The Department of Chemical Engineering is proud to announce the Dissertation Defense of Ph.D. candidate, Zhonghua Zhan.
“Ash Deposition and Ash Aerosol Formation Mechanisms during Oxy-coal Combustion.”
Advisor: Jost Wendt
Wednesday, March 25th, 2015
11am-1pm in the Eccles Boardroom (WEB)


Climate change is a challenging issue for humans, eliciting much attention on CO2 emission control from coal combustion sources. Oxy-coal combustion, that is, the process of burning coal with pure O2 and recycled flue gas (RFG) rather than air, has been considered a promising technology for CO2 capture for existing coal fired power plants. Numerous studies have been conducted to evaluate furnace performance when converting from air combustion to oxy-coal combustion. This work is concerned with the effects of retrofit on both the fly ash produced and the deposits laid down on heat transfer surfaces. The research targets include: 1) To find out the difference of ash aerosol and ash deposits 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 deposits formation characteristics during oxy-coal combustion under various RFG stream cleanup options and various RFG amounts. 4) To build up a model to predict ash deposition rate on 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 (OFC), firing a Powder River Basin (PRB) coal. The combustion conditions included air combustion and oxy-coal combustion under various RFG stream cleanup options and various RFG amounts. A Berner low pressure impactor (BLPI), a scanning mobility particle sizer (SMPS), and an aerodynamic particle sizer (APS) were used to obtain size segregated ash aerosol samples and to determine the particle size distributions (PSD). An uncooled probe was used to collect slagging deposits. A novel surface temperature controlled ash deposition probe system was developed and used to collect the fouling deposits.

The results showed that, it was necessary to treat the deposits separately (inside, outside, vertical and side) other than a bulk, because the deposits from different locations of the probe showed different characteristics in both compositions and PSD. Deposits from the vertical and side surfaces were more similar to the inside deposits on the horizontal surface than they were to the bulk deposits. The main formation mechanism for vertical (inside) deposits was thermophoresis because its deposition rate was found to be proportional to the temperature gradient. Further exploration on ash deposition formation under oxy-coal combustion at high inlet O2(OXY50) and air combustion showed that, the compositions of the inside deposits were consistent with the compositions of the vaporization mode aerosols. This implied that, the main formation mechanism of the inside (initial) layer deposits was transportation of the vaporization mode ash aerosols through thermophoresis. Therefore, the compositions of both the size segregated ash aerosols and the spatially resolved deposits must be determined in order to understand how the deposits were constructed. Furthermore, cases of oxy-coal combustion under various RFG stream cleanup options and various RFG amounts were also conducted to identify ash aerosols and ash deposits formation characteristics. The exhaustive measurements of the ash aerosol under various RFG cleanup options showed that the extent of RFG cleanup 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 vertical surface within a laminar flow field to evaluate the role of thermophoresis. 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 criteria that a particle would be captured onto the vertical surface was Tp < 1. Based on this criteria, a three-dimensional capture zone and a two-dimensional deposition region, in which particle would be captured onto the vertical surface, were calculated. The predicted ash deposition rates showed high consistence with the experimental data. That was, ash deposition rates on vertical surface increased linearly as gas-probe temperature difference increased, and this was because the area of the deposition region increased when the difference of gas-probe temperature increased. Comparing to air combustion, a larger area of deposition region and a higher ash aerosols concentration in OXY50 case contributed equally to the higher deposition rate on vertical surface of OXY50 case.