NOx formation and abatement in the Circulating Fluidized Bed Combustor (CFBC) of biomass at Caledonian Paper (Ayr, UK)
Mahmoudi, S. , Seville, J.P.K. and Baeyens, J., University of Warwick, UK
(free)
Caledonian Paper (CaPa) is a major paper mill, located in Ayr, Scotland.
For its energy supply (steam), it relies on the use of a Circulating Fluidized Bed Combustor (CFBC) of 58 MWth, burning coal, wood bark and wastewater treatment sludge.
It currently terminates the erection of a new fluidized bed combustor of 102 MWth to generate steam at 99 bar, superheated to 465 °C. The boiler is followed by a steam turbine and 15 kg/s steam circuit at 5 and 10 bar into the paper mill. Whereas previously coal, wood bark and wastewater treatment sludge were used as fuel, currently only wood (mainly spruce), demolition wood, wood bark and sludge are used.
Since these biosolids contain nitrogen and in view of combustion at 850-900 °C, mainly fuel-NOx is being formed (the contribution of thermal NOx to the emissions is not significant). NOx emissions (NO + NO2) vary between 300 and 600 mg/Nm³ (dry gas). The current emission standard is 350 mg/Nm³, hence making a NOx-abatement necessary.
The proposed paper will firstly review the NOx-formation mechanisms and the
de-NOx techniques presented in the literature, with special focus upon the techniques that are applicable at CaPa. From these techniques, Selective Non-Catalytic Reduction using ammonia or urea emerges as the most appropriate solution. The paper will thereafter present data of SNCR-experiments obtained at CaPa. It will finally discuss these results, present recommendations for design and operation, and assess the economic impact of implementing a SNCR-technology.
The operating cost of applying NH3-SNCR and urea-SNCR to the CFBC at CaPa are £15,000/year and £45,000/year respectively. This costing is based upon an average 30% NOx reduction in order to meet the 350 mg/Nm³ emission standard. Although the purchase cost of urea is cheaper than NH3, the overall operating cost when using urea is 3 times higher due to the poor NOx abatement efficiencies at the operating temperatures. Higher tempratures will improve the removal efficiency when using urea and will consequently reduce the operating costs, but such temperatures are inpractical for CaPa.
