Design and operation of a 20 kWth fluidised bed combustor for biomass oxy-fuel combustion

Sher, Farooq (2017) Design and operation of a 20 kWth fluidised bed combustor for biomass oxy-fuel combustion. PhD thesis, University of Nottingham.

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Due to growing concerns about climate change, the heat and power sector is continuously facing challenges to reduce CO2 emissions. Carbon capture and storage (CCS) is one of the short-medium term measures that can mitigate CO2 emissions emitted from fossil fuels utilisation. Oxy-fuel combustion is a promising technology for CSS that can be integrated into the new and the current fleet of power plants. Biomass is a carbon neutral renewable source of energy that can replace fossil fuels. If the biomass is utilised as a fuel in oxy-fuel combustion it could lead even to negative CO2 emissions. Although the sintering and agglomeration problems associated with the combustion of non-woody biomasses in the fluidised beds are still major issues, fluidised beds have emerged as one of the best among the other proven biomass combustion technologies, mainly due to their fuel flexibility, low SOx and NOx emissions. However, oxy-fuel combustion technology in fluidised beds is in the early stages of development and still needs a lot of research for improvement before its application on full-scale power plants.

In this work basic combustion fundamentals of different biomass fuels in terms of energy production were studied using thermogravimetric analysis (TGA) under air, N2, CO2 and selected oxy-fuel (30%O2/70%CO2) reaction environments. Then a 20 kWth bubbling fluidised bed combustor (BFBC) was designed, manufactured and successfully tested for a range of biomass fuels under air and oxy-fuel combustion environments. The agglomeration and sintering behaviour of these biomass fuels during combustion under air was also investigated using different analytical techniques such as SEM-EDX, XRD and XRF. The biomass fuels investigated in this study include domestic wood, industrial wood, miscanthus, wheat straw and peanut shell pellets. The BFBC testing of these biomass fuels focused on the influence of operating conditions, the effect of excess air level and fuel feed rate on the hydrodynamics, temperature profiles and emissions, NOx, CO2 and CO within the BFBC. Air staging can be very effective in reducing NOx emissions of non-woody biomass fuels especially when the secondary air was injected at the higher level with an overall low excess air level. A maximum NOx reduction percentage of 30% was achieved for the non-woody biomasses during air staging combustion.

The non-isothermal TGA analyses under N2 and CO2 showed almost identical weight loss (R), reactivity (RM) and activation energy (Ea) profiles in devolatilisation zones. However, when devolatilisation occurred under CO2 conditions at temperatures higher than 700 oC, an additional weight loss was observed for all biomass fuels, being indicative of the contribution of CO2-char gasification reactions. Under air and oxy-fuel (30%O2/70%CO2) results showed almost similar profiles for R, RM and Ea. In oxy-fuel atmospheres, by replacing N2 with CO2 a slight increase in the maximum rate of weight loss (RMax) was observed in both reaction zones for all studied biomasses.

The unstaged and staged air combustion experiments in the 20 kWth BFBC showed that higher excess air always led to higher NOx emissions for any of the biomass fuels tested because less CO and char were available in the reactor to promote NOx reductions. Due to the consequence of the high volatile matter content of the biomass fuels, the maximum temperatures were achieved at the top of the dense bed and/or beginning of the freeboard, which suggests that the main combustion reaction takes place in this part of the combustor. Air staging leads to higher temperatures in the freeboard, especially at low excess air levels, as a result of additional combustion in the freeboard under staged air conditions. Air staging can be very effective in reducing NOx emissions of non-woody biomass fuels especially when the secondary air was injected at the higher level with an overall low excess air level. A higher percentage of carbon in ash was obtained while working under air staging conditions than that of without air staging combustion.

The results of oxy-fuel combustion tests in the 20 kWth BFBC showed that oxy-fuel combustion was different from air combustion in several ways, including reduced gas temperatures, delayed flame ignition, increased CO emissions under 21%O2/79%CO2 and 25%O2/75%CO2 oxy-mixtures. Many of these parameters were associated with differences in properties of the main diluting gases CO2 and N2 in oxy-fuel and air combustion respectively. In order to match the biomass oxy-fuel combustion gas temperatures to those of air combustion, the oxygen concentration in the mixture of O2/CO2 has to be increased to 30% or higher. Moreover, the oxy-fuel combustion with 30%O2/70%CO2 has shown higher efficiencies than air, which indicates biomass fuels can be successfully combusted in the BFBC under oxy-fuel combustion conditions.

The agglomeration and sintering behaviour was observed under continuous air combustion conditions in the 20 kWth BFBC. The analysis using SEM-EDX, XRD and XRF concluded that the potassium present in wheat straw was mainly responsible for agglomeration, which was detected in the form of KCl and K2O in the bed material and cyclone ash samples.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Liu, Hao
Sun, Cheng-Gong
Snape, Colin
Keywords: Global warming, Sustainable energy, Oxy-fuel technology, CO2 capture, Biomass fuels, Fluidised bed combustion, Air staging combustion, Oxy-fuel combustion and Pollutants emission.
Subjects: T Technology > TJ Mechanical engineering and machinery
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 40606
Depositing User: Sher, Farooq
Date Deposited: 13 Jul 2017 04:40
Last Modified: 12 Oct 2017 14:37

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