1 Introduction
Similar to thermochemical conversion of coal, biomass gasification via fluidized bed has great advantages for industrial application. To explore the benefits of a fluidized bed in biomass conversion, an in-depth understanding of the hydrodynamics of beds of different particle types is required. This thesis investigates the effect of different particle properties on bubbling fluidized bed behaviour. The investigation also includes the mixing and segregation patterns of biomass particles in different mixtures with sand particles. In addition, the performance of a biomass gasification process under different operating parameters including the particle size of the bed material and biomass-loading rate is studied. The finding of this study can be a useful tool for the initial design phase, operational control and parameter optimization of bubbling fluidized bed reactors.
1.1 Research overview
In recent years, there is increasing number of researches in the field of biomass conversions destined for different uses, which include direct combustion for heat production, and gasification for power generation and synthesis of chemicals such as methanol, biodiesel and bioethanol. Biomass, as a source of energy, is an ancient technology where wood was burned in homes, primarily for heat production. The conversion of biomass such as grains and oil into ethanol and biodiesel can be traced as far back as the Second World War.
Generally, biomass includes all the energy sources, which are derived from animal and plant matters. In this definition, the so-called “first generation” of bioenergy technology was described to include different feedstock such as food grains, forest wastes, forest plants, soya bean and palm oil. The forest-based biomass are widely distributed across the globe as shown in Figure 1.1, and are the most commonly used due to their consistent properties. The municipal solid wastes and animal manure are also regarded as biomass. The growing interest in biomass research and technology today is widely attributed to the belief that biomass is a greener energy source when compared to the fossil fuels. Biomass is also believed to be a renewable source of energy because it can be re-grown after used. The plant-based biomass is grown all year round and once removed for food or energy, new ones are re-grown artificially or naturally. As a renewable energy source, the plant-based biomass (which are the most reliable form of biomass) remove approximately the same amount of carbon dioxide (CO2) they emit when burned during their lifecycles. Biomass is widely available and can easily be stored and transported. It is ranked among the top four-world energy sources, providing more than 10% global energy supply [6, 7] as shown in Figure 1.2.
___
2
Figure 1.1. Global map showing the distribution of forest-based biomass [8].
Figure 1.2. Contribution of different energy sources to the world energy consumption. [IEA World Energy Outlook 2014].
Biomass powered plants can be found in some countries today. Some chemical companies also use biomass as feedstock. Biomass makes up 4.8% of United States (US) total energy consumption and 12% of all the renewable energy sources, where wood is the largest biomass energy resource. In US, there are 227 plants running on biomass, while in the United Kingdom, about 35 plants exist [9]. Globally, biomass is viewed as a solution to the world projected energy crisis due to depletion of fossil fuels. Because of this, a large number of researches has been devoted to improving on the overall usage of biomass. For efficient use, biomass is converted into gaseous form by means of gasification. As shown in Figure 1.3, the main steps involved in the biomass conversion and utilization are classified into upstream processing, gasification and downstream processing. In the upstream processing, the biomass is made suitable for the gasification process by means of drying, size reduction and densification [10]. Biomass gasification involves drying and thermochemical degradation of the feedstock by pyrolysis, partial
___
3
oxidation and gasification of the resulting char particles. The gasification of char particles is achieved through their reactions with air, pure oxygen, carbon dioxide, steam or their combination. The energy value and quality of the product gas depend on the gasifying agent, biomass properties, temperature, pressure and reactor design [11].
With the use of catalyst and sorbents, the gasification process can also be improved.
Figure 1.3. Different process steps for conversion and utilization of biomass [10].
Biomass gasification can accept a wide variety of feedstock, thus generating multiple useful products. For gasification with air, the calorific value of the product gas is in the range 4 – 7 MJ/Nm3, and with pure oxygen a value of 12 – 28 MJ/Nm3 can be achieved.
The gasification process reduces the carbon to hydrogen mass ratio, thereby increasing the calorific value of the product gas [12]. A gasification reactor is usually designed for a specific feedstock type classified into woody biomass, herbaceous biomass, manures and marine biomass. The first biomass gasification plant was constructed and installed in US in 1999 under the Wabash River Coal Gasification project [13]. Since then, more advancement has been recorded in the gasification projects towards achieving the global energy demands and reduction in the greenhouse gas emissions.
One major challenge in biomass gasification is the tar content of the product gas, which degrades the gas quality and often results in reduction of the process efficiency. Tar is a thick viscous liquid of aromatic hydrocarbon with some traces of heavy metals [14]
formed during biomass pyrolysis. The yield of tar can be minimized by thermal cracking, partial oxidation and reforming processes. To some extent, the quality of product and efficiency of the process depend on the type of gasification technology employed. The most common technologies used are the fixed bed, fluidized bed and the entrained flow gasifiers, depicted in Figures 1.4 – 1.6.
___
4
In a fixed bed gasifier, biomass is fed from the top of the reactor, and as shown in Figure 1.4, the different stages of gasification can clearly be distinguished in this type of gasifier. As a single column reactor, air is often used for the gasification process such that the partial combustion of char particles provides the necessary heat required during the reduction stage. Depending on the flow of air in relation to the direction of the biomass flow, fixed bed reactors are classified into downdraft and updraft gasifiers. Air is blown upwards through the biomass bed in the updraft design and downwards in the downdraft configuration. The updraft gasifier operates within 750 – 1000 ֯C, resulting in a high tar yield in the range 10 - 20 wt% of the product gas compared to the yield of about 5 g/Nm3 in the downdraft gasifier. The low tar content in a downdraft fixed bed system is due to its higher operating temperature 1200 – 1400 ֯C that enhances the cracking of the heavy hydrocarbon [15]. However, the ash content of the product gas from a downdraft gasifier is on a high side and the requirement for the moisture content of the feedstock is very low (< 25 wt.%) compared to other technologies, limiting the use of variety of biomass types.
Fluidized bed gasifiers employ inert bed material that is fluidized to aid the distribution of heat and fuel particles. In the fluidized state, the superficial velocity of the incoming gas is greater than the minimum gas velocity required to lift the bulk material against the bed weight. As shown in Figure 1.5, fluidized bed gasifiers are divided into bubbling fluidized bed (BFB), circulating fluidized bed (CFB) and dual fluidized bed (DFB) gasifiers.
In a BFB gasifier, biomass is fed from either the top or side of the bed. The gas velocity is usually within twice the minimum fluidization velocity to reduce particle elutriation effects, and a wide distribution of particle size can be used. On the other hand, a CFB gasifier requires a higher gas velocity, a lower bed height and a smaller particle size. The solid particles in a CFB reactor are circulated through a cyclone system to increase their contact time with the gasifying agents. In a dual fluidized bed configuration, two interconnected reactor columns (BFB column and CFB riser) are used. The biomass gasification takes place in the BFB column while combustion of char residue and additional fuel takes place in the CFB riser. While the bed material is circulated between the separate reactors, it transfers the heat released during combustion in the riser to the bubbling bed column to aid the gasification process. This reactor design is usually applied for steam gasification, as the process is highly endothermic.
An entrained flow gasifier as shown in Figure 1.6 is highly energy efficient, operating above 1000 ֯C and has the least tar yield among the known gasification technologies. For coal gasification, the reactor design is widely applied. However, the requirement that the feedstock must be pulverized poses some operational challenges when biomass is used as the feed.
___
5 Figure 1.4. Updraft and downdraft configuration of fixed bed biomass gasifier[16].
(a) (b) (c)
Figure 1.5. Fluidized bed biomass gasifier showing different configurations (a) bubbling bed (b) cirgulating bed [17] (c) dual-fluidized bed [18].
Figure 1.6. Configuration of entrained flow reactor as applied for coal gasification [From http:// biofuelsacademy.org, retrieved on March 25, 2019].
This thesis focuses on the bubbling fluidized bed reactors, which are common among the three different fluidized bed designs shown in Figure 1.5. The fluidized bed
___
6
technology offers a number of advantages for thermochemical conversions, and thus has a wide industrial application. Due to rapid mixing of solids and better heat distribution, a continuous feed and operational control can be achieved easily in a fluidized bed reactor.