Abstract
Air staging is a well-known effective method to control NOx emissions from solid fuel combustion boilers. However, further research is still needed to clarify the effect of air staging at different injection locations on the gaseous emissions of Fluidised Bed Combustion (FBC) boilers that fire 100% biomass fuels, particularly non-woody biomass fuels. The main objective of this work is to investigate the effect of the staging air injection location on the gaseous emissions (NOx and CO) and temperature profiles of a 20 kWth bubbling fluidised bed combustor firing three non-woody (straw, miscanthus and peanuts) and two woody biomass fuels. The experimental results showed that injecting the secondary air at the higher location could lead to a greater NOx reduction due to the fact that the biomass combustion reaction mainly took place in the splash zone and/or beginning of the freeboard. Up to 30% of NOx reduction, compared with no air staging, was achieved for the non-woody fuels when the staging air was injected at the higher position. Air staging also significantly reduced the CO emissions as a result of the higher temperatures in the freeboard and longer residence time in the primary combustion zone.
Highlights
The UK government recently announced its intention to close all of the UK's coal-fired power plants within 10 years [1]
The main conclusions are: (1) Higher overall excess air always leads to higher NOx emissions for any of the tested biomass fuels as the combustion condition with higher excess air favours the conversion of fuel-N to NOx and there are less CO and char available in the reactor to promote NOx reductions
NOx emissions depend directly on the fuel-N content e the higher the fuel-N content, the higher the NOx emissions; (2) As a consequence of the characteristically high volatile matter contents of the biomassfuels, the maximum temperatures are reached above the dense bed in the splash region and/or at the beginning of the freeboard, which suggests that the main combustion reaction takes place in this part of the combustor
Summary
The UK government recently announced its intention to close all of the UK's coal-fired power plants within 10 years [1]. As a renewable and carbon-neutral source of energy, biomass can play an important role in CO2 emissions mitigation and be an alternative to the carbon-intensive and ‘dirty’ fossil fuel, coal, for power generation. Woody biomass has been widely used as a fuel in the energy sector [2e4]. Fluidised bed combustion (FBC) boilers have been increasingly used to burn difficult to use fuels such as biomass waste materials over the past decades. In a FBC boiler, the fuels are burned in a turbulent bed of an inert solid bed material, ensuring high heat transfer rates, excellent gas-solid mixing and good combustion efficiency, at typical combustion temperatures of 800e900 C. Biomass combustion systems emit relatively high levels of NOx and particulates in comparison to the combustion systems of light fuel oil or natural gas. The life cycle assessment (LCA) indicates that almost 40% of the environmental impact of a modern automatic wood furnace is associated with NOx emissions [6]
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