Applied Energy 9 (1981) 211-222

EFFECTS OF WATER VAPOUR A N D SOLID CATALYSTS ON THE GASIFICATION OF CELLULOSE AT ELEVATED

PRESSURES

PETER FONG and ROBERT A. ROss

Department of Chemistry, Lakehead University, Thunder Bay, Ontario, P785EI (Canada)

SUMMARY

The gasification of'cellulose was examined from 350 to 650 °C in helium and helium/ water vapour mixtures up to total pressures of 25OOkPa. Particular attention was paid to the effects on the [(CO 2 + H2)/CO ] molar ratio in the product gases of the selected additives--iron(Ill)oxide, zinc( lI)chromite and potassium carbonate. The results are interpreted in terms of catalytic influences on key steps in the reaction sequence. Thus, the most effective additive in realising the highest gaseous fuel potential was potassium carbonate which may act as a catalyst for the carbon~steam reaction. Although the calorific value of the gases produced was not altered much by pressure, it did slightly affect the distribution of the product gases, probably by influencing the secondary reactions of tars. Scanning electron micrographs of the various solid samples are presented.

INTRODUCTION

There has been a growing interest recently in many aspects of the production of synthetic gaseous and liquid fuels from carbonaceous solids. Examples include the renewed activity in studies of coal gasification1'2 and the several assessments of the potential of various renewable biomass sources as feedstocks for interconversion reactions to gaseous or liquid fuels. With particular reference to the gasification of cellulosic materials, several investigations have been reported regarding the effects of a number of physical and chemical parameters on fuel potentials and thermal decomposition mechanisms. 3 - 7

Some important differences between coal and biomass, emphasising calorific values and chemical properties, have been enunciated s and biomass has been shown to offer some advantages over coal as a synfuel feedstock. Its low sulphur and ash contents could reduce the cost of emission control and ash disposal. Further, the higher volatile matter content of biomass, from 70 to 90 %, in contrast with a

211 Applied Energy 0306-26 ! 9/81/0009-0211/$02.50 © Applied Science Publishers Ltd, 1981 Printed in Great Britain

2 1 2 PETER FONG, ROBERT A. ROSS

maximum of around 50~o for coal, promises less intensive operating plant conditions.

Current coal gasification technology usually involves the addition of oxygen or steam to the operation which lowers the yields of tar and oil and optimises gas production. The latter is favoured by pressure increase 9 and these phenomena may also be linked to the reported effects of certain catalytic additives in largely fundamental studies of the gasification of carbon. The additives include transition,lO 12 alkali13 15 and alkaline earth 16'1~ metal compounds.

The present paper is intended to contribute to studies of these effects, with particular emphasis on the role of water vapour and some selected solid additives 4 on the gas composition and fuel potential of the gases produced from cellulose gasification from 350 to 650°C under the influence of pressure up to 2500kPa.

EXPERI MENTAL

Materials Whatman column chromedia CFI fibrous cellulose powder (maximum 0.015 '!/o

ash, W. and R. Ralston Ltd), reagent grade iron(III)oxide, zinc(II)oxide, chromium(III) oxide and potassium carbonate were used as supplied. The preparation of the zinc(I I)chromite, dry-mix and impregnated cellulose samples has been described previously.18

Apparatus and method Gasification experiments were conducted in the earlier high-pressure apparatus 5

and the gaseous products were analysed by means of a Beckman GC-5 chromatograph.

Sample mass was around 100 mg as weighed in the stainless steel bucket of the reactor. The latter was flushed with helium to remove air before any water vapour was admitted in the appropriate studies. The required pressure of the system (101 to 2533 kPa) was attained by admitting helium or helium through a water saturator (2-1 kPa) to the reactor. The preselected temperature was obtained in 5 min and kept constant for a period of 30 rain. The gases formed were then analysed.

A Cambridge Stereoscan 600 instrument was used for scanning electron microscopy.

RESULTS

The gases analysed were methane, hydrogen, water vapour, carbon monoxide and carbon dioxide.

The [(CO z + H/)/CO] ratios for the gasification of the various cellulose samples at 1013 k Pa in helium and helium/water vapour (2.1 k Pa) from 400 to 650 °C are given in Figs 1 and 2. With the exception of samples containing 5 ~ KzCO 3, all

CATALYTIC GASIFICATION OF CELLULOSE AT ELEVATED PRESSURES 213

B et

#

90 / ' ,o / ! i t

i / i I i I

i / Ic#

s •

l / l l / I t

7C /~ ' t , ,

t ¢

ii / i i I t

+~ 50 ,~ oo ,',

0 / I-- i I n-- <~ ? /

30 / / I / !

I ! I I s. "S ~x i i /

. tz ~o *

I :575 450 550 650

TEMPERATURE (°C)

Fig. 1. The effect of iron(Ill)oxide, zinc(II)chromite and water vapour (2.1kPa) on the ratio of [(CO2 + H2)/CO] for cellulose gasification at a total pressure of 1013kPa from 400 to 650°C in helium. • = cellulose; • = • + 10.2 %w/w iron(Ill)oxide; • -- • + 14.2 %w/w zinc(lI)chromite;

O = • + w a t e r v a p o u r ; A = O + • ; V = O + • .

showed an increase of the [(CO s + H2)/CO ] ratio in the presence of water vapour. In general, the values of the ratio for samples with different concentrations of KxCO 3 were comparatively lower than those for plain cellulose.

Figure 3 shows the effect of K2CO 3 concentration on the ( [ C O 2 + H2)/CO ] ratio in both atmospheres at 1013kPa. In helium, samples containing 0"7~o K2CO 3 decreased the ratio at any given temperature, whilst cellulose containing 5 ~ KECO 3 increased the ratio at temperatures below 600 °C. In the presence of water vapour, the ratio decreased with increasing concentration of K2CO 3.

The effects of pressure (101 to 1013 k Pa) on the component gas ratios--H2/CO, CH4/CO and CO2/CO--for plain cellulose in helium and helium/water vapour at 650 °C are given in Figs 4 and 5. The corresponding plots for samples containing 5 ~o KzCO a are shown in Figs 6 and 7.

214 P E T E R F O N G , R O B E R T A. ROSS

m

4 0

f 3O 'f

,,..) q a ,;/ £ ,/ + ,I I , ~ z o z',. ,'J ~, o ,: , O_ • IJ / '

d , / / / / , 0

10

, I i I i I J 375 450 550 650

TEMPERATURE (°C)

Fig. 2. The effect of K2CO 3 concentration, preparation procedure and water vapour (2.1 k Pa) on the ratio of [(CO2 + H2)/CO] for cellulose gasification at a total pressure of 1013 kPa from 400 to 650°C in helium. O=cellulose; m=Q+5.0%w/w K2CO3; A=O+0-7~ow/w K2CO 3 impregnated; • =•+bal l -mil led; O=cellulose+water vapour; [ ] = O + • ; / x = O + A ; ~ = O + V ;

tD= O +0"7~ow/w K2CO3 dry mix.

Scanning electron micrographs for cellulose heated at 650 °C at different pressures in helium and helium/water vapour atmospheres are shown in Fig. 8. No significant structural differences among the fibres were noted. Fibre twists and crack formations along the main fibre axis were observed, as noted in previous studies. 5,t 8

Calorific values of the gas mixtures formed at the various pressures and temperatures were calculated by summing the products of the percentage composition of each of the components and their known calorific values (reference 19, Tables 1 and 2). Within these limits no significant effects of pressure or additives on the calorific values of the gas mixtures were observed.

D I S C U S S I O N

The principal gases evolved from cellulose gasification were hydrogen, carbon monoxide, methane and carbon dioxide. In systems of the present design, absolute gas yields cannot be determined accurately and hence emphasis is placed on the

CATALYTIC GASIFICATION OF CELLULOSE AT ELEVATED PRESSURES 215

9o~

*-- in He/H20 0t 650°C

30 %" / I'',, 2 0 - 'k

o

o in He 0t 5,50 °C ~E - - - m .

10 \"\ ~ _ ........ ~ - ~ " " in He at 650°C ~ ~ -

i J I o I 4 5

K2CO 3 IN CELLULOSE (%W/W)

Fig. 3. The effect of K2CO 3 concentration on the ratio of [(CO 2 + H20)/CO ] for cellulose gasification at a total pressure of 1013 kPa in helium and helium/water vapour (2'i kPa).

product ratios--H2/CO, C H J C O and CO2/CO. These are readily determined and provide important information on the key reactions which take place during gasification.

Figure 1 shows effects of the solid additives and water vapour on the [(CO2 + H2)/CO)] ratio obtained from cellulose in helium at 1013kPa. In pure helium, this ratio was slightly increased by the addition of iron(Ill)oxide. A displacement of the shift reaction (1) to the right would clearly explain the observation:

CO(g) + H20(g)~--ICO2(g ) + H2(g ) AH~98 = - 4 0 kJ mo1-1 (1)

In the presence of water vapour, the [(CO2 + H2)/CO] ratio increased substantially with all cellulose samples from 500 to 650°C in the following order:

Cellulose + iron(III)oxide > cellulose > cellulose + zinc(II)chromite (97) (86) (14)

216 PETER F O N G , ROBERT A. ROSS

m

2O

H2/CO

o

n,- 10

cH4/co

o 2;0 ooo aoo ,ooo PRESSURE ( kPo )

Fig. 4. Theef foctofpressure( ]01 to ]013kPa)on theproduc t ra t i os fo rce l l u ]osegas i f i ca t i ona t650°C in he l ium.

The values of the [(CO 2 + H2)/CO ] ratio at 650°C and 1013kPa are shown in parentheses.

It is felt that the key reaction influencing the final gas composition in the various cellulose gasifications was reaction (1), especially in the presence of water vapour. As could be anticipated, iron(III)oxide behaved as a catalyst for reaction (1). 21 The decrease of the [ (CO/+ H2)/CO ] ratio in the presence of zinc(II)chromite suggests that this additive may inhibit reaction (1). Similar effects for these additives have been noted in the gasification of white birch sawdust. 22

Gasification patterns were markedly related to the concentration level of K /CO 3. In helium, a lower [(CO2 + H2)/CO] ratio was obtained for the samples containing 0.7 ~o KzCO3 compared with that obtained from cellulose (Figs 2 and 3). On the other hand, 5~o K2CO3 addition increased the ratio below 600°C. Previous investigations is of cellulose gasification at 101 kPa showed that K2CO 3 addition decreased CO, CO z and CH~ yields. The extent of the decrease increased progressively with increase in the concentration of K2CO 3. A greater decrease in C O concentration relative to that of CO z with increasing concentration of KECO 3 could readily account for the increase of [(CO2 + H2)/CO] in the presence of 5 ~o K2CO3.

CATALYTIC GASIFICATION OF CELLULOSE AT ELEVATED PRESSURES 217

B

60

5O 0

4O

_(2

. . . 1

o

I C O 20

I0

o 200 ,,60 6ao 10o0 8O0 PRESSURE ( kPo ) Fig. 5. The effect of pressure (l 01 to 1013 k Pa) on the product ratios for cellulose gasification at 650 °C

in helium + 2.1 kPa water vapour.

With added water vapour, the usual pattern of [(CO 2 + H2)/CO ] increase was also observed in all the samples containing 0.7 ~o K2CO3, although such increases occurred to a lesser extent than those measured with plain cellulose. The unusual decrease of the [(CO 2 + H2)/CO ] ratio demonstrated by cellulose containing 5 K2CO 3 suggests that a reaction other than the water-gas shift was intervening to a significant degree. K2CO 3 has been reported to catalyse the carbon-steam reaction (reaction (2)): 14'15

C(s) + H20(g),~ CO(g) + H2(g ) A H ~ 9 8 = 130 kJ tool-1 (2)

Catalysis of the gasification of jack pine chars by K2CO 3 commences above 500 °C and was directly proportional to the concentration of K2C 03. 22 Thus, the lowering of [(CO 2 + H2)/CO] by the addition of 5 ~ K2CO 3 is consistent with catalysis of reaction (2). The higher [(CO 2 + H2)/CO ] ratio obtained from cellulose with 0.7 K 2 C O 3 agrees with the dependence of catalytic activity on catalyst loading, which

218 PETER FONG, ROBERT A. ROSS

y ..J o

4

COi/CO

CH4/CO

i .

0 5 10 15 20 25

PRESSURE (kPQ) xlO z

Fig. 6. The effect of pressure (101 to 2533 kPa) on the product ratios for cellulose + 5.0 % w/w K2CO 3 at 650 °C in helium.

has also been observed in the gasification of chars in the presence of calcium, sodium and nickel compounds. 23'z4 It also suggests that both reactions play an important role in the gasification of samples with 0.7 ~ K2CO 3 whereas reaction (2) may be more important in the gasification of samples containing 5 ~ K2CO 3. With respect to the effect of preparation procedure, samples impregnated with 0.7 ~ K2CO 3 gave a higher [(CO 2 + H2)/CO] molar ratio than the dry-mix counterpart (Fig. 2). This effect, mainly due to the increase in hydrogen composition, is similar to the previous observation on cellulose gasification in helium at 101 k Pa.~8

In general, gasification rates tend to increase with increasing pressure. 2s Moreover, the Wright-Malta process, operated under pressure (2756kPa) with Na2CO 3 as the catalyst, has shown promising results in reducing char formation and increasing the amount of fuel gas by reforming the liquid volatiles. 26

Previously, it has been reported that pyrolysis of cellulosic material in nitrogen at atmospheric pressure provided less tar and more char than the corresponding pyrolysis under vacuum. 20 Similar pressure effects in increasing the yield of char on the pyrolysis of coal tar and petroleum pitches from 300 to 600°C and 200 to 15,000 k Pa have been observed 27 and it has also been shown that in the gasification

CATALYTIC GASIFICATION OF CELLULOSE AT ELEVATED PRESSURES 219

- H2/co ~o ,~

8

6 O

O

4 CO2/CO

2

o%/co

0 10 15 20 25

PRESSURE ( kPo ) x 102

Fig. 7. The ¢ff'ect of pressure (607 to 2533 kPa) on the product ratios for cellulose + 5.0 % w/w K2CO 3 at 650°C in helium + 2.1 kPa water vapour.

of cellulose, char production increased from 12 % weight of cellulose feedstock at 100 k Pa to 16 % at 600 k Pafl s Pyrolysis products are not immediately separated from each other and hence secondary reaction between the products can take place. 29

Secondary reactions are thus important when operating at low temperature, high pressure and low heating rates, as in liquefaction or in the Wright-Malta gasification process. These previous observations show that secondary reactions of tars to form gases or chars are significantly affected by pressure.

Figure 4 shows that no trend of pressure effect in the various product ratios was obtained from the gasification of cellulose at 650 °C in helium. The yield of CH 4 from:

C(s) + 2Hz(g ) ~- CH4(g ) AH~98 = - 75 kJ mol- 1 (3)

would be expected to increase with increasing pressure and thus, for any CH4/CO decrease, there should be an increase in [CO] relative to [CH4]. Reaction (2), along with:

C(s) + CO2(g) ~ 2CO(g) AH~98 = + 170 kJ mol- 1

220 PETER FONG, ROBERT A. ROSS

Fig. 8. Scanning electron micrographs of cellulose after heat treatment at 650 °C at different pressures and in various atmospheres. A and B=cellulose at 101kPa in he l ium+2.1kPa water vapour;

C = cellulose at 1013 kPa in helium + 2.1 kPa water vapour; D = cellulose at 1013 kPa in helium.

would not yield an increase of [CO] as pressure increased. However, secondary reactions of tars could increase the CO yield. Similarly, increasing pressure may initiate or enhance further reactions of tars to form more H 2 and CO 2. It appears that the relative increase in CO, CO 2, H 2 and CH 4 concentrations is not directly proportional to increasing pressure. Thus no monotonic trend in the Hz/CO, CH4/CO and CO2/CO ratios with pressure was observed.

In the presence of water vapour, H / /CO and CO2/CO ratios increased with pressure increase (Fig. 5). The subsequent shift of CO, produced from the secondary reactions of tars, to CO 2 and H 2, via reaction (1) could account for the upward trend of the product ratios with increasing pressure.

In helium, no pattern in the product ratios from samples of cellulose containing 5 ~o K2CO3 was apparent with a pressure increase at 650 °C (Fig. 6). As in cellulose gasification, an irregular increase of the gaseous products with increasing pressure from the secondary reactions of tars can be invoked to explain these observations. Reaction (2), catalysed by KECO 3, would reduce the CO yield with pressure increase and this reaction is likely to be limited in extent by the small amount of water formed on pyrolysis.

With the addition of water vapour, the H2/CO ratio increased slightly with increasing pressure (Fig. 7), consistent with the CO yield from reaction (2) decreasing in step with increasing pressure. Reaction (1) is not dependent on

CATALYTIC GASIFICATION OF CELLULOSE AT ELEVATED PRESSURES 221

TABLE 1 CALORI~C VALUES OF GAS MIXTURES FROM GASIFICA~ON OF PLAIN CELLULOSE

Temperature Calorific value (kJ m - 3) (°C) Helium atmosphere Helium + 2.1 kPa water vapour

Total pressure (kPa) Total pressure (kPa) 101 607 1013 101 607 1013

400 6166 6478 5708 7036 6593 - -

450 7930 - - - - 8433 - - - - 460 - - 9900 10308 - - 10100 8431

500 9922 - - - - 9088 - - - - 520 - - 10022 9498 - - 9981 10439

550 8936 - - - - 9312 - - - - 580 - - 9663 8931 - - 9911 10620

600 9384 - - - - 9221 - - - - 620 - - 9263 8386 - - 9282 8705

650 9545 - - - - 9336 - - 660 - - 9621 8304 - - 8837 7993

TABLE2 CALORIFIC VALUES OF GAS MIXTURES FROM GASIFICA~ON OF CELLULOSE +5 ~o K2CO3

Temperature Calorific value (kJ m -3) (o C) Helium atmosphere Helium + 2.1 kPa water vapour

Total pressure (kPa) Total pressure (kPa) 607 1013 2533 607 1013 2533

400 7669 7974 4230 6274 5773 6664 450 17840 15024 13848 8280 6686 6661 500 13910 14814 16323 9225 9513 8208 550 10682 10894 9679 10242 9670 9452 600 10043 9996 7497 9946 9761 9330 650 9760 9569 7282 9039 9695 10128

p r e s s u r e a n d it m a y , in effect , a c t to k e e p t h e h y d r o g e n yie ld a t a r e l a t i ve ly c o n s t a n t

level . T h u s , w h e n p r e s s u r e w a s i n c r e a s e d f r o m 608 to 2 5 3 3 k P a , t h e C O yie ld

dec reased , i w h i c h , in t u r n , i n c r e a s e d t h e H 2 / C O ra t io .

S c a n n i n g e l e c t r o n m i c r o g r a p h s (Fig~ 8) s h o w t h a t p r e s s u r e a n d w a t e r v a p o u r h a d

n o a p p a r e n t effect o n t h e f ib res o f t h e r e s i d u e p r o d u c e d b y h e a t i n g ce l l u lo se a t

650 °C. T h i s s u p p o r t s t h e s u g g e s t i o n t h a t t h e f ina l g a s c o m p o s i t i o n is l ike ly to

d e p e n d o n s e c o n d a r y r e a c t i o n s o f t a r s a n d i n d i v i d u a l gases .

T h e ca lo r i f i c v a l u e s o f t h e g a s e s f r o m 500 to 600 °C r a n g e l a rge ly f r o m 8000 t o

10 ,000 k J m - 3 . S u c h m i x t u r e s c a n be c o n s i d e r e d to b e m e d i u m e n e r g y s o u r c e s ,

s i m i l a r t o w a t e r ga s in h e a t c o n t e n t . P r e s s u r e u p to 2533 k P a h a d l i t t le effect o n t h e

ca lo r i f i c v a l u e a n d h e n c e , i f t h e p r o d u c t ga se s a r e u s e d as a fuel , t h e o p e r a t i n g

p r e s s u r e w o u l d b e d e t e r m i n e d p r i m a r i l y b y t h e ga s y ie lds a n d t h e k i n e t i c s o f

g a s i f i c a t i o n . O n t h e o t h e r h a n d , i f t h e g a s e s w e r e r e q u i r e d as c h e m i c a l f e e d s t o c k , t h e

o p t i m u m p r e s s u r e s h o u l d b e t h a t w h i c h w o u l d give t h e r e q u i r e d s t o i c h i o m e t r i c r a t i o

222 PETER FONG, ROBERT A, ROSS

for the s u b s e q u e n t syn the t i c process . T h e h ighes t ca lor i f ic v a l u e - - 1 7 , 8 4 0 k J m - 3

was o b t a i n e d f r o m samples c o n t a i n i n g 5 % p o t a s s i u m c a r b o n a t e at 450 °C in h e l i u m

b u t the ex t en t o f gas i f ica t ion was low at t h a t t e m p e r a t u r e and w o u l d have to be

c o n s i d e r e d in the c h o i c e o f a w o r k i n g o p t i m u m .

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