Stoichiometric Relations for Neutral Emissions in the Generation of
Electricity and Motor Fuels Produced from Coal and Biomass
Linked to Sequestration in Coal Seams

John H. Walsh
Energy Advisor

Note: The following text does not include the seven figures which may be obtained in hard copy form by contacting the author at


This paper is the second in a series dealing with coal-based processes for the generation of electricity linked to the production of Coal Bed Methane (CBM) with the possible co-production of hydrogen or carbon-bearing liquid fuels. These processes are assumed equipped for the capture of carbon dioxide which is then sequestered in coal seams not expected to be mined causing the displacement of methane from the surface of the coal which is returned to the process. Since the capture of carbon dioxide is rarely complete, biomass grown sustainably is co-charged with the coal in quantities chosen to just off-set these fugitive losses so that the products of the process, whether electricity, hydrogen or liquid fuels, results in null net emissions of this gas to the atmosphere when consumed.

Because the carbon in the biomass is generally more expensive than the carbon in the coal, the objective is to minimize the quantity needed. For this reason the results in this paper are calculated in terms of the ratio of the carbon-in-coal to the carbon-in-biomass required to achieve neutral emissions. When 80% of the carbon charged to the process was captured as the dioxide for sequestering in coal seams, this ratio ranged from 2.0 to 3.5 for the case where electricity is generated or hydrogen is produced as the Exchange Ratio of carbon dioxide sequestered to methane released in the CBM operation varied from its theoretical low of two to a high of eight. These values are corresponding greater at higher capture efficiencies and lower when carbon-bearing liquid fuels are co- produced. An Approximate Power Index was devised to give some indication of the effect of biomass additions and a range of values of the Exchange Ratio on the generation of electricity.


Methane derived from coal seams (CBM) not expected to be mined already provides some five per cent of the current U.S. production of natural gas. The National Energy Board expects this non-conventional energy source to become an important new supply of gas in Canada in the period 2008-2013.1 Carbon dioxide captured from fuel consuming processes may serve as a displacing gas to release methane in CBM operations where it becomes sequestered in the coal. A field trial is now underway in Alberta of this potentially important option to control greenhouse gas emissions by linkage to CBM operations.2

This paper extends the calculations reported earlier in two different ways.3 First, biomass is introduced as a co-fuel under the assumption it is grown sustainably such that the release of its carbon content does not lead to a net increase in carbon dioxide emissions to the atmosphere. Such carbon is termed `eligible' here to reflect the terminology arising in the present international negotiations aimed at controlling emissions of greenhouse gases. Because carbon from this source is generally more costly than that derived from the fossil fuels, it is important that it be consumed as effectively as possible. In this paper, the maximum proportion of eligible carbon co- processed with coal is calculated such that the biomass source of carbon off-sets the inevitable fugitive emissions of carbon dioxide escaping from generation or other processes equipped for the capture of carbon dioxide. When the quantity of eligible carbon consumed equals the loss of carbon dioxide to the atmosphere, the overall process may be said to be atmospherically-neutral in the sense that all the carbon entering with the fossil fuels is sequestered. The eligible biomass carbon thus accounts for the fugitive loss. When this condition is met, the electricity generated (and/or hydrogen or liquid fuels co-produced) are themselves neutral to the atmosphere and may be consumed anywhere and at any time and still meet this null criterion.

The second extension to the previous paper involves the ratio of the carbon dioxide sequestered in the Coal Bed Methane (CBM) operations to the methane displaced termed here the Exchange Ratio. This value is extended from the theoretical value of two used in the previous paper over a range with a high of eight to account for experimental data now arising from the trials in sub-bituminous coal beds of which extensive resources not likely to be worked by conventional mining methods exist in Alberta.

In essence, three feeds are assumed to electrical generation facilities (or to processes for the production of hydrogen/synthesis of liquid fuels) equipped for the capture of carbon dioxide-coal, biomass and the methane recovered from the coal seam resulting from the sequestering of the captured carbon dioxide. If the capture stages worked perfectly such that there were no emissions of carbon dioxide, there would be no need for the consumption of the biomass to achieve null net emissions. But even if complete capture could be achieved, for the case when a carbon-containing liquid fuel is produced, the carbon in the biomass fed to the process would have to equal the carbon in the liquid fuel to meet the criterion of no net emissions. The advantage of this approach in this case is that this is the maximum required. In free-standing biomass processes to produce liquid fuels, the carbon requirement is generally higher for process reasons. In most of the following calculations, an 80% overall capture efficiency was assumed but with sensitivity cases of 85 and 90% in Case A.

The biomass source is envisioned as any supply of eligible carbon which may include the important case of sewage wastes charged to gasification processes or possibly a specially grown field crop. There is, however, a complication when the object is to produce liquid fuels alone with no co-generation of electricity. The quantity of biomass fed may have to be increased to cover emissions arising in the generation of the electricity purchased to operate the process to achieve full life-cycle atmospheric neutrality of the fuel synthesized, but this case is not considered here.


Two basic equations were used in the calculations. The first concerned atmospheric neutrality where the carbon in the biomass feed is set to just equal the fugitive carbon dioxide emissions.

(x + y + z)(Fugitive Loss Factor) = y

where x = moles of C in coal, y = moles of C in eligible biomass and z = moles of C in CH4 recovered by exchange from CBM operations. The Loss Factor was usually taken as 20% of the total feed carbon but sensitivity cases of 10 and 15% were also calculated in Case A.

The second equation involved the CBM Exchange Ratio.

(x + y + z)(Recovery Factor)= (Exchange Ratio)(z)
Values of the Exchange Ratio in CBM operations were chosen from two to eight to cover the range of main interest. The Recovery Factor was usually taken as 80% with sensitivity cases of 85 and 90% in Case A to correspond with the Loss Factor (100% - Recovery Factor) used in the first equation.

There are thus two equations with three unknowns. In this situation, the calculations were based upon the unit moles of carbon entering with the coal (usually one) because it is the independent variable in the sense that both the quantity of biomass consumed and methane produced in CBM operations depend upon the quantity of coal fed to the process. The results were expressed as the ratio of moles of carbon-in-coal to the moles of carbon-in-biomass for values of the CBM Exchange Ratio ranging from two to eight.

An Approximate Power Index (API) was also calculated based upon the assumption that the electrical output should be roughly proportional to the energy in the three fuels fed. The carbon in the biomass was assumed to bring with it one half the energy in the carbon of the coal due to its high oxygen content. Similarly, each carbon in the methane recovered from CBM operations was assumed to contribute twice the energy content entering with each carbon of the coal due to its high hydrogen content. The API was calculated for each case as the ratio of the total of the input energy from the three fuels so estimated divided by the energy input of the coal and biomass summed together with the latter value calculated as bringing one-half the energy of the coal per unit of carbon as before. The API is thus an index of the energy equivalent output including CBM methane per equivalent unit input of coal and biomass fed. The purpose was to provide at least a crude estimate of how the electrical output might be expected to vary with the CBM Exchange Ratio.

Case A - Generation of Electricity and/or Production of Hydrogen

The block diagram for Case A appears in Figure 1. As neither product-electricity and/or hydrogen-contains carbon, this element in the feed to the process appears only in the captured flow sent to CBM operations or in fugitive emissions to the atmosphere. Three sensitivity values (80, 85 and 90%) were assumed for the capture efficiency. The pronounced effect of increasing the capture efficiency on the ratio of moles of carbon-in- coal to carbon-in-biomass is evident in this Figure. At 80% capture efficiency this ratio increased from 2.0 to 3.5 whereas at 90% efficiency, the range was 4.5 to 7.9. This ratio increased with the CBM Exchange values with the effect stronger at the lower values. The Approximate Power Index was also plotted for the three cases over the same range of Exchange Ratios. The API fell as the Exchange Ratio increased which is somewhat counter-intuitive. The API was not strongly affected by increasing the capture efficiency but it did increase slightly.

Case B - Generation of Electricity and Co-Production of an Atmospherically-Neutral Liquid Fuel

The block diagram for Case B appears in Figure 3. Methanol was chosen as the transportation fuel synthesized to simplify calculations in that each molecule of this liquid contains exactly one carbon but the technique would apply to any hydrocarbon. The three input fuels may be gasified together in processes such as those developed by the Shell or Texaco companies. Following purification, the resulting gas may be used directly to generate electricity or for a synthesis step before which stage its composition may be adjusted as required by `shifting' with steam. A co-production stage may be used instead such as the emerging Liquid Phase Synthesis process as developed by the Air Products Company. In this scheme, an intermediate synthesis stage is introduced into the Integrated-Gasification Combined-Cycle Process (IGCC). To achieve atmospheric neutrality for both the electricity generated and the liquid fuel synthesized, the carbon of the biomass must account for that in the methanol produced as well as the fugitive emissions of carbon dioxide. Possible equilibrium limitations are not considered in the calculations here.

The calculations were conducted based upon the production of one mole of methanol (MeOH). One mole of carbon in the MeOH was deducted from the total carbon fed in the three fuels and the remaining captured as carbon dioxide with a recovery of 80% assumed in all cases. The ratios of moles of carbon-in-coal to moles of carbon-in- biomass were then computed over the range of CBM Exchange ratios used before in Case A-two to eight. As plotted in Figure 4, these values ranged from 0.67 with one mole of carbon in the coal per mole of MeOH synthesized to 2.59 when ten moles of carbon in coal were fed per mole of MeOH synthesized over CBM Exchange Ratios of two to eight which was assumed the range of interest. The maximum ratio of carbon-in- coal to carbon-in-biomass increases with the CBM Exchange Ratio and with the ratio of carbon-in-coal fed to MeOH produced. As compared to stand-alone methods of producing methanol from biomass, the eligible carbon is used more effectively because less is needed because the only incremental requirement is to meet the carbon content of the methanol: the fugitive losses from the generation of the electricity would occur whether methanol was synthesized or not.

The Approximate Power Index appears in Figure 5 for the ranges of value assumed. The API fell as the Exchange Ratio increases. This value increased as the ratio of carbon- in-coal to methanol synthesized is increased as expected.

Case C - Addition of Exogenous Carbon Dioxide

In Case C, it is assumed that a source of exogenous carbon dioxide captured from fossil- fuel facilities at other locations is available to the processing and CBM operations as in the block diagram in Figure 6. One illustrative example was calculated despite the several assumptions that have to be made because it is a likely case in practice. An addition ranging from one to ten moles of carbon dioxide was made to the total flowing to CBM operations for a mid-range value of Case B. It was assumed that five moles of carbon were fed in the coal when one mole of MeOH was produced with the Exchange Ratio set at four and with 80% capture of carbon dioxide.

In Figure 7 it may be seen that the ratio of carbon-in-coal to carbon-in-biomass falls from 1.82 to 1.43 with an increase in the exogenous moles of carbon dioxide sent to CBM operations. The decline is, however, not great. As expected, the Approximate Power Index steadily increases with the addition of exogenous carbon dioxide to that captured from the local production facility.

Summary and Conclusions

A generic process based upon coal and equipped for the capture of carbon dioxide was envisioned linked to Coal Bed Methane operations in such a way that the methane produced was fed together with the coal to generate electricity and/or produce hydrogen and carbon-bearing liquid fuels. A quantity of biomass grown sustainably such that it could be used with no net increase in carbon dioxide emissions to the atmosphere was also fed with the coal and the recovered methane to exactly off-set the fugitive emissions of this gas. When this balance was achieved, the products of the process-the electricity generated or the hydrogen or the carbon-bearing liquid fuels produced-were also neutral to the atmosphere and consequently may be consumed anywhere and at any time with null net emissions.

In Case A, it was assumed that the products were electricity and/or hydrogen so that the total carbon fed to the process arising from the coal, the methane from CBM operations and the biomass has only two ultimate destinations-a captured supply for the CBM step or fugitive losses. The capture efficiency was varied from 80 to 90%. The ratio of the carbon-in-coal to carbon-in biomass was calculated over a range of interest with the Exchange Ratio of carbon dioxide fed to methane produced in the CBM stage varied from the theoretical minimum of two to a high of eight to reflect the experimental results experienced in current trials. This ratio was chosen because the carbon in the biomass is more costly than the carbon in the coal and thus the process objective should be to maximize this value. The ratio increased strongly with an increase in capture efficiency but more weakly with the Exchange Ratio. An Approximate Power Index (API) was also calculated to give at least a first estimate of the change in power generation with the Exchange Ratio. This index fell slightly with an increase in this ratio.

In Case B it was assumed that a liquid fuel containing carbon (here methanol) was also co-produced as a fuel for vehicles in mobile applications with null net emissions. The ratio of carbon-in-coal to carbon-in-biomass increased with the moles of carbon-in- coal consumed per mole of methanol synthesized and with an increase in the Exchange Ratio. The API fell with an increase in Exchange Ratio.

In Case C, exogenous carbon dioxide from other sources equipped for the capture of carbon dioxide was assumed fed along with that captured from the process for sequestering in the CBM activity. In a mid-range set of values chosen for the calculation, the ratio of carbon-in-coal to carbon-in-biomass fell gradually as the Exchange Ratio increased but the API increased substantially.

The choice of the most efficient linkage of sequestering of carbon dioxide captured from coal-using processes with CBM operations is a complex question worthy of further study. It is far from clear which process combination will prove the most efficient. An adroit use of neutral carbon derived from biomass grown sustainably provides an opportunity for the generation of electricity and/or the production of hydrogen or carbon- containing liquid fuels with null net emissions of carbon dioxide. The relatively more costly biomass requirement can be reduced by increasing the the efficiency of capture of the carbon dioxide preduced in the process.


  1. National Energy Board, Canadian Energy Supply and Demand to 2025, Calgary, Alberta, T2P 0X8, June 1999 (ISBN 0-662-27950-6). Web Site:
  2. S. Wong, W.D. Gunter, D. Law and M.J.Mavor. Economics of Flue Gas Injection and Carbon Dioxide Sequestration in Coalbed Methane Reservoirs, Fifth Conference on Greenhouse Gas Technologies, Cairns, Australia, August 2000. To be published in Energy Conversion and Management. Paper from the Alberta Research Council, 250 Karl Clark Road, Edmonton, Alberta, T6N 1E4.
  3. J.H. Walsh, The Linkage of Coal Bed Methane Production in Energy Conversion Technologies Equipped for Capture of Carbon Dioxide, January 2000. (Web -
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