Effect of ash components on devolatilization behavior of coal in comparison with biomass – Product yields, composition, and heating values

Fuel 114 (2013) 64–70

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Effect of ash components on devolatilization behavior of coal in comparison with biomass – Product yields, composition, and heating values Denise Reichel a,b,⇑, Mathias Klinger a,b, Steffen Krzack a, Bernd Meyer a a b

TU Bergakademie Freiberg, Department of Energy Process Engineering and Chemical Engineering, Freiberg, Germany German Center for Energy Resources, Freiberg, Germany

a r t i c l e

i n f o

Article history: Received 14 December 2011 Received in revised form 21 January 2013 Accepted 22 January 2013 Available online 13 February 2013 Keywords: Coal Biomass Pyrolysis Fixed bed Demineralization

a b s t r a c t The influence of inorganic constituents on the pyrolysis behavior of different biomass materials and brown coals was studied in a fixed bed reactor within a temperature range of 250–700 °C. The tests were carried out with untreated and demineralized samples. Inorganic elements dominant in the used brown coals are calcium, silicon, iron, magnesium, and for some samples also sodium and aluminum. Biomass inorganic constituents mainly involve potassium, silicon and calcium. Nearly total demineralization was accomplished for brown coals via HCl and HF treatment. To prevent exceeding structural changes for biomass materials only HCl was used to remove elements like potassium, magnesium, calcium, and phosphorus. Yields, product composition and HHV were determined for each pyrolysis temperature to create mass and energy balances. Demineralization causes an increase in total liquid yields, while gas yield and char yield (only slightly) decrease. Biomass materials show a stronger effect and main decomposition stage is shifted to higher temperatures. Gas composition is also affected by acid treatment, whereas differences occur between the various fuels. Furthermore, the pyrolysis process becomes more endothermic for the demineralized samples. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Pyrolysis plays an important role within thermochemical conversion processes for the production of heat, power, and chemicals. Due to the complex macromolecular structure of coal and biomass a multitude of reactions take place during thermal decomposition and the occurring mechanism are actually not known in detail. But not only fuel structure and their relating composition and properties influence the pyrolysis behavior. The generated product yields and their properties are also affected by process parameters (e.g. temperature, pressure, heating rate) as well as the amount and type of embedding of the present inorganic elements. Some research works deal with the catalytic effect of alkali and earth alkali metals (Na, K, Ca) or transition metals (Ni, Co, Fe) on brown coal pyrolysis [1–4]. However, much more research effort was spent on influence of various inorganic species on pyrolysis behavior of different kinds of biomass [5–8] and the biopolymers cellulose, hemicellulose (xylan), and lignin [7,9–11] for upgrading of the produced liquid and gas phase. ⇑ Corresponding author. Address: TU Bergakademie Freiberg, Department of Energy Process Engineering and Chemical Engineering, Fuchsmühlenweg 9 - Haus 1, 09596 Freiberg, Germany. Tel.: +49 3731 39 4450; fax: +49 3731 39 4555. E-mail address: [email protected] (D. Reichel). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.01.045

In this paper a comparison of the effect of ash constituents in different brown coals and biomass materials on the mass and energy balance of pyrolysis process is presented. The influence of temperature is discussed as well.

2. Experimental 2.1. Sample characterization Three German brown coals (Lusatia – LBC, Rhineland – RBC, Central Germany – CGBC) as well as three biomass materials (spruce wood chips incl. bark – SWCs, wheat straw pellets – WS, maize silage pellets – MS) were used in a raw (untreated) and a demineralized state for the investigations into pyrolysis behavior in a fixed bed reactor. Distinctive for the brown coals is a high bitumen and ash content (Central Germany), high sodium content (Rhineland) and xylitol content (Lusatia). Structural differences between the various biomass materials occur within the yields and structure of biopolymers (lignin, hemicellulose, and cellulose). Ultimate and proximate analyses of raw samples are given in Table 1. These materials were chosen due to their different ash content (1.45 wt.% for spruce wood up to 17.67 wt.% for Central Germany brown coal) and composition (see Table 2) as well as

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nomenclature A AP,i b.d.l. CEN/TS CGBC d Dechem di daf de DE DIN FC HHV

HHVFeed HHVP,i l LBC M MS r RBC STP SWCs TG VM WS xi,raw xi,de XRF

ash content yield of pyrolysis product i below detection limit European committee for standardization/technical specifications central Germany brown coal dry state of samples difference between chemically bound energy content within products and feed inner diameter of pyrolysis reactor dry and ash free state of samples demineralized level of demineralization German institute for standardization fixed carbon higher heating value

higher heating value of feed higher heating value of pyrolysis product length of pyrolysis reactor Lusatian brown coal moisture content maize silage raw state (delivery state of sample) Rhenish brown coal standard temperature and pressure spruce wood chips (incl. bark) thermogravimetry volatile matter content wheat straw element content by weight of original sample element content by weight of demineralized sample X-ray fluorescence

(1:1) for 1 h first. To reach nearly complete demineralization (removal of SiO2, Al2O3) diluted hydrofluoric acid (8%) was added to the sample for 1 h. Afterwards another washing step with HCl for 1 h was enclosed to remove residues of HF. Demineralization of the biomass samples was done according to Vamvuka et al. [13] with diluted HCl (1:1) for 1 h. Washing with distilled water was accomplished after the acid treatment until the chlorine concentration in the removed liquid could be neglected. Total demineralization cannot be reached for biomass since treatment with HF will lead to strong changes in biomass structure.

various contents of biopolymers in case of biomass. With regard to potential catalytic activity of inorganic constituents differences mainly occur in sodium, potassium, calcium, and iron but also in silicon content. Please note that determination of ash is done under oxidizing conditions according to DIN 51719 for coal and DIN CEN/ TS 14775 for biomass. Due to the fact that the inorganic components are usually present in the highest oxidation state, ash contains a lot of oxygen. Under inert or rather reducing conditions during pyrolysis other mineral phases than the pure oxides are present. For further considerations and balancing of the pyrolysis process the sum of inorganic elements (XRF analysis), exclusive total sulfur, is used instead of ash content. Thus, the oxygen content (calculated as difference) is higher as usual. This means the values are not comparable to ultimate analysis data reported by other authors.

2.3. Pyrolysis equipment The pyrolysis reactor set-up consists of a fixed bed reactor (di = 20 mm, l = 335 mm), a vertically movable tube furnace (max 1.15 kW), two cooling traps designed like washing flasks (1st ice water cooled, 2nd cooled by a saturated NaCl solution, which was pre-cooled in a freezer to 18 °C), a mass flow controller to set the purge gas stream, a gas collection system composed of six

2.2. Demineralization procedure The brown coal samples were demineralized according to the procedure of Samaras et al. [12] using diluted hydrochloric acid Table 1 Results of ultimate, proximate analysis, and heating value determination of raw samples. Proximate analysis in wt.%

Ultimate analysis in wt.% (daf)

Heating value in kJ/kg (d)

M (r)

A (d)

VM (d)

FC (d)

C

H

N

SC

O

HHV

LHV

Brown coal RBC LBC CGBC

51.12 54.00 50.60

5.47 11.99 17.67

50.70 52.08 51.78

43.83 35.93 30.55

69.04 69.54 72.19

5.01 5.41 5.81

0.79 0.64 0.64

0.05 1.40 2.45

25.11 23.01 18.91

25,267 24,603 24,289

24,233 23,565 23,245

Biomass SWC WS MS

12.53 6.81 13.07

1.45 8.16 4.16

78.54 73.17 80.91

20.01 18.67 14.93

52.06 49.62 48.32

5.92 6.06 6.38

0.25 0.69 1.12

0.11 0.14 0.11

41.66 43.49 44.07

20,154 17,939 18,559

18,881 16,724 17,225

Table 2 Inorganic constituents present in brown coal and biomass samples (results of XRF analysis). Data given in wt.% (d). C (carbonate)

Na

Mg

Al

Si

P

S (total)

Cl

K

Ca

Fe

Sum traces

Sum w/o S (total)

Brown coal RBC LBC CGBC

b.d.l. 0.05 b.d.l.

0.26 b.d.l. b.d.l.

0.40 0.25 0.17

0.05 0.09 0.96

0.12 1.70 1.91

b.d.l. b.d.l. 0.00

0.64 2.64 4.00

0.03 0.02 0.01

0.03 0.01 0.05

1.21 1.83 2.99

0.43 0.90 0.93

0.04 0.07 0.11

2.57 4.92 7.13

Biomass SWC WS MS

0.11 0.41 0.01

0.00 b.d.l. 0.02

0.04 0.23 0.12

0.01 0.02 0.08

0.04 1.46 0.81

0.02 0.03 0.22

0.11 0.20 0.18

0.00 0.16 0.03

0.13 1.75 0.70

0.41 0.38 0.23

0.01 0.01 0.09

0.04 0.01 0.02

0.81 4.46 2.33

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gas sampling tubes (each with a volume of 1 l), and two filters. It is possible to take a gas sample of each gas sampling tube for analysis. The second cooling trap is filled with a defined amount of tetrahydrofuran acting as an absorbent to realize a nearly complete precipitation of condensable products. 2.4. Experimental procedure Pyrolysis investigations were carried out with untreated brown coal and biomass samples within a temperature range between 250 and 700 °C. The increments were set to 50 K in the main decomposition stage. Afterwards the temperature was raised in 100 K steps. For the demineralized samples pyrolysis experiments were run between 300 and 700 °C in 100 K increments. The samples were grinded to a particle size <2 mm and dried at 105 °C until weight constancy. The fixed bed reactor was prepared with 20– 30 g of dried sample to reach nearly the same sample volume for each experiment. The furnace preheating temperature was set to a value of 200 K above desired pyrolysis temperature to achieve a high heating rate (45 to 122 K/min in dependence on desired pyrolysis temperature). The same pyrolysis temperatures for different samples result in similar heating rates. After flushing the reactor with Argon to remove air from the system the furnace was pulled over the reactor and the sample was heated up. The furnace was removed slightly before the sample thermocouple inside the reactor reached the desired temperature. The whole system is flushed with a constant Argon stream of 50 ml/min (STP) during the whole test run. After cool down all the parts of the system were detached, weighed, and char as well as liquid product were recovered for further analyses. The experiments at each temperature were carried out once. For selected samples and temperatures the experiments were repeated several times (e.g. WS at 500 °C, 8 times) to test repeatability of the results. In this case the mean values of the experiments are used. The obtained errors within mass balance are in the range of ±2 wt.%.

3. Results and discussion 3.1. Level of demineralization Results of XRF analysis of raw and demineralized (de) samples were used to calculate the level of demineralization (DE) according to Eq. (1) obtained by acid treatment (see Fig. 1). Carbonate, sodium, magnesium, and for the brown coal samples also phosphorus were completely removed.

DE ¼

xi;raw  xi;de  100% xi;raw

ð1Þ

Differences in level of demineralization for the used fuels are the result of the nature of inorganic constituents (bound to organic matter or as minerals like carbonates, sulfates, silicates, and so on) and their variable resistance against acid treatment. Compared to brown coals (>91.6%) the biomass samples reach a significantly lower level of demineralization (WS: 34.1%; MS: 47.7%; SWC: 84.3%) because no treatment with HF was done to remove the silicon and aluminum containing species. The relatively high value for wood is related to the fact that calcium carbonate is the dominant inorganic species, while less silicon is present. However, the elements which are supposed to have a catalytic activity, like alkaline, alkaline earth or transition metals, are removed to a satisfying amount. Chlorine, which, in general, can be easily washed out from coals or biomass, is not mentioned up to now. Table 3 shows an increase in chlorine content for the demineralized samples up to several times compared to the original one (except for wheat straw). This is a result of the demineralization procedure. Washing of the samples with diluted hydrochloric acid, as mentioned before, leads to an entry of chlorine to the sample. The subsequent flushing with distilled water for several times was elaborate and it was not possible to remove all the chlorine from the samples. Hence, no suggestion can be made about the chlorine reduction caused by the demineralization process. 3.2. Product yields

2.5. Product analysis The composition of pyrolysis gas was analyzed via gas chromatography (Micro GC Agilent 3000A, 3 columns: mole sieve, PLOTU, alumina; detector: heat conductivity, carrier gases: He, Ar). For char characterization ultimate, proximate, and XRF analysis as well as heating value determination was done according to German standardization. The obtained results for char were not presented within this paper. The removed tar from the cold traps was characterized via ultimate analysis and heating value determination. The water content of the generated tar/oil was determined by Karl Fischer titration. The titrant used was HYDRANAL Composite 5 K together with HYDRANAL KetoSolver as solvent.

In general, comparison of pyrolysis behavior of brown coals and biomass shows significant differences, due to the fact of varying Table 3 Chlorine content in original (raw) and demineralized samples (given in wt.% (d)).

RBC LBC CGBC SWC WS MS

raw

de

de–raw

0.028 0.019 0.012 0.001 0.164 0.032

0.384 0.110 0.107 0.045 0.079 0.188

+0.356 +0.091 +0.095 +0.043 0.086 +0.156

Fig. 1. Demineralization level obtained by acid treatment. Left: brown coals. Right: biomass.

D. Reichel et al. / Fuel 114 (2013) 64–70

compositions (see Table 1). While brown coals possess a very continuous decrease in char yield according to rising temperatures within the investigated temperature range (see Fig. 2a), biomass shows a main decomposition stage within a narrow temperature range, starting at 200–250 °C and ending at 350–400 °C with a mass loss of up to 70 wt.% (daf) (see Fig. 2 b). Wheat straw starts pyrolysis first, followed by maize silage and spruce wood. The influence of demineralization on pyrolysis product yields is exemplary shown for Rhenish brown coal and wheat straw in Fig. 2, since similar trends occur within both material groups. Brown coals possess only a slight decrease in char yield by demineralization (on a dry and ash free basis), which is not steady within the investigated temperature range (Fig. 2a). Less char amounts after acid treatment are also reported from [13], but no effect during TG experiments was found in [3]. In contrast, liquid yields increase to the disadvantage of gas, whereas reaction water shows an increase (Fig. 2c) and in case of tar/oil no similar behavior can be observed (RBC ;, LBC and CGBC "). The increase in tar/oil yield for CGBC and LBC corresponds to [4]. The biomass materials show a stronger influence caused by acid treatment. For the herbaceous ones (WS, MS), containing high amounts of hemicellulose, pectin, and cellulose, the main decomposition stage is shifted to higher temperatures (strong influence for WS (see Fig. 2b)). Due to high amounts of potassium in raw biomass, cellulose pyrolysis is shifted to lower temperatures as already reported [7,8]. Finally, less char remains for demineralized samples (700 °C: WS 7.7%; MS 10.4%; SWC 10.2%), since potassium is supposed to catalyse polymerization reactions [11]. Production of total liquid amount is significantly risen by acid treatment (700 °C: WS +24.9%; MS +10.2%; SWC +9.3%), while gas yields decrease about 11% for SWC and up to 30.0% for WS at 700 °C (see Fig. 2b and d). The obtained trends correspond to the work of [8] and partly to that of [5,6]. The strong increase of liquids is caused by an enhancement of tar/oil, whereas reaction water production is nearly unaffected except for SWC which shows a slight rise up to 500 °C. 3.3. Gas yields and composition Main gas components being released during pyrolysis of brown coal and biomass are CO2, CO, H2, and CH4. Furthermore, saturated

67

and unsaturated hydrocarbons (C2–C4) as well as H2S and COS, the latter especially in the case of brown coals, are produced. In general, demineralization leads to lower CO2 and H2 yields during pyrolysis of all investigated samples, while CH4 and CO are increased for brown coals (see Fig. 3). The different biomass fuels show various trends for CH4 and CO release with subject to temperature. Acid treatment causes a slight shift to higher temperatures for the start of CO2 production in brown coal (except for LBC) and biomass pyrolysis, whereas the trend according to temperature is similar for raw and demineralized samples, but as already depicted at lower yields. Especially in case of wheat straw pyrolysis the effect on CO2 yield caused by demineralization is very strong and the released amount from the raw sample is bisected. On contrary, CO production increases for demineralized brown coals, whereas start temperature is not affected. With subject to increasing temperatures CO is released in two phases, which can be seen especially for LBC and RBC (Fig. 3b). The temperature for the first plateau is shifted to about 100 K higher values by acid treatment. The increase in CO and decrease in CO2 yields during brown coal pyrolysis corresponds to results of Zou et al. [1], who found that the presence of calcium (present in high amounts in the used brown coals) leads to lower levels in CO and aliphatic hydrocarbons. The latter fact is also closed to the results of this study, where a rise of saturated hydrocarbons (C2–C4) was obtained. On the contrary, Yang and Cai [2] confirm only the behavior of CO2 by adding Ca and Fe to untreated lignite. CO release during biomass pyrolysis is shifted to higher temperatures. Demineralized SWC as well as WS produce a similar amount of CO which is below the value for raw samples up to 600 °C and rises further for increasing temperatures while the raw feeds show a plateau phase. These characteristics are also reported by Di Blasi et al. [6]. However, the obtained CO yield for demineralized maize silage was below the raw sample. Methane yields are also affected by demineralization (Fig. 3d), but the trends vary for brown coal and biomass. CH4 production starts in significant amounts at about 300 °C for raw and demineralized brown coals whereupon the trends with subject to temperature are very similar. But, in case of demineralized brown coals higher CH4 yields are produced, which is contrary to the results of [2] and to the behavior of biomass samples. The demineralized

(a)

(b)

(c)

(d)

Fig. 2. Pyrolysis product yields using the examples of Rhenish brown coal (RBC, left column) and wheat straw (WS, right column). Solid lines for raw samples, dashed lines for demineralized samples. (a,b) Char and gas yields. (c,d) Liquid product yields (tar/oil, reaction water, and total liquid).

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D. Reichel et al. / Fuel 114 (2013) 64–70

(a)

(b)

(c)

(d)

Fig. 3. Yields of main gas components during pyrolysis of German brown coals. Solid lines for raw samples, dashed lines for demineralized samples. (a) CO2 yield. (b) CO yield. (c) H2 yield. (d) CH4 yield.

biomass materials produce a significantly lower amount of CH4 above 400 °C for SWC and above 500 °C for WS. Differences range between 12 l (STP)/kg feed (daf) for SWC and 20 l (STP)/kg feed (daf) for WS. However, the demineralized maize silage delivers a higher methane yield than the raw sample. In dependence on pyrolysis temperature the methane yield shows a maximum at around 600 °C for WS and MS. Yields of saturated gaseous hydrocarbons (C2H6, C3H8, C4H10) decrease for demineralized biomass and behave the other way around for brown coal. In case of unsaturated hydrocarbons (C2H4, C3H6, C3H4) yields decline for CGBC and LBC, but rise for the other fuels. This may be attributed to missing alkali metals present in the raw biomass samples and RBC.

3.4. Tar/oil composition The composition of the tar/oil-fraction produced during biomass and brown coal pyrolysis was characterized by ultimate analysis. Results are exemplarily shown for a pyrolysis temperature of 500 °C in Tables 4 and 5. The obvious difference in composition of tar/oil from biomass compared to that from brown coal is the lower carbon (60–65 vs. 67–73 wt.% daf) and the higher oxygen content (26–33 vs. 10–19 wt.% daf), which is related to the different structures of the fuels. Although only a slight change could be observed in case of tar/oil yields between raw and demineralized samples, a variation in tar/oil composition occurs. The main effect

Table 4 Results for tar/oil ultimate analysis obtained during brown coal pyrolysis at 500 °C. Values given in wt.% (daf). RBC

C H N SC O

LBC

CGBC

raw

de

raw

de

raw

de

70.40 11.91 0.68 0.88 16.13

68.38 10.30 0.35 1.55 19.42

69.51 11.70 0.21 6.27 12.31

66.86 8.26 0.17 7.52 17.19

73.21 11.06 0.27 5.82 9.64

69.58 10.61 0.15 5.54 14.12

Table 5 Results for tar/oil ultimate analysis obtained during biomass pyrolysis at 500 °C. Values given in wt.% (daf). SWC

C H N SC O

WS

MS

raw

de

raw

de

raw

de

59.84 7.58 0.34 0.30 31.94

63.14 6.17 0.14 0.49 30.06

63.72 8.21 1.47 0.39 26.21

60.95 5.86 0.21 0.48 32.50

62.07 7.95 1.56 0.42 28.00

64.58 6.77 0.96 0.52 27.17

caused by demineralization of brown coal samples is obvious for carbon and oxygen content. Changes in hydrogen, nitrogen, and sulfur content are also observed. Demineralization effect on tar/oil composition for biomass samples varies. While wheat straw shows the same trends due to acid treatment like brown coals (C;, H;, O"), maize silage and spruce wood behave contrary with respect to carbon and oxygen content. The changes in absolute values are similar for the brown coals and wheat straw (C:2.0 to 3.6 wt.%, O: 3.3–6.3 wt.%). Demineralization of spruce wood and maize silage causes only a slight decrease in oxygen (0.8 to 1.9 wt.%) and hydrogen content (1.2 to 1.4 wt.%), which is nearly equal to the rise in carbon content. Slight changes in sulfur (%) and nitrogen (&) content within tar/ oil of the treated samples are observed as well. The results of rising tar/oil oxygen content and decrease in CO2 release during pyrolysis of demineralized brown coals indicate that under the absence of inorganic elements cleavage of oxygen bearing functional groups via decarboxylation is limited. Maybe the formation of some low molecular carboxylic acids is favoured. Furthermore, decarbonylation to form CO seems to be limited for demineralized wheat straw pyrolysis. Although the same trend of decreased CO and CO2 release is obvious for demineralized spruce wood and maize silage the oxygen content in tar/oil shows a declining trend. This leads to the conclusion that maybe other mechanisms are relevant for the decomposition of this fuels due to other existing structures.

D. Reichel et al. / Fuel 114 (2013) 64–70

product as well as the difference between chemically bound energy within products and feed Dechem can be calculated by Eq. (2).

3.5. Product heating values and energy distribution Changes in pyrolysis mechanisms due to demineralization of the used fuels are reflected in the product heating values as well. Results are exemplarily shown for brown coals at a pyrolysis temperature of 500 °C in Table 6 and for biomass at 400 °C in Table 7. Demineralization of brown coals leads to an increase in HHV of char and gas, while tar/oil HHV decreases. The rise within gas energy content is related to the observed higher CO and CH4 yields as mentioned above. The decline in tar/oil HHV, which is strong especially for LBC and CGBC, is caused by a higher formation of oxygen containing, condensable species and a corresponding reduction of carbon and hydrogen content (see Table 4). In case of biomass a decrease in HHV caused by demineralization can only be observed for char derived from wheat straw and maize silage pyrolysis. A really strong increase in HHV can be seen for tar/oil from wheat straw and spruce wood pyrolysis. In contrary to the effect of demineralization during brown coal pyrolysis, the tar/oil from biomass pyrolysis shows a higher carbon and lower oxygen content for the acid treated samples (except WS). The comparatively high HHV for tar/oil derived from wheat straw pyrolysis cannot be explained by the changes in ultimate analysis results. The increase is probably attributed to some compounds with high HHV, which are present in tar/oil from wheat straw pyrolysis. Based on the determined yields (A) and higher heating values (HHVs) of pyrolysis products (P, i) the energy distribution to each

Table 6 Higher heating values of char, tar/oil and gas in kJ/kg (daf) for pyrolysis of brown coal samples (raw and demineralized) at 500 °C. RBC

Char Tar/oil Gas

LBC

CGBC

raw

de

raw

de

raw

de

28,295 35,261 4,476

30,007 34,764 6,196

29,593 34,891 6,549

29,911 31,642 7,773

25,802 37,007 6,940

29,456 33,727 8,600

Table 7 Higher heating values of char, tar/oil and gas in kJ/kg (daf) for pyrolysis of biomass samples (raw and demineralized) at 400 °C. SWC

Char Tar/oil Gas

WS

69

MS

raw

de

raw

de

raw

de

28,828 24,104 3,376

28,896 26,231 4,258

28,031 25,247 3,358

26,778 31,163 4,734

29,946 27,722 3,451

28,667 28,023 3,878

Dechem ¼

n X ðAP;i  HHV P;i Þ  HHV Feed

ð2Þ

i¼0

Results are shown exemplarily in Fig. 4 for pyrolysis at 500 °C for brown coals and at 400 °C for biomass materials. For liquids an increase in chemically bound energy occurs from raw to demineralized samples, but no general trend could be observed for the other products. This is a result of different effects caused by acid treatment on product yields and composition between and within the various material groups. The general trend, which arises for the effect of demineralization on Dechem is that occurring reactions seem to be more endothermic than for untreated samples within the investigated temperature range. At 400 °C the values change from partly strongly exothermic (e.g. LBC, WS) to slightly endothermic for all samples except CGBC. Due to the fact of missing data for e.g. heat capacities, enthalpy of evaporation of tar and so on, the creation of a complete energy balance for the pyrolysis process including chemically bound and physically enthalpies is actually not possible. It should be noted that the given values for Dechem can only be considered as trends. 4. Conclusions Amount and composition of inorganic species have a significant impact on the thermal degradation characteristics of brown coals as well as biomass materials. Due to the fact of different macromolecular structure, varying ash content, and dominant inorganic species, the results differ between brown coals and biomass fuels and furthermore within the two material groups. The higher tar/oil yields produced during pyrolysis of demineralized biomass, to the disadvantage of gas (mainly decrease of CO2, CO, H2), indicate that inorganic matter (K, Ca) may act as catalysts for the breakdown of oxygen containing functional groups via decarboxylation or decarbonylation. Thermal decomposition of acid treated biomass produces higher amounts of light oxygen bearing species e.g. phenol, organic acids or methanol (see [7]) leading to an increase in tar/oil oxygen content. The latter can in this study only be confirmed for wheat straw pyrolysis. The influence of mineral matter on degradation characteristics for brown coal results mainly from high amounts of calcium, iron, and sodium (the latter only for RBC). The main effect is an increasing production of reaction water caused by demineralization. This leads to higher amounts of total liquids to the disadvantage of gas and char. Strong influence due to demineralization was also observed in gas composition. The decrease in CO2 and the increase in CO yield by acid

Fig. 4. Difference in chemically bound energy between pyrolysis products and feed (Dechem). Basis for calculation: HHV, daf. Full bars for raw samples, patterned bars for demineralized samples. (a) Pyrolysis of brown coal samples at 500 °C and (b) pyrolysis of biomass at 400 °C.

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treatment may be related to missing calcium, acting as catalyst for the formation of acidic oxygen bearing species (see [1]). This is confirmed by the observed higher oxygen content in tar/oil of acid treated brown coals. The obtained increase in endothermic reactions after demineralization is associated with rising liquid yields. This indicates the effect of inorganic constituents, present in brown coal and biomass, on the occurring decomposition mechanisms, too. In general, the change in product yields between pyrolysis of raw and demineralized samples is not only related to the total amount of constituents acting as catalysts (Na, K, Mg, Ca, Fe), because the effect is comparatively higher for biomass samples, although the content of possible catalysts is lower. Acknowledgements The investigations were financially supported by the German Federal Ministry of Agriculture, Food and Consumer Protection (biomass pyrolysis) as well as from the German Federal Ministry of Education and Research, Vattenfall, RWE, Mibrag and Romonta within the research project ‘‘German Centre for Energy Resources’’. References [1] Zou X, Yao J, Yang X, Song W, Lin W. Catalytic effects of metal chlorides on the pyrolysis of lignite. Energy Fuels 2007;21(2):619–24.

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