Mathematical modeling of staged combustion in an AFBC

Staged combustion can be used as a means to limit NO_x emissions from a reactor. In an AFBC, staged combustion may be realized by decreasing primary air flow to create reducing conditions in the bed, and supplying secondary air in the freeboard in order to maintain complete combustion. Under reducing bed conditions, reaction mechanisms of the several processes in the combustor may change. For instance, combustion may partly shift from the bed to the freeboard, NO_x reduction may be strongly accelerated by the presence of reducing agents in the bed such as CO and H₂, and the sulfated limestone may decompose via a reductive reaction mechanism. Furthermore, the secondary air injected in the freeboard may influence the flow and temperature pattern in the combustor. The above mentioned phenomena have been modeled and incorporated in an overall AFBC system model, denoted by FBCSIM. This overall reactor model describes combustion efficiency and emissions such as NO_x, SO₂ and CO as a function of combustor geometry, coal and lime properties, and process conditions. In this paper, much attention has been paid to a number of new subroutines, viz. a char combustion model, a NO-char reaction model and a sulfur capture model. Model calculations of the FBCSIM model have been favourably compared with data obtained from staged combustion experiments carried out in the 4 MW_th AFBC of TNO in the Netherlands. Investigated parameters were: primary/secondary air ratio, height of the secondary air nozzles and coal type. Finally, the FBCSIM overall model has been combined with an optimization package to simultaneously minimize emissions and maximize efficiencies by estimating optimum process conditions.