.. 65 (1966) 16726b. 152 P-1.Bei’kcsichand KA. Gaiduk. Vestsi Akad. Navuk B. SSR Ser. Khim. Navuk, No. 1 (1968) 94; C. A.. 69 (1968) 29163r. 153 P-1.Bel’kcvich.K-A. Gaiduk. EV. Trubilko and EA. Yurkcvich,Vestsi Akad. Navuk B. SSR Ser. Khim. Navuk, No. 6 (1969) 87; C. A., 73 (1970) 5793c154 P-1.Bcl’kcvichand K-A Gaiduk, Khim. Twrd. Topl, No. 6 (1971) 27; C. A.. 76 (1972) 36049L 155 P-1.Bcl’kcvich.M-1. Minkevich and KA. Gaiduk. Doll. Akad. Nauk B. SSR 16 (1972) 128; C A., 76 (1972) 129764a. 156 P-L Bci’kcvichand K.A. Gaiduk, ThcrmoIysisof Peat and its Components,Proc ht. Peat Congr, 4th, ht. Peat Society,Helsinki,Finland, 4 (1972) 143. 157 P-1. Bel1cvich. K-A Gaiduk, EV. Trubilko and V-E Tushinskaya,Vestsi Akad. Navuk B. SSR Ser. Khim Navuk, No_ 4 (1973) 79; C. A.. 81 (1974) 138410~ 158 H. Juutgu~and K-H, van He& Fuel, 48 (1968) 103. 159 YaA. Bclikhmacr,V.M. Ikrin and S.I. Smol’yaninov, Therm. Anal, Proc. Int Cons 4th, 3 (1974) 265; C. A.. 87 (1977) 120352j. 160 YaA_ Bclikhaer axd LV. shishmina,Solid Fuel Chcm., II (1977) 86. 161 YaA. BeEkhmaer.S.I. Smol’yaninov,V.M. Ikrin and LV. Shishmina Deposited Dot_. VINITI 3994-77(1977) 2; C A.. 90 (1979) 171256a 162 YaA. &likhmaer, V-M. Ikrin and S.1, Smol’yaninov.Minerah Syr’e i Ncft&hhiya, Tom&, (1977) 105: C & 91 (1979) 41761~. 163 Yak Bclikhmacr, SJ. Smol’yaninov and I-V_ Shishmina, Tcor. Prakt Podgot. KoksovanieUglci, No. 9 (1980) 28; C. A, 93 (1980) 2423811~ 164 G-1. Kravtsova and S.I. Smol’yaninov,Izv. Tomsk_Politekh.Inst.,238 (1977) 83; C A, 89 (1978) 182139vw 165 RF. Weimcr and D.Y. Ngan. Am. Chun. Sot. Div. Fuel Chem. Prepr, 24. No. 3 (1979) 129. 166 P_ Haubaba H Juntgenand W. Peters,Brcnnst Ghan, 49 (1968) 368. 167 E Paulik and M. Wehncr, Acta Cbim. Acad. Sci Hung., 16 (1958) 159; C. A, 53 (1959) 5639c. 168 V-1. Lo&ii and S.I. Smol’yaninov,IN- Tom& Politdrh Inst., 198 (1974) 38; C. A., 83 (1975) 118377~. 169 M. Lcwque and H. DineI. Geodtrma. 20 (1978) 201. 170 SM. Reprintseva,Torfe Promst, 45 (l%S) 18; C. A., 69 (1968) 108505g. 17l PL Falyushinand 0.1. mazina,Vestsi Akad. Navuk B. SSR Ser. Khim. Navuk, No. 4 (1970) 94; C. A., 74 (1971) 55362~~. 172 VA_ Fhonov and V-E Rakovskii,Inzh. FE Zh., Akad. Nauk B. SSR, 4 (1961) 18; C. & 55 (l%l) 21545h.

330 173 P-l._ FaIlushin. N.S. Pankratov and 1.1.Lishtvan. Torf. Promst No. 5 (1976) 13: C. A, 86 (1977) 19315d. 17< N.S. ?ankratov. SS. .%I’_ P-L. FaIyushin and S.S. Povarkova_ Solid Fuel Chcm. 11 (1977) 121. 175 G. Keppeler and E E&r. Brennst. Chem.. 21 (1940) 97; C. A 35 (1941) 3061. 176 P. Pascal Tech. Mod, 28 (1936) 833. 177 F_ Iliichot-Dupont. BuII. Sot Encour. Ind. NatI.. 136 (1937) 227; C. A.. 31 (1937) 6442 178 E Ramotowki. Trzan- Chcm, 22 (1938) 471: C A_ 33 (1939) 3563. 179 R-D. Drozhalina. V.E Rakovskii and X.0. Bulgakova. Solid Fuel Chem.. 11 (1977) 14. IS0 AX Khidasheli. Tr. Gruz Politekh. In%. No. 5 (1957) 81: C. A.. 55 (1961) 4928. 181 S.I. Smol’ytninov. K.K. Stramkovskap and G-1. Kravtsova. Perspekt IspoIL Torfa MetaIl.. Mater. Vses. Nauchno-Tekh. Soreshch.. (1969) 113: C. A, 77 (1972) 8055. IS2 S-1. Smol’pninov. V.I. Lo&n and YaA. Bdikhmacr. IN. Tomsk. PoIitekh. Inst.. 233 (19742 53: C. A, 83 (1975) 1006ooc. IF3 K.K. Stramkovska~x and V-D. Ivano~z~ In= Tomsk. Politekh. Inst. 126 (1964) 12: C. A. 64 ( 1966) 486d. I84 K.K. StramkovskaJa. V-D. Ivanova and G-G. Volkov. Izv. Tomsk. Politekh. Inst.. 136

(1965) 51: C_ A_ 65 (1966) 12Oa 185 G-1. Kravtsora. K.K. Stramkovskay. S.I. Smol’janinov. LB. Terekhova and V-P. Bashorina. I;n= TomsL Politekh. Inst.. 238 (1977) 148: C. A, 89 (1978) 200166h. 186 S.D. DrozhaIina. EE Bakovskii. V-A. Asmkhov and S-0. Bulgakova. Solid Fuel Chem, 12 (1978) 70. IS7 ZK_ Luk’yanova. S.D. Drozhalina. V.K. Zhukov. 0.1. Ma&a V.E Rakovskii and AV. Chebotarev. Vestsi Akad Saruk B. SSR Ser_ Khim Sauk. Ko. 6 (1979) 49; C. A.. 92 (1952) 131825v_ IS8 0.1. %azina. N.D. Drozhalina. V.K. Zhukov. Z.K. Luk’ynovz G-P. Makeeva and V-E Rakovskii. Solid Fuel Chum, 11 (1977) 52 189 P.I. BeI’kaich. K.A. Gaiduk. V.E Tushinskaya and EA_ Yurkaich Vestsi Akad- Navuk B. SSR. Ser. Khim_ Savuk. So. 5 (1978) 24; C. A, 90 (1979) 57656L I90 P-1. Bel’kexich. K.A. Gaiduk. Yu.Yu. Savoshs V-P. Strigutskii. ES. Cherepanova. A-1. KiseIcv-rr.MI. Minkerich. EV. Trubilko and V.E Tushinskaya. Vestsi Akad. Savuk B. SSR Ser. Khim. Sawk. So. 4 (1978) 55: C. A.. 89 (1978) 182174c 191 0-L Mazina. G.P. Makeew. S-D_ DrozhJina. V-E B&or-&ii and X.0. Bulgakoxt. Solid Fuel Chcm. 14 (1980) 53. I92 G.P. Makeeva. 0-I. Marina. V-K_ Zhukov and V_E Rako\skii. Solid Fuel Chem, 13 r1979) 81. 103 P-1. BeTkevich. KA Gaiduk. AL KiseIeva. .X1. Minkevich. Yu.Yu. Navosha. V.P.

Strigutskii. V-E Tushinskaya and EA Yurkevich Vestsi Akad. Nawk Khim. Sawtk. So_ 5 (1979) 41: C. A.. 92 (1980) 96564k.

B. SSR Ser.

194 P.I. Bcpkcrich. KA_ Gaiduk OL Mazina and LR Chisto\-a. New Trends in ChemicaI Processing of Puts. Proc. 6th Int Peat Congress_ Duiuth. MX August 17-23. 1980. Intern. Peat Socict_v.HcIsinki. Finland. (1980) 513-519_ 195 C. Roy and E Chomer Can_ J. Chem_ Ens 60 (1982) 393. IS6 P-1. BeI’kcvich. KA. Gaiduk. V_E Tushinskaya and EA. Yurkevich. Vestsi Akac!_ Xaruk B. SSR. Ser- Khim. Savuk. Ir;o_2 (1977) 86: C. h 87 (1977) 104301m. 197 P-1. Bel’kevich. i;A. (1979)

Gaiduk and AL

Kiseleva. Torf. Promst.. No. 2 (1979) 23: C_ A.. 91

76656t

198 P-L BeI’kevich. KA. Gaiduk. V-E Trubilko. EA. Yurkevich and V.P. Strigutskii. Vestsi Akad- Navuk B. SSR Ser_ Khim. Navuk. No. 3 (1979) 21: C_ A, 91 (1979) 125862t. 199 P.I. Bcl‘kevich. M.1. Minkmich. A-E Tomson and V-K_ Zhukov. Vestsi Akad Navuk B. SSR Ser. Khim Eavuk, So_ 6 (1980) 49: C. A, 94 (1981) 19475Od. 200 C-H. Fuchsman. Peat: Industrial Chemistry and Technology. Academic Press. New York. 1980. Ch. 13.

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332 228 229 230 231 232 233 234

235 236 237 238 239

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