Ash deposition and ash aerosol formation mechanisms during oxy-coal combustion.

摘要

Oxy-coal combustion has been considered a promising technology for CO2 capture for existing coal-fired power plants. This work is concerned with the effects of retrofit on both the fly ash and the deposits. The research targets include: 1) To find out the difference between ash aerosol and ash deposit formation between oxy-coal combustion and air combustion. 2) To ascertain the relationships between deposits composition and size segregated ash aerosol composition. 3) To find out ash aerosol and ash deposit formation characteristics during oxy-coal combustion under various recycled flue gas (RFG) cleanup options and various RFG amounts. 4) To build up a model to predict ash deposition rate on a vertical surface.;Ash aerosol and ash deposits formation during oxy-coal combustion were explored through experiments in a self-sustained 100 kW rated down-fired oxy-fuel combustor, firing a Powder River Basin coal. The results showed that it was necessary to treat the deposits separately (inside, outside, vertical and side) other than in bulk, because the deposits from different locations of the probe showed different characteristics in both composition and particle size distribution. Deposits from the vertical and side surfaces were more similar to the inside deposits than to the bulk deposits. The main formation mechanism for vertical (inside) deposits was transportation of the vaporization mode ash aerosols through thermophoresis. Cases of oxy-coal combustion under various RFG cleanup options and various RFG amounts were also conducted to identify ash aerosols and ash deposit formation characteristics. The results showed that the extent of RFG cleanup options had little effect on the ash aerosol compositions or size distributions. However, OXY50 cases produced more vaporization mode particles due to higher combustion temperature. Finally, a model was built up to predict ash deposition rate on a vertical surface within a laminar flow field. A dimensionless number, Thermophoresis number (Tp), was defined, which was the ratio of travel time by thermophoresis force and travel time by drag force. The criterion that a particle would be captured onto the vertical surface was Tp < 1. The predicted ash deposition rates showed high consistence with the experimental data.