Numerically Study on Combustion and NOx Emission Characteristics in Tangentially Fired Boiler Co-Firing Semi-Coke

Semi-coke is a specific solid fuel, which is mainly produced by upgrading low-rank coal. The poor reactivity of semi-coke makes a difficulty to its practical utilization in utility boilers. Previous research was mainly focused on the combustion behavior of semi-coke, while the industrial application has to be understood. In this paper, the effect of co-firing semi-coke and bituminous coal on the operation performance of pulverized boiler was numerically studied. The work was conducted on a 300 MW tangentially fired boiler, and the temperature distribution, the char burnout and NOx production were mainly examined. The results indicate that the incomplete combustion heat loss drops with the increase in semi-coke blending ratio. The NOx concentration increases from 186 mg/Nm3 for only firing the bituminous coal to 200, 214, and 255 mg/Nm3, when the blending ratio was 17%, 33% and 50%, respectively. With enhancing excess air coefficient for the co-firing condition, the combustion efficiency got improved, while NOx production increased very slightly. In general, the boiler is well adapted to co-firing semi-coke, and the semi-coke blending ratio of 1/3 with an excess air coefficient of 1.235 is recommended.

Studies on Combustion Characteristics of Two Interacting Coal Particles at Different Separation Distance

To understand the interactions of coal particles on devolatilization, volatile burning, and char combustion, the combustion characteristics of two interacting equal-sized coal particles placed in the upstream and downstream configuration in a hot laminar flow are numerically investigated. A two-dimensional mathematical model was developed based on the wall surface reaction theory in the commercial software FLUENT. The numerical results show that the particle interaction has different effects on the coal ignition time and combustion characteristics of the upstream and downstream particles. The upstream coal particle undergoes a faster temperature rise, earlier coal devolatilization, and faster char burnout than the downstream one. With increasing the particle separation distance within a certain range, the temperature rise, coal devolatilization, and char combustion processes are further enhanced and weakened for the upstream particle and the downstream particle, respectively. There exists a critical particle separation distance beyond which the combustion processes of the two particles are similar to those of a single particle.

Coal Char Kinetics of Oxidation and Gasification Reactions

Experimental investigations and numerical simulations have been conducted in this study to derive and test the values of kinetic parameters describing oxidation and gasification reactions between char carbon and O2 and CO2 occurring at standard air and oxy-fuel combustion conditions. Experiments were carried out in an electrically heated drop-tube at heating rates comparable to fullscale pulverized fuel combustion chambers. Values of the kinetic parameters, obtained by minimization of the difference between the experimental and modeled values of char burnout, have been derived and CFD simulations reproducing the experimental conditions of the drop tube furnace confirmed proper agreement between numerical and experimental char burnout.

Numerical Simulation of Combustion Performance of Pulverized Coal Burner

The combustion mechanism of pulverized coal in a DRB-4Z burner are analyzed and the temperature distribution, char burnout and CO production in the burner outlet area are obtained. The gas phase turbulence model is the Realizable k-ε two equation model, and radiation heat transfer model is P-1 radiation model. The discrete phase model is used to simulate the force and motion trajectory of the pulverized coal particles, and the stochastic model is used to simulate the flow of coal particles. The combustion model is non-premixed combustion model, and the devolatilization model is two competing rates model; char combustion model is kinetics/diffusion-limited model. Numerical results revealed the mechanism of pulverized coal devolatilization and char combustion, and the solution may give reference to air arrangement of the same type of burners.