Presented at ASME-IGTI TURBO-EXPO 2001, New Orleans June 4-7 and published in the proceedings

WHAT TO DO WITH CO2

Alex E. S. Green and Greg P. Schaefer,ICAAS-Clean Combustion Technology Laboratory (CCTL)

PO Box 112050 University of Florida, Gainesville, Florida, 32611-2050Tel: 352-392-2001, Fax: 352-392-2027, email: aesgreen@ufl.edu

ABSTRACT: Indirectly heated gas liquid and char converters (IHGLCCs) are employed to investigate several means of using or sequestering CO2. As a gasification agent CO2 is used with IHGLCCs to substantially increase gaseous energy output for a given carbonaceous feedstock. IHGLCCs are also used for mild oxidation of coal and for mild pyrolysis of biomass to produce humate type soil amendments. These soil amendments can indirectly sequester CO2 by enhanced plant growth and the atmospheric scrubbing action of plants (photosynthesis-respiration). Results of attempts to convert coal-biomass blends into an activated charcoal that can scrub CO2 and also become useful soil organic carbon are inconclusive as yet but appear promising. Countries that must import oil or have agriculturally depleted lands need "omnivorous" feedstock converters to upgrade available domestic feedstock into fuels, chemicals and chars that serve their energy, agricultural and other needs. The results of exploratory research directed at applying IHGLCC forms of omnivorous feedstock converters to using or sequestering CO2 are reported. IHGLCC technology should also be useful in mitigating potential global greenhouse problems and CO2 and waste disposal problems on space missions.

BackgroundEnvironmental, energy and economic (EEE)

strategies have been the main concern of the Interdisciplinary Center for Aeronomy and other Atmospheric Sciences (ICAAS) since its founding at the University of Florida in 1970. Early studies made it clear that most anthropogenic atmospheric emission problems are associated with energy use. A state supported interdisciplinary Coal Burning Issues (CBI) study [Green, 1980] examined from many perspectives Florida’s EEE problems that might be associated with replacing oil with coal in industrial and utility boilers. The 1979 oil crises, the CBI study and a study An Alternative to Oil, Burning Coal with Gas [Green, 1981] led to the establishment of the Clean Combustion Technology Laboratory (CCTL) to pursue R&D on related technical problems. Considerable progress was made in 1985 in co-firing coal water slurries with natural gas in a 20,000 lb/hr steam boiler designed for oil. However, when oil prices plummeted interest in oil-back-out declined [Green, 1986]. The CCTL’s focus then shifted to waste disposal and co-firng of domestically available solid fuels including coal, biomass, municipal solid waste, agricultural wastes and bio-solids in existing capital facilities [Green, 1988]. To mitigate environment problems, particularly greenhouse gas emissions, potential solutions were identified as Energy Efficiency, Renewable Energy, Nuclear Energy, CO2 Sequestration and

Population Stabilization [Green, 1989]. To mitigate energy and economic problems, particularly oil imports, it was clear that available solid fuels must be converted to fuels suitable for the transportation sector [Green, 1991]. At the millennium carbon dioxide emissions, toxic emissions and other environmental concerns as well as economic impacts have become the drivers of energy projects. Systems are needed that make use of all available domestic fuels while mitigating anthropogenic emissions. Figure 1 conceptually illustrates an omnivorous feedstock converter to meet national EEE needs.

The CCTL’s efforts over the last decade have focused on applications of laboratory scale IHGLCC systems. The initial intent was to develop processes needed to convert coal and other solid fuels into clean gaseous or liquid fuels suitable for transportation use and for combustion turbines and fuel cells. In this study we examine the use of IHGLCCs with CO2 to improve the quantity and quality of the gaseous output, to indirectly or directly assist in the global management of CO2 and to seek solid outputs (charcoal) that can serve both to sequester CO2 and as valuable chars (i.e. soil organic carbon, humates, peat, activated carbon, charcoal, chelating and detoxifying agents etc). In effect we have expanded our co-utilization R&D with IHGLCC technology to include thermo-chemical studies with CO2 in the input blend and useful chars as an output.

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Co, Bm, RDF, Bs CO2, NG, St, Air, O2Processor, Ct, Ab, Re

Generator

Steam Turbine

Co=Coal, Bm=Biomass, RDF=Refuse Derived Fuel, Bs=Biosolids, Ct=Catalyst, Ab=Absorbents, Re= Reactants, CO2=Carbon Dioxide, St=Steam, NG=Natural Gas, O2= Oxygen, GCU=Gas Clean Up, HRSG=Heat Recovery Steam GeneratorAC=Activated Carbon, Hu=Humates, CO2Sc=CO2 scrubber

HRSGSpecialty ChemicalsAC,Hu,CO2Sc, Coke, Ash Liquid

Fuels

GCU Gasifier/LiquifierGenerator Filter-DistillerCombustion Turbine

Figure 1. Conceptual Illustration of an Omnivorous Feedstock Converter

IHGLCC TechnologyThe technical options for front-end gasifiers to

“cook” solid fuels singly or in blends at high temperatures, together when needed, with carbon dioxide, steam, natural gas, air, oxygen, catalysts, adsorbants, reactants and even liquids appear relatively few. Fluidized bed reactors are receiving the major attention for large units. However, disparate feedstock densities, textures and reactivity rates can be problems. Thus for microturbine generators and other applications IHGLCCs with mechanical (auger) transport might provide a viable option.

The technical approach taken in this paper is to blend disparate feedstock including CO2 in IHGLCCs and seek useful gases, liquids and chars including chars that can restore SOC to agriculturally depleted lands. This approach unfortunately involves a number of complex sub-disciplines that still appear unsettled or poorly understood even by specialists. For example, pyrolysis, a basic step in thermo-chemical processes is still far from a predictive science [Solomon et al., 1992]. Other involved subjects, catalysis, activated carbon, humic acid etc. are also at primitive levels. Some general features of the response of coals, biomass and other solids to increasing temperatures might eventually come from basic quantum chemistry [Green and Zanardi, 1998]. In the meantime simple experiments with simple adaptable apparatus and the development of models to fit the results can serve to identify the major controlling variables and their impacts. Essentially the scientific method should be applied to these complex systems just as operations or systems analysis applies the scientific method to large organizational problems [Green, 2000].

Most of the CCTL’s prior quantitative work with IHGLCCs has been carried out with batch fed units that have given reproducible yield data with a variety of feedstock or blends for several high temperature conditions [Green et al. 1995, 1996abc, 1997; Peres and Green, 1998; Green and Mullin, 1998; Green and Schaefer, 1999; Schaefer and Green, 2000]. We now have about a dozen of these systems with inner diameter tube furnaces ranging from 1 cm to 10 cm that can be used with samples ranging from 1 to 25 grams. Eight use furnaces wound or rewound on various sizes of mullite tubes and four use commercial tube furnaces. These simple systems can be used with particle sizes comparable to those that might be used in commercial systems. To address the question “What to Do with CO2?" we have: 1) incorporated rear end gas injection in our continuously fed (CF) gasifier; 2) activated and adapted an old kiln for gas injection; 3) activated an old high temperature oven with an air fan for drying, air oxidation and weight loss on ignition (WLOI) studies; 4) activated an old minigasifier for proximate analysis; 5) built a new microgasifier, for 1- 2 gram samples; 6) built a new minigasifier for 5-10 gram samples with rear end gas injection intended for careful mass balance and gas analysis studies and for two temperature treatment investigations. Our CF- IHGLCC system as shown in Figure 2 is a thermo-chemical-mechanical (TCM) system that has evolved from several prior auger driven CF systems. The incorporation of rear end gas injection was included a number of times earlier for several purposes. Using CO2 injection increases the safety of our operation since it can serve as a fire suppressant. Potential use of CO2 from combustion stack gases using near term separation technology is one application under consideration

Oxidizing Activity of CO2

In CCTL gasification studies with batch fed micro-gasifiers we have found convincing evidence for the oxidizing

action of pyrolysis generated CO2 at high temperatures [Green et al., 1997; Peres and Green, 1998; Green and Mullin, 1998].

Figure 2. The CCTL- Continuously Fed IHGLCC.This was in the form of increasing heating values of our output products as we proceeded from 700ºC to 1000ºC, increasing yields of CO, decreasing yields of CO2, increasing total energy in gaseous form and decreasing yields of char. This appears to be due to the interaction of pyrolysis generated CO2 with char as in the Boudouard reaction

CO2 + C 2CO. (1)This reaction should also apply with externally introduced CO2 from combustion of hydrocarbons such as petroleum or coals. Professor Raphael Kandiyoti’s group at the Imperial College of London has examined the pyrolysis of various coal rank samples when heated to temperatures in the 850-1000ºC range under helium pressure (‘pyrolysis conditions’) and CO2

pressure (“gasification conditions’). In all cases they obtain increased volatile yields under gasification conditions confirming that CO2 at high temperatures is oxidizing the coal and releasing extra CO [see for example Collot et al. 1998; Messenbock et al. 1999, 2000].

For direct applications of CO2 with any hydrocarbon feedstock we must consider generalizations of Eq. 1. For simplicity let us consider the feedstock of interest to be polymers of a fundamental monomer CnHmOp where m/n and p/n have the approximate hydrogen/carbon and oxygen/carbon ratios of various energetic fuels that lie along the natural coalification curve shown in Figure 3.

Figure 3. The O/C vs H/C coordinates of solid fuels. The smooth coalification curve is the logistic function (the learning curve). The contours give the ratio of the heating value relative to carbon according to a Dulong formula.

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The formation of a compound molecule i.e.CO2 + CnHmOp Cn+1 HmOp+2 (2)

would strongly oxidize the original feedstock. The potential decay products of the compound molecule can be very large. An important path is represented by the net reaction CO2+CnHmOpCn+1HmOp+2CnHmOp+1+CO (3) that produces a useful gaseous fuel while mildly oxidizing the incoming feedstock. Using CO2 as a gasifying agent has advantages over air with its dilution of the output by nitrogen. An advantage with respect to oxygen as the gasifying agent is the avoidance of a costly gas separation plant. With respect to steam the advantages are the avoidance of a boiler and the use of water that has other purposes.

IHGLCCs used for pyrolysis de-oxidize (reduce) the feedstock via dissociation reactions such as CnHmOpCn-1HmOp-1+CO orCn-1HmOp-2+CO2 (4)

In CCTL studies with indirectly heated gasifiers we have long wondered whether the moisture in our biomass and the pyrolysis H2O are helpful or harmful in gasification. It appears possible that the H2O as well as the CO2 can be useful under optimum temperature conditions. The synergistic effects observed at KTH Stockholm Sweden [Sjöström et al., 2000] might be explained by the action on coal of the H2O and CO2 generated in biomass pyrolysis as well as the greater production of free radicals. The role of H2O with IHGLCCs is also important. Conceivably pyrolysis H2O from

CnHmOp CnHm-2Op-1 + H2O (5)might be a useful source of water. On the other hand H2O can be used to produce H2, a desired fuel gas by the reaction

H2O + CnHmOp CnHmOp+1 + H2 (6)There are many other break up modes of the original feedstock at high temperatures. By blending solids, gases and even liquids in IHGLCCs one can foster a great variety of break-up products and reactions between these products. It would be helpful to have some overall picture of the energetics of the thermo-chemical reactions in IHGLCCs.

Reaction EnergeticsTo estimate reaction energies one might use

Dulong”s approximate formula for HHV, the higher heating value per unit mass of unknown reactants or h, the heating value per mole. We let HHV = A N/M in MJ/kg where M =12n + m + 16p in kg and h = AN in MJ/kgmol.Here A is in MJ and N is a dimensionless combination of n, m and p. The coefficients used in Dulong formulas are sometimes quoted to four or five significant figures but differ between

the coal, biomass, chemistry and nutrition communities in the second figure. For an initial ballpark overview we might simplify Dulong’s approximate formula by rounding the relative coefficients to integers or half integers. We find that

the coefficients used in coal circles (CC) [Babcock and

Wilcox, 1992] can be approximated with N ~ 3n + m – 2p with A = 136 MJ :CC (7)However, the coefficients of Gaur and Reed [1998] for biomass applications are closer to

N ~ 3.5n + m –1.5p with A = 120 MJ :BC (8)As a compromise we have made use of N ~ 4n + m – 2p with A = 108 MJ :G (9)This combination approximately maintains the usual carbon coefficient and has the virtue of giving zero h and H for CO2

and H2O, the end products of complete hydrocarbon combustion. With the CC coefficients (Eq.7) the reaction for

forming the compound molecule Cn+1 HmOp+2 is exoergic (-136 MJ/kgmol) whereas with the BC coefficients (Eq.8) it is endoergic (+60MJ/kgmol). The GC coefficients (Eq. 9) gives a neutral (zero) result for CO2 and also H2O capture. In any case the reaction energies for compound molecule formation and for several possible bi-molecular decay products are usually small compared to the heating values of the original coal monomer. Accordingly it would only take a small portion of the energy in the input coal or possibly even the gas, liquid or char outputs to provide the heat of conversion. With the GC coefficients the use of CO2 as a gasifying or liquifying agent becomes as logical as the more familiar use of steam (H2O). While H2O is a more reactive gasification agent, it is a valuable resource with many uses whereas CO2 is the world's largest waste product. Thus improving technologies for separation and use of CO2 is important.

Of course, a barrier or activation energy might limit the rate of conversion of the solid feedstock to desirable gases, liquids or chars. However, high temperatures and/or catalysts could overcome these barriers.

IHGLCC Experiments with CO2 as a Gasification Agent A total of 24 successful experiments were

performed by operating our continuous flow (CF) IHGLCC in semi-continuous mode; that is, loading in a known amount of biomass into the inner hopper and running the gasifier until all the material was reacted. The reproducibility of the results was at the 5% level. Mass balance closures were between 60-80% in the pyrolysis mode and 80-90% in the gasification mode. While the addition of CO2 led to an increase in gas yield and reduction in the char yield, the liquid yield data was inconclusive, although substantial in all runs. For almost all runs there was an increase in gas heating value with increasing temperature. Due to the diluting effects of unreacted CO2, the specific energy of the gas (essentially the product of the gas yield and heating value) is a better performance index. This quantity increases with increasing temperature, and also increases with the addition of CO2. This increase in gas energy comes largely at the expense of the solid char product, as expected from Eqs. 1 and 3. However, the marginal increases in yield and specific energy vary with feedstock. In general, feedstock with higher volatile contents are more reactive and thus exhibit a larger response to temperature and CO2 addition. Bagasse was actually more reactive than the volatile content would indicate. Coal blends and pine wood/bark, with relatively low volatile contents, exhibit less reactivity with CO2. An analysis of the gas composition results shows a large increase in CO yield when CO2 is added. This conversion of CO2 to CO is accelerated by higher temperatures and is faster with more reactive biomass chars, a conclusion corroborated by the literature [Kaupp and Goss, 1984; Collot et al. 1998; Messenbock et al. 1999, 2000].

An analysis of the transient (time-dependant) gas characteristics (flow rate and composition) of our CF- IHGLCC suggests that some initial combustion of the feedstock occurs at the beginning of the run, with air in the gasifier prior to being sealed. The gas flow rate is in general quite unsteady, suggesting that either the biomass feed rate should be stabilized or a flow damper would need to be installed for gas turbine applications.

As expected, a substantial decrease in char yield was observed with CO2 runs. A decrease in heating value of the residue was observed suggesting the residue is more oxidized but this may also be due to an increase in the ash fraction of the char. In pyrolysis there is, of course, a strong correlation between char yield and fixed carbon content of the

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feedstock. For relatively non-reactive feedstock such as coal blends, the char yield change when CO2 is added is comparatively small. Thus for energy purposes a catalyst might be needed to foster reactions with these feedstock chars.

Extensive studies of catalytic gasification of carbon [McKee and Chatterji, 1975], and many other references [see Green et al., 1996c] have been made to understand the mechanism of catalysis in fuel conversion applications and to seek a catalyst that will help in the production of fuel gases from graphite and coal at lower temperatures. In this respect alkali metals and their salts have been found to be efficient catalysts in oxygen, carbon dioxide and water vapor atmospheres. Alkali metal carbonates have been suggested in conjunction with the cycle of reactions K2CO3 + 2C 2K + 3CO, 2K + CO2 K2O + CO,

K2O + CO2 K2CO3 (10)

The net result of this cycle is that 2C and 2 CO2 are converted to 4 CO, a useful middle heating value gaseous fuel. The energy that the furnace needed to sustain the process increased in the case of CO2 injection. The pyrolysis reactions required little or no net energy, whereas the energy required for the CO2 experiments represented a significant fraction of the gas energy. In all cases, however, the increase in gas energy between pyrolysis and CO2 gasification was larger than the increase in furnace energy. Calculations indicated that the extra furnace energy consumed during the CO2 experiments is largely attributable to the energy needed to sustain the Boudouard reaction, rather than an increase in sensible heat of the reactants or products. Table 1 summarizes some of the key results of the CO2 blending cases.

Table 1. IHGLCC Feedstock Blending Results, Pyrolysis and CO2 Gasification.

Bag. Pine Pine

Feedstock Oak Chips Oak Chips & Bit. Coal Bagasse &

Coal W/B & Coal

Fdstck. HHV (MJ/kg) 19.20 25.37 19.30 25.61 21.43 26.69

Reactor Temp. (°C) 700 850 1000 1100 850 1000 1100 1000 1100 1000 1000 1000

CO2 GasificationCO2 Flow Rate 6.2 11.1 8.7 10.0 9.4 10.0 9.4 9.7 9.3 9.8 10.1Specific Gas 1.62 7.07 8.76 3.22 5.29 6.15 7.36 10.61 5.39 4.42 4.70Energies Char 8.75 1.99 0.64 13.63 12.50 9.53 3.36 1.31 12.06 7.15 13.95

(MJ/kg) Furnace 0.26 1.97 1.93 0.59 1.32 1.12 2.27 3.21 1.58 0.72 1.05

Liquid* 9.1 12.1 11.7 9.4 8.9 11.1 10.9 10.6 9.8 10.6 9.1Product Gas ResultsVol. Yield (L/kg) 316 969 912 466 541 634 981 1238 595 503 482HHV (MJ/m3) 5.12 7.30 9.61 6.92 9.78 9.71 7.51 8.57 9.07 8.79 9.74LHV (MJ/m3) 4.84 7.13 9.28 6.41 9.15 9.15 7.25 8.34 8.49 8.29 8.66Density (kg/m3) 1.52 1.35 1.18 1.41 1.16 1.15 1.31 1.25 1.20 1.23 1.18PyrolysisSpecific Gas 2.51 3.41 4.86 2.72 3.56 1.80 4.00 2.97Energies Char 4.71 5.48 5.04 13.35 13.58 6.80 5.95 7.80

(MJ/kg) Furnace 0.03 0.64 0.00 0.00 0.02 0.00 0.64 0.58

Liquid* 10.1 10.7 7.4 9.3 8.6 10.7 10.0 11.2Product Gas ResultsVol. Yield (L/kg) 209 259 354 164 218 160 306 222HHV (MJ/m3) 11.98 13.18 13.75 16.58 16.38 11.28 13.04 13.38LHV (MJ/m3) 11.17 12.30 12.79 15.03 14.84 10.48 12.12 12.39Density (kg/m3) 1.10 1.03 0.98 0.77 0.73 1.10 0.97 0.96* Liquid results are by difference. All results are dry basis.

IHGLCCs and Sequestering CO2

DOE’s recent report on Carbon Sequestration [DOE, 2000] divides the major strategies into: Ocean, Terrestrial, Geologic, Advanced Biological and Advanced Chemical. The Terrestrial strategy is based upon the valuable role soil organic carbon (SOC) can serve in indirect sequestering of CO2 via plant growth i.e. photosynthesis – respiration. Pursuing the possibility of sequestering CO2 by using coal to scrub stack gases from coal burning plants while converting it to SOC or peat (also called turf

in some countries) was the starting point of this investigation. This

is basically a combination of the Terrestrial and Advanced . Chemical Sequestering strategies.

We have previously noted that peat could be the ideal gasification feedstock [Green et al., 1996b]. It has a high volatile content (~ 70%) and its residual carbon has enough energy to supply the heat required for indirectly heated gasification-liquification (IHG/L) and to generate the steam needed to gasify the remaining carbon. On the other hand in the USA peat is more

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Figure 4. The difference between FTIR spectra of exposed and unexposed high volatile bituminous (HVB) coal. The exposed coal was subjected for one hour at 400ºC to a mixture of about 75% CO2 and 25% air (the muffle furnace had a small leak). The difference spectra for coal exposed to air at 200ºC for 16 hours was quite similar. The appearance of peaks at 1747, 1545 and 1328 cm-1 indicates an increase in carbonyl and carboxylic groups characteristic of humic acids. This is consistent with our calorimetric measurements. Thus by mild oxidation we appear to have changed the coal to humates

valued as SOC than as fuel. The intermediary position of peat on the coalification curve between coal and biomass has suggested that for some combustion or gasification purposes blending biomass together with lignite or bituminous coal might be as effective as using peat as the feedstock. During our pursuit of coal as a CO2 scrubber the important role of peat as soil organic carbon (SOC) to foster plant growth and thus indirectly scrubs CO2

(photosynthesis – respiration) became very apparent. Thus we arrived at the simple strategy of changing abundant coal or available waste biomass (agricultural and forest residues, bio-solids, paper component of MSW etc) to peat with an omnivorous feedstock converter. If we include some air in the input we could mildly oxidize coal into peat thus reversing naturesconversion of peat to coal. If we mildly pyrolyze biomass into peat we, in effect would move the geological clock forward. In both cases we could produce a peat-like output that could enhance soil fertility and indirectly scrub CO2.

While recognition that soil fertility can be sustained or improved by organic matter goes back almost to the dawn of agriculture [Burns et al., 1986] the scientific understanding of how, when and why this occurs has still not been achieved. During the past century enhanced fertility has been associated with soil organic carbon (SOC) and particularly humic substances. There are a number of monographs describing humic acids [Stevenson, 1982; Burns et al., 1986]. Leonardite is a naturally oxidized form of lignite [Hoffman et al., 1993]. Humalite is a weathered product of sub-bituminous coals and carbonaceous shales.. Both contain humic acids that are valued as soil amendments. The fact that the natural weathering process in going from lignite to leonardite takes eons raises the question as to whether the process can be speeded up. J. Green and Manahan [1979] have prepared humic acids by the partial oxidation of bituminous coals using nitric acid. They noted that the humate salts are of interest because of their chelation, detoxifying, floculation, acid neutralization and acid gas sorption properties. Other chemical or solvent extraction methods are available with moderate time scales (hours). Walia of ARCTECH [1999] has developed coal bioconversion technologies based upon microorganisms isolated from termites. Their Humasorb product has been successful in remediating ground water contaminated with mining waste at the Berkeley Pit in Butte, MT [Schwartz, 1999].

Biochemical processes usually require weeks to produce useful gases or soil amendments. Since high temperatures processes are some thousand times faster than biochemical processes, power plants and other high fuel-use systems might find it more practical to proceed thermo-chemically in producing valuable humates, fuels and chemicals. Additionally it appears worthy to systematically investigate: converting coals or coal mine debris to more valued solids or liquids including soil amendments, absorbing waste heat, and sequestering CO2, while converting the solid input to more valued products.

Experiments using IHGLCCs to make HumatesIn preliminary studies with IHGLCC we have used

heating value determinations made with our Parr calorimeter as an oxidation indicator. With the GC Dulong coeficients the HHV is approximately

HHV = A{4n + m – 2p}/{12n + m + 16p} (11)where A = 108 MJ/kg. An increase in p (say by 1 i.e. single oxidation) decreases the numerator and increases the denominator thus strongly lowering the heating value. On the other hand a decrease in p (i.e. reduction) raises the heating value. Thus, if allowance is made for the mineral matter (ash), HHV is a sensitive indicator of degree of oxidation. The change in heating value of runs with bituminous and lignite

coal and two chars by air oxidation at two temperatures and times are given in the Table 2. The results suggest that IHGLCCs can humificate lignite and bituminous coal by mild oxidation (accelerated weathering) and also humificate biomass by mild pyrolysis (accelerated composting). The last two rows of Table 2 show the effect of paired experiments in which a feedstock blend that included coal, biomass and some K2CO3 is first converted to a char by pyrolysis at 800ºC. Then the sample is quickly moved to a lower temperature

Table 2 Heating Values.   Heating Value (MJ/kg dry basis)    Air CO2/Air Pyrol. 800°C    200°C 400°C CO2 300°C

Feedstock Untreated16 hours 1 hour 7 minutes

Lignite Coal 24.7 19.2 20.6 -Bituminous Coal 32.0 24.6 26.8 -Biomass Char 27.1 25.4 - -Biomass/Coal Char 29.4 27.5 - -Bit. Coal/Biom. Char 26.3 - - 25.2Lig. Coal/Biom. Char 22.0 - - 20.6

region of the tube furnace and a) left untreated b) exposed to CO2. This explored the use an IHGLCC system to accomplish CO2 scrubbing with gram level samples of coal-biomass char. This represented an attempt to extrapolate work on strong chemisoption of CO2 by coal chars prepared by pyrolysis at 800ºC [Molina et al., 1999]. Milligram char samples were treated with KOH and then exposed to CO2 at 300ºC using thermogravimetric (TGA) apparatus. Our two-temperature results are encouraging but very preliminary. Figure 4 show a difference FTIR spectral result of rapid mild oxygenation of coal.

877.

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0.39

1095

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1328

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1456

.31

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.63

1747

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2858

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2932

.04

2951

.46

3642

.72

-0.1

0.0

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0.2

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0.5

Abs

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1000 2000 3000 4000 Wavenumbers (cm-1)

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Although oxidation, or weathering, of coal with air is traditionally viewed as a negative, with the increasing value of humates as environmental clean-up agents and to restore soil organic carbon (SOC) the potential net positive aspects of mild coal oxidation are large. Coal is the world’s most widely mined material and almost every continent has an abundance of low grade coal that could be directed towards these environmental purposes.

We have also found that by mild pyrolysis wood char develops similar spectral features to mildly oxidized coal suggesting that we have accomplished accelerated composting. In using the continuously fed (CF) IHGLCC to demonstrate this possibility essentially wood chips are transported by auger at lower temperatures (~350ºC) to accelerate nature’s process of decreasing the volatile and oxygen content of the wood. The solid products was peat-like in appearance and the heating value was increased from 19 to 21 MJ/kg, consistent with that of natural peat. This result has been duplicated with batch IHGLCCs. The features in the 1.6 cm-1 region (6-micron) from biomass and its char samples also suggested that we had formed some aromatic-COOH molecules similar to those in humates or weathered coal [Joseph and Mahajan, 1989].

Thus it appears that we can also advance forward nature’s geological clock. With our CF-IHGLCC we have control of enough independent variables (feedstock blend, temperature, pressure, particle sizes, transport time, catalysts, CO2 air and/or H2O flows etc) to simulate almost all of the parameters suggested by CCTLs batch experiments or TGA experiments at Universidad de Antioquia in Columbia by Professor Fanor Mondragons group [Molina et al., 1999].

Colorimetric, and weight loss on ignition (WLOI) tests were also tried to assess humification of char samples from mild coal oxidztion or nild biomass pyrolysis. Systematic tests, however, remain to be executed.

Sequestering Potential The CCTL has sought thermo-chemical conditions

and catalysts to use CO2 as a mild oxidizer to artificially weather coal, biomass or blends into more valuable solids (e.g. accelerated conversion of lignite to leonardite, sub-bituminous coal to humalite, biomass to compost, waste fuel blends to valuable humate type soil organic carbon (SOC), chelating and detoxifying agents, activated charcoal, or other valuable chemicals). If the SOCs are long lived in soils this would provide a direct means for sequestering carbon while improving land fertility. Restoration of land fertility would provide an indirect means of CO2 sequestering by the atmospheric scrubbing action of plants. Exploratory results of tests already indicate that we can thermo-chemically humificate bituminous, lignite and biomass chars by mild air oxidation with our IHGLCC systems. Thus indirect sequestering of CO2 via enhanced plant growth is conceptually assured. Tests for direct sequestering by coal or biomass chars made by adding CO2 to the input of thermo-chemical humification processes gave promising preliminary results but are not yet confirmed with systematic tests.

It would appear probable, however, that thermo-chemical systems will evolve in which inclusion of CO2 as an

input will enhance the production of a medium heating value gas. It also appears certain that IHGLCCs can be used to mildly oxidize coal and mildly pyrolize biomass into humates. The composite use of CO2 both as a gasification agent and a char oxidizer seems promising but it would be premature to claim that we have demonstrated this. If by a combined use of coal and CO2 we can, with the help of biomass, simply reduce carbon emissions per MJ of useful energy to that of natural gas this already would be an important carbon management strategy.

If the SOC produced by IHGLCC systems resists microbial decomposition for long periods and has useful humification qualities, its need to restore depleted agricultural lands would be very large. Globally about one billion tons of SOC are lost each year to erosion [DOE, 2000] and some 40-60 billion tons of carbon have been lost due to forest clearings and agriculture in the past two centuries. Improved cropland management can restore some losses while lowering CO2

emission and sequestering CO2 via the atmospheric scrubbing action of plants [Lai et al., 1998]. Adding SOC can help substantially as illustrated by a simple calculation.

If as an initial goal the USA plans to add say 0.1 kg/m2 of SOC each year to 100 million hectares of the 563 Mha or so of its agricultural lands the requirement would be for 108x 104x10-1= 100 million metric tons (MMT) of SOC (where 1MT =1 metric ton =1000 kg=0.984 long tons =1.10 short tons). With USA production of coal at about 1000 MMT this level of a new carbonaceous product should be within the capabilities of the coal industry. The biomass sector could also contribute about 100 MMT of biomass by simply collecting one extra metric ton of agricultural or forest residue or energy crop per hectare from say 100 Mha. The MSW industry could also contribute about 10 MMT of waste paper. Thus coal and biomass availability and productivity in the USA seems feasible [see also DOE-Fossil, 1999; DOE-Bioenergy, 1999].

Related Technical Literature Our exploratory experiments to date led to an

intensified literature search for external evidence in support of our quest for an answer as to "What to Do with CO2." First we note two recent publications that fit into the scope of the development of an omnivorous feedstock converter. These include a study in Spain in which residual biomass and poor coal blends are used as feedstock for gasification [Pan. et al., 2000]. In a study in Turkey cellulosic waste material and lignite coal are co-liquified [Karaca and Bolat, 2000]. We have also found works that support our thermo-chemical humification of coal and biomass although these works had entirely different objectives. Indications that we can thermo-chemically transform coals back to humates or peat were mostly directed toward finding ways of minimizing partial oxidation of coal. [Chang and Berner, 1998; Kruszewska et al., 1996; Doyle, 1992; Green J. and Manahan, 1979; Fowkes and Frost, 1959]. We have not found prior studies on the thermo-chemical conversion of biomass to humates nor of the conversion of coal or biomass to CO2 scubbers. However a number of articles or books [Barton et al., 1983; Bansal et al., 1998; Molina et al., 1999; Radovic, 1997] on the preparation of activated carbon give information on micropore development and chemisorption that suggest that it might be feasible to convert coal to a selective form of activated charcoal that could capture significant quantities of CO2.

SummaryThis exploratory study indicates that one can use

CO2 with IHGLCCs to make more fuel gas from hydrocarbon

6

feedstock. It also indicates that coal and biomass can be converted with IHGLCCs to peat-like material to accelerate the restoration of soil organic carbon (SOC). The enhanced plant growth could convert atmospheric CO2 into potentially long-lived timber. The possible conversion of coal, biomass or blends or their chars into scrubbing agents that capture stack gas CO2 separated from nitrogen by developing molecular sieve technology should be examined. It would appear that the charcoal could be simultaneously converted into useful peat-like SOC. However, many more experiments are needed to establish this.

In final summary our exploratory experiments and literature search indicate that thermo-chemical-mechanical (TCM) production of synthetic turf (SOC, peat, humates) using biomass or coal or blends with IHGLCCs seems assured. If confirmed the coal sector could help the agricultural sector by providing useful gaseous fuel and restoring the vitality of great land areas that have been agriculturally depleted of organic carbon. The agriculture sector could help the coal sector by terrestrial sequestering CO2 in SOC and plant matter forms. Clearly greenhouse mitigation is a global operations and systems analysis problem in which all potential measures must be considered in relation to the benefits to the whole system rather than only parts. A number of complex alchemies would have to be made into sciences and technologies before understanding of essential processes could be achieved. If the fuel, industrial and governmental sectors and countries involved can apply teamwork and literally and figuratively overcome turf problems this global problem might well be manageable. With the world's recoverable reserves of coal at about one million MMT [DOE/EIA-0384 (99) July 2000] applying a billion or so tons per year for this application could solve an important environmental problem and save this major world-wide industry.

It might be noted that the development of advanced thermo-chemical-mechanical (TCM) methods for using or sequestering CO2 can also have applications in long space missions. Currently the International Space Station (ISS) plans to use mature technologies for their typical 45-day missions. However, advanced methods for management of CO2 and the disposal of solid wastes that do not involve so much back and forth transportation of food and waste could be considered in the later missions or as part of zero gravity tests for longer missions. For the planned missions to Mars some of the ISS technologies will certainly not be adequate for nominal to and from travel times of 200 days and a stay of 200 days. Using or sequestering the CO2 generated in the spacecraft and in the sealed ground living quarters and the conversion of the solid waste into sterile liquids or useful gaseous fuel or chars are essential for the success of such long missions. Extracting oxygen from the natural carbon dioxide in the Martian atmosphere also poses a great challenge. For space travel the extrusion system used in our TCM continuously fed gasifier/liquifier/char reactor should be more adaptable to zero gravity than fluidized bed systems that require gravity as a restoring force. The fact that TCM conversion of solid waste can be accomplished in minutes whereas biochemical conversion requires weeks should be advantageous when time is of the essence in space missions. The rapid manufacture of humates from food residues and bio-solids should be helpful for plant growth needed to provide food for the long travel time Mars mission and for the Mars stay. By seeking ways to avoid global climate change and preserve Planet Earth perhaps we will also find answers to questions relative to Manned Mission to Mars and vice versa.

In final summary, R&D devoted to changing CO2

from a waste output to useful input may well hold the final answer to “What to Do with CO2”.

Acknowledgements This work was supported in part by the Mick A

Naulin Foundation and the College of Engineering. We also thank Professor Fanor Mondragon Director of the Coal Chemistry and Catalysis Research Group at the Universidad de Antioquia in Medellin-Colombia for his advice and for processing and measuring the IR spectra of our samples.

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