[attachment=9521]
ABSTRACT
At present, carbon dioxide (CO2) is the largest contributor among greenhouse gases. This article addresses the potential application of photo catalysis to the reduction of CO2 emissions from industrial flue gas streams. Not only does this process remove CO2, but it can also convert CO2 into other chemical commodities such as methane, methanol, and ethanol. In addition, the photo catalytic process can consume less energy than conventional methods by harnessing solar energy. Given these advantages, photo catalysis is an attractive alternative for CO2 capture. This report reviews the principle of photo catalysis; existing literature related to photo catalytic CO2 reduction; and the effects of important parameters on process performance, including light wavelength and intensity, type of reductant, metal-modified surface, temperature, and pressure. Finally, we discuss various system configurations for UV and solar photo catalytic reactors. The advances in photo catalysis technology indicate a promising application potential for significant reductions of CO2 emissions and a positive impact on climate change effects.
1. INTRODUCTION
According to the Intergovernmental Panel on Climate Change the Earth’s surface temperature has risen by approximately 0.6 K in the past century, with particularly significant warming trends over the past two decades. The primary contributor to this phenomenon is carbon dioxide (CO2) emissions from fossil fuel combustion. A great deal of effort has been expended to reduce CO2 emissions from the industries where the largest percentages of fossil fuels are used.
The reduction of CO2 emissions can be achieved by three approaches:
(1) Efficient use of carbon-based energy sources,
(2) Use of alternative or carbon-free energy sources, and
(3) Use of a post treatment carbon-capture technology.
With the abundanceof fossil fuels, the continued reliance of global markets on thisenergy source, and the current absence of a cost-effectivealternative energy source, post treatment carbon-capture technologyis a viable means of reducing CO2 releases into theatmosphere.
Carbon capture refers to the removal of CO2 from industrialflue gas by a gas separation process prior to release to theatmosphere. The captured CO2 can be stored in depleted oil and gas wells, the Deep Ocean, or aquifers. It can also be utilizedin one of two ways. First, the captured CO2 can be used as achemical commodity for meat freezing, a component of carbonatedbeverages, or a reactant for methanol production. Second,the captured CO2 can be injected into geological formationsfor the enhanced recovery of fossil fuel products in processessuch as Enhanced oil recovery (EOR), Enhanced coal bedmethane recovery (ECBM) and Enhanced gas recovery (EGR).
Currently, many technologies are available for the captureof CO2 from flue gas. Such technologies include gas absorptioninto chemical solvents, permeation through membranes, cryogenicdistillation, and gas adsorption onto a solid sorbent (Table11).
Gas absorption into chemical solvents refers primarily tochemical absorption. This is the most promising technologybecause of its capacity to handle a large volume of flue gasand its efficiency in CO2 capture at atmospheric pressure.Nevertheless, chemical absorption is costly, with significant energy required for CO2 stripping and solvent regeneration.
Gas absorption membranes are also available for flue gas treatment. By combining chemical absorption with membrane separation techniques, the efficiency of CO2 capture can be improved. However, the development of effective membranes with highCO2 selectivity and permeability presents a great challenge.
Cryogenic processes, although technically feasible, are noteconomically viable because of the considerable energy inputrequired for phase transformation from gas to liquid. Furthermore,the cryogenic process must be operated at high CO2 partialpressure, which is not applicable for a typical flue gas and is costly.