The deashed charcoal—oil—water mixture: a liquid fuel for biomass energetical valorization

THE CHEMICAL ENJGd;W;lt(lG ELSEVIER

The Chemnxl Engmeenng Journal 60 ( 1995) 49-54

The deashed charcoal-oil-water mixture: a liquid fuel for biomass energetical valorization Adopo N’kpomm a, Adlpoh Bon1 a, GQard Antomm b, Ohvler Fraqols

b

aUnwers~te’ Nahonalede C8te d’lvowe, Dipartement de Physrque, 22 BP 582, AbrdJan 22, Ivory Coast b Unrversrtt?de Technologre de Compkgne, D~vrsrondu G&e des Transferts et Energitlque, BP 649. 60206 Complegne Cedex, France

Received 10 December 1993, revwed 9 September 1994

Abstract Biomass IS often considered an important altematwe resource for the world’s growing demand for energy resources The problem of producing a high calonfic value and low ash content charcoal slurry IS dlscussed m tins paper The process presented 1s a deashmg-slurry formation process based on selective 011agglomeration to prepare a ternary deashed charcoal-oil-water mixture (DCOWM) Non-lomc ethoxylated surfactants have been tested successfully for DCOWM preparation A DCOWM of 45% solid content and 27% 011content has been prepared with a Merantl tree charcoal deashed to 0 90 wt % ash content The charcoal, containing 29% volatile matter, was ground to 8 pm mean diameter The slurry obtained has 24 671 kJ kg-’ low calonfic value and 0 41 wt % residual ash content Keywords Deashmg-slurry formatIon, Llqmd fuels, Biomass energetxal valonzatlon

1. Introduction Liquid fuels present several advantages over sold fuels - they are easier and cheaper to transport owing to their energy density, - they are easily mamtamed and stocked, which 1s important for mstallatlons of small ( 10-20 kW) and medmm (500 kW5 MW) power plants, - their combustion uses a simple and efficient technology with a low production of pollutants The search for techmques to transform solid fuels into liquid thus seems Justified Several studies have been done using suspensions of finely ground charcoal m either heavy fuel oil or water [ l-51 Charcoal-heavy fuel 011 mixtures have a maximal sohd concentration of 40 wt % [ 1,2,4] This type of fuel has an apparent dynamic vlscoslty higher than that of the heavy fuel 011 and 1s difficult to pump A charcoal-water mixture with 57% solid content has been obtained [5] This fuel, even though easy to pump, 1s not efficient, smce its calorlfic value 1s too low (less than 19 MJ kg-‘) The combustion of charcoal-water mixture fluids has encountered the same difficulties as those of coal-liquids It has been observed that the accepted hmltatlons on dust emlsslons of ash and unburnt materials are rapidly exceeded when mineral coal-liquid mixtures with 30% solid content are burnt [ 6-91 The massive use of these suspensions has to be hm0923-0467/95/$09 50 0 1995 Elsewer Science S A All nghts reserved SSDIO923-0467(95)02986-9

lted In fact, if charcoal-hqmd mixtures are to be used, they should not produce major changes m the technical envlronment of the user However, the conversion of a heavy fuel 011 boiler mto a coal-water boiler needs very expensive eqmpment to reduce the dust m the smoke and thus to reduce the amount of ash sent into the atmosphere Furthermore, the boiler has to be modified to receive and evacuate the remammg ash Thus the problem to be solved before producing charcoal-liquid mixtures 1s the deashmg of charcoal The results obtained m deashmg by selective 011 agglomeration were interesting m this respect [ 10-131 After the agglomeration procedure one obtains agglomerates of deashed charcoal contammg both water and the agglomerant (bridging liquid) The objective now 1s to obtain from these agglomerates a liquid suspension with a mixture of deashed charcoal, agglomerant (domestic 011) and water Like coal-water-o11 mixtures [ 14,151, deashed charcoal-all-water mixtures (DCOWMs) flow easily m pipes, burn directly m the systems for fuel combustion and have more efficient combustion than binary charcoal-fuel mixtures

2. The production of the deashed charcoak%water mixture The DCOWM was prepared according to the following procedure The surfactant (1 wt % maximum) was added to

A N’kpomm et al /The Chemical Engrneenng Journal 60 (1995) 49-54

50

wet agglomerates obtained from the deashmg process (selective 011agglomeration) and mixed with a mechanical stirrer Then, m order to bring the slurry to a uniform temperature, the DCOWM 1s placed m a water bath at the required temperature DCOWMs may be generally represented by C-A-E-T, where C (wt %) represents the charcoal concentration m the total mixture, A (wt %) 1s the agglomerant concentration m the total mixture, E IS the liquid concentration and T IS the amount of surfactant (weight per cent) m the total mixture For example the product 40-10-49 2-O 8 corresponds to a mixture containing 40% of charcoal, 10% of domestic fuel 011, 49 2% of water and 0 8% of surfactant However, to simplify we will adopt the notation C-A, so the previous mixture will be represented by 40-10 A DCOWM is acceptable when its apparent maximal dynamic viscosity 1s 2000 CP (2 Pa s) at a shear rate of 100 s-l This value of the viscosity 1s generally sard to be the upper limit that allowed a pumping transportation of pseudohomogeneous mixtures When the slurry vlscoslty reached 2000 cP, its charcoal concentration was measured and defined as the maximum charcoal concentration In this state, DCOWMs are obtained with high reproduablhty, at least 95% A Contraves concentric cylinder vlscoslmeter (Rheomat 30) was then used to obtain the rheologlcal data The appropriate measuring cups and bobs were equilibrated to the required temperature prior to use The cup was then filled with slurry and the bob was carefully immersed so as to prevent entrapment of air bubbles The filled measurmg system was placed in the bath and then attached to the VISCOSImeter Shear stress measurements were then made This study has been made on charcoals received from &ad-Forst (Nogent-sur-Marne, France) We have crushed them m crushing rolls Then we ground the crushed charcoals m a hammer mill to obtain the desired particle sizes The ultrafine particles (4 pm mean diameter) came from the flying dust collected m a filter durmg the grmdmg process The size distributions were measured with a Coulter counter (model TAII) and are presented m Fig 1 They are characterized by the mean diameter

has oriented our research towards non-lomc surfactants with a linear chain of oxide of ethylene - Chosen among non-lomcs were surfactants containing no alkali metal which would be harmful on combustion It was found that the non-ionic surfactants were superior to the others m high loading ability of coal-liquid mixtures, also, they did not contam sulphur or alkali metal which can corrode a boiler [ 171 - Variations m the ethoxylate adduct distribution also affect surfactants’ properties m such a way that products with relatively narrow dlstrlbutlons possess features which are highly desirable m many household and mdustrlal apphcations [ 18,191 0 narrow range ethoxylates have an inherent efficiency advantage m practical apphcatlons where surfactants generally are used on a weight basis, 0 narrow range ethoxylates are more efficient as wetting agents than broad range surfactants, 0 narrow range ethoxylates lower mterfaclal tension more effectively than their broad range counterparts - Non-lomc surfactants are used m many apphcatlons to improve wettablhty of solids They can be used as dlspersmg agents [ 20,211 - The most commonly used non-lomc surfactants have a polyoxyethylene (POE) chain as the hydrophlle [ 221 - POE chains have been shown to blodegrade [ 20,221 - Adsorbed non-ionic macromolecules are known to stablhze suspensions m both aqueous and non-aqueous media Adsorbed macromolecules provide a bamer which effectively prevents the net attraction of the particles by the van der Waals forces [23] Thus, two families of non-lomc surfactants whose commercial names are Sapogenat T and Arkopal N have been tested These tests have been conducted with a 37-15 DCOWM prepared with the 11 pm mean diameter charcoal sample Sapogenat T and Arkopal N are polyglycol ethers Several surfactants have been tested in each family (the first two numerals indicate the Sapogenat T, number of molecules of oxide of ethylene) TO60, TOSO, TlOO, TllO, T180, Arkopal N, NOSO, N060, NO80

Sapogenat T 3. Results and discussion 3 1 Research on surfactants As m all solid-hqmd tion of ternary mixtures (chemical additive) to obtain minimal vlscoslty

suspensions [ 14,16,17] the formarequires the addition of a surfactant the wet agglomerates m order to and slow sedimentation character-

1st1cs

Previous tests with anionic, catlomc and amphoterlc surfactants have not given satisfactory results The work of N&a [17l,Dlllan [18,19],Chanderetal [20],andParfitt [21]

-(CH2

-CH,

-0)nH

Arkopal N

In comparing the performance of different surfactants m mterfaclal phenomena it 1s usually necessary to dlstmgmsh between the amount of surfactant required to produce a given amount of change m the phenomenon under mvestlgatlon and the maximum change m the phenomenon that the surfactant can produce, regardless of amount used [24] The former parameter, called the efficiency of the surfactant, 1s consldered m this study

A N’kpomtn et al /The Chermcal Engtneenng Journal 60 (1995) 49-54

Table 1 Surfactants

51

nummum concentrations

Mmlmum concentration

TO60

TO80

TlOO

TllO

T180

NO50

NO60

NO80

09

08

1

08

07

1

08

1

555

410

600

611

963

700

650

650

(%) Viscosity at 100 s-1, 2oac

(CP)

*cent Cumulabve

Sonplt A Samfle 0 -SOmpkC Sample D -____ sample E

The results show that Sapogenat TO80 and Arkopal NO60 give the lowest apparent vlscoslty m each family As seen from Table 1, they both have a minimum mass concentration of 0 8% with respect to the mixture total mass (beyond this concentration of surfactant there 1s no significant decrease m the 37-15 slurry’s vlscoslty) Among these effective additives TO80 gives the lowest vucosltles, and thus will be used m the production of concentrated DCOWMs 3 2 Effect of partlcle size m the productton of deashed charcoal-o&water mixtures

Porhcle aze (mlcrons] I 100

10

I(

Fig 1 Charcoal particle size distnbutlons

Table 2 Birch tree charcoal charactensttcs Mean diameter

80 wt % passing through (Fm)

It 1s well known that particle size dlstrlbutlon 1s an lmportant parameter m the preparation of solid-liquid suspensions [ 16,25,26] An optimal particle size dlstrlbutlon will lead to a better packing of charcoal particles However, m this study which examines the effect of particle mean diameter, the charcoal is ground m a dry grmdmg process and used m a given dlstrlbutlon as shown m Fig 1 For all samples (Table 2) there is 21% volatile matter and 5 59% head ash The results shown m Table 3 and represented m Fig 2 show that a more concentrated suspension 1s obtained when the mean diameter of the particles 1s smaller This result can be explained by the fact that charcoal 1s porous, and large

(pm) Sample Sample Sample Sample

Table 3 Vanation m deashed charcoakd-water of particle size Charcoal mean diameter

35 33 30 9

28 24 21 8

A B C D

0

nuxture solid content as a function

DCOWM sohd content (%)

DCOWM domestIc 011 content (%)

DCOWM water content (%)

DCOWM calonfic value

40 47 45 38 30 24

12 14 1 135 114 9 72

48 38 9 415 506 61 68 8

17384 20426 19557 16515 11507 10480

(kJ kg-‘)

(fim) 4 8 11 21 24 28

20’ 0

I

I

x)

20

Pattlcle mean diameter (mvxons) Fig 2 Effect of particle size on the productlon

on DCOWMs

A N’kpomm et al /The Chemical Engrneermg Journal 60 (1995) 49-54

52

Table 4 Deashed charcoal-ad-water amount of volatde matter

mixture sohd content as a function of the

Charcoal volatile mattq (%)

DCOWM sohd content (%)

DCOWM domestic oil content (%)

DCOWM surfactant + ;;tT content 0

DCOWM calonfic value (kJ kg-‘)

177 29 30 39 3

35 40 50 26 5

10 12 15 8

56 5 48 35 65 5

14815 16862 21359 10970

(a)

(Table 3) show that for the size the charcoal concentration decreases considerably The reduced concentration of the slurry with extreme fine particles is due to network formation which 1s enhanced by larger surface forces, thus trapping more liquid and lowering the slurry concentration In fact, the ultrafine particles present a larger specific area and larger mterpartlcle forces [ 27,281 Interpartlcle forces can give rise to structure formation, even in suspensions That particle network formed entrapped the liquid causing the vlscoslty to increase [ 29-321 (b)

(cl Fig 3 (a) 35 grn mean diameter charcoal (b) 14 pm mean diameter

charcoal (c) 4 pm mean diameter charcoal (Magmficatlons (a) 200 X , (b) 1400X,(c) 5000x) the liquid used to make the mixture fluid When the particle size 1s smaller, the pore size 1s smaller, liquid absorption 1s less significant, and there 1s a higher concentration of charcoal m the mixture This explanation 1s confirmed by the followmg electron microscope photographs of the birch tree charcoal samples - Sample 1 (35 pm mean diameter, Fig 3(a) ) which 1s the coarsest, conserves the porous structure of charcoal - Sample 2 ( 14 pm mean diameter, Fig 3 (b) ) , much finer, has hardly any porous particles, but the particles are elongated - Sample 3 (4 pm mean diameter, Fig 3(c) ), the finest, does not contam any porous particles Even m an enlargement, 5000 times the ongmal, no pores were observed and the particles are more compact However, m the case of DCOWMs, charcoal powder with a mean diameter close to 4 pm gave poor results The results

3 3 ESfect of amount of volatde matter m the productron of deashed charcoal-o&water mixtures Samples of M&anti tree charcoals ground to a 15 pm mean diameter (see Fig 1, sample E) with varying volatde matter have been used m these tests The results are given m Table 4 and are represented m Fig 4 It 1s shown that the amount of charcoal m the suspension increases with the amount of volatile matter It passes through a maximum at about 30% and decreases thereafter We can see from Table 5 that the DCOWMs’ maximum solid content (50 wt %) IS obtained with charcoal which contains the higher proportion of hydrogen (4 05%), corre60r

pores absorb

30-

20 10

20

30

Charcoal volatde matter content (%I Fig 4 DCOWMs’ maxzmum sohd content vs volatile matter

, 1

A N'kpomm et al / The Chemwal Engmeenng Journal 60 (1995) 49-54 Table 5 M6ranU charcoal charactensUcs Sample number

Volatde matter (%)

C (%)

H (%)

O (%)

N (%)

1 2 3 4

17 70 28 95 30 00 39 30

85 20 80 90 79 48 76 89

244 324 4 05 3 84

11 79 15 56 16 16 19 17

0 58 0 30 0 31 0 10

53

6) show that the apparent wscoslty of the D C O W M o f 43% (sohd content) increases with the ratio of fuel od to charcoal It is known that the dispersion of sohd particles m a llqmd by a surfactant and the resulting decrease m viscosity of the suspension are due to the decrease m interaction between particles due to the surfactant It can thus be thought that the mcrease m the D C O W M ' s apparent viscosity related to the increase m the fuel oll charcoal rauo is due to the increased interaction between charcoal particles The llqmd c a r e e r is trapped by the flakes, and the effective llqmd volume is reduced S o h d - s o h d mteraction takes the place of s o h d - h q u l d interaction, mcreasmg the shear and thus the mixture's viscosity

Analysis by Clrad-For~t,France Table 6 Deashed charcoal-off-water mixture apparent viscosity as a function of domestic o11to charcoal rauo DCOWM sohd content (%)

Domestic fuel off to charcoal ratio (%)

DCOWM viscosity (cP) at 100 s -~ and 25 °C

4. Optimal production of deashed charcoal-oil-water mixtures

43 43 43

20 25 30

1389 1818 1950

W e have prepared three D C O W M s using the optimal condmons obtained from the preceding parametric study (part~cle's mean diameter, 8 /zm, volatde matter, 29%) Their characteristics are shown m Table 7 The slurries prepared with deashed charcoal (0 9% residual ash content) have 0 41% of ash They are low polluting fuels, their combusuon produces at most 0 18 g dust MJ -~ The D C O W M with a calorific value of 23 834 kJ k g - ~ contains 70% of combusUble matter The contribution of charcoal to the calorific value is 56%, which lS qmte slgmficant

spondlng to the opUmal volatile matter content of charcoal To explain this phenomenon we have to refer to the selective oll agglomeration process In fact the D C O W M s were prepared with agglomerates obtained from the above process with aqueous suspension of fine coal parhcles, added lmmlscthle orgamc hqmds will displace the suspending hqmd from the normally hydrophoblc coal particles to form agglomerates held together by the orgamc wetting hquld The mineral part~cles such as calcite, for example, are enveloped m a water film attached firmly to the surface through hydrogen bonding [33] which wall not permit the hydrocarbon to spread over the surface, consequently such minerals remain dispersed m water [ 34 ] The surface of coal is a patchwork assembly of hydrophoblc and hydrophlhc sites Thus a higher proportion of hydrogen m charcoal will allow the hydrophdlc sites to be enveloped m a water film attached firmly to the particle surface m the oll selectlve agglomerauon process Then when D C O W M s prepared with such particles are sheared the hqmd phase is sufficient fo fill the void between particles, lowermg the viscosity of the suspension

5. Conclusions W e have demonstrated the posslblhty o f producing s o h d hqmd suspensions of deashed charcoal by the selectwe agglomeration process These ternary mixtures are produced by the addmon of 0 8% (mass ratio with respect to the total mixture) of a non-ionic surfactant enclosing a hnear chain of e~ght molecules of oxide o f ethylene The study shows that D C O W M should be prepared with charcoal having around 30% of volatile matter Non-lomc surfactants with a hnear chain of eight molecules of oxide of ethylene are more efficient m D C O W M producUon Charcoal parttcles with mean diameter between 7 and 11 /zm allow the producUon of more concentrated suspensions The domestic od (agglomerant) concentrauon depends on the amount of agglomerant used to prepare the agglomerates

3 4 Effect o f mass ratio between agglomerant and charcoal tn the productwn o f deashed c h a r c o a l - o d - w a t e r mtxtures M6rantl tree charcoal of 10/xm mean diameter with 29% of volatde matter has been used The results obtained (Table Table 7 CharactensUcs of three optimal deashed charcoal-oil-water mixtures Sample number

DCOWMsohd content(%)

DCOWMfuel oll content (%)

DCOWMwater content (%)

DCOWM ash content (%)

DCOWMdust emlsston (g MJ -t)

DCOWMvlscos~ty at 100 s- l and 25 *(2 (cP)

DCOWMcalonfic value (kJ kg- 1)

1 2 3

45 45 45

25 27 30

30 28 25

0 41 0 41 0 41

0 172 0 166 0 158

1253 1778 2861

23834 24671 25926

54

A N’$omtn

et al /The Chemrcal Engrneenng Journal 60 (1995) 49-54

However, 13-23 wt % of domestic 011 m the mixture is enough to obtain fuels with a calorlfic value of at least 21 000 kJ kg-’ The combustion of such fuels emlts Cl18 g dust MJ- ‘, thus respecting the French norms on pollution (0 25 g MJ-‘) By combmmg the optimal values of the different parameters we are able to obtain fluid DCOWM 45-27 containing 72% of combustible matter respecting pumping condltrons and having a 24 671 kJ kg- ’ low calorlfic value

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