Coal and biomass may be combusted together in conventional facilities for the generation of electricity equipped for the capture of carbon dioxide. The recovered gas is then sequestered in Coal Bed Methane (CBM) operations which results in the displacement of methane bound to the surface of the coal. This methane is then recovered and converted to liquid fuels useful for transportation purposes. In this paper, methanol is synthesized in the conventional process except that carbon dioxide is also captured from the combustion gases exiting the underfiring of the reforming stage. This carbon dioxide augments the flow of this gas captured from the generation facilities and is sent together to the CBM operations. Sufficient biomass carbon is combusted along with the coal to equal the carbon content of the methanol synthesized and to account for fugitive losses of carbon dioxide from the two processes. When this condition is met, both the electricity and the liquid fuel may be consumed with null net emissions of carbon dioxide to the atmosphere.
The carbon requirement from both coal and biomass were calculated on the basis of the synthesis of one mole of methanol. Fugitive losses in the capture processes were assumed to range from twenty down to zero percent. The Exchange Ratio of carbon dioxide fed and methane produced in CBM operations were assumed to vary from the theoretical minimum of two to a high of eight as might be encountered with the sub-bituminous coals. The requirement for biomass carbon rose with increasing losses in the capture processes and with the Exchange Ratio.
For the purposes of the calculations that follow, the percent recovery of CO2 from the two exiting combustion gas streams is assumed to be equal over a range of interest chosen to be 80-100%. There is no reason for this percent recovery to be exactly the same from the two separate stack gas streams but these values may not be very different from each other in practice. This simplification permits an easier and more transparent calculation and only in extreme cases will it lead to major differences in the results that follow.
Methanol is chosen for the neutral transportation fuel to be synthesized for two reasons. First, as a unique chemical compound of fixed composition, it always contains one mole of carbon which simplifies calculations. Nothing in principle, however, prevents the synthesis of other hydrocarbon fuels from the recovered methane such as gasoline or diesel fuel. The second reason is that MeOH is a candidate fuel for the supply of hydrogen to fuel cells mounted in cars. In this application, it serves essentially as a convenient hydrogen carrier. Hydrogen may be produced from the MeOH by conversion with steam in on-board reformers but, in the future, fuel cells of the homogenous type now in the early development phase may also be used which accept MeOH directly. To reduce the number of cases calculated, the recovery of carbon in the standard MeOH synthesis process was assumed to be 68%. This number is the percentage ratio of the carbon content of the MeOH synthesized to the carbon in the total methane feed entering the process across its boundary limits. This approach avoids a possible complication. When CO2 is captured or otherwise available, some could be added to the process stream of methane fed to the reformer so as to decrease the ratio of hydrogen to carbon monoxide fed to the synthesis stage with the object of improving MeOH recoveries. Despite this possible internal plant re-circulation, an overall ratio of carbon in the product MeOH to that in the entering methane may be computed. In essence, carbon enters only with the methane and leaves only in the MeOH synthesized, and in the captured and fugitive loss (the capture is not perfect) streams of CO2. The value of 68% is taken here as a probable recovery factor, but this percentage may well be different according the specifics of a given situation.
The net emissions are neutral when the carbon in the biomass combusted equals that in the fugitive losses in the two combustion exhaust streams together with the one mole of carbon in the MeOH produced. When this condition is met, the moles of fossil-fuel carbon both in the coal consumed and the methane recovered in CBM operations are equal to the moles of carbon sequestered. Both the electricity generated and the MeOH synthesized are thus neutral with respect to CO2 changes in the atmosphere. The MeOH may then be consumed as a transportation fuel for vehicles at any place and at any time with no net increase in CO2 emissions.
An Approximate Power Index (API) was also calculated. It was assumed as a first approximation that the quantity of electricity generated was proportional to the energy fed to the combustion stage and that the carbon in the biomass brought half as much energy to this step as that in the coal due to its greater oxygen content. This quantity was then divided by the lowest value for the adjusted fuel input in all five cases which was found at an Exchange Ratio of 2 in Case E when a fugitive CO2 loss of zero was assumed. The API was calculated to give an approximation of the power that could be generated for each unit of MeOH synthesized over the range of conditions chosen here as of probable interest.
There are two reasons for determining the minimum quantity of biomass needed for neutral emissions. First, the carbon of the biomass is more costly than the carbon of coal. Secondly, the maximum quantity of biomass economically available at any one site is frequently limited by its `catchment' area. With a high ratio of carbon-in-coal to carbon- in-biomass, it is easier to achieve a more efficient scale of operations.
Because the location of the source of the biomass is likely to be restricting, the possibility of virtual operation of this sequence of processes should be explored. The single combustion stage illustrated in Figure 1 could be considered a number of such stages at different locations added together. At some of these locations, biomass may be more relatively plentiful or easier to gather than others. The only requirement is that the combustion stages be equipped to capture CO2 and that this gas be piped to CBM operations.
Even the location of the methanol synthesis facility is not critical. There is no reason that methane recovered from the CBM operations could not be exchanged for supply from conventional sources. If it is not feasible to capture and pipe CO2 captured from the underfiring gases, null net emissions can still be achieved but at the expense of higher biomass use per unit of MeOH.
Virtual operations are possible when there is a CO2 pipeline gathering system serving many facilities. Only some of the facilities may be involved in the collaboration to produce liquid fuels. As long as the conditions for neutrality are met overall by the various partners in such a project, the same gathering system for captured CO2 may serve other facilities which may not be part of the partnership. Such a virtual agreement is within the spirit of the trend of present negotiations in international fora on climate change.
The ratio of the carbon-in-coal to the carbon-in-biomass fed to the combustion stage for neutral emissions increases with the Exchange Ratio is shown in Figure 3. The Approximate Power Index increases with the Exchange Ratio as plotted in Figure 4 but falls modestly with a decline in the fugitive loss. These results are consistent with the requirement of more CO2 at high Exchange Ratios to release a given quantity of methane and thus MeOH.
Over the ranges assumed in this paper, the requirement for biomass carbon ranged from one to 3.9 moles per mole of methanol synthesized. The lowest biomass carbon requirement was found at high efficiencies of capture of carbon dioxide and low values of the Exchange Ratio. Ratios of carbon-in-coal to carbon-in-biomass ranged from a low of 0.85 to 10.3.
An Approximate Power Index was calculated which indicated maximum power output is obtained with high values of the Exchange Ratio and low values of the capture efficiency although the API is less sensitive to the latter variable.
Provided a pipeline system is available to move captured carbon dioxide to CBM operations, virtual arrangements are possible to link the process units. The only overall condition is that the biomass consumed by any or all of the cooperating partners must equal the carbon content of the fuel synthesized and account for the fugitive process losses of carbon dioxide.
|Line||Calculation in Moles of C|
per Mole of MeOH
Ratio = 3
Ratio = 4
Ratio = 6
|a||MeOH Output - Moles of C||1||1||1||1||1|
|b||C Input to MeOH Synthesis
Carbon Recovery 68%
|c||CO2 Fugitive Loss from
Gas at 20%
|d||CO2 Captured from
Gas at 80%
|e||CO2 for CBM Operations
(b) x Exchange Ratio
|f||C Input to Combustion Stage
|g||Loss of Fugitive CO2 in Combustion
(f) x 0.2
|h||Total C in Biomass Needed
1.0 + (g) + (c)
|i||C in Coal for Combustion
(f) - (h)
|j||C in Coal/C in Biomass
|k||Approx. Power Index