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Energy & Fuels 2005, 19, 298-304
Trace Element Distribution in Sewage Sludge Gasification: Source and Temperature Effects G. P. Reed,* N. P. Paterson, Y. Zhuo, D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College London, South Kensington Campus, London SW7 2BY, United Kingdom Received March 3, 2004. Revised Manuscript Received September 27, 2004
Constraints within European Union (EU) countries on sewage sludge disposal routes are growing as former options meet with increasing environmental, legislative, and economic pressure. Gasification of sewage sludge for heat and power generation in combined heat and power (CHP) applications is an attractive concept that provides an environmentally acceptable, efficient, and economically viable means of generating energy from a waste disposal problem. The final solid residues are pathogen-free but may contain toxic elements such as barium, copper, mercury, lead, and zinc at levels that could make their disposal to landfills costly as well as environmentally unsound. Elements such as barium, copper, mercury, lead, and zinc are present in sewage sludges at levels significant to the disposal of the residual streams from a gasifier. The distribution of barium, copper, mercury, lead, and zinc to the ash residue streams has been studied in an airblown laboratory-scale spouted-bed gasifier that is fueled by crushed, dried sewage sludge pellets. The gasifier was operated at temperatures of 770-960 °C, and samples of the solid residues were collected. In this study, measurements of trace element concentrations have been used to determine their overall retention in the solid streams, as well as their relative depletion from the coarser bed residue and enrichment in the fines carried to the gas-cleaning system. The effect of the gasifier bed temperature and the type of sewage sludge has been investigated. Under all of the conditions studied, no mercury retention in the solid residues was observed. Cobalt, copper, manganese, and vanadium were neither depleted from the bed residue nor enriched in the fines. The extent of barium, lead, and zinc depletion from the bed residue varies with sludge type, and the enrichment of lead in the fines seems to be enhanced by gasifier bed temperatures in excess of 900 °C. The observed behavior of these elements is discussed in relation to their speciation, as predicted by thermodynamic equilibrium modeling. The potential implications of these findings for process design, operating conditions, and residue disposal are discussed.
Introduction The production of sewage sludge in the European Union (EU) has been forecasted to grow as the population increases and becomes increasingly urbanized. In the past, the main disposal routes for sewage sludge have been marine disposal, application to agricultural land, landfill, and incineration. Sewage sludge disposal has been the focus of much attention within the water industry in recent years, as some of these disposal routes have been banned or become subjected to greater constraints. Disposal at sea was banned in 1998,1 because of concern about the marine environment. Any expansion of agricultural land application will be limited, because of concerns about the possibility of crop contamination by pathogenic bacteria and the buildup of toxic or phytotoxic trace elements. The shortage of landfill capacity and taxation increases have combined to make landfills an increasingly expensive option. Incineration has met with increasing local resistance, in regard to planning applications, and is affected by * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Urban Waste Water Treatment Directive No. 91/271/EEC, EUROPA, European Commission, 1976.
government measures that are intended to reduce CO2 emissions by extending the use of wastes as fuel for energy production. Consequently, gasification has attracted considerable interest from water utilities as an alternative technology with the same advantages of destruction of pathogenic bacteria and volume reduction, and the additional benefits of energy recovery and lower-cost atmospheric emissions control. Trace elements (defined as elements with a fuel concentration level of 900 °C. There are also indications of barium and zinc depletion in some of the higher-temperature tests. The UK1 sludge shows stronger evidence of barium depletion than any of the others. The trends observed for lead and barium exceed the estimated error and warrant an explanation. The enrichment data in Figure 3a-e show that lead was especially enriched in the fines, with enrichment factors in excess of 20 being observed in some tests. The enrichment of lead is also observed to increase markedly as the temperature increases. The temperature plotted in Figure 3 is the temperature inside the bed; the temperature where the fines sample was collected is estimated to be ∼500 °C. Although enrichment might
be caused by condensation on the fines, the apparent dependency of enrichment on bed temperature might be better explained by a chemical reaction between lead vapor in the gas and the fly ash to form involatile lead aluminosilicates; the removal of lead vapors from gases in this way has been observed by Lachas et al.8 Alternatively, increasing the bed temperature has been observed to increase the apparent rate of fines production; this is attributed to greater production of soot by the cracking and dehydrogenation of VM released by the initial pyrolysis of the fuel. The soot is collected with (8) Lachas, H.; Herod, A. A.; Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. Trace Element Removal from Hot Gases: Screening Sorbents for Performance and Product Leachability. Energy Fuels 2003, 17, 521531.
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Table 5. System Compositiona and Constraints Used in Modeling
b
component
content (g-mol)
component
content (g-mol)
carbon hydrogen oxygen nitrogen sulfur
3.32 9.26 3.94 10.1 0.03
chlorine calcium barium lead zinc
0.002b 0.144 0.0004 0.0001 0.0015
a Taken from test 28. Data obtained for a pressure of 1 atm. Assumed value.
Table 6. Species Allowed to Form in Model Predictions gas phase
condensed phase
Major and Minor Elements C, H, O, N, CH4, CO, CO2, COS, CS2, C, CaCO3, Ca, CaCl2, S, Cl, Ca CaCl2, CaO, CaS, Cl2, HCl, CaO, CaSO4, H2, NH3, H2O, H2S, N2, O2 CaS, NH4Cl Ba Pb Zn
Trace Elements BaCl, BaCl2, BaClHO, BaO, BaS PbCl, PbCl2, Pb, PbS ZnS, Zn, ZnCl, ZnCl2
Ba, BaCO3, BaCl2, BaO, BaS PbCl2, Pb, PbS ZnCl2, ZnS, Zn
the fines; it may be that this additional soot is involved in the capture of lead. The experimental data does not allow the relative importance of these mechanisms to be assessed. Modeling. It is instructive to examine whether the trends in bed char depletion and fines enrichment observed could have been predicted by thermodynamic equilibrium modeling. The absence of cobalt, copper, manganese, and vanadium depletion or enrichment is consistent with previous predictions of low volatility for similar systems.7 The typical system composition, containing barium, lead, and zinc, shown in Table 5 therefore has been modeled using the multiphase module of the MTDATA software from the National Physical Laboratory (NPL),9 which applies a free-energy minimization approach and uses thermodynamic data from the SGTE database. An ideal gas phase and pure condensed-phase models were assumed, together with global equilibrium. The system composition used is that determined for the exit gas during test 28,6 with the exception of chlorine, where a typical value that was determined in other tests was assumed. The system incorporates all of the major components (carbon, hydrogen, nitrogen, and oxygen) and the minor components sulfur, chlorine, and calcium, which are known to be important to the speciation of trace elements in a reducing atmosphere. A calculation protocol that was outlined in a previous paper5 was used to construct a simplified database that contained only those species that were significant; the species that were allowed to form are shown in Table 6. Predictions of the gas-phase speciation at a pressure of 1 atm (in moles of gas-phase trace element species, as a function of system temperature) for each of barium, lead, and zinc are shown in Figures 4a, 4b, and 4c, respectively. The low level of chlorine is observed to be important to the speciation of all three elements, which will compete with hydrogen to associate with it. In this (9) Davies, R. H.; Dinsdale, A. T.; Gisby, J. A.; Robinson, J. A. J.; Martin, S. M. MTDATAsThermodynamics and Phase Equilibrium Software from the National Physical Laboratory. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2002, 26 (2), 229-271 (http:// www.calphad.org/).
Figure 4. Predicted gas-phase speciation for composition in Table 5: (a) barium, (b) lead, and (c) zinc.
system, both lead and zinc should be entirely in the gas phase (as Pb(g) and Zn(g), respectively) at temperatures of >700 °C, i.e., well below the range of the experimental parameters. It is likely that the absence of thermodynamic data for more-complex involatile species (such as aluminosilicates, phosphates, and titanates) from the database may account for some of the differences between the predictions and measurements. Barium should be the least volatile of the three trace elements, requiring temperatures of >900 °C before any gas-phase species should form; this is in agreement with the observed bed depletion and fines enrichment behavior observed for four of the sludges but not that of the UK1 sludge. Barium is known to form phosphates of low volatility at temperatures of
