Fate and Remediation of Biomass Gasification Gas Contaminants

Abstract

The development of alternative and renewable energy and fuels is at the forefront of research because of the forecasted depletion of crude oil reserves and the growing concern of irreversible environmental damage caused by the greenhouse gases. As lignocellulosic biomass is a renewable feedstock, its conversion to energy and fuels is particularly very attractive as it has the potential to replace fossil fuels derived liquid fuels, chemicals and energy while recycling carbon dioxide (CO2) into plant material by photosynthesis. Lignocellulosic biomass conversion can be achieved thermally by pyrolysis, gasification or combustion and biologically by fermentation. Biomass gasification is well suited among other thermochemical technologies due to its higher efficiency and the plethora of applications that could increase efficiency as well as synthesize value-added by-products. Energy and fuel production from biomass is well adapted to southern states in the United States due to abundant biomass resources. Of the woody biomass feedstocks, pine is the most likely feedstock for large-scale production as it accounts for 83 % of tree species in the aforementioned states. Therefore, this dissertation project focuses on pine gasification with an emphasis on the fate of syngas contaminants. In addition, means of remediating these contaminants were also explored. In Chapter 3, the effects of equivalence ratios (0.15, 0.25 and 0.35 at 934°C) and temperature (790, 934 and 1078°C at 0.25 equivalence ratio) on primary gases and contaminants were investigated. It was observed that carbon monoxide (CO) and hydrogen (H2) increased while carbon dioxide (CO2) and methane (CH4) decreased from 790 to 1078°C. Opposite trends were observed for equivalence ratio (ER). Based on overall contaminant weight, tar was highest at all temperatures (7.81, 8.24 and 8.93 g/kg dry biomass) and ERs (13.08, 8.24 and 2.51 g/kg dry biomass). Ammonia (NH3) varied from 1.63 to 1.00 g/kg dry biomass between 790 and 1078°C and 1.76 to 1.47 g/kg dry biomass between 0.15 to 0.35 ERs. Hydrogen sulfide (H2S) ranged between 0.13 and 0.17 g/kg dry biomass from 790 to 1078°C and 0.15 to 0.18 g/kg dry biomass from 0.15 to 0.35 ER. Finally, hydrogen chloride (HCl) yields ranged from 13.63 mg/kg dry biomass to below detection limit and 11.51 to 0.28 mg/kg dry biomass over the range of temperature and ER, respectively. For primary gases, the major finding was that the endothermic Boudouard and water gas reactions play a key role in controlling CO and H2 in the product gas as temperature is increased. As oxygen supply (i.e. ER) increases, char and homogeneous oxidation reactions largely explain increasing trends of fully oxidized major species in the producer gas (CO2 and water (H2O)) while CO and H2 decreased. While higher temperatures are known to promote hydrocarbon cracking, it also leads to secondary and tertiary tar formation. It was observed that increasing oxygen availability to induce tar oxidation is more effective than increasing temperature to reduce tar content. NH3 and HCl both decrease as temperature increased, the first due to its thermal decomposition to nitrogen (N2) and H2 and the latter due to speciation to metal chloride. Increasing oxygen supply did not significantly affect H2S and NH3 because of oxygen consumption by major species. HCl significantly decreased likely because of reactions with biomass trace metals released as biomass is decomposed. Chapter 4 looks at air gasification of pine from a performance angle by carrying out mass and energy balances as well as energy analysis at 790, 934 and 1078°C and 0.15, 0.25 and 0.35 ERs. Mass balance closures ranged from 94.73 to 96.72 % and from 89.82 to 96.93 % as temperature and ER increased. Over the same range of temperature and ER, carbon closures ranged from 80.77 to 92.29 % and from 79.09 and 87.13 %. Carbon conversion efficiencies increased from 72.26 to 84.32 % and 72.26 to 84.66 % as temperature and ER increased. The carbon flow analysis showed that the char product streams retained 10.26 to 6.94 % and 8.82 to 2.13 % of the total carbon feed to the gasifier as temperature and ER increases, respectively. The carbon content in the liquid condensate was minimal compared to the carbon in other product streams and accounted for less than 0.1 % of the carbon input to the gasifier at all conditions. The cold and hot gas efficiencies increase from 56.12 to 67.45 % and from 67.51 to 83.83 % as temperature is increased and in contrast decreased from 63.85 to 52.84 % and from 78.06 to 73.00 as equivalence ratio increase. The air equivalent heating value of producer gas increased from 4.93 to 5.73 MJ/m3 with increasing temperature and decreased from 7.11 to 3.28 MJ/m3 with increasing ER. Chapters 5 and 6 investigate contaminant removal using solid sorbents and catalysts in a fixed bed reactor. Chapter 5 deals H2S removal on vanadium oxide (V2O5) and zinc oxide (ZnO) and SrO supported sorbents. Five sorbents (V2O5, 5 wt % ZnO/V2O5, 10 wt % ZnO/V2O5 and 10 wt % ZnO-10 wt % SrO/ V2O5) were investigated for H2S removal. In the absence of syngas constituents (CO, CO, CH4, H2) V2O5 was an effective H2S sorbent in temperature ranges of interest in cold and warm gas cleanup. At 50˚C, H2S was removed below detection limit by V2O5 for more than 5 days at 1500 ml/g h WHSV. As temperature increased and gas space velocity was increased to 12,000 ml/h g, the breakthrough time increased to 36, 95 and 140 min at 50, 150 and 250°C, respectively for V2O5. Bulk V2O5 was less effective in H2S removal when syngas constituents were introduced along with H2S due to competitive chemisorption of other syngas constituents as H2S adsorption was not observed. However, subsequent ZnO and SrO impregnation to produce 10wt % ZnO/V2O5 and 10wt % ZnO-10wt % SrO/V2O5 improved the performance and increased the breakthrough time from 0 to 20 min in the presence of syngas constituents. This sorbent formula outperformed ZnO, commonly considered one of the best H2S sorbent, at the same temperature. In the presence of syngas, the breakthrough times were shorter than when inert gas (argon) alone was used. The following order was observed for sorbents: V2O5 < ZnO < 10wt % ZnO/V2O5 < 10wt % ZnO-10wt % SrO/V2O5. XRD characterization of spent V2O5 sorbent interestingly revealed that adsorption rather than sulfidation is the mode of H2S removal. This mechanism of removal can be potentially advantageous for sorbent regeneration as H2S is merely adsorbed on the sorbent. The major findings of this chapter are: 1) on bulk V2O5, the mode of sulfur removal appears to be by chemisorption rather than by sulfidation to metal sulfide and 2) competitive adsorption of syngas constituents hinder H2S removal on V2O5 and other V2O5 based sorbents and 3) supported catalyst of V2O5 are effective in H2S removal at warm gas cleanup temperature range (< 300°C). In Chapter 6, the catalytic removal of naphthalene, as model tar compound, on strontium oxide (SrO) was investigated from 300 to 900°C. Naphthalene concentration was reduced from 6628.47 to 6392.70, 4787.97, 1562.43 and 43.68 mg/m3 at 300, 500, 700 and 900°C, respectively corresponding to 3.56, 27.77, 76.43 and 99.34 % conversion at the same temperature in the presence of syngas. It was observed that in the presence of syngas, SrO was more active in naphthalene decomposition suggesting the occurrence of dry reforming. When helium was used as a carrier gas, the concentration of naphthalene was reduced from 6628.47 to 6512.29, 5383.03 and 3677.51 mg/m3 at 500, 700 and 900°C corresponding to 1.75, 18.79 and 44.52 % conversion. The activation energies were 45.24 kJ/mol when syngas was used as a carrier gas and 61.23 kJ/mol when helium was used. At higher temperature, SrO was equally as active as nickel based catalysts tested in this study. The major findings in this chapter are: 1) the presence of syngas constituents improve the catalyst activity by enhancing dry reforming, 2) 700 to 900°C is required to achieve conversions higher than 80 % and 3) SrO is as effective as nickel based or other basic mineral catalysts that regarding as the best tar removal catalysts at 900°C with 99.34 % conversion.