Environ. Sci. Technol. 2009 43, 9030–9037
CO2 as a Carbon Neutral Fuel Source via Enhanced Biomass Gasification HEIDI C. BUTTERMAN AND MARCO J. CASTALDI* Department of Earth and Environmental Engineering (HKSM) Columbia University, 500 West 120th Street, 927 S.W. Mudd, New York, N.Y. 10027
Received July 7, 2009. Revised manuscript received October 10, 2009. Accepted October 12, 2009.
The gas evolution, mass decay behavior and energy content of several woods, grasses, and agricultural residues were examined with steam and CO2 gasification using thermogravimetric analysis and gas chromatography. CO2 concentrations were varied between 0 and 100% with steam as a coreactant. Carbon conversion was complete with 25% CO2/75% steam compared to 90% conversion with pure steam in the temperature range of 800-1000 °C. The largest effect was from 0-5% CO2 introduction where CO concentration increased by a factor of 10 and H2 decreased by a factor of 3.3 at 900 °C. Increasing CO2 from 5 to 50% resulted in continued CO increases and H2 decrease by a factor of 3 at 900 °C. This yielded a H2/CO ratio that could be adjusted from 5.5 at a 0% CO2 to 0.25 at a 50% CO2 concentration. Selection of the gasification parameters, such as heating rate, also enabled greater control in the separation of cellulose from lignin via thermal treatment. 100% CO2 concentration enabled near complete separation of cellulose from lignin at 380 °C using a 1 °C min-1 heating rate. Similar trends were observed with coal and municipal solid waste (MSW) as feedstock. The likely mechanism is the ability for CO2 to enhance the pore structure, particularly the micropores, of the residual carbon skeleton after drying and devolatilization providing access for CO2 to efficiently gasify the solid.
Introduction Increasing energy demands coupled with a heightened awareness of the influence of various pollutant species on the global climate system, particularly that due to the production of greenhouse gases, have resulted in incentives to develop alternative sources of energy and chemicals to address energy security in an environmentally responsible manner. Biomass fuels constitute promising renewable resources for meeting a significant portion of the demand for energy and transportation fuel production. By suitable control of the gasification process, the gasification medium, and the type of feedstock, the possibility exists for the production of a spectrum of liquid hydrocarbons. An important attribute of biomass, along with its carbon neutral status, is its versatility from a dual polysaccharide (cellulose/ hemicellulose) and aromatic (lignin) structure. While one method of dealing with commercial scale release of CO2 has been through the development of carbon capture and sequestration technologies, we are investigating * Corresponding author phone: 212-854-6390; fax: 212-854-7981; e-mail:
[email protected]. 9030
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an alternative strategy for dealing with the large CO2 streams produced as a result of industrial processes. This beneficial use for CO2 involves its recycling by incorporation into the fuel making process. A potential market for these applications can be created that can globally process tens to hundreds of megatons of CO2 per year (1, 2). Additionally, thermal processing through CO2 rather than steam avoids the use of large quantities of water. CO2 gasification of biomass offers a distinct advantage in enabling a more complete conversion to volatiles at a lower temperature. An immediate reduction of greenhouse gases can be realized in the production of hydrocarbon liquid fuels from renewable resources. If 20% of liquid fuels were produced from carbon neutral sources, such as biomass, the resulting CO2 emissions reduction would be 15%. Total global CO2 emissions amount to nearly 29.5 billion metric tons with nearly one-third arising from the transportation sector (3). With the use of 20% biofuels, 15% of this CO2 production or 1.4 billion metric tons, can be averted through fuel replacement alone. For low temperature gasification of beachgrass we found that incorporation of CO2 into the fuel making process to satisfy 20% of the 1.07 × 1020 kJ estimated 2008 total transportation energy demand (4), could create a beneficial use for an additional 437 million metric tons of CO2. For a typical automobile producing 6 t of CO2/year this would be equivalent to removing 308 million vehicles from the road. The mass decay curves for both CO2 and steam gasification of grasses, woods and agricultural and forestry residues all showed gasification behavior intermediate between the extremes of low conversion lignin and high conversion cellulose. Actual feedstocks high in cellulose showed pyrolytic mass decomposition characterized by thermal processing at lower temperatures with smaller quantities of pyrolytic char. Feedstocks high in lignin showed more significant levels of residual pyrolytic char with significant mass decomposition occurring into the gasification range, behavior characteristic of the thermally resistant lignin structural component. However, pyrolysis behavior was not framed by the two extreme decay rates of the cellulose and lignin structural surrogates due to the impurities inherent in any natural lignocellulosic feedstock. Many of the biomass fuels, particularly the herbaceous feedstocks, underwent significant depolymerization much earlier than the pure structural components indicating a possible coupling of the degradation mechanisms and the presence of a catalytic effect similar to that observed by Ye et al. (5). They found catalytic effects to be significant in the CO2 gasification of coal chars, where the mineral components were chemically bound to the functional groups of the hydrocarbon feedstock. Research using CO2 as a gasification medium has been done with biomass and coal samples. The main performance comparison was with those results using steam to understand the impact on the kinetics and reactivity. Ye et al. (5) found that CO2 gasification had a lower activation energy than steam but overall reactivity was higher using steam for low-rank coal samples. Zhang et al. (6) also found similar performance for anthracite coal. They determined that char reactivity with steam gasification was significantly higher than with CO2 and concluded that CO2 gasification was more dependent on catalytic effects from the mineral content in the coal. Marquez-Montesinos et al. (7) reported similar trends in activation energy and char reactivity for gasification of grapefruit skin char. They also attributed CO2 gasification reactivity to a catalytic effect of minerals from the biomass. It should be noted that these studies examined char that was produced via heat-treatment in a nitrogen atmosphere 10.1021/es901509n CCC: $40.75
2009 American Chemical Society
Published on Web 11/05/2009
FIGURE 1. Representative data set: (A) 0% CO2 walnut shells steam gasification, 10 °C min-1, 22-980 °C, (B) 30% CO2 walnut shells steam gasification, 10 °C min-1, 22-980 °C, (C) Lignin, H2O/N2, 1 °C min-1, 22-860 °C, (D) Lignin, CO2, 1 °C min-1, 22-860 °C, (E) SEM of douglas fir char fiber showing enhanced micropore structure (10.0 kV, 12.8 mm ×1000). prior to their investigations. For example, the anthracite coal char was produced by a 900 °C heat treatment for 0.5 h, the low-rank coal char was formed from a 600 °C heat treatment for 15 min. The char produced from the grapefruit skins was done at 700 °C for 2 h. These heat treatments result in removal of moisture and volatile hydrocarbons that significantly contribute to the pyrolysis reactions during the gasification of raw biomass. Therefore, a char produced under an inert compared to the char produced during the gasification with different reactive media (i.e., steam or CO2) will be very different. We have found that CO2 is a more reactive medium from initial calculations that have been previously published (8). To confirm the calculation results, an experimental investigation was undertaken to better understand the influence of CO2 on syngas production from biomass processing and the limitations on incorporation of CO2 during the thermal treatment process. We are researching a gasification process that uses CO2 as a reactant to produce syngas for hydrocarbon fuels. Using CO2 the opportunity exists to adjust the ratio of H2/CO in the syngas produced by selecting the concentration of CO2 fed into the reactor system resulting in operational and economic advantages. Finally, coupled with slow heating rates, a more efficient separation of the lignin and cellulose structural components was possible. Cellulose decomposition to liquid and gaseous fuels occurs at low pyrolysis temperatures (375-450 °C). At slower heating rates a larger fraction of the residual lignin is retained that can be incorporated into a chemical manufacturing process using CO2.
Experimental Section Supporting Information (SI) Figure S1 shows a schematic diagram of the gasification test facility used. It consists of a thermal analyzer (Instrument Specialists) that can regulate the temperature and heating rate of the quartz furnace in the Dupont 951 thermogravimetric analyzer. N2 and CO2 (Bone dry CO2, UHP N2 TechAir) cylinders feed gases via Teflon tubing regulated by Gilmont GF 1060 Rotameters. A custom steam generator is fed by a kd-Scientific 780-100 syringe pump using distilled water with N2 carrier gas. The slightly heated delivery steam, whose temperature is monitored by an Omega digital E-type thermocouple readout, along with the CO2 and N2 gas, flows directly into the furnace where the biomass sample sits in an inert pan. Real time monitoring of mass decay and sample temperature are done by TPI software (Instrument Specialists). Data processing is
done by Acquire software (Instrument Specialists). Gaseous products leaving the TGA pass through a condensation column to remove moisture before entering a micro-gas chromatograph (Agilent 3000). Data acquisition software allows the collection of mass loss data at an appropriate sampling frequency (10-2 - 1 samples sec-1) corresponding to the furnace heating rate (1-100 °C min-1). Online GC analysis of gas evolution concentrations as a function of temperature (time) occur at a rate of one chromatogram every 4 min. The biomass samples tested appear in SI Table S1. They include various hardwoods, softwoods, grasses, needles, shells, bark, pit, and hull. Samples of approximately 20 mg were weighed on a Mettler balance to the nearest (0.1 mg. In addition, two different sub-bituminous coal samples (Wyoming, Montana) were tested and appear in SI Table S2. The inlet flow was set to the desired CO2 concentration so that the total (steam + CO2 + N2) process flow rate remained constant at 90 mL min-1. A calculation of the flow lag time from when the gas exited the furnace until it reached the GC inlet enabled identification of gas evolution as a function of furnace sample temperature to be synchronized. The micro-GC was calibrated using a certified mixture grade of CO, O2, CH4, CO2, H2, and N2 (Matheson Tri-Gas NIST weight certificate no. 105003). Finally, heat content (HHV) of the feedstocks were determined using a Parr 1108 oxygen bomb calorimeter. The samples were prepared according to the protocol specified by Parr.
Results and Discussion The influence of CO2 is visible when comparing the steam gasification of walnut shells with and without CO2 in the gasification medium. Figure 1A shows 0% CO2 injected resulting in a large black char residue after the heating cycle at a rate of 10 °C min-1. Figure 1B shows the enhanced char burnout using 30% CO2 injected as a cofeed with a resultant small light mineral residue. The Boudouard reaction, between CO2 and the porous carbon skeleton, was identified as the most important reaction responsible for completely processing the feedstock at high temperatures. The chemical and physical mechanisms of char conversion are modified by introducing CO2 during gasification resulting in the creation of a more reactive char. Quantification of char reactivity in the current investigation is based on visual observations as well as measurements taken of the pore size and its distribution. The porosity data was determined from electron micrographs of the char residues VOL. 43, NO. 23, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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produced in H2O/N2 and CO2 atmospheres. Many investigators have found that though a H2O/N2 environment may prove to be more reactive at low pyrolysis temperatures (5-7) (400 °C), particularly in the presence of chemically bound catalytic agents in the form of mineral impurities, a more complete processing results during CO2 gasification. Mineral impurities were identified as the single most important factor in processing the feedstocks by Marquez-Montesinos et al. (7). They explained the inability of the H2O/N2 environment in accessing the microporous channel network as being due to the fact that either (a) the channel was too narrow so as to allow the CO2 but not the N2/H2O molecules to travel through and adsorb onto or react with the channel surface or, what they felt was more likely, that (b) the bound mineral impurities would block channel access during H2O/N2 processing but would not impede transport of CO2 molecules within the channels. Then, even though the apparent activation energy may be higher for CO2 gasification as compared to thermal treatment in the H2O/N2 environment, a more porous char structure as measured by a higher adsorption surface area with a more intricate channel network would correspond to a more reactive char surface during CO2 gasification. Marquez-Montesinos et al. (7) calculated an activation energy of ∼200-250 kJ mol-1 for CO2 gasification of grapefruit skin char compared to ∼130-170 kJ mol-1 for H2O/N2. The CO2 char surface area calculation was 635 m2 g-1 as compared to a BET determination of only 10 m2 g-1. The discrepancy between the two was attributed to the inability of N2 to access the narrow micropore structure. The calculated reactivity for CO2 processing measured from the specific weight loss rate (dw/wdt) became more pronounced at higher processing temperatures and higher extent of conversion with mineral impurities playing a very significant role. Ye et al. (5) also measured reactivity of the char in terms of time necessary for 50% fixed carbon conversion. Their kinetic calculations for gasification of a high mineral content low rank coal gave a conversely higher activation energy for processing the coal in steam (131 kJ mol-1) as compared to 91 kJ mol-1 for its processing in CO2. In the current study, the choice was made to characterize reactivity based on porosity observations and measurements. CO2 was observed to be a more reactive medium than H2O at gasification temperatures since it could better access the biomass components by creating a highly porous network of channels in the char structure. The total number of pores observed during CO2 thermal treatment was an order of magnitude greater than that observed during H2O/N2 processing and the range in pore sizes was much greater with typical sizes between 2-50 µm in CO2 compared to 10-20 µm in H2O. A hard, low porosity surface coating that is created during steam gasification results in less available area for continued exposure to the steam and thus a lower biomass conversion (Figure 1C). The micropore char structure of lignin is accessible to the CO2 molecule while species such as O2 cannot easily diffuse into the micropore channels but primarily adsorb onto sites in the meso/ macroscale porous network (10, 11). During CO2 enhanced steam gasification the H2O molecule can access the larger macropores created by the reactive CO2 medium. The macropore char structure created in the CO2 gasification environment has a much higher surface to volume ratio that enables the CO2 to penetrate the inner volume of the biomass sample and more completely process the char (Figure 1D). The SEM of a Douglas fir char fiber processed with CO2 shows the increased porosity of this vascular structure and the microscopic basis for enhanced access to the inner volume of the biomass sample (Figure 1E). Tracheid cells composed of thick lignified walls for vertical water transport and structural support have narrow helical pits for horizontal 9032
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water seepage. Following CO2 activation the pits enlarge and the entire wall becomes a spongy, more porous structure. CO2 was found to significantly increase the conversion of char residue to volatiles at high gasification temperatures and to result in conversion of a portion of the inorganic ash to volatile mineral oxides that was not observed using conventional steam gasification. Cellulose (HHV ∼ 4168 cal/ g) and hemicellulose (HHV ∼ 4171 cal/g) have a polysaccharide structure and begin to thermally degrade at low pyrolysis temperatures, ∼225 and 175 °C respectively. Lignin (HHV ∼ 5062 cal/g) is aromatic, highly cross-linked, and characterized by its methoxy-to-phenylpropanoid ratio. Though it begins to degrade at ∼200 °C, its thermal decomposition is much slower and continues past pyrolysis into gasification temperatures of ∼550-650 °C. While pyrolysis involves thermal cracking of the molecular structures, gasification involves the conversion to volatile products. Starting with low gasification temperatures between 500 and 650 °C, the water gas shift reaction (WGS) is important and results in a portion of the low temperature production of H2 during steam gasification. CO + H2O f H2 + CO2 ∆H ) -41.2 kJ mol-1
(1)
While radical reactions are critical in maintaining all thermal degradation processes, hydroxyl, methyl, and methoxy radicals characteristic of the lignocellulosic structure are continually produced during decomposition. During pyrolysis the steam introduced for gasification combines with any available CO in the reactor through the WGS reaction to produce H2 that can, through direct hydrogenation reactions, produce a continuous low concentration of CH4.
2CO + 2H2 f CH4 + CO2
∆H ) -246.9 kJ mol-1 (2)
CO + 3H2 f CH4 + H2O
∆H ) -205.7 kJ mol-1 (3)
C + 2H2 f CH4 ∆H ) -74.4 kJ mol-1
(4)
At high temperatures (700-1200 °C) the dominant reactions that govern steam gasification are the reforming reaction, Boudouard reaction and reverse water gas shift reaction (rWGS) C + H2O f CO + H2 ∆H ) +131.3 kJ mol-1
(5)
C + CO2 f 2CO ∆H ) +172.5 kJ mol-1
(6)
H2 + CO2 f CO + H2O ∆H ) +41.2 kJ mol-1
(7)
In addition, decomposition reactions break down the biomass lattice and result in O2 release from the oxygenated minerals, and the oxidation of the char. C+
1 O (from biomass structure) f CO∆H ) 2 2 110.5 kJ mol-1 (8)
A set of coupled mechanisms involving molecular and radical species arising from the thermal degradation of the lignocellulosic material result in pyrolytic species that are characteristic of the polysaccharide (cellulose, hemicellulose) and methoxyphenylpropanoid (lignin) biomass structures. As the thermal processing proceeds, depolymerization, cleavage, and condensation reactions result in the creation of a porous graphitic char structure whose surface is coated with free radicals, the evolution of volatiles mainly composed
of CO, H2, and CH4, and final conversion to residual mineral ash (12, 13). Several biomass feedstocks were heated from 25 to 1000 °C at rates of 1-100 °C min-1 in H2O/N2/CO2 and pure CO2 environments. As the heating rate was increased (1, 2, 5, 10, 20, 50, and 100 °C min-1), a uniform shift in the mass decomposition to higher temperatures was noted. Since all of the decomposition curves, in each of the lignin and cellulose sets, could be framed by the lowest (1 °C min-1) and highest (100 °C min-1) heating rate graphs, these were the ones chosen for presentation in the current paper. We have previously identified two regimes (9) representing distinct mass decay and gas evolution profiles: low temperature pyrolysis and high temperature gasification, transitioning near 400 °C. The introduction of CO2 into the gasification medium produced very different gas evolution profiles. A definite CO enhancement and H2 and CH4 depression were observed in the grasses and woods as the percent CO2 introduced was increased. The influence of CO2 on chemical species evolution during gasification is shown quantitatively in Figure 2 for poplar and beachgrass (10 °C min-1 heating rate) that are representative of all biomass samples. As reported by Butterman and Castaldi (14, 15), introduction of CO2 as a cofeed into the reactor has been shown to enhance the production of CO during high temperature gasification while depressing the H2 and CH4. The steam reforming reaction becomes important at temperatures above 550 °C due to high concentrations of CO2 and H2O relative to CO and H2. This gasification reaction is responsible for the abrupt increase in H2 production that occurs at about 600 °C, whereas H2 evolution depression is clearly visible with increasing percent of CO2 injection as seen in Figure 2A. Alternatively, an increase in CO concentration was observed in the CO2 enhanced gasification of the biomass. At high temperatures, above 700 °C, CO production rises due to the Boudouard reaction in which CO2 reacts with the solid carbon residual. Figure 2B shows the strong CO evolution enhancement that occurs with increasing levels of CO2 introduced into the reactor. During pyrolysis any available oxygen in the gasifier, from H2O, CO2, or CO evolving from the biomass structure itself, can be adsorbed by the highly reactive char. These sources of oxygen can react with the carbon char structure at pyrolytic temperatures to produce surface-bound CO. Upon continued heating, this char-adsorbed CO is subsequently released that can account for the uniformly increasing concentrations of CO near 400 °C and above 800 °C in Figure 2B. By varying the heating rate and percent CO2 introduced into the reactor, conditions that could separate lignin from the cellulosic biomass components were identified. Therefore, one can thermally process the cellulosic component at low temperature and then treat the remaining lignin component thermally and chemically via CO2. A combination of both processes can optimize the percent of lignin in the pyrolytic char that remains available for subsequent conversion and the maximum extent of conversion to volatiles during gasification. The distinction between thermal processing using steam compared to CO2 can be seen in Figure 3A. While the slow 1 °C min-1 heating rate for steam gasification, appearing in Figure 3A, left nearly 40% of the lignin still unprocessed to volatiles by 930 °C, the slow heating of the lignin and cellulose components in a CO2 gasification medium resulted in complete conversion by 930 °C. While the slow 1 °C min-1 heating rate for steam gasification left nearly 40% of the lignin still unprocessed to volatiles by 930 °C, the slow heating of the lignin and cellulose components in a CO2 gasification medium resulted in complete conversion by 930 °C with only a 3% ash/mineral
FIGURE 2. (A) H2 evolution depression using CO2 injection (representative: poplar). (B) CO evolution enhancement using CO2 injection (representative: beachgrass). residue remaining from the lignin and only 0.4% remaining from the cellulose. The mass decomposition curves for two representative biomass feedstocks processed under steam (0%) and CO2 (100%) gasification is shown in Figure 3B. Though the low temperature pyrolysis behavior is similar for either CO2 or H2O, the high temperature processing of the poplar and blue fir needles is distinctly different for the two gasification media. Blue fir needles, higher in lignin content, begins pyrolytic degradation earlier (200 °C) and has a slower rate of decay. Poplar wood, higher in cellulose content, begins pyrolytic degradation at a later temperature (250 °C) and shows a much faster rate of decomposition during pyrolysis. The greatest difference between H2O and CO2 gasification is seen after 900 °C where, rather than leaving 22% of the needles unprocessed, less than 2% ash remains using CO2. Similarly, rather than 12% of the wood remaining unprocessed, less than 1% ash results using CO2. The relative percents of the lignocellulosic structural components available to enter into gasification above 400 °C can be adjusted by varying the VOL. 43, NO. 23, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Syngas adjustment as a function of CO2 reactant.
FIGURE 3. (A) Mass decomposition curve for lignin and cellulose, 1 °C min-1, comparing steam (0%) and CO2 (100%) gasification. (B) Real feedstock decomposition curve, 10 °C min-1, comparing blue fir needles and poplar. (C) lignin and cellulose CO2 gasification, 1 °C min-1 and 100 °C min-1. 9034
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furnace heating rate and the CO2 level in the gasification medium, and it is in this control of the gasification parameters, coupled with the selection of the biomass feedstock, that optimization of the thermochemical conversion process can be achieved for the production of the desired group of chemical products. The decomposition profiles of lignin and cellulose shift to lower temperatures as the heating rate is decreased (Figure 3C). At the slowest rate in the present study (1 °C min-1), using CO2 as the gasification medium, the cellulose decomposed at the lowest temperature leaving the highest residual lignin fraction (∼70%). At the highest heating rate, 100 °C min-1, less differential isolation was possible with only 50% of the lignin remaining following a 90% degradation of the cellulose that was completed at a temperature of 450 °C. Slower heating rate during CO2 gasification offers a selective advantage when lignin isolation and conversion is the goal. It enables processing of the cellulosic component within a narrow temperature window during which the decomposition rate of the recalcitrant lignin permits a greater fraction of the lignin component to survive. The ability to perform the thermal treatment at a lower gasification temperature using CO2 without the cost and energy consumption necessary to heat water used during thermal treatment with steam offers an additional advantage. Furthermore, every percent CO2 used enables the opportunity to offset an equal percent H2O. The CO enhancement and H2 depression behavior resulting from CO2 introduction during gasification allows the tailoring of the H2/CO ratio by varying the amount of CO2 injected as a reactant. Figure 4 shows H2/CO ratios for several different biomass feedstocks that include beachgrass (540 °C S/C ) 5.5, 570 °C S/C ) 18, 640 °C S/C ) 18), poplar (680 °C S/C ) 21), and Douglas fir (680 °C S/C ) 48, 720 °C S/C ) 48), taken at different gasifier temperatures and operating at different steam/carbon (S/C) ratios as described in an earlier study (14). Overlaid on that data are application regions that are suited to a specific H2/CO ratio. By adjusting the level of CO2 introduced into the gasification environment, we can gain greater control in determining the final H2/CO ratio produced. These products can serve as a feedstock for Fischer-Tropsch (FT) hydrocarbon synthesis, or as a fuel for devices such as gas turbines or solid oxide fuel cells. H2/ CO ratios between 4 and 6 allow operation of solid oxide fuel cells (SOFC) (16) and H2/CO ratios near 2 are more suited for the FT synthesis of liquid fuels (17, 18). While high temperature gasification, with CO2 injection, resulted in a syngas having a H2/CO ratio of about 0.8-1.4, suitable for catalyst-based FT synthesis, the low temperature H2/CO ratios for the grasses with no CO2 injection were substantially higher from 2.8 to 5. The addition of as little as
FIGURE 5. (A) CO evolution enhancement from Wyoming coal using CO2 and (B) from Montana coal using CO2.
FIGURE 6. (A) H2 evolution depression from Wyoming coal using CO2 and (B) from Montana coal using CO2. 5% CO2 into the influent stream could decrease the H2/CO ratio to about 1.8-2.5 which is suitable for FT processes or a gas turbine combustor (19, 20). CO2 injection during thermal processing will enable production of a syngas with an optimum range in H2/CO ratios particularly suited for the production of a specific biomass-derived liquid chemical (21, 22). This ability to manipulate the syngas ratio provides a strong measure of control in the thermal treatment process. It ensures that the feedstock has less of a direct impact on the syngas produced and offers greater fuel flexibility while retaining control in the production of a suitable syngas that can meet the requirements of a desired industrial application. Therefore, using CO2 can enable the decoupling of the final outcome from the initial feedstock, resulting in a more universal process for the conversion of carbon neutral feedstocks to energy or energy containing products. Figure 5A and B show the CO evolution for Wyoming and Montana coal, respectively. As seen in the figures, CO evolution increased at temperatures greater than 700 °C and became more pronounced as temperature increased. For the Wyoming coal, a 40% CO2 injection yielded a factor of 4 increase in CO production, whereas a 50% CO2 injection resulted in nearly a factor of 9 increase in CO production at 940 °C. The Montana coal displayed even more pronounced effects with the injection of CO2 at 940 °C. For example, a 40% CO2 injection led to a factor of 4 increase and a 50% injection yielded a 13-fold increase that rose to a 15-fold increase in CO evolution by 980 °C. While the trends are similar, one difference is that the CO evolution does not diminish as CO2 is increased as it did with the biomass feedstock. Moreover, the biomass feedstock showed the most pronounced enhancement with a slight addition of CO2 whereas the coal samples yield the most enhancement with large CO2 addition. This is likely due to the fact that a greater portion of the coal mass survives into gasification temper-
FIGURE 7. Mass decomposition curve for MSW under various CO2 concentrations. atures, the coal has a higher mass fraction of carbon (70-80% C on a dry basis) than the biomass (45-55% C on a dry basis) though it is mostly the lignin component (∼68% C) that survives into the high temperatures, and that the enhancement of CO is primarily due to the Boudouard reaction between the remnant carbon skeleton and the CO2 injected during high temperature processing. Figure 6A and B show the expected trends of hydrogen suppression with CO2 addition. Here, the addition of 40% CO2 nearly eliminated H2 production. The suppression of H2 followed more closely the behavior of biomass in that a 10% CO2 addition resulted in a 6-fold decrease in H2 evolution from Montana coal and a 7-fold decrease in H2 from Wyoming coal at 940 °C. A representative sample of MSW was tested using this process. Figure 7 depicts characteristic data over a temperVOL. 43, NO. 23, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Comparison of CO2 vs. Steam Gasification of Biomass Fuels CO2 addition to gasification Utilizes CO2 (generally an unwanted end product of various industrial processes) no heat requirement for phase change produces a more reactive char to enhance thermochemical conversion results in higher levels of CO at gasification temperatures produces a greater conversion of char to volatiles - more efficient gasification decreases the H2/CO ratio of grasses at low gasification temperatures to a value (1.5-2.5) more suitable for a Fischer-Tropsch fuel synthesis permits separation of lignin from holocellulose (cellulose + hemicellulose) fraction at slow heating rates while creating a more reactive lignin char residual following pyrolysis more easily accesses both micropores and macropores to create a more porous char structure CO2 is a less corrosive gasification medium
ature range from ambient to 1000 °C at a heating rate of 10 °C min-1 under various concentrations (0-20% CO2/N2). As shown in Figure 7, there are two main thermal degradation steps and there is ∼20% residual char from MSW thermal processing in a N2 atmosphere. The first thermal degradation step occurs between temperatures 280-350 °C and consists of the decomposition of the biomass component in MSW where light C1-C3 hydrocarbons are observed in the product gas. The second thermal degradation step between temperatures ∼380-450 °C is mainly attributable to the polymer components of MSW, such as plastics (polystyrene) and rubber (styrene-butadiene) (23). A residual char was observed without CO2 injection that decreased significantly with increasing CO2 concentration in the reaction gases. The inset in Figure 7 is a magnification of the high temperature decay region in which the CO2 exhibits the most significant effect. The decomposition and gas evolution behavior of the MSW shows similarities to that of the biomass and coal. The initial devolatilization below 100 °C involves oils and moisture in the case of coal whereas it is due to dehydration in the case of biomass. Though both coal and MSW exhibit an intermediate pyrolytic mass loss step, the percent weight loss as a function of temperature shows a much better correlation between MSW and lignin decomposition as can be seen by comparison of Figure 7 with Figure 3A and C. CO2 processing of the MSW feedstock showed enhanced gasification as was observed in the other hydrocarbon feedstocks. For example a CO2 concentration of 1% leads to a char reduction at 950 °C of about 10%. However, with 2.5% CO2, the char is reduced by nearly a factor of 5. Further addition of CO2 has only a limited impact on the char. This is likely a result of the MSW being composed of a mixture of components such as biomass and other hydrocarbons that, when heated, shows pyrolytic decomposition and gas evolution behavior characteristic of its large biomass fraction (∼60%) with characteristic mass loss intervals in the pyrolytic decomposition curve attributable to the polymer component. It is possible that the CO2 is converting the feedstock at temperatures below the high temperature gasification regime but the nature of the thermochemical conversion cannot be completely elucidated since the MSW is a highly heterogeneous sample. Finally, for comparison, a summary table has been assembled to highlight the major differences between CO2 9036
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Steam (experimental N2 carrier) only Utilizes H2O (an increasingly rarer resource) requires 2260 J g-1 to vaporize prior to introduction into the gasifier produces less reactive char yet still allows good thermochemical conversion results in higher levels of H2 at gasification temperatures results in a larger char residual - feedstock for various other processes increases the H2/CO ratio of woods at moderate gasification temperatures to a value (5-8) more suitable for use in a SOFC permits separation of lignin from holocellulose (cellulose + hemicellulose) fraction at slow heating rates easily accesses macropores but less easily diffuses into micropores creating a less porous char structure H2O vapor is conducive to a more corrosive gasification environment
gasification and steam gasification. Though Table 1 is not exhaustive, it does help to put into perspective some advantages that could be realized using CO2 as a gasification medium. Clearly, depending on the feedstock and source of CO2, an analysis needs to be done to ensure that a net CO2 reduction and energy efficiency increase is obtained.
Acknowledgments We thank Dr. Eilhann Kwon, a post-doctoral researcher in the Columbia Combustion and Catalysis Group, for providing the MSW mass decomposition data that appears in Figure 7.
Supporting Information Available A schematic of the gasification facility, tables listing the biomass fuels and sub-bituminous coals that were tested and their characterization, the biomass database sources used. This material is available free of charge via the Internet at http://pubs.acs.org.
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