Nitrogen oxides emission from a circulating fluidized bed combustor

Abstract

A Circulating Fluidized Bed Combustor (CFBC) is a highly efficient combustor. It can handle various types of solid fuels such as coal, biomass or agricultural wastes. Coal and biomass have been used as fuel to generate heat for a boiler in many industries. To predict the proper amount of mixed fuel and to reduce the emission from coal burning, a rigorous mathematical model for the CFBC is needed. This paper describes the CFBC model developed as additional subroutines working with ASPEN PLUS version 11.1. The model was divided into three parts: reaction, hydrodynamic, and gas emission. In the first part, the reactions in the combustor were represented by a Continuous Stirred Tank Reactor (CSTR) module. The module was modified by adding the shrinking core model for calculating the size and the weight fraction of particles in each region. In the second part, the hydrodynamics in CFBC were divided into two regions: a lower region with one interval, and an upper region with three intervals. In each region the characteristics, such as the height of the bed, void, and volume, were calculated and sent to CSTR module for adjusting reaction rates in the regions. In the third part, gas emission models were used to calculate the kinetic rates of NO, N 2 O and the conversion of SO 2 to predict the emission to the environment.

Simultaneous catalytic reduction of N2O and NOx produced in a fluidized bed incineration of sewage sludge

“…Figure shows that more N 2 O is produced at lower bed temperatures, whereas more NO emissions occur at high bed temperatures. In the fluidized bed combustion of coal, a similar phenomenon has been generally observed, which might be a result of the decreasing amounts of char and CO available for NO reduction at high bed temperatures . On the other hand, the decrease in the N 2 O emissions at high temperatures could be explained by the effective destruction of N 2 O on solid surfaces and their homogeneous decomposition by means of H and OH radicals .…”

“…In the fl uidized bed combustion of coal, a similar phenomenon has been generally observed, which might be a result of the decreasing amounts of char and CO available for NO reduction at high bed temperatures. 8 On the other hand, the decrease in the N 2 O emissions at high temperatures could be explained by the eff ective destruction of N 2 O on solid surfaces 14 and their homogeneous decomposition by means of H and OH radicals. 13 It was found that N 2 O emissions during fl uidized bed incineration can be controlled by increasing the bed temperature.…”

“…First, the reduction reaction of NO emitted is decreased, as the amounts of char and CO in the bed decrease with increasing temperature. Secondly, while the conversion from HCN to NO increases, that to N 2 O decreases with increasing temperature . The emission of N 2 O is mostly controlled by operating conditions such as the combustion temperature and the choice of sorbent for capturing SO x .…”

“…Secondly, while the conversion from HCN to NO increases, that to N 2 O decreases with increasing temperature. 8 Th e emission of N 2 O is mostly controlled by operating conditions such as the combustion temperature 4 and the choice of sorbent for capturing SO x . 9,10 In contrast to chemical plants for N 2 O abatement during nitric acid production, 11 the catalytic abatement technology for N 2 O has not been developed yet for fl uidized bed combustion.…”

Trends in the control of NOx and SOx combustion emissions: Implications to the design of fluidised bed combustion operations

“…So, to circumvent the problem of N 2 O emissions, there is need to design an afterburner for CFB technologies. Apart from this, the effect of ash recirculation ratio versus the concentration of NOx and N 2 O investigated by Feng et al 17 noted that by increasing the ash recirculation ratio, the particle density of char in the upper part of the furnace increased, resulting in the reduction of the flame temperature which positively resulted in a lower yield of thermal NOx. 27 The trend of these results imply that a high ash recirculation ratio is desirable for NOx emission reduction; in view of this, CFB engineers need to focus on designs that can sustain operations under high ash recirculation schemes.…”