| Description |
Oxy-fuel combustion is a promising technology for CO2 capture and sequestration in power plants. First generation oxy-fuel combustion with air-like flame temperature and heat transfer rate has been widely studied, but it is not economically efficient. This dissertation is concerned with second generation atmospheric pressure oxy-fuel combustion, a process that employs minimum recycled flue gas (or high temperature oxy-combustion). The research targets of this work include the following: 1) To investigate the formation mechanisms of ash aerosol and ash deposit under high temperature oxy-fuel combustion condition. 2) To determine the effect of fuel properties on the formation of ash aerosol and ash deposition rates for both air- and oxy-combustion. 3) To predict growth rates of both the tightly bound inside deposits and also the loosely bound outside deposits, through real-time measurement of ash aerosol in flue gas. Eight different solid fuels ranging from coals and petroleum coke to biomass and biomass with coal blends were burned in a 100 kW down-fired oxy-fuel combustor in this work. Two representative conditions were tested for all these fuels: 1) air combustion and 2)high temperature oxy-combustion with 70% O2 and 30% CO2 in the oxidant gas(OXY70). The particle size distributions of ash aerosols were measured in real time using online electrical mobility (SMPS) and light scattering techniques (APS). Size segregated aerosols were collected using a Berner Low Pressure Impactor (BLPI). The ash deposits were collected in a series of sampling times (namely 10, 20, 30, 60, 120 minutes) using a iv specially designed temperature controlled deposition probe. The experimental results suggested that particle size distributions of ash aerosol can be divided into submicron nucleation and accumulation modes and supermicron fragmentation modes. Compared to air combustion, OXY70 cases generated more submicron particles, but the same amount of supermicron particles. The diameter of sampled submicron accumulation particles could be predicted through coagulation theory and certain assumptions. Coagulation occurred within a diffusion layer near the fuel particles. The thickness of this diffusion layer strongly depended on fuel properties and oxidant gas environment. The weight of inside deposits increased rapidly initially and then ceased growing, while the weight of outside deposits increased in a constant rate over the entire time span covering the measurement. Outside deposition rates are normally 10 times higher than inside deposition rates. For all tested fuels and combustion conditions, inside deposition rates were proportional to the PM1 concentration in the flue gas regardless of the PM1 composition. In contrast to prevailing theory, this so-called "glue effect" of submicron particles was not affected by the bulk or surface concentration of alkali contents. The outside deposition rates were proportional to total alkali concentration in flue gas. |
