A method to target and correct sources of unburned carbon in coal-fired utility boilers

• Published in: Fuel (Guildford) 2013-06, Vol.108, p.484-489
• Main Authors: Gao, H., Majeski, A.J., Runstedtler, A.
• Format: Article
• Language: English
• Subjects: Applied sciences; Availability; Boilers; Burnout; Carbon; CFD; CIA; Coal; Computational fluid dynamics; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Fuel consumption; Fuels; Mathematical models; Residence time; Utilities; Utility boiler
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2012.12.086

► A general method was developed to identify sources of unburned carbon in utility boilers. ► Oxygen availability was found to play the key role in burnout rather than residence time. ► A simple strategy was designed to improve burnout by targeting air to specific regions. ► 80% of the unburned carbon is from the 6 out of the 16 coal burners. ► Coal particles larger than 140 μm contribute 70% of the total unburned carbon. This study presents a novel method to examine particle burnout in coal-fired utility boilers by extracting information from computational fluid dynamics (CFD) model results. The direct targeting of underlying causes enabled by this method can provide dramatic improvement in the unburned carbon in fly ash (carbon-in-ash or CIA), improving the commercial value of the ash as an additive in the production of concrete and cement and improving boiler efficiency. Data for thousands of particles are extracted and summarized for engineering characteristics such as injection burner, particle size, residence time, and exposure to oxygen. As an example of the method, unburned carbon is studied for a 200MWe tangentially-fired utility boiler with CIA issues. The analysis, which is consistent with boiler operation data, reveals that a majority of CIA can be contributed by a disproportionately small number of sources, hence the advantage of a targeted approach. The analysis of the 200 MW boiler reveals that fewer than half of the burners contribute about 80% of the CIA and the two largest coal particle size classes contribute 70% of the CIA. Most importantly, however, the oxygen availability for the coal particles is found to be the key factor for coal burnout. Based on this result, a simple and targeted strategy to improve burnout by improving oxygen availability is designed. This method is predicted by the model to reduce CIA from 3.27% to 1.3% and NOx from 588 ppm to 503 ppm.