Flue gas decarbonization vs. fuel gas decarbonization

A technically feasible but costly option for achieving deep reductions in greenhouse gas emissions is to extract the CO₂ from flue gases of large fossil fuel combustors (e.g. at fossil fuel power plants) and to isolate the CO₂ so recovered from the atmosphere (Blok et al. 1989). This "flue gas decarbonization" strategy is costly largely because of the expenses associated with separation of the CO₂ from flue gases (in which the concentration of CO₂ is only 8-15 per cent); once the CO₂ is separated out, the incremental cost of isolating the recovered CO₂ from the atmosphere can often be relatively modest (van Engelenburg and Blok 1993; Hendriks 1994).

A much more promising approach involves fuel decarbonization: the production of hydrogen or a hydrogen-rich fuel from a carbon rich fuel, in the process of which a stream of essentially pure CO2is separated as a byproduct at low incremental cost - a process that might more appropriately be labelled "fuel gas decarbonization" (see Appendix A). Pioneering work on fuel gas decarbonization has been carried out at the University of Utrecht (Blok et al. 1989; Hendriks 1994) and at Shell in the Hague (van der Burgt et al. 1992) in conjunction with the production of electricity from coal via integrated gasification/combined cycle power plants. Although the cost penalty for fuel decarbonization and sequestration of the separated CO₂ with this approach is far less than that for various flue gas decarbonization schemes, the electricity produced this way would nevertheless be about 30 per cent more costly than with a conventional coal integrated gasifier/combined cycle power plant (Hendriks 1994), simply because there are no direct economic benefits (only environmental benefits) arising from fuel gas decarbonization.

Because the production of hydrogen is inherently costly, it is desirable to use it in conversion equipment where it is worth more than conventional fluid fuels- especially because there is little prospect that the prices of conventional hydrocarbon fuels will rise high enough in the foreseeable future to the point where hydrogen will be able to compete on a $-per-GJ-equivalent basis. The true value of hydrogen should be determined not by a comparison of fuel costs but by a comparison of the costs of providing an energy service such as the cost per vehicle km of travel.

The use of hydrogen in low-temperature fuel cells for transport and distributed combined heat and power applications could provide the needed high value. Fuel cells offer high thermodynamic efficiency and zero or near zero local pollutant emissions without the need for pollution control equipment. Moreover, for combined heat and power applications, the absence of scale economies for production units, the lack of need for operating personnel, low maintenance requirements, and low noise levels make it possible to site low-temperature fuel cells near users where the produced energy is more valuable than at centralized facilities.

Until recently it has not been practical to take advantage of these attributes. The only commercial fuel cell is the phosphoric acid fuel cell. Its power density is too low for it to be considered for automotive applications, and its prospective costs in mass production are not especially low. However, recent advances relating to the proton exchange-membrane (PEM) fuel cell indicate a hopeful future for this technology for both distributed combined heat and power (Little 1995; Dunnison and Wilson 1994) and transport (Williams 1993, 1994b; Mark et al. 1994) applications.⁵ When mass produced for transport applications, its costs could be low, approaching the costs of internal combustion engines.⁶

Low-temperature PEM fuel cells can very efficiently utilize hydrogen or methanol that is re-formed with steam to produce a gaseous H₂/CO₂ mixture onsite or, in transport applications, onboard the vehicle.⁷ Such fuels have good prospects for becoming major energy carriers in the "post-combustion" era, when electrochemically based fuel cells will have become well established in the energy economy. The least costly ways of producing these energy carriers are from chemical fuel feed stock's - initially natural gas and later coal and biomass (Williams et al. 1995 a,b).

Whereas the alchemists failed in their attempts to transmute base metals into gold, the technology for making hydrogen from carbon is well established. Specifically, a carbon-rich fuel feedstock can be processed to produce hydrogen or methanol (a hydrogen carrier) by first converting the feedstock into "syugas" (a mixture of CO and H₂) via steam re-forming (in the case of natural gas) or via thermochemical gasification (in the case of coal or biomass) and then shifting the energy contained in the CO to H₂ by reacting the CO with steam - a process requiring very little net energy input (see Appendix A). In the final stages of the manufacturing process, CO₂ is separated from the fuel product (e.g. using pressure swing adsorption [PSA] in the case of hydrogen production or Selexol in the case of methanol production) in a virtually pure stream that is available as a by-product at low incremental cost (see table 6.1).⁸

If there were no greenhouse problem, this stream of pure CO₂ would be vented to the atmosphere. In a greenhouse-constrained world, consideration might be given to isolating this CO₂ from the atmosphere because of the large potential and relatively low costs involved. If this stream of separated CO₂ could be stored in isolation from the atmosphere, CO₂ emissions would be sharply reduced.