Gu, Quan
(2020)
Biomass combustion in domestic boilers with low emissions.
PhD thesis, University of Nottingham.
Abstract
Compared with conventional fossil fuels, biomass has its unique advantages in terms of wide availability, potential high energy generation and carbon neutrality. Up to now, biomass combustion has made huge contributions to meet the heating demands of both commercial and domestic fields. However, one of the main challenges of biomass combustion is its pollution emissions, including NOx, CO and hydrocarbons. Meanwhile, the release of CO2 from biomass combustion which contributes to the greenhouse effect, should be paid more attention as well. The present PhD study aims to explore the combustion behaviours of biomass fuels by burning individual biomass pellets under different combustion atmospheres (air and oxy-fuel) and develop catalytic methods of NOx emission abatement for the potential use in small-scale and domestic biomass boilers.
In this study, five different types of biomass (wood, straw, miscanthus, peanut and torrefied wood) pellets were applied as the fuels in a visible drop tube furnace (V-DTF) to investigate the combustion behaviours under both conventional air and oxy-fuel (O2 fraction of 21%, 25% and 30%) combustion atmospheres. Four combustion phases, namely pre-heating, volatile combustion, volatile/ char overlap combustion and char combustion were observed and identified. As expected, a shorter total burnout time of a biomass pellet was observed when increasing the combustion temperature (from 800°C to 900°C), which was mainly contributed more rapid char combustion. Increasing oxygen fraction from 21% to 30% also accelerated the pellet combustion, shortening the total burnout time in oxy-fuel atmospheres while the total burnout time under conventional air combustion condition was between those of 21% O2-79% CO2 and 25%O2-75% CO2 atmospheres for untreated (raw) biomass fuels. Due to the impacts of C-CO2 gasification reaction and torrefaction, torrefied wood was observed to have shorter char burning time under 21% O2-79% CO2, hence resulting in shorter total burning time than the air combustion. By means of high-speed camera and MATLAB software processing the flame images, the volatile flame brightness was found to increase with O2 fraction in oxy-fuel atmospheres while the brightness of volatile flame in air was close to the one in 30% O2-70% CO2 atmosphere.
The tests to investigate the combustion performance and gaseous emissions (e.g. NOx, CO and CH4) were carried out in a domestic 50kW biomass boiler with different primary and secondary air settings by using typical wood pellets. The results showed that the boiler had good performance in terms of NOx emissions under all conditions. Continuously fluctuating CH4 and CO were detected in the combustion chamber during normal combustion processes due to insufficient mixing and limited chamber space. More importantly, both CH4 and CO concentrations followed the same variation trend. The generation of CH4 and CO was found to be determined by the amount of primary air while that of NOx was more dependent on the nitrogen content of the fuel. Both CH4 and CO can potentially be utilized to reduce NOx emissions in suitable SCR processes.
Performance of CH4-SCR and catalytic NOx-CO reactions were experimentally tested in a lab-scale fixed bed reactor with simulated flue gas atmospheres. For CH4-SCR tests, Co-ZSM-5 displayed great activity to reduce NOx with the help of excess oxygen. The optimum NOx conversion rate reached over 60% when using 2.89% or 4.89% Co-ZSM-5 (Si/Al ratio of 12.5) under the furnace setting temperature of 400℃. The catalytic performance of NOx reduction over Co-ZSM-5 was suppressed significantly in the absence of oxygen although there was activity between NOx and CH4 at high temperatures due to thermal reactions. Fe-supported catalyst was inactive in CH4-SCR system, but it could promote catalytic NOx-CO reactions in the absence of oxygen (with the maximum NOx reduction of 92.62% @ 500℃). However, the catalyst lost its activity when there was O2 in the feed gas.
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