Effects of Operating Conditions and Dusty Fuel on the NOx, N₂O, and CO Emissions in PFB Co-combustion of Coal and Wood

The substitution of a part of subbituminous coal by biomass in pressurized fluidized bed combustion (PFBC) can offer lower NO x emissions as an advantage, but simultaneously some adverse accompanying issues, e.g., problematic feeding, ash sintering, CO emissions, etc., have to be solved. The objective of the experimental study is to assess the effects of co-combustion of crushed wood chips and fuel dust presence on the NO x, N 2 O, and CO emissions and agglomeration behavior. The study employed operating pressures in the range 0.1−1 MPa, temperatures 800−950 °C, and a broad range of up to 40 mass % of the biomass in the inlet fuel under. Wood chips addition in PFBC of a subbituminous, medium ash coal has a beneficial effect in moderate reduction of the NO x emissions, a very small influence on N 2 O emissions, and the only negative effect is an increase of CO emissions under comparable conditions of the combustion. The dependences of the NO x and N 2 O emissions on the operating pressure and temperature are similar in both cases, i.e., the subbituminous coal combustion and co-combustion with biomass. The presence of coal dust (0−0.5 mm) in PFBC of coal (0.5−1.6 mm) leads to slightly higher N 2 O emissions, lower NO x emissions, and higher carbon content in cyclone ash, especially at fluidized bed temperatures below 850 °C, caused by entrainment of char and by CO-coupled effects. At lower oxygen concentrations (below 6 vol %), the combustion of dusty subbituminous coal is accompanied by higher CO concentration in flue gas. Influence of wood chips, wood dust, and operating pressure in PFB co-combustion on the content of unburnt carbon in the cyclone ash is only weak. The presence of coal dust has a decisive effect on carbon content in cyclone ash in PFB combustion and co-combustion of coal with wood. The feeding of the mixed coal/wood fuel should be more regular/uniform than that of the coal, because of higher content of volatiles and faster burning of the biomass. The suitable coal ash composition, with kaolinite for example, enables sufficiently high temperature for wood co-combustion (even up to 900 °C) without agglomeration of ash particles.