Modelling and simulations of bubbling fluidized bed and entrained flow biomass gasification reactors

Abstract

The world needs sustainable energy solutions to replace fossil fuels. Climate change is the defining challenge of our time. New reports from the International Energy Agency (IEA, 2021)1 have developed a roadmap for max 1.5°C global warming and net zero emissions by 2050 from the energy sector. The roadmap recommends increasing efforts and a clear political action to invest in renewable energy extensively and immediately. Biomass contributes to more than 10% of the global energy demand and has the potential to contribute to a renewable energy supply. Gasification is a thermal conversion of biomass into higher energy gases such as carbon monoxide (CO), hydrogen (H2), and methane (CH4), and the gases can be used directly or synthesized into biofuel and higher value chemicals.

This thesis contains the experimental results for a cold flow model for bubbling fluidized bed (BFB) and a pilot scale BFB gasifier. The reactor hydrodynamics and the mixing behaviour of the bed were investigated using the cold flow model study. The experimental studies in BFB gasifier include the gasification of wood chips, wood pellets and grass pellets at different air flow rates and biomass feed rates. The product gas compositions and the gasifier performance (based on the mass balance of N2 in the inlet and outlet gas) were measured and analyzed. Increasing the equivalence ratio (ER) gave an increased gas yield per kilogram of biomass, however, the lower heating value (LHV) of the product gas decreased due to the dilution of the product gas with N2. Gasification of grass pellets was challenging due to the formation of agglomerates and gave a low carbon conversion. Wood chips showed reasonable results at a temperature of around 850°C with a carbon conversion of around 60%.

Computational particle fluid dynamics (CPFD) models were developed for the cold flow model for BFB, BFB gasifier and entrained flow (EF) gasifier. The models were validated against the experimental results from the corresponding reactor/gasifiers. The results from the model showed that bed hydrodynamics plays a significant role in biomass conversion in the BFB gasifier. The bubbling behaviour of the bed influenced the heat and particle distribution, thus affecting the gasification behaviour. For a case with birch wood, the CO concentration decreased from 25 to 13.2 mole % and the CO2 concentration increased from 17 to 19.5 mole % when the ER increased from 0.2 to 0.3.

Simulation results for the EF gasifier showed that the Char- O2 and char-H2O reactions are significant in the gasifier entrance region, whereas the char-CO₂ reaction is prevalent throughout the reactor elevation. Particles in the central region show high carbon conversion compared to the particles in the other zones. The ratio of product gas to biomass was calculated as 3.61 Nm³/kg of biomass. The average gas fractions on a volume basis were 0.038 of CH4, 0.457 of CO, 0.226 of CO2, and 0.275 of H2. The lower heating value of the product gas is calculated as 7.8 MJ/kg.

A process simulation model was developed to study the BFB biomass gasification in Aspen Plus. The model was used to predict the gasifier performance for different operating conditions, i.e., temperature, steam to biomass ratio (STBR), biomass types, and biomass loadings. Hydrogen production was around 50% for all types of biomass while CO production varies from 8% (Pig manure) to 24.5% (Olive residue) at 700°C. H2/CO ratio increased with an increase in STBR for all types of biomass. H2 concentration increased from 46 % to 54% and CO concentration decreases from 30% to 20% with an increase in STBR from 0.6 to 1 for the wood residue.

The results obtained from this study can be useful for the operational control and the optimization of the biomass gasification reactors. The proposed model for the BFB gasifier can be extended into a dual circulating fluidized bed (DCFB), which gives the product gas free from nitrogen (N2). The models for gasifiers accept different possible inputs to the gasifiers, which can be useful in determining the optimal operating conditions for efficient biomass conversion.