Review of gas/particle flow, coal combustion, and NO_x emission characteristics within down-fired boilers

Low-volatile fuels such as anthracite and lean coal are widely used in power generators throughout the world. In comparison with tangential-fired and wall-arranged furnaces, down-fired boilers are thought to be more suitable for firing anthracite and lean coal. Currently, down-fired boilers are widely in service and have developed rapidly in China over the past 20 years. In this paper, a comprehensive review of investigations into the gas/particle flow, combustion and NO_x emission characteristics within various types of down-fired boilers is presented. The published work disclosed that down-fired boilers suffered similarly from various problems such as late coal ignition, poor combustion stability, low burnout (carbon in fly ash typically in the range 7-15%), heavy slagging, high NO_x emissions (typically in the range 1100-2100mg/m³ at 6% O₂), and asymmetric combustion. Again, the causes of these problems and various solutions in dealing with them were introduced in turn. Although causes of these problems are complicated, the reported deficiencies such as the premature mixing between high-speed secondary air and low-speed fuel-rich coal/air flow, short coal/air flow penetration depth, downward coal/air flow washing over walls, shallow air-staging conditions, and asymmetric flow-field formation contribute great efforts to develop these problems. To summarize experiences and the lessons in those reported solutions, a series of suggestions for organizing reasonable combustion in down-fired furnaces have been provided so as to achieve timely ignition, symmetric and stable combustion, weak slagging, good burnout, and low NO_x emissions.