Title: Oxy-Combustion of Different Coals in a Circulating Fluidized Bed
Abstract: Combustion of three Polish and one South African bituminous coal particles in air versus O2/CO2 mixtures with oxygen concentrations in the range from 21% to 60% vol. was conducted at temperature of 850°C in a 12 kW bench-scale CFB combustor. Combustion in air was proceeded at ~50 C higher centre temperatures and was slightly shorter in time compared to combustion in O2/CO2 mixture with 21% vol. O2. Larger heat capacity of CO2 compared to that of N2 also retards the ignition of volatiles in O2/CO2 mixtures with 21% O2. However, when the concentration of oxygen in O2/CO2 mixtures is larger than 30%, the ignition time decreases and surface and centre temperatures increase significantly with increasing O2 content. INTRODUCTION Nowadays, greenhouse gases emissions from coal-fired systems, particularly CO2, become more and more important. Oxy-fuel combustion is one of the promising options for power generation with carbon dioxide capture. This technology can reduce significantly emissions of NOx and improve the thermal efficiency of the combustion process through the reduction of flue gas volume. In the oxy-fuel combustion, coal particles are burnt in a mixture of pure oxygen and recycled flue gas. Because nitrogen is eliminated from the oxidizing gas, the flue gas leaving the combustion chamber is highly enriched in CO2 which means that the combustion process takes place in an O2/CO2 environment. Partial recycling of flue gas helps to control the flame temperature in the combustion chamber. Extensive studies in both pilot-plant and lab scales have pointed out the pronounced influence of gas composition (air versus O2/CO2) on coal combustion performance. The heat transfer and temperature distribution in a furnace are greatly affected by the large specific heat capacity of CO2. Coal ignition is delayed in O2/CO2 in comparison to in O2/N2 with the same O2 concentration. To match the flame/particle temperature in air, a large amount of O2 in CO2, typically around 30%, is required. Coal conversion rate, char properties, and reactivity are also affected by the replacement of air with an O2/CO2 mixture, Zhang et al. (1). Buhre et al. (2) and Toftegaard et al. (3) summarized the literature on oxy-fuel combustion of pulverized coal and discussed a number of operational concerns, including ignition, heat transfer, environmental issues and flame stability. Oxy-fuel combustion has now been well studied for pulverized coal combustion, but to date has received relatively little attention for oxyfuel circulating fluidized bed combustion (CFBC), Jia et al. (4). Work in this field has been conducted by: Foster Wheeler Energia Oy and VTT (5), ALSTOM (6), CANMET (4) and Czestochowa University of Technology (7). In the present work, oxy-fuel CFB combustion tests were conducted in a 12-kW bench-scale CFB combustor. The main objective of this study is to investigate the combustion behaviour of three Polish and one South African bituminous coal particles, in air and O2/CO2 mixtures, in terms of particle temperature profiles, ignition time, devolatilization time and the total combustion time.
Publication Year: 2011
Publication Date: 2011-01-01
Language: en
Type: article
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Cited By Count: 8
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