Carbon-capture and storage benefits: NOx reduction in O2/CO2 pulverized fuel combustion

Fuel combustion in a mixture of O₂/CO₂-recycled is an emerging technology for cost-effective power generation with carbon capture and sequestration that has extra benefits of NO_x emission reductions and lower flue gas cleanup cost. This paper presents experimental results of pulverized coal combustion in a 20 kW furnace on combustion in air and O₂/CO₂ mixtures. The effectiveness of oxidant staging on reducing NO_x emissions was investigated for combustion in O₂/CO₂ mixtures in comparison to that in air combustion. The fate of recycled NO_x in combustion with various oxidants and combustion conditions was also investigated. The results show that simply replacing the N₂ in the combustion air with CO₂ will result in a significant decrease of combustion gas temperatures. However, with a same firing rate and combustion stoichiometry, coal combustion in 30:70-O₂/CO₂ produced a similar flame temperature profiles to those in air combustion, while producing a significantly lower furnace NO_x emission and a higher char burnout. The staged combustion tests show that oxidant-staging is a very effective method in reducing NO_x emissions for coal combustion in 30:70-O₂/CO₂, and can be more effective than in staged-air combustion. The recycled NO_x tests results show that the reduction efficiency of the recycled NO depends on the combustion media, combustion conditions and NO_x recycling injection locations, and is influenced by the coal properties but not by the recycled NO_x concentrations. Compared to those in air combustion, NO_x reduction efficiency in 30:70-O₂:CO₂ is more sensitive to coal properties, particularly the coal fuel ratio.