Waste as feed to a biomass gasification reactor

Abstract

Solid waste management has been a global challenge. With the growing population, an increase in solid waste from households, agriculture, and industries is inevitable. At the same time, rapid industrialization and technological advancements due to increasing population have increased the energy demand significantly. The predominant reliance on fossil fuels has led to severe environmental consequences, including greenhouse gas emissions, global warming, rising sea levels, extreme weather conditions, and depletion of fossil fuel reserves. Consequently, the need for alternative energy sources has become crucial. This study explores the utilization of waste for energy production, aiming to address two global problems: waste management issues and supply for the clean energy demand. Thermochemical conversion technology, such as gasification, has gained attention when it comes to converting wastes into energy. The main combustible gas components from the gasification process are methane, hydrogen, and carbon monoxide, which can be useful for producing biofuels and chemicals or for power generation. The major challenge in utilizing wastes in a gasification reactor is to convert solid wastes and their mixture as fuel and operate the reactor economically to produce high-quality synthesis gas. The choice of a gasification reactor is critical for economical operation as it should allow to handle a wide variety of feedstock. In this regard, a bubbling fluidized bed reactor (BFB) is a better option among various thermal reactors available since BFB provides the flexibility to use a wide variety of feedstocks and provides uniform heat and mass transfer for thermal degradation.

This project investigates various challenges when converting waste to high calorific value feedstock (fuel) and utilizing such feedstock for energy recovery in a BFB gasification reactor. The task has been accomplished using a combination of experimental work and CPFD simulations. For the experimental work, three different BFB reactors were employed to gain better understanding of the fluid dynamic behaviour coupled with thermochemical conversion phenomena in a BFB gasifier. The fluidized bed setups used in this work include: (i) a cold flow BFB reactor equipped with pressure sensors, (ii) a cold flow BFB reactor equipped with electrical capacitance tomography (ECT) sensors, and (iii) a 20 kW BFB gasifier. The cold models were used to investigate the fluid dynamics behaviour of the bed, such as mixing and segregation, bubble dynamics and fluidization regime. The results from the cold model test were used to select operating parameters for the gasification experiments. For the gasification experiments, the 20 kW BFB gasifier was operated in autothermal and allothermal modes with several waste feedstocks and air as the gasifying medium. The feedstocks from various sources, such as municipal solid wastes (MSW), agricultural wastes, garden residue, and industrial wastes, were used as fuel for the gasifier. All the waste feedstocks, paper, fish, garden residue, sawdust, coffee grounds, barley straw, bark, and grass were used in the pellet form except for wood chips. Among these feedstocks, barley straw, saw dust, bark and coffee grounds were pelletized using a lab-scale pellet mill. The syngas production potential of the feedstocks and the reactor performance at different reactor operating conditions were assessed based on the carbon conversion efficiency, chemical conversion efficiency, thermal conversion efficiency, and gas yield strengths.

Similarly, computational particle fluid dynamic (CPFD) models were developed to investigate the thermal and hydrodynamic behaviour of the reactor and determine the operational parameters suitable for running the gasifier at optimal conditions. The CPFD simulation were developed in the commercial software Barracuda VR 21.1.1 and the results were validated with experimental data obtained from the 20 kW gasifier and cold bed rigs. The CPFD models were used to address challenges such as slug in the bed, excess steam in the product gas, unconverted carbon, reduced gas residence time, and inadequate distribution of gas and biomass in the bed. The simulations also explore the influence of bubble properties, biomass feeding position, and air supply patterns on the reactor performance.

The air-gasification tests with the feedstocks, wood chips, grass pellets and wood pellets revealed that at equivalence ratio (ER) <0.16, the syngas quality was enhanced with lower nitrogen yield at reactor temperature above 800 ℃ . About 70-75% of the carbon conversion efficiency was achievable at allothermal reactor conditions. However, grass pellets showed agglomerations in the bed at reactor temperature 800 ℃ and wood chips caused blockage in the feeding system. Similarly, the low-grade feedstocks, fish pellets (animal waste origin), garden waste pellets (plant origin) and paper pellets (MSW) were gasified in an autothermal mode with air as gasifying medium and at reactor initial temperature of 650 ℃. The reactor initial temperature was selected by considering the pyrolysis temperature and ash melting temperature of the feedstock. The result depicted that overall conversion efficiency of the paper pellets reached approximately 80-90%, garden waste pellets exhibited a conversion efficiency of 50-60% and the fish pellets demonstrated a lower conversion efficiency of 22- 40%. The reduced conversion efficiency of the fish pellets was ascribed to the animal-based origin of the feedstock, containing fats and proteins, necessitating higher temperatures for degradation. Notably, despite the high moisture content of paper pellets (26 wt.%), the results indicated a higher conversion efficiency. This suggested that the moisture content had little to no influence on the conversion process when the feedstock was injected on the top of the bed. Pelletization experiments with coffee grounds, sawdust, bark and barley straw revealed that optimal conditions for the pelletization of the feedstocks were at the die temperature of 50℃- 60℃, a moisture content of 9-14 wt.%, and a roller-to-die distance of 4 mm. Mechanically durable pellets with enhanced compression strength improved the conversion efficiency of the feedstock during gasification tests. Over the ER of 0.15-0.32, the carbon conversion efficiency of the feedstock coffee grounds pellets and straw pellets reached about 75%, demonstrating that both wastes are potential feedstock for energy recovery.

CPFD simulations with two different feeding positions of the biomass (on the top of the bed and in the bed near to the bottom of the reactor) in the gasifier showed that in-bed feeding of the biomass was better compared to top bed feeding. The result demonstrated that higher carbon conversion, uniform reactor temperatures and better gas quality yield were achieved with in-bed biomass feeding. Similar, different modes of air supply such as: (i) uniform air distribution (with air distributor), (ii) with side nozzles, and (iii) multiple injection points were investigated in a BFB reactor. The result demonstrated that air can be injected in the fluidized bed with the side nozzles while maintaining similar fluid dynamics properties of the bed compared to air distributor as flow boundary conditions. Thus, side nozzles as flow boundary conditions can be employed to minimize the cost and avoid operational challenges related to the presence of air distributor in the gasifier. Also, the CPFD model results illustrated that for an auto-thermal operation of the reactor with maximum hydrogen yield in the product gas, the steam-to-air ratio was 0.05. Based on the experimental measurements and CPFD model simulations, a simple correlation (𝑔𝐷𝑏/𝑢0 = 3.0), based on superficial gas velocity (𝑢0) and average bubble diameter over the bed (𝐷𝑏) for efficient biomass gasification in a bubbling fluidized bed was developed. The result revealed that at the optimum gas residence time, the concentration of hydrogen is maximum while the concentrations of carbon dioxide and water vapor are minimum in the product gas. This proposed model can, therefore, be used to size the reactor or set the operating gas velocity to achieve optimum gasification