Experimental investigation on controlling of airflow trajectories and flow-field of down-fired boiler by adding on arch secondary air

To effectively control the flow and combustion of pulverised coal in down-fired boilers and improve their thermal efficiency while ensuring low NO x emissions, on arch secondary air (OASA) combustion technology and a smoke-tracing analysis were proposed. The aerodynamic characteristics of a 300-MWe Foster Wheeler (FW) down-fired boiler with the OASA ratios (R-OASA) of 0%, 10%, 20%, and 30%, the flow trajectories and mixing characteristics of OASA, and fuel-rich coal/air flow (FRA) and fuel-lean coal/air flow (FLA) were studied experimentally. For the R-OASA of <10%, FRA and FLA flow to furnace centre soon after they are injected into the furnace. OASA and FLA may mix with FRA before ignition. For the R-OASA of 20% or higher, FRA and FLA flow to the near-wall area soon after being injected into the furnace. FRA mixes with OASA after ignition. With the increase in R-OASA, the mixing distance between OASA and FRA increases first and then decreases. The penetration depth of coal/air flow and furnace airflow fullness increases. For the R-OASA of 30%, the dimensionless penetration depth of coal/air flow can exceed 0.7. Furnace dimensionless airflow fullness at all the dimensionless heights can exceed 0.6. In real operation, the recommended R-OASA is between 20 and 30%.