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Effect of Experimental Conditions on Gas Quality and Solids Produced by Sewage Sludge Cogasification. 2. Sewage Sludge Mixed with Biomass Filomena Pinto,* Rui Neto Andre´, Helena Lopes, Ma´rio Dias, I. Gulyurtlu, and I. Cabrita Instituto Nacional de Engenharia, Tecnologia e InoVação (INETI), Departamento de Engenharia Energética e Controlo Ambiental (DEECA), Est. Pac¸o do Lumiar, 22, 1649-038 Lisboa, Portugal ReceiVed December 19, 2007. ReVised Manuscript ReceiVed March 3, 2008
This is the study of environmental implications of sewage sludge cogasification mixed with biomass wastes (straw pellets). NH3, H2S, and HCl contents of syngas were compared to those produced during cogasification of sewage sludge with coal. In the presence of straw pellets, lower concentrations of NH3 and higher contents of HCl were obtained, probably due to its low and high contents of N and Cl, respectively. However, NH3 and HCl contents in the syngas were very low and similar for both sewage sludge blends with straw pellets or with coal, because most of these compounds were retained in the condensation system of the gasification installation. Lower H2S levels were also measured when blends of sewage sludge with straw pellets were used, probably due to the lower sulfur contents of straw pellets. The results obtained show that it is possible to cogasify sewage sludge with either biomass, such as straw pellets, or coal, without major modifications in previously existing gasification installations. However, because the release of undesirable S, N, and Cl compounds is most affected by the contents of these elements in the fuels, special care should be taken when substituting one fuel for another and an adjustment of experimental conditions is most advisable. The results obtained showed that sewage sludge should be cogasified with straw pellets to take profit of its diluting effect, although effective sorbents could be added to the gasification medium to achieve further reductions of sulfur compound release. Gasification solid residues were analyzed to evaluate possible reuses and leachability behavior. The leachability tests performed according to European regulations showed that metals leachability was within the limits for landfilling inert residues, but chlorine release could be very high, especially in the presence of straw pellets, due to its high chlorine content.
1. Introduction The energy content of sewage sludge, around 24 MJ/kg daf, makes its use for energy production an interesting issue. On the other hand, sewage sludge may present considerable amounts of sulfur, chlorine, and especially nitrogen, together with other elements, such as heavy metals, which may lead to the release of pollutants. Therefore, their deposition in landfills and the combustion of these wastes is not advisable, and special care should be taken in the selection of the thermal treatment process. Wang et al.1 compared cogasification and coincineration of sewage sludge mixed with coal, and the results obtained proved that cogasification was a better option than coincineration. The work carried on by Paterson et al.2 showed that sewage sludge gasification produced a syngas rich in methane, which led to a fuel gas with a high calorific value. According to these authors, the presence of sewage sludge during coal gasification increased both the calorific value of the fuel gas and the fuel conversion. Folgueras et al.3 pyrolysis studies of coal mixed with sewage sludge showed that sludge was more reactive than coal, since it decomposed and devolatilized at lower temperatures. * To whom correspondence should be addressed. Telephone: 351-21092-4786. Fax: 351-21-716-6569. E-mail:
[email protected]. (1) Wang, X.; Xiao, Y. ASME Turbo Expo 2004sLand, Sea and Air, Vienna, Austria, June 2004; GT2004-54077. (2) Paterson, N.; Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. ASME Turbo Expo 2002sLand, Sea and Air, Amesterdam, The Netherlands, June 2002; GT2002-530013. (3) Folgueras, M. B.; Dı´az, M. R.; Xiberta, J. Energy 2004, 30 (7), 1079.
Due to the reducing conditions used for gasification, it is expected that most of the gaseous compounds of nitrogen, sulfur, and chlorine appear as H2S, NH3, and HCl. The presence of these compounds is undesirable for most syngas applications; therefore, their formation should be minimized and controlled. The formation of these compounds during gasification has been studied by several authors.1–13 Sewage sludge also contains variable quantities of other elements, including heavy metals, depending on the origin of waste waters. These elements may be volatilized to the gas phase at high temperature, or they may be retained by the solid bed residue, trapping some of the nitrogen, sulfur, and chlorine introduced by the feedstock.14,15 (4) Drift, A.; van der Doorn, J.; Vermeulen, J. W. Biomass Bioenergy 2001, 20, 45–56. (5) Liu, H.; Gibbs, B. M. Fuel 2003, 82, 1591–1604. (6) Kuramochi, H.; Wu, W.; Kawamoto, K. Fuel 2005, 84, 377–387. (7) Khan, M. R. Fuel 1989, 68, 1439–1449. (8) Li, W.; Lu, H.; Chen, H.; Li, B. Fuel 2005, 84, 1874–1878. (9) Nichols, K. M.; Hedman, P. O.; Smoot, L. D.; Blackman, A. U. Fuel 1989, 68, 243–248. (10) Tian, F.-J.; Li, B.-Q.; Chen, Y.; Li, C.-Z. Fuel 2002, 81, 2203– 2208. (11) Tian, F.-J.; Yu, J.; Mckenzie, L. J.; Hayashi, J.; Chiba, T.; Li, C. Fuel 2005, 84, 371–376. (12) Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. ASME Turbo Expo 2002sLand, Sea and Air, Amesterdam, The Netherlands, June 2002; GT2002-30672. (13) Zhou, J.; Masutani, S. M.; Ishimura, D. M.; Turn, S. Q.; Kinoshita, C. M. Ind. Eng. Chem. Res. 2000, 39 (3), 626–634. (14) Frandsen, F.; Dam-Johansen, K.; Rasmussen, P. Prog. Energy Combust. Sci. 1994, 20, 115–138.
10.1021/ef700767q CCC: $40.75 2008 American Chemical Society Published on Web 06/11/2008
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Several authors reported that NH3 formation and destruction during sewage sludge gasification depended on several parameters, such as gasification temperature, steam and oxygen contents in the gasification medium, feedstock nitrogen content, and even feedstock inorganic matter or reactor material, as they may interact in NH3 formation reactions.5,10,11 Zhou et al.13 stated that most of the fuel nitrogen is released as NH3 and N2, as less than 1% of the nitrogen in the feedstock is detected as HCN and NO. Drift et al.4 gasified different biomass fuels in a circulating fluidized-bed gasifier and reported that the concentration of NH3 in the product gas depended upon the amount of nitrogen present in the fuel. On average about 60% of the fuelbound nitrogen was converted to NH3. Probably, a small part was converted to HCN and other N-containing components, while the remaining released part was reduced to N2. The rise of temperature leads to a decrease in NH3 release in the gas phase, because the thermodynamic decomposition of NH3 is an endothermic reaction, favored by temperature increase.2,5,10 The increase of oxygen flow rate also decreases NH3 release, probably due to the formation of more NO, which would decompose to N2 under the reducing conditions of the bed. Therefore, less NH3 would be formed, as observed by Liu et al.5 Kuramochi et al.6 measured H2S contents of around 4000 ppmv in the gasification gas, due to high sulfur levels in sewage sludge and also to the forms in which sulfur was present. On the other hand, Drift et al.5 reported maximum H2S contents of 230 ppmv during gasification of biomass samples at 800 °C; these low concentrations were probably due to smaller sulfur contents in the biomass used. Sulfur may react with other elements present in the feedstock inorganic matter, such as Fe and Ca, which may retain sulfur in the char.6 Khan et al.7 showed that sulfur distribution between char, tars, and the fuel gas depended upon the form in which sulfur was present, either as organic or inorganic compounds. These authors also observed that H2S formation was also dependent on gasification temperature, as higher temperatures increased the H2S formation and its release in the gas phase. The increase of oxygen flow rate also allowed decreasing H2S, probably due to the formation of more SO2 from fuel-S when oxygen rate was increased; therefore, lower fuel-S was available to form H2S. Li et al.8 coal pyrolysis studies showed that the volatility of chlorine might be different, depending upon the chlorine form, organic or inorganic. However, an increasing temperature led to higher chlorine release in the gaseous phase, independently of the coal type or chlorine form. HCl formation was also dependent upon the content of other elements, such as Si, Al, and Ca, and heavy metals6 that may retain chlorine. Kuramochi et al.6 also stated that the variation of HCl release with temperature also depended on the presence of elements, such as K, Al, Na, and Si, and the competition among them to react with chlorine. Cogasification studies of sewage sludge mixed with coal15 showed that the use of sewage sludge increased the energy conversion of coal gasification and allowed rising gas yield. However, the syngas produced presented concentrations of H2S, HCl, and especially NH3 higher than those allowed by the Synthesis Gas Rule defined by the Environmental Protection Agency (EPA) for a syngas to be used as a fuel22 unless the gas was cooled in a condensation system. The Synthesis Gas Rule is a regulatory benchmark for classifying synthesis gas produced from hazardous waste as a fuel rather than as a hazardous waste. Therefore, sewage sludge was mixed with (15) Pinto, F.; Lopes, H.; Andre´, R. N.; Dias, M.; Gulyurtlu, I.; Cabrita, I. Energy Fuels 2007, 21, 2737–2745.
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straw pellets before being gasified, to dilute the effect of these undesirable species. This paper studies cogasification of sewage sludge blended with straw pellets and compares it to sewage sludge gasification, analyzing gas yields and composition and the environmental impact of the processes studied. The results obtained were also compared to data for the cogasification of sewage sludge blended with coal.15 Heavy-metal content in bed char residues were measured, together with their leachability according to European regulations,16 to evaluate measures needed to minimize any harmful consequences during landfilling or possible further reuse. Due to the significant energy content of sewage sludge, its use for energy production should be studied. However, due to its contents of sulfur, chlorine, and especially nitrogen, the formation of some undesirable S, N, and Cl compounds during gasification of this material mixed with straw pellets or with coal needs to be measured and controlled by adjusting experimental conditions. As a decrease in these compounds release is directly related to their retention in the solids inside the gasifier, special care should be taken in their analysis to select the best options for these solids disposal or reuse. 2. Experimental Section Cogasification tests were performed on a bench-scale atmospheric fluidized-bed gasifier previously described,15,17 whose schematic diagram is shown in Figure 1. The gasifier is circular in crosssection with an inside diameter of 0.08 m and total height of 1.5 m. Feedstock was fed inside the bed. Fluidizing velocities of around 10 m/s were used to fluidize the silica sand bed. Granulated sewage sludge was thermally dried and blended with straw pellets to obtain different compositions that changed between 0 and 100% (w/w). Both sewage sludge and straw pellets analysis are shown in Table 1. The gasification medium was a mixture of air and steam; steam flow rate was 5.0 g/min, and the air flow rate varied to correspond to equivalent ratio (ER) values in the range of 0-0.6. The feedstock flow rate was kept constant to the value 5 g daf/min. The range of temperatures varied between 750 and 900 °C. Each experiment was performed during around 60 min. The syngas, after leaving the gasifier, went through a cyclone, a condensation system, and filters and then was collected in bags to be analyzed by gas chromatography to determine the composition of CO, CO2, H2, O2, N2, CH4, and CnHm (gaseous hydrocarbons heavier than CH4, with the number of carbon atoms between 2 and 4). Gas composition values were converted to a dry-inert-free basis, to eliminate the effect of nitrogen and moisture on concentrations of the gas obtained. The use of an inert free basis allows a better comparison between experimental results when N2 contents are very dissimilar, as it happens when the ER effect is studied. In the presence of high N2 contents and due to its diluting effect, variations in gas composition may be not clear and may be more difficult to detect and explain, unless a dry-inert-free basis is used. Results presented in this basis may also be easily compared to those obtained in gasification with oxygen, instead of air. For each cogasification experiment, gas yield was calculated on the basis of the production of inert free gas per weight of dry-ashfree feedstock, excluding nitrogen and water from the gas composition. The higher heating value (HHV) was also determined, defined as the gross calorific value of the dry-inert-free gas on a volumetric basis, based on gas composition. Sets of test runs were repeated under the same experimental conditions, to ensure the reproducibility of experimental results; when deviations higher than 5% in experimental determinations were observed, more tests were performed to check the results and to maintain uncertainty lower than 5%. After the condensation system, syngas samples were also collected to analyze the contents of H2S, HCl, and NH3. H2S was analyzed using method 11 of the EPA; NH3 was sampled on the basis of method CTM-027 of the EPA; and chlorine was analyzed
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Figure 1. Schematic representation of gasification installation.
using EPA method 26. Some sulfur, nitrogen, and chlorine compounds were retained in the condensation system, which were also analyzed. Chlorine trapped in the condensation system was analyzed using capillary ion electrophoresis, and condensed NH3 was measured potentiometrically. Sulfur held in the condensation system as SO42- was analyzed simultaneously with chlorine using capillary ion electrophoresis. The content of heavy metals in bed char residues was determined as described previously.15 The residual solids collected from the fluidizing bed were separated from the sand by sieving and then were analyzed by atomic absorption spectrometry, after dissolution with HF, HNO3, and H3BO3 in a microwave oven. Hg, C, and S were analyzed in the solid matrix using automatic analyzers. Leachability tests were performed according to EN 12457, and ultrapure water with a liquid/solid ratio of 10 was used without a pH adjustment.
3. Discussion of Results 3.1. Effect of Experimental Conditions on Gas Yield and Main Compounds. The addition of sewage sludge to straw pellet blends led to an increase in CH4 and CnHm concentrations (16) Council Decision 2003/33/EC of Dec 19, 2002. Off. J. Eur. Communities, number L 011, Jan 16, 2003.
of around 48 and 62%, respectively (Figure 2). As straw pellets presented a higher content of volatiles, the faster release of these compounds might have contributed to their cracking and reforming reactions, those leading to lower hydrocarbons contents. The rise of hydrocarbon concentrations was accompanied by a decrease in H2 release of around 20%, up to sewage sludge contents of 80%; further increases of this waste did not lead to further H2 variations. Similar variations in H2 concentrations were also obtained when sewage sludge was mixed with coal.15 If hydrocarbons had further reacted, it was expected that they might have contributed to an increase in H2 formation. These experimental results agree fairly well with those of the literature,15,17–19 when different wastes were cogasified with coal. The increase of wastes content during cogasification of coal with wastes, such as plastics, pine, olive oil industry wastes, and edible oil wastes, always led to an increase in hydrocarbons release and to a reduction in H2 (17) Pinto, F.; Franco, C.; Lopes, H.; Andre´, R. N.; Gulyurtlu, I.; Cabrita, I. Fuel 2005, 84 (17), 2236–2247. (18) Pinto, F.; Andre´, R. N.; Franco, C.; Tavares, C.; Dias, M.; Gulyurtlu, I.; Cabrita, I. Fuel 2003, 82, 1967–1976. (19) Andre´, R. N.; Pinto, F.; Franco, C.; Dias, M.; Gulyurtlu, I.; Matos, M. A. A.; Cabrita, I. Fuel 2005, 84 (12-13), 1635–1644.
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Table 1. Fuels Composition straw pellets Elemental Analysis (% daf) carbon content 49.27 hydrogen content 6.21 nitrogen content 0.77 sulfur content 0.15 chlorine content 0.28 oxygen content 43.60 Proximate Analysis (% w/w) fixed carbon 16.16 volatiles 68.52 mineral 6.26 moisture 9.06 HHV (MJ/kg daf) 19.8 ash composition iron (% w/w) 0.01 aluminum (% w/w) 0.01 calcium (% w/w) 0.30 potassium (% w/w) 1.09 sodium (% w/w) 0.03 magnesium (% w/w) 0.07 phosphorus (% w/w) 0.09 fluor (% w/w)
