Clean Coal Technology: A Novel Process for the Combustion of Coal Using an Oxygen Carrier

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The global supply of electricity accounts for ~38% of total anthropogenic carbon emissions to the atmosphere or ~2,400 Mte/y (carbon basis), a figure projected to exceed 4,000 Mte/y by 2020. Globally, coal generates the largest share of world electricity production (39% of total delivered energy) followed by renewables (principally hydroelectricity) (20%), nuclear (17%), gas (15%) and oil (10%). Electricity production will increase by ~80% over the period 2000 to 2020, with the fraction generated from coal remaining at ~39%, due to increased exploitation of coal in India and China and steady growth in the USA. In the U.K., the most recent White Paper only envisages a future for coal provided ways can be found materially to reduce its carbon emissions , which therfore requires the sequestration of the CO2 arising from the combustion of the coal, or fuels derived from it, in the earth. The cost of sequestration is small (~ $4-8/ te C) compared to the costs of separation of CO2 from typical flue gases (~ $100 - 200/te C)so that such disposal only approaches viability if the CO2 is available in almost pure form, largely free of nitrogen and other inert gases.We wish to use chemical looping for the in situ gasification and combustion of coal in a process to produce CO2 and steam as pure products, without significant contamination by N2. In our proposed scheme, there would be one reactor, containing a bubbling fluidised bed of oxygen carrier, most likely a Cu-based oxide on a titania or alumina support, the durability of which has been demonstrated by other workers. The reactor would be operated in a cycle of three consecutive periods, t1, t2 and t3. During t1, the bed would be fluidised by steam, (or steam and CO2) and coal would be fed steadily to the bed, the temperature of which would be ~ 800 - 1000 C. Two events would occur:(1) the coal would undergo gasification (endothermic) by the steam to yield a synthesis gas containing CO and H2 (plus smaller amounts of CH4 and higher hydrocarbons): C(s) + H2O(g) = CO(g) + H2(g) (enthalpy of reaction: +131 kJ/mol),(2) the syngas would react with the surrounding CuO particles to give CO2 and steam by: CuO(s) + H2(g) = Cu(s) + H2O(g) and CuO(s) + CO(g) = CO2(g) + Cu(s) (enthalpies of reaction -86 kJ/mol and -127 kJ/mol, respectively).Copper has the only oxides which give exothermic reactions in (2); the heat produced exceeds that needed for the endothermic gasification reaction in (1). In effect, the metal oxide has been used in place of air, or cryogenically-produced O2, so that the products of combustion do not contain N2. Of course, this system can only function down to a certain degree of reduction of the metal oxide. Thus, after time t1, the feed of coal ceases and the remaining inventory of bed carbon is allowed to gasify and combust for a further period of time, t2, until the inventory is sufficiently small. At the end of t2, the bed is fluidised by air instead of steam for a period of time, t3, during which the reduced metal oxide carrier is regenerated in Cu + 0.5O2 = CuO (enthalpy of reaction -156 kJ/mol Cu). During t3 some carbon will be burnt off, originating either from coked metal oxide or from residual carbon inventory remaining after t2, so that there would be a small release of CO2 with the regenerating air, but this would be very much less than that emitted by direct combustion of the coal in air. Once the metal oxide has been regenerated, the cycle starts again at t1. Thus, the concept enables coal to be burnt cleanly with a rather smaller reduction in thermal efficiency than is obtained with other schemes for isolating the CO2, using e.g. cryogenically-separated oxygen from air.