Article pubs.acs.org/EF
Enhancing Emissions Reduction and Combustion Processes for Biomass in a Fluidized Bed F. Okasha,*,†,‡ M. El-Naggar,† and E. Zeidan†,‡ †
Power Mechanical Engineering Department, Mansoura University 35516, Mansoura, Egypt Department of Mechanical Engineering, Faculty of Engineering, Taif University, Al-Haweiah, P.O. Box 888, 21974, Saudi Arabia
‡
ABSTRACT: The present paper reports on an experimental study on staged-air combustion of rice-straw pellets in a new fluidized-bed configuration. The conventional method and the staged-air technique of secondary air feeding are applied for comparison. The results confirm that the new fluidized bed is more effective in controlling NOx for all considered operating conditions. SO2 emission exhibits reduction at lower secondary air; however, it increases at higher ratio for the conventional method. With the new configuration, the concentration of exhausted SO2 decreases as the amount of secondary air increases. The results also show that CO emission does not change with the amount of secondary air for the new configuration, while it increases as the amount of secondary air increases in the conventional method. With the conventional method, fixed carbon loss tends to decrease at lower secondary air ratios while combustion efficiency suffers a decrease at higher values. Alternatively, in the case of the new configuration, fixed carbon loss steadily decreases while combustion efficiency improves monotonically as the amount of secondary air increases.
1. INTRODUCTION Current scenarios and projections indicate that there will be an increase in worldwide energy demand while fossil fuel resources will continue to dominate.1,2 However, two major concerns arise, including the depletion of fossil fuel resources and the environmental impact of their use. Fossil fuels are indicated as the main source of air pollutants and greenhouse gas emissions. Development of renewable energy utilization is in progress to compensate fossil fuels and to mitigate pollutant emissions. Among the alternative renewable energy sources, biomass is the only carbon-based sustainable option.3,4 Biomass is a renewable fuel and almost CO2-neutral.5,6 Moreover, utilization of biomass in energy production helps to overcome the waste disposal problem and avoids landfilling materials that eventually decompose, yielding both carbon dioxide and methane, two of the most harmful greenhouse gases.7 Open burning of agriculture residuesin particular, rice strawposes a serious problem where smoke is obviously recognized during the harvest season. Instead of burning of rice straw and rice husk in the field without any energy reward, many works have been performed on rice byproducts combustion for energy generation.8−13 Many other works have been successfully carried out on the co-combustion of rice byproducts with coal14−18 or with bitumen.19−21 Although biomass is recognized as a potential energy resource to alleviate carbon dioxide release, attention should be given to nitrogen oxides (NOx) emissions. NOx emissions play an important role in the atmospheric reactions that yield harmful products of negative environmental impacts. These include, for instance, lung and eye irritation, acid rain formation, and generation of photochemical smog. In fluidized-bed systems, the formation of thermal NOx is very unlikely, since the operating temperature is usually
