- Email: [email protected]
Co-gasification of wet sewage sludge and forestry waste in situ steam agent
Bioresource Technology 114 (2012) 698–702
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Co-gasification of wet sewage sludge and forestry waste in situ steam agent Lixin Peng a, Yongxiu Wang a, Zhihong Lei a, Gong Cheng a,b,⇑ a b
Shenzhen Academy of Environment and Science, Shenzhen 518001, PR China School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 19 January 2012 Received in revised form 22 March 2012 Accepted 24 March 2012 Available online 30 March 2012 Keywords: Co-gasification Wet sewage sludge (WSS) Forestry waste (FW) Fuel gas
a b s t r a c t The co-gasification of wet sewage sludge (80 wt.% moisture, WSS) and forestry waste (FW) blends was studied. The thermogravimetric analysis showed that weight loss and the maximum weight loss rate of the sample increased with the increase in FW content. The co-gasification process was performed in a lab-scale fixed bed gasifier to investigate the effects of WSS content and reactor temperature on product yields, gas composition and gasification performance. The results indicated that steam generated from the moisture content in WSS took part in the gasification with char. The gas yield decreased with the increasing WSS content. And the concentrations of H2 and CO reached the maximum when the WSS content was 50%. The LHV of fuel gas ranged from 11.89 MJ/Nm3 to 12.72 MJ/Nm3 when the reactor temperature increased from 700 °C to 900 °C. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Sewage sludge, an unavoidable by-product of municipal wastewater treatment plants, is known to contain numerous harmful chemicals (Ji et al., 2010). In the coming years, the amount of sewage sludge will increase considerably due to municipal development and escalating populations in cities (Sánchez et al., 2007). The presence of harmful substances such as heavy metals, poorly biodegradable organic compounds, viruses, pharmaceuticals and hormones, may represent a complication in the disposal management strategies. Currently, sewage sludge has mainly been handled via landfill, incineration, anaerobic digestion and application to agricultural land (Pokorna et al., 2009; Lei et al., 2010). The landfill disposal has become much less acceptable in many countries (Seggiani et al., 2012), because it requires a lot of space and the soil has to be sealed adequately to prevent the leaching of toxic compounds. The harmful substances in sewage sludge would enter the food chain again by the utilization as fertilizers on farmland (Groß et al., 2008). In addition, incineration was accompanied with the emission of secondary contaminant materials (Chun et al., 2011). Gasification of sewage sludge was considered as the potential technology, due to the advantages of converting the sludge into combustible gaseous products by reducing its volume, preventing from the toxic organic compounds, and fixing the heavy metals in the solid residue. Gasification of sewage sludge was associated
⇑ Corresponding author at: Shenzhen Academy of Environment and Science, Shenzhen 518001, PR China. Tel./fax: +86 0755 25589688 2042. E-mail address: [email protected] (G. Cheng). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.03.079
with the extensive experience in biomass gasification. Therefore, it was natural to build on this experience (Nipattummakul et al., 2010a; Werle and Wilk, 2010). However, wet sewage sludge (WSS) from the wastewater treatment plant, after dewatered by filter pressing or centrifugation, still contained 80 wt.% of water (Zhang et al., 2011). The high moisture content was one of the major drawbacks for the gasification of sewage sludge. Domínguez et al. (2006) suggested that long gas residence times and high heating rates were required to maximize gas production during the pyrolysis and gasification of WSS. Zhang et al. (2011) showed that the gas yield was less than 40% in WSS pyrolysis with situ steam. The excessive steam generated from WSS condensed into liquid fraction, which resulted in 50–70% of liquid yield. Also, many researchers studied fuel gas or hydrogen-rich syngas produced from the gasification of dried sludge feedstock (Nipattummakul et al., 2010b; Andrés et al., 2011). But the drying process used to remove the water, consumed a large amount of energy and increased the cost of disposal considerably. Co-gasification based on two different materials has received much attention in recent years, since it may compensate their weakness each other (Mastellone et al., 2010; Aigner et al., 2011; Howaniec et al., 2011). Biomass was abundant, renewable, and low cost. Forestry waste, one of the important biomass sources, was considered as a suitable feedstock for gasification. Compared with sewage sludge, forestry waste contained high contents of volatile matter and fixed carbon, but low ash and moisture contents. The addition of forestry waste to wet sewage sludge in appropriate proportion could adjust the moisture content of the blends. The presence of water also generated the steam in situ, which would increase the production of hydrogen by gasification reaction with char.
699
L. Peng et al. / Bioresource Technology 114 (2012) 698–702
The aim of this study was to investigate the performance of co-gasification of wet sewage sludge (WSS) and forestry waste (FW) in a lab-scale fixed bed gasifier. Experimental tests were carried out at different WSS contents and reactor temperatures. The co-gasification performance was evaluated in terms of dry gas yield and gas composition, and H2 yield. 2. Methods 2.1. Materials The wet sewage sludge (80% of moisture content) produced in a Wuhan urban wastewater treatment plant fed with municipal waters was dewatered by filter press and then collected as original material. Forestry waste (FW) was from a farm in Wuhan City, Hubei Province, China. It mainly consisted of 92 wt.% pine sawdust, 6 wt.% branches and 2 wt.% leaves. The material was dried under the sun for a period of 7 days to reduce the moisture content and then was shredded into particle size between 0.125 and 0.25 mm. The proximate and ultimate analyzes of two materials were listed in Table 1. Ultimate analysis was obtained with a CHNS/O analyzer (Vario Micro cube, Elementar). Such analysis gives the weight percent of carbon, hydrogen, nitrogen, and sulfur in the sample simultaneously, and the weight percent of oxygen is determined by difference. The thermogravimetric analyzer was used to carry out the proximate analysis which was expressed in terms of moisture, volatile matter, fixed carbon and ash (Lua and Guo, 1998). The blend samples were prepared by different mixing ratios of WSS and FW. The WSS content added in blend was 0%, 30%, 50%, 70% and 100% in the test. 2.2. Thermogravimetric analysis (TGA)
Experiments were conducted at different reactor temperatures from 700 to 900 °C in 50 °C increment. At the start of each test, the electric furnace heater was turned onto heat the reactor to desired temperature. The gasification run started when pre-set temperature conditions were achieved and temperature of the gasifier stabilized. And the holding time of the feedstock was controlled at 45 s. The volatiles evolved from the feedstock flowed out the reactor and passed through a four sequential ice-cooled quencher. Condensable gas changed into liquid product was captured by tar collectors, while non-condensable gas passed through a metallic sieve to remove solid particles and then was sampled for analysis. Residue char was collected from the outlet and directly weighed as solid fraction. All runs were conducted for 30 min. The total volume of the gas was measured by gas flow meter. The gaseous products of H2, CH4, N2, CO, CO2, C2 (C2H4, C2H6) were analyzed by GC 9800T with a thermal conductivity detector (TCD). The columns used were TDX-01 for the analysis of H2, CH4, CO, CO2 and 5A, porapak Q for the analysis of C2H4, C2H6. The temperatures of injector, oven and detector were at 200, 85 and 90 °C, respectively. The carrier gas was argon in all analyzes. The gas standards were mixtures of H2, CH4, CO, CO2, C2H4 and C2H6. With further dilutions by different volumes of pure N2 standard, different concentrations of each gas compound were obtained to generate a calibration curve. 2.4. Methods of data processing The lower heating value (LHV) of fuel gas is defined as (Lv et al., 2004; He et al., 2009),
LHVðkJ=Nm3 Þ ¼ ðCO 126:36 þ H2 107:98 þ CH4 358:18 þ C2 H4 590:36 þ C2 H6 637:72Þ
ð1Þ
where, CO, H2, CH4, C2H4 and C2H6 are the molar percentages of components of the product gas. The carbon conversion efficiency is calculated by,
Previous study showed that the higher moisture content of WSS resulted in a lower measurement accuracy of TGA. Thus, the sample was dried to reduce the moisture content at 105 °C for 24 h before this test. Thermogravimetric analysis of the sample was carried out by Diamond TG/DTA (PerkinElmer Instruments, Shanghai, China). A sample mass of 3.5 ± 1.0 mg was used for the analysis in each experiment. Nitrogen was used as a carrier gas. The heating rate was controlled at 20 °C/min from 20 to 800 °C.
where, Y is the product gas yield (Nm3/kg), C% is the mass percentage of carbon in ultimate analysis of the sample, and CO, CO2, CH4, C2H4 and C2H6 are the molar percentages of components of the product gas.
2.3. Apparatus and procedures
3. Results and discussion
As shown in Fig. 1, the experimental apparatus mainly consists of electric furnace heater, gasifier, two-stage screw feeder, temperature controller, gas cleaning/drying system, tar collector and gas analyzer. Its typical installation area is about 2 m2 and the treatment capacity is designed to be 1.2 kg/h. The gasifier, is a horizontal cylindrical stainless steel tube, has a length of 1.2 m and an i.d. of 90 mm. It is heated by an electrical furnace heater to insure reaction region isothermal and the effective length of reaction region is 600 mm. Three thermocouples are inserted the heater to record reactor temperature. Feedstock was continuously fed into gasifier by two-stage screw feeder with a 1.2 kg/h of feed rate.
3.1. Thermogravimetric analysis
X C ð%Þ ¼ ½12YðCO% þ CO2 % þ CH4 % þ 2 C2 H4 % þ 2 C2 H6 %Þ=22:4 C% 100%
ð2Þ
In this study, TG and DTG of sewage sludge (SS), FW and SS/FW blends at the heating rate of 20 °C/min were investigated. Several characteristic parameters from the TGA data were discussed, such as the maximum weight loss rate (DTGmax), final weight loss (Dw) and temperature of the maximum weight loss rate (Tp). A clear difference between SS and FW was observed in TGA curves. This was depended on the characteristics of the organic and inorganic matter of the two materials. SS contained the higher amount of ash and lesser amount of volatile matter, which resulted
Table 1 Ultimate and proximate analyzes of the samples. Proximate analysis (dry basis, %)
WSS FW a
Ultimate analysis (dry basis, %)
Moisture content
Volatile matter
Fixed carbon
Ash
C
H
Oa
N
S
76.0 8.6
15.6 83.1
15.9 15.8
68.5 1.1
12.99 49.97
2.54 7.91
16.3 40.6
2.37 0.36
<0.05 0.06
By difference.
700
L. Peng et al. / Bioresource Technology 114 (2012) 698–702
Fig. 1. Experimental apparatus. 1- motor, 2- feedstock hopper, 3- control panel, 4- screw feeder, 5- gasifier, 6- electric heater, 7- thermocouple, 8- char outlet, 9- gas outlet, 10- tar condenser, 11- cotton filter, 12- gas flow meter, 13- valve, 14- gas analyzer.
3.2. Effect of WSS content Fig. 2 indicated the product distribution and gas composition at different WSS contents in the blends. As shown in Fig. 2(a), the gas yield dramatically decreased with the rapid increase in liquid yield (tar and water) when the WSS content varied from 0% to 100%. Meanwhile, a slight decrease from 18.9% to 6.6% was obtained for the char yield. The variation was because the addition of WSS led to the decrease in dry basis matter in the blends. Also, the steam generated from the moisture content of WSS was partly condensed into liquid fraction. When the feedstock was fed in the gasifier, the initial drying process occurred and the moisture content of WSS generated a
Product yield/%
(a)
80
60 Gas Char Tar+water
40
20
0
0
20
40
60
80
100
WSS ratio/%
(b)
Gas composition/%
in the lower Dw of 29.6% and lower DTGmax of 1.62%/min. The organic matter of SS was formed by complex fractions included biodegradable materials (such as hemicelluloses), dead bacteria cell and non-biodegradable polymers (Font et al., 2005). Thus, the TGA curve presented a stepwise degradation in a broad temperature range (200–700 °C) with an overlapping peak at Tp of 336 °C. In contrast, the TG curve of FW showed a major weight loss between 220 and 370 °C. The Dw was relatively high in comparison of the value of SS. And the DTGmax reached 16.1%/min, 9 times higher than that of the SS material. It can be interpreted by the fact that lignocellulosic biomass mainly consists of cellulose, hemicellulose, and lignin, and these compositions can be easily decomposed in the temperature range from 200 to 400 °C. For the blends, TG and DTG curves lie between the ones of the isolated materials. TG curves show a behavior similar to FW before 370 °C, due to the high contribution of volatile matter in FW. After 370 °C, it was easy to see the main contribution of the two materials. The DTGmax and Dw simultaneously increased with the increase of the FW content in the blends. It suggested that the thermal decomposition property of the blends would be improved by adding FW in appropriate proportion. Similar results were observed for the co-pyrolysis behavior of sewage sludge and rice straw Zhang et al. (2009). The TGA showed that rice straw significantly affected the release of volatile matter in the co-pyrolysis. With the increase of rice straw amount, the volatile matter release accelerated.
40
30
20 H2
CO
CO2
CH4
60
80
C2
10
0
0
20
40
100
WSS ratio/% Fig. 2. Effect of WSS content. (a) product distribution; (b) gas composition. Experimental conditions: feed rate, 1.2 kg/h; reactor temperature, 800 °C.
steam-rich atmosphere in the gasifier. Meanwhile, the feedstock was rapidly pyrolyzed to gas, char and tar Eq. (3). Then the second-
701
L. Peng et al. / Bioresource Technology 114 (2012) 698–702
ary reactions Eqs. (4)–(8) proceed, e.g. tar cracking, char gasification with steam, water gas shift and steam reforming reactions with intermediate products. The final gas composition of the cogasification process was depended on a series of complex and competing reactions (Lv et al., 2007).
Table 2 Effect of WSS content on gasification performanceb. WSS content (%) 3
Dry gas yield (Nm /kg) H2 yield (mol/kg) H2/CO LHV (MJ/Nm3) CCE (%)
Feedstock ! char þ tar þ waterðsteamÞ þ gasesðH2 ; CO;CO2 ; CH4 ; H2 O;Cn Hm Þ
ð3Þ b
C þ H2 O ! CO þ H2 131kJ=mol
ð5Þ
CO þ H2 O ! CO2 þ H2 þ 41kJ=mol
ð6Þ
C þ 2H2 ! CH4 75kJ=mol
ð7Þ
CH4 þ H2 O ! CO þ 3H2 206kJ=mol
ð8Þ
From Fig. 2(b), the concentrations of major gaseous products such as H2, CO, CO2, CH4 and C2, appeared a nonlinear trend with the WSS content. When the WSS content was from 0% to 50%, the H2 concentration significantly increased from 20.5% to 35.8%. In addition, the rise of WSS content led to a slight increase in CO concentration but a marked reduction in CO2 concentration. According to Zhang et al. (2011), steam generated from WSS took part in the Boudouard, water–gas shift and steam reforming reactions which contributed to the production of H2 and CO. However, the concentrations of H2 and CO appeared a slight decrease trend with the further increasing of WSS content from 50% to 100%. The increasing of WSS meant the decrease of organic matter and fixed carbon contents in blends, which resulted in small H2 and CO fractions released from pyrolysis and water–gas reactions. In addition, high WSS content resulted in the excess steam, which was favorable for converting CO to CO2 through the water–gas shift reaction. It may be proved by the significant increase in CO2 concentration from 25.3% to 38.6%. CmHn was mainly produced by the cracking of aromatic-ring aliphatic side chains and aliphatic hydrocarbons during the pyrolysis process (Zhang et al., 2011). Thus, the concentration of CmHn in product gas was relatively low, and slightly decreased. The effect of WSS content on co-gasification performance was also listed in Table 2. WSS presented the relatively low dry gas yield and H2 yield in comparison with the FW. The addition of FW improved dry gas yield and H2 yield during the co-gasification. When the WSS content was 30%, the dry gas yield and H2 yield respectively reached the maximum values of 0.62 Nm3/kg and 8.97 mol/kg. It seems that synergetic effects occurred in the cogasification of WSS/FW in the experimental conditions. The carbon conversion efficiency increased with the increasing WSS content, while the LHV of the fuel gas gradually decreased. 3.3. Effect of reactor temperature Fig 3(a) indicates the distribution of products from co-gasification of WSS and FW at different reactor temperatures. As expected, the increase in temperature resulted in the increasing gas yield with the reduction in the yields of char and liquids. The variation was mainly attributed to two reasons: (1) gaseous product from the tar cracking and the char gasification, which were favorable at elevated temperatures, (2) higher gas production in the initial pyrolysis step at the higher heating rate due to the higher temperature. The effect of reactor temperature on the gas composition is presented in Fig. 3(b). As shown in Fig. 3(b), the elevating reactor temperature led to the increase in H2 and CO concentrations. However,
70
50
30
0
0.07 0.86 1.47 11.27 72.21
0.30 4.03 1.33 11.65 70.33
0.48 7.67 1.35 12.03 68.67
0.62 8.97 1.30 12.17 68.55
0.59 5.40 0.83 14.95 57.86
Feed rate: 1.2 kg/h, reactor temperature: 800 °C.
(a)
80
60
Product yield/%
ð4Þ
Gas Char Tar+water
40
20
0 700
750
800
850
900
T/ºC
(b)
Gas composition/%
C þ CO2 ! 2CO 172kJ=mol
100
40
30
20 H2
CO
CO2
CH4
C2
10
0 700
750
800 T/ºC
850
900
Fig. 3. Effect of reactor temperature. (a) product distribution; (b) gas composition. Experimental conditions: Feed rate, 1.2 kg/h; WSS content, 30%.
the content of CO2 decreased from 33.1% to 20.4% with the increasing reactor temperature. The endothermic Boudouard and water gas reactions (Eqs. (4) and (5)) strengthened by increasing temperature, were the main factors responsible for the increase in H2 and CO contents in the co-gasification process. Moreover, steam reforming reaction Eq. (8) was favored at high temperature, which accounted for a slight decrease in CH4 concentration as temperature increased. CmHn concentration was also observed to decrease owing to the further cracking of hydrocarbons at high temperature. The evolution of the main gaseous products was similar to the trend observed in steam gasification of dry sewage sludge (Nipattummakul et al., 2010b). The effect of reactor temperature on co-gasification performance was presented in Table 3. It can be seen that the higher temperature led to the higher dry gas yield, H2 yield and carbon
702
L. Peng et al. / Bioresource Technology 114 (2012) 698–702
sity of Science and Technology for carrying out the ultimate analysis of the samples.
Table 3 Effect of reactor temperature on gasification performancec. Reactor temperature (°C) 3
Dry gas yield (Nm /kg) H2 yield (mol/kg) H2/CO LHV (MJ/Nm3) CCE (%) c
700
750
800
850
900
0.46 4.70 0.93 12.72 58.89
0.54 6.12 0.94 12.40 65.91
0.61 9.10 1.16 12.13 66.36
0.66 10.67 1.19 12.07 69.46
0.70 11.67 1.23 11.89 70.38
Feed rate: 1.2 kg/h, WSS content: 30%.
conversion efficiency. Furthermore, the LHV of fuel gas slightly decreased from 12.72 MJ/Nm3 to 11.89 MJ/Nm3, when the reactor temperature increased from 700 to 900 °C. It was attributed to the remarkable decreases in CH4 content from 9.6% to 4.7% and C2 content from 5.9% to 3.6%. The LHV of the fuel gas mainly depended on the concentrations of methane and hydrocarbon, because they had much higher heating value than other gases components such as H2, CO. The LHV of the product gas in this study was relatively high, compared with the values obtained from previous sewage sludge gasification processes. The LHV of gas was less than 4 MJ/Nm3 in air–steam gasification of dried sewage sludge with catalysts (Andrés et al., 2011). Xie et al. (2010) studied the effect of moisture content on the air gasification of three kinds of sewage sludge. The results showed that the LHV of the product gas ranged from 6 MJ/ Nm3 to 8 MJ/Nm3 during the gasification at different moisture content from 0% to 50%. Chun et al. (2011) reported that the LHV of gas was about 10 MJ/Nm3 from the steam gasification of dried sewage sludge in a combined screw and rotary kiln gasifier. 4. Conclusions Co-gasification of wet sewage sludge and forestry waste was investigated in a lab-scale fixed bed gasifier. The addition of FW to WSS decreased the moisture content and improved the volatile matter content in the blends. The steam in situ generated from the moisture content of WSS promoted the concentrations of H2 and CO in the product gas during the gasification. The optimal WSS content and reactor temperature were 30% and 900 °C, respectively. Under the optimal condition, the dry gas yield was 0.70 Nm3/kg, the H2 yield 11.67 mol/kg, the LHV of product gas was 11.89 MJ/Nm3. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 20876066) and Hubei Province Scientific and Technological Project (No. 2007AA204B01). The authors are grateful to the Analytical and Testing Center of Huazhong Univer-
References Aigner, I., Pfeifer, C., Hofbauer, H., 2011. Co-gasification of coal and wood in a dual fluidized bed gasifier. Fuel 90, 2404–2412. Andrés, J.M., Narros, A., Rodríguez, M.E., 2011. Behaviour of dolomite, olivine and alumina as primary catalysts in air–steam gasification of sewage sludge. Fuel 90, 521–527. Chun, Y.N., Kim, S.C., Yoshikawa, K., 2011. Pyrolysis gasification of dried sewage sludge in a combined screw and rotary kiln gasifier. Appl. Energy 88, 1105– 1112. Domínguez, A., Menéndez, J.A., Pis, J.J., 2006. Hydrogen rich fuel gas production from the pyrolysis of wet sewage sludge at high temperature. Int. J. Hydrogen. Energy. 77, 127–132. Font, R., Fullana, A., Conesa, J., 2005. Kinetic models for the pyrolysis and combustion of two types of sewage sludge. J. Anal. Appl. Pyrol. 74, 429–438. Groß, B., Eder, C., Grziwa, P., Horst, J., Kimmerle, K., 2008. Energy recovery from sewage sludge by means of fluidised bed gasification. Waste. Manag. 28, 1819– 1826. He, M.Y., Xiao, B., Hu, Z.Q., Liu, S.M., Guo, X.J., Luo, S.Y., 2009. Syngas production from catalytic gasification of waste polyethylene: influence of temperature on gas yield and composition. Int. J. Hydrogen. Energy. 34, 1342–1348. Howaniec, N., Smolin´ski, A., Stan´czyk, K., Pichlak, M., 2011. Steam co-gasification of coal and biomass derived chars with synergy effect as an innovative way of hydrogen-rich gas production. Int. J. Hydrogen. Energy 36, 14455–14463. Ji, A.M., Zhang, S.T., Lu, X.B., Liu, Y., 2010. A new method for evaluating the sewage sludge pyrolysis kinetics. Waste. Manag. 30, 1225–1229. Lei, Z.F., Chen, J.Y., Zhang, Z.Y., Sugiura, Norio., 2010. Methane production from rice straw with acclimated anaerobic sludge: effect of phosphate supplementation. Bioresour. Technol. 101, 4343–4348. Lua, A.C., Guo, J., 1998. Preparation and characterization of chars from oil palm waste. Carbon 36, 1663–1670. Lv, P.M., Xiong, Z.H., Chang, J., Wu, C.Z., Chen, Y., Zhu, J.X., 2004. An experimental study on biomass air–steam gasification in a fluidized bed. Bioresour. Technol. 95, 95–101. Lv, P.M., Yuan, Z.H., Ma, L.L., Wu, C.Z., Chen, Y., Zhu, J.X., 2007. Hydrogen-rich gas production from biomass air and oxygen/steam gasification in a downdraft gasifier. Renew. Energy 32, 2173–2185. Mastellone, M.L., Zaccariello, L., Arena, U., 2010. Co-gasification of coal, plastic waste and wood in a bubbling fluidized bed reactor. Fuel 89, 2991–3000. Nipattummakul, N., Ahmed, I., Kerdsuwan, S., Gupta, A.K., 2010a. High temperature steam gasification of wastewater sludge. Appl. Energy 87, 3729–3734. Nipattummakul, N., Ahmed, I., Kerdsuwan, S., Gupta, A.K., 2010b. Hydrogen and syngas production from sewage sludge via steam gasification. Int. J. Hydrogen. Energy 35, 11738–11745. Pokorna, E., Postelmans, N., Jenicek, P., Schreurs, S., Carleer, R., Yperman, J., 2009. Study of bio-oils and solids from flash pyrolysis of sewage sludges. Fuel 88, 1344–1350. Sánchez, M.E., Martínez, O., Gómez, X., Morán, A., 2007. Pyrolysis of mixtures of sewage sludge and manure: a comparison of the results obtained in the laboratory (semi-pilot) and in a pilot plant. Waste. Manag. 27, 1328–1334. Seggiani, M., Vitolo, S., Puccini, M., Bellini, A., 2012. Cogasification of sewage sludge in an updraft gasifier. Fuel 93, 486–491. Werle, S., Wilk, R.K., 2010. A review of methods for the thermal utilization of sewage sludge: the Polish perspective. Renew. Energy 35, 1914–1919. Xie, L.P., Li, T., Gao, J.D., Fei, X.N., Wu, X., Jiang, Y.G., 2010. Effect of moisture content in sewage sludge on air gasification. J. Fuel. Chem. Tech. 38, 615–620. Zhang, S.Q., Yue, X.M., Yin, Z.Y., Pan, T.T., Dong, M.J., Sun, T.Y., 2009. Study of the copyrolysis behavior of sewage-sludge/rice-straw and the kinetics. Earth. Planet. Sci. 1, 661–666. Zhang, B.P., Xiong, S.J., Xiao, B., Yu, D.K., Jia, X.Y., 2011. Mechanism of wet sewage sludge pyrolysis in a tubular furnace. Int. J. Hydrogen. Energy. 36, 355–363.
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Co-gasification of wet sewage sludge and forestry waste in situ steam agent Lixin Peng a, Yongxiu Wang a, Zhihong Lei a, Gong Cheng a,b,⇑ a b
Shenzhen Academy of Environment and Science, Shenzhen 518001, PR China School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 19 January 2012 Received in revised form 22 March 2012 Accepted 24 March 2012 Available online 30 March 2012 Keywords: Co-gasification Wet sewage sludge (WSS) Forestry waste (FW) Fuel gas
a b s t r a c t The co-gasification of wet sewage sludge (80 wt.% moisture, WSS) and forestry waste (FW) blends was studied. The thermogravimetric analysis showed that weight loss and the maximum weight loss rate of the sample increased with the increase in FW content. The co-gasification process was performed in a lab-scale fixed bed gasifier to investigate the effects of WSS content and reactor temperature on product yields, gas composition and gasification performance. The results indicated that steam generated from the moisture content in WSS took part in the gasification with char. The gas yield decreased with the increasing WSS content. And the concentrations of H2 and CO reached the maximum when the WSS content was 50%. The LHV of fuel gas ranged from 11.89 MJ/Nm3 to 12.72 MJ/Nm3 when the reactor temperature increased from 700 °C to 900 °C. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Sewage sludge, an unavoidable by-product of municipal wastewater treatment plants, is known to contain numerous harmful chemicals (Ji et al., 2010). In the coming years, the amount of sewage sludge will increase considerably due to municipal development and escalating populations in cities (Sánchez et al., 2007). The presence of harmful substances such as heavy metals, poorly biodegradable organic compounds, viruses, pharmaceuticals and hormones, may represent a complication in the disposal management strategies. Currently, sewage sludge has mainly been handled via landfill, incineration, anaerobic digestion and application to agricultural land (Pokorna et al., 2009; Lei et al., 2010). The landfill disposal has become much less acceptable in many countries (Seggiani et al., 2012), because it requires a lot of space and the soil has to be sealed adequately to prevent the leaching of toxic compounds. The harmful substances in sewage sludge would enter the food chain again by the utilization as fertilizers on farmland (Groß et al., 2008). In addition, incineration was accompanied with the emission of secondary contaminant materials (Chun et al., 2011). Gasification of sewage sludge was considered as the potential technology, due to the advantages of converting the sludge into combustible gaseous products by reducing its volume, preventing from the toxic organic compounds, and fixing the heavy metals in the solid residue. Gasification of sewage sludge was associated
⇑ Corresponding author at: Shenzhen Academy of Environment and Science, Shenzhen 518001, PR China. Tel./fax: +86 0755 25589688 2042. E-mail address: [email protected] (G. Cheng). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.03.079
with the extensive experience in biomass gasification. Therefore, it was natural to build on this experience (Nipattummakul et al., 2010a; Werle and Wilk, 2010). However, wet sewage sludge (WSS) from the wastewater treatment plant, after dewatered by filter pressing or centrifugation, still contained 80 wt.% of water (Zhang et al., 2011). The high moisture content was one of the major drawbacks for the gasification of sewage sludge. Domínguez et al. (2006) suggested that long gas residence times and high heating rates were required to maximize gas production during the pyrolysis and gasification of WSS. Zhang et al. (2011) showed that the gas yield was less than 40% in WSS pyrolysis with situ steam. The excessive steam generated from WSS condensed into liquid fraction, which resulted in 50–70% of liquid yield. Also, many researchers studied fuel gas or hydrogen-rich syngas produced from the gasification of dried sludge feedstock (Nipattummakul et al., 2010b; Andrés et al., 2011). But the drying process used to remove the water, consumed a large amount of energy and increased the cost of disposal considerably. Co-gasification based on two different materials has received much attention in recent years, since it may compensate their weakness each other (Mastellone et al., 2010; Aigner et al., 2011; Howaniec et al., 2011). Biomass was abundant, renewable, and low cost. Forestry waste, one of the important biomass sources, was considered as a suitable feedstock for gasification. Compared with sewage sludge, forestry waste contained high contents of volatile matter and fixed carbon, but low ash and moisture contents. The addition of forestry waste to wet sewage sludge in appropriate proportion could adjust the moisture content of the blends. The presence of water also generated the steam in situ, which would increase the production of hydrogen by gasification reaction with char.
699
L. Peng et al. / Bioresource Technology 114 (2012) 698–702
The aim of this study was to investigate the performance of co-gasification of wet sewage sludge (WSS) and forestry waste (FW) in a lab-scale fixed bed gasifier. Experimental tests were carried out at different WSS contents and reactor temperatures. The co-gasification performance was evaluated in terms of dry gas yield and gas composition, and H2 yield. 2. Methods 2.1. Materials The wet sewage sludge (80% of moisture content) produced in a Wuhan urban wastewater treatment plant fed with municipal waters was dewatered by filter press and then collected as original material. Forestry waste (FW) was from a farm in Wuhan City, Hubei Province, China. It mainly consisted of 92 wt.% pine sawdust, 6 wt.% branches and 2 wt.% leaves. The material was dried under the sun for a period of 7 days to reduce the moisture content and then was shredded into particle size between 0.125 and 0.25 mm. The proximate and ultimate analyzes of two materials were listed in Table 1. Ultimate analysis was obtained with a CHNS/O analyzer (Vario Micro cube, Elementar). Such analysis gives the weight percent of carbon, hydrogen, nitrogen, and sulfur in the sample simultaneously, and the weight percent of oxygen is determined by difference. The thermogravimetric analyzer was used to carry out the proximate analysis which was expressed in terms of moisture, volatile matter, fixed carbon and ash (Lua and Guo, 1998). The blend samples were prepared by different mixing ratios of WSS and FW. The WSS content added in blend was 0%, 30%, 50%, 70% and 100% in the test. 2.2. Thermogravimetric analysis (TGA)
Experiments were conducted at different reactor temperatures from 700 to 900 °C in 50 °C increment. At the start of each test, the electric furnace heater was turned onto heat the reactor to desired temperature. The gasification run started when pre-set temperature conditions were achieved and temperature of the gasifier stabilized. And the holding time of the feedstock was controlled at 45 s. The volatiles evolved from the feedstock flowed out the reactor and passed through a four sequential ice-cooled quencher. Condensable gas changed into liquid product was captured by tar collectors, while non-condensable gas passed through a metallic sieve to remove solid particles and then was sampled for analysis. Residue char was collected from the outlet and directly weighed as solid fraction. All runs were conducted for 30 min. The total volume of the gas was measured by gas flow meter. The gaseous products of H2, CH4, N2, CO, CO2, C2 (C2H4, C2H6) were analyzed by GC 9800T with a thermal conductivity detector (TCD). The columns used were TDX-01 for the analysis of H2, CH4, CO, CO2 and 5A, porapak Q for the analysis of C2H4, C2H6. The temperatures of injector, oven and detector were at 200, 85 and 90 °C, respectively. The carrier gas was argon in all analyzes. The gas standards were mixtures of H2, CH4, CO, CO2, C2H4 and C2H6. With further dilutions by different volumes of pure N2 standard, different concentrations of each gas compound were obtained to generate a calibration curve. 2.4. Methods of data processing The lower heating value (LHV) of fuel gas is defined as (Lv et al., 2004; He et al., 2009),
LHVðkJ=Nm3 Þ ¼ ðCO 126:36 þ H2 107:98 þ CH4 358:18 þ C2 H4 590:36 þ C2 H6 637:72Þ
ð1Þ
where, CO, H2, CH4, C2H4 and C2H6 are the molar percentages of components of the product gas. The carbon conversion efficiency is calculated by,
Previous study showed that the higher moisture content of WSS resulted in a lower measurement accuracy of TGA. Thus, the sample was dried to reduce the moisture content at 105 °C for 24 h before this test. Thermogravimetric analysis of the sample was carried out by Diamond TG/DTA (PerkinElmer Instruments, Shanghai, China). A sample mass of 3.5 ± 1.0 mg was used for the analysis in each experiment. Nitrogen was used as a carrier gas. The heating rate was controlled at 20 °C/min from 20 to 800 °C.
where, Y is the product gas yield (Nm3/kg), C% is the mass percentage of carbon in ultimate analysis of the sample, and CO, CO2, CH4, C2H4 and C2H6 are the molar percentages of components of the product gas.
2.3. Apparatus and procedures
3. Results and discussion
As shown in Fig. 1, the experimental apparatus mainly consists of electric furnace heater, gasifier, two-stage screw feeder, temperature controller, gas cleaning/drying system, tar collector and gas analyzer. Its typical installation area is about 2 m2 and the treatment capacity is designed to be 1.2 kg/h. The gasifier, is a horizontal cylindrical stainless steel tube, has a length of 1.2 m and an i.d. of 90 mm. It is heated by an electrical furnace heater to insure reaction region isothermal and the effective length of reaction region is 600 mm. Three thermocouples are inserted the heater to record reactor temperature. Feedstock was continuously fed into gasifier by two-stage screw feeder with a 1.2 kg/h of feed rate.
3.1. Thermogravimetric analysis
X C ð%Þ ¼ ½12YðCO% þ CO2 % þ CH4 % þ 2 C2 H4 % þ 2 C2 H6 %Þ=22:4 C% 100%
ð2Þ
In this study, TG and DTG of sewage sludge (SS), FW and SS/FW blends at the heating rate of 20 °C/min were investigated. Several characteristic parameters from the TGA data were discussed, such as the maximum weight loss rate (DTGmax), final weight loss (Dw) and temperature of the maximum weight loss rate (Tp). A clear difference between SS and FW was observed in TGA curves. This was depended on the characteristics of the organic and inorganic matter of the two materials. SS contained the higher amount of ash and lesser amount of volatile matter, which resulted
Table 1 Ultimate and proximate analyzes of the samples. Proximate analysis (dry basis, %)
WSS FW a
Ultimate analysis (dry basis, %)
Moisture content
Volatile matter
Fixed carbon
Ash
C
H
Oa
N
S
76.0 8.6
15.6 83.1
15.9 15.8
68.5 1.1
12.99 49.97
2.54 7.91
16.3 40.6
2.37 0.36
<0.05 0.06
By difference.
700
L. Peng et al. / Bioresource Technology 114 (2012) 698–702
Fig. 1. Experimental apparatus. 1- motor, 2- feedstock hopper, 3- control panel, 4- screw feeder, 5- gasifier, 6- electric heater, 7- thermocouple, 8- char outlet, 9- gas outlet, 10- tar condenser, 11- cotton filter, 12- gas flow meter, 13- valve, 14- gas analyzer.
3.2. Effect of WSS content Fig. 2 indicated the product distribution and gas composition at different WSS contents in the blends. As shown in Fig. 2(a), the gas yield dramatically decreased with the rapid increase in liquid yield (tar and water) when the WSS content varied from 0% to 100%. Meanwhile, a slight decrease from 18.9% to 6.6% was obtained for the char yield. The variation was because the addition of WSS led to the decrease in dry basis matter in the blends. Also, the steam generated from the moisture content of WSS was partly condensed into liquid fraction. When the feedstock was fed in the gasifier, the initial drying process occurred and the moisture content of WSS generated a
Product yield/%
(a)
80
60 Gas Char Tar+water
40
20
0
0
20
40
60
80
100
WSS ratio/%
(b)
Gas composition/%
in the lower Dw of 29.6% and lower DTGmax of 1.62%/min. The organic matter of SS was formed by complex fractions included biodegradable materials (such as hemicelluloses), dead bacteria cell and non-biodegradable polymers (Font et al., 2005). Thus, the TGA curve presented a stepwise degradation in a broad temperature range (200–700 °C) with an overlapping peak at Tp of 336 °C. In contrast, the TG curve of FW showed a major weight loss between 220 and 370 °C. The Dw was relatively high in comparison of the value of SS. And the DTGmax reached 16.1%/min, 9 times higher than that of the SS material. It can be interpreted by the fact that lignocellulosic biomass mainly consists of cellulose, hemicellulose, and lignin, and these compositions can be easily decomposed in the temperature range from 200 to 400 °C. For the blends, TG and DTG curves lie between the ones of the isolated materials. TG curves show a behavior similar to FW before 370 °C, due to the high contribution of volatile matter in FW. After 370 °C, it was easy to see the main contribution of the two materials. The DTGmax and Dw simultaneously increased with the increase of the FW content in the blends. It suggested that the thermal decomposition property of the blends would be improved by adding FW in appropriate proportion. Similar results were observed for the co-pyrolysis behavior of sewage sludge and rice straw Zhang et al. (2009). The TGA showed that rice straw significantly affected the release of volatile matter in the co-pyrolysis. With the increase of rice straw amount, the volatile matter release accelerated.
40
30
20 H2
CO
CO2
CH4
60
80
C2
10
0
0
20
40
100
WSS ratio/% Fig. 2. Effect of WSS content. (a) product distribution; (b) gas composition. Experimental conditions: feed rate, 1.2 kg/h; reactor temperature, 800 °C.
steam-rich atmosphere in the gasifier. Meanwhile, the feedstock was rapidly pyrolyzed to gas, char and tar Eq. (3). Then the second-
701
L. Peng et al. / Bioresource Technology 114 (2012) 698–702
ary reactions Eqs. (4)–(8) proceed, e.g. tar cracking, char gasification with steam, water gas shift and steam reforming reactions with intermediate products. The final gas composition of the cogasification process was depended on a series of complex and competing reactions (Lv et al., 2007).
Table 2 Effect of WSS content on gasification performanceb. WSS content (%) 3
Dry gas yield (Nm /kg) H2 yield (mol/kg) H2/CO LHV (MJ/Nm3) CCE (%)
Feedstock ! char þ tar þ waterðsteamÞ þ gasesðH2 ; CO;CO2 ; CH4 ; H2 O;Cn Hm Þ
ð3Þ b
C þ H2 O ! CO þ H2 131kJ=mol
ð5Þ
CO þ H2 O ! CO2 þ H2 þ 41kJ=mol
ð6Þ
C þ 2H2 ! CH4 75kJ=mol
ð7Þ
CH4 þ H2 O ! CO þ 3H2 206kJ=mol
ð8Þ
From Fig. 2(b), the concentrations of major gaseous products such as H2, CO, CO2, CH4 and C2, appeared a nonlinear trend with the WSS content. When the WSS content was from 0% to 50%, the H2 concentration significantly increased from 20.5% to 35.8%. In addition, the rise of WSS content led to a slight increase in CO concentration but a marked reduction in CO2 concentration. According to Zhang et al. (2011), steam generated from WSS took part in the Boudouard, water–gas shift and steam reforming reactions which contributed to the production of H2 and CO. However, the concentrations of H2 and CO appeared a slight decrease trend with the further increasing of WSS content from 50% to 100%. The increasing of WSS meant the decrease of organic matter and fixed carbon contents in blends, which resulted in small H2 and CO fractions released from pyrolysis and water–gas reactions. In addition, high WSS content resulted in the excess steam, which was favorable for converting CO to CO2 through the water–gas shift reaction. It may be proved by the significant increase in CO2 concentration from 25.3% to 38.6%. CmHn was mainly produced by the cracking of aromatic-ring aliphatic side chains and aliphatic hydrocarbons during the pyrolysis process (Zhang et al., 2011). Thus, the concentration of CmHn in product gas was relatively low, and slightly decreased. The effect of WSS content on co-gasification performance was also listed in Table 2. WSS presented the relatively low dry gas yield and H2 yield in comparison with the FW. The addition of FW improved dry gas yield and H2 yield during the co-gasification. When the WSS content was 30%, the dry gas yield and H2 yield respectively reached the maximum values of 0.62 Nm3/kg and 8.97 mol/kg. It seems that synergetic effects occurred in the cogasification of WSS/FW in the experimental conditions. The carbon conversion efficiency increased with the increasing WSS content, while the LHV of the fuel gas gradually decreased. 3.3. Effect of reactor temperature Fig 3(a) indicates the distribution of products from co-gasification of WSS and FW at different reactor temperatures. As expected, the increase in temperature resulted in the increasing gas yield with the reduction in the yields of char and liquids. The variation was mainly attributed to two reasons: (1) gaseous product from the tar cracking and the char gasification, which were favorable at elevated temperatures, (2) higher gas production in the initial pyrolysis step at the higher heating rate due to the higher temperature. The effect of reactor temperature on the gas composition is presented in Fig. 3(b). As shown in Fig. 3(b), the elevating reactor temperature led to the increase in H2 and CO concentrations. However,
70
50
30
0
0.07 0.86 1.47 11.27 72.21
0.30 4.03 1.33 11.65 70.33
0.48 7.67 1.35 12.03 68.67
0.62 8.97 1.30 12.17 68.55
0.59 5.40 0.83 14.95 57.86
Feed rate: 1.2 kg/h, reactor temperature: 800 °C.
(a)
80
60
Product yield/%
ð4Þ
Gas Char Tar+water
40
20
0 700
750
800
850
900
T/ºC
(b)
Gas composition/%
C þ CO2 ! 2CO 172kJ=mol
100
40
30
20 H2
CO
CO2
CH4
C2
10
0 700
750
800 T/ºC
850
900
Fig. 3. Effect of reactor temperature. (a) product distribution; (b) gas composition. Experimental conditions: Feed rate, 1.2 kg/h; WSS content, 30%.
the content of CO2 decreased from 33.1% to 20.4% with the increasing reactor temperature. The endothermic Boudouard and water gas reactions (Eqs. (4) and (5)) strengthened by increasing temperature, were the main factors responsible for the increase in H2 and CO contents in the co-gasification process. Moreover, steam reforming reaction Eq. (8) was favored at high temperature, which accounted for a slight decrease in CH4 concentration as temperature increased. CmHn concentration was also observed to decrease owing to the further cracking of hydrocarbons at high temperature. The evolution of the main gaseous products was similar to the trend observed in steam gasification of dry sewage sludge (Nipattummakul et al., 2010b). The effect of reactor temperature on co-gasification performance was presented in Table 3. It can be seen that the higher temperature led to the higher dry gas yield, H2 yield and carbon
702
L. Peng et al. / Bioresource Technology 114 (2012) 698–702
sity of Science and Technology for carrying out the ultimate analysis of the samples.
Table 3 Effect of reactor temperature on gasification performancec. Reactor temperature (°C) 3
Dry gas yield (Nm /kg) H2 yield (mol/kg) H2/CO LHV (MJ/Nm3) CCE (%) c
700
750
800
850
900
0.46 4.70 0.93 12.72 58.89
0.54 6.12 0.94 12.40 65.91
0.61 9.10 1.16 12.13 66.36
0.66 10.67 1.19 12.07 69.46
0.70 11.67 1.23 11.89 70.38
Feed rate: 1.2 kg/h, WSS content: 30%.
conversion efficiency. Furthermore, the LHV of fuel gas slightly decreased from 12.72 MJ/Nm3 to 11.89 MJ/Nm3, when the reactor temperature increased from 700 to 900 °C. It was attributed to the remarkable decreases in CH4 content from 9.6% to 4.7% and C2 content from 5.9% to 3.6%. The LHV of the fuel gas mainly depended on the concentrations of methane and hydrocarbon, because they had much higher heating value than other gases components such as H2, CO. The LHV of the product gas in this study was relatively high, compared with the values obtained from previous sewage sludge gasification processes. The LHV of gas was less than 4 MJ/Nm3 in air–steam gasification of dried sewage sludge with catalysts (Andrés et al., 2011). Xie et al. (2010) studied the effect of moisture content on the air gasification of three kinds of sewage sludge. The results showed that the LHV of the product gas ranged from 6 MJ/ Nm3 to 8 MJ/Nm3 during the gasification at different moisture content from 0% to 50%. Chun et al. (2011) reported that the LHV of gas was about 10 MJ/Nm3 from the steam gasification of dried sewage sludge in a combined screw and rotary kiln gasifier. 4. Conclusions Co-gasification of wet sewage sludge and forestry waste was investigated in a lab-scale fixed bed gasifier. The addition of FW to WSS decreased the moisture content and improved the volatile matter content in the blends. The steam in situ generated from the moisture content of WSS promoted the concentrations of H2 and CO in the product gas during the gasification. The optimal WSS content and reactor temperature were 30% and 900 °C, respectively. Under the optimal condition, the dry gas yield was 0.70 Nm3/kg, the H2 yield 11.67 mol/kg, the LHV of product gas was 11.89 MJ/Nm3. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 20876066) and Hubei Province Scientific and Technological Project (No. 2007AA204B01). The authors are grateful to the Analytical and Testing Center of Huazhong Univer-
References Aigner, I., Pfeifer, C., Hofbauer, H., 2011. Co-gasification of coal and wood in a dual fluidized bed gasifier. Fuel 90, 2404–2412. Andrés, J.M., Narros, A., Rodríguez, M.E., 2011. Behaviour of dolomite, olivine and alumina as primary catalysts in air–steam gasification of sewage sludge. Fuel 90, 521–527. Chun, Y.N., Kim, S.C., Yoshikawa, K., 2011. Pyrolysis gasification of dried sewage sludge in a combined screw and rotary kiln gasifier. Appl. Energy 88, 1105– 1112. Domínguez, A., Menéndez, J.A., Pis, J.J., 2006. Hydrogen rich fuel gas production from the pyrolysis of wet sewage sludge at high temperature. Int. J. Hydrogen. Energy. 77, 127–132. Font, R., Fullana, A., Conesa, J., 2005. Kinetic models for the pyrolysis and combustion of two types of sewage sludge. J. Anal. Appl. Pyrol. 74, 429–438. Groß, B., Eder, C., Grziwa, P., Horst, J., Kimmerle, K., 2008. Energy recovery from sewage sludge by means of fluidised bed gasification. Waste. Manag. 28, 1819– 1826. He, M.Y., Xiao, B., Hu, Z.Q., Liu, S.M., Guo, X.J., Luo, S.Y., 2009. Syngas production from catalytic gasification of waste polyethylene: influence of temperature on gas yield and composition. Int. J. Hydrogen. Energy. 34, 1342–1348. Howaniec, N., Smolin´ski, A., Stan´czyk, K., Pichlak, M., 2011. Steam co-gasification of coal and biomass derived chars with synergy effect as an innovative way of hydrogen-rich gas production. Int. J. Hydrogen. Energy 36, 14455–14463. Ji, A.M., Zhang, S.T., Lu, X.B., Liu, Y., 2010. A new method for evaluating the sewage sludge pyrolysis kinetics. Waste. Manag. 30, 1225–1229. Lei, Z.F., Chen, J.Y., Zhang, Z.Y., Sugiura, Norio., 2010. Methane production from rice straw with acclimated anaerobic sludge: effect of phosphate supplementation. Bioresour. Technol. 101, 4343–4348. Lua, A.C., Guo, J., 1998. Preparation and characterization of chars from oil palm waste. Carbon 36, 1663–1670. Lv, P.M., Xiong, Z.H., Chang, J., Wu, C.Z., Chen, Y., Zhu, J.X., 2004. An experimental study on biomass air–steam gasification in a fluidized bed. Bioresour. Technol. 95, 95–101. Lv, P.M., Yuan, Z.H., Ma, L.L., Wu, C.Z., Chen, Y., Zhu, J.X., 2007. Hydrogen-rich gas production from biomass air and oxygen/steam gasification in a downdraft gasifier. Renew. Energy 32, 2173–2185. Mastellone, M.L., Zaccariello, L., Arena, U., 2010. Co-gasification of coal, plastic waste and wood in a bubbling fluidized bed reactor. Fuel 89, 2991–3000. Nipattummakul, N., Ahmed, I., Kerdsuwan, S., Gupta, A.K., 2010a. High temperature steam gasification of wastewater sludge. Appl. Energy 87, 3729–3734. Nipattummakul, N., Ahmed, I., Kerdsuwan, S., Gupta, A.K., 2010b. Hydrogen and syngas production from sewage sludge via steam gasification. Int. J. Hydrogen. Energy 35, 11738–11745. Pokorna, E., Postelmans, N., Jenicek, P., Schreurs, S., Carleer, R., Yperman, J., 2009. Study of bio-oils and solids from flash pyrolysis of sewage sludges. Fuel 88, 1344–1350. Sánchez, M.E., Martínez, O., Gómez, X., Morán, A., 2007. Pyrolysis of mixtures of sewage sludge and manure: a comparison of the results obtained in the laboratory (semi-pilot) and in a pilot plant. Waste. Manag. 27, 1328–1334. Seggiani, M., Vitolo, S., Puccini, M., Bellini, A., 2012. Cogasification of sewage sludge in an updraft gasifier. Fuel 93, 486–491. Werle, S., Wilk, R.K., 2010. A review of methods for the thermal utilization of sewage sludge: the Polish perspective. Renew. Energy 35, 1914–1919. Xie, L.P., Li, T., Gao, J.D., Fei, X.N., Wu, X., Jiang, Y.G., 2010. Effect of moisture content in sewage sludge on air gasification. J. Fuel. Chem. Tech. 38, 615–620. Zhang, S.Q., Yue, X.M., Yin, Z.Y., Pan, T.T., Dong, M.J., Sun, T.Y., 2009. Study of the copyrolysis behavior of sewage-sludge/rice-straw and the kinetics. Earth. Planet. Sci. 1, 661–666. Zhang, B.P., Xiong, S.J., Xiao, B., Yu, D.K., Jia, X.Y., 2011. Mechanism of wet sewage sludge pyrolysis in a tubular furnace. Int. J. Hydrogen. Energy. 36, 355–363.