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Since the world is gradually drifting toward sustainable development, renewable energy technologies are gaining traction and gasification technology is one of many renewable energy technologies that have gained popularity in recent times. The gasification technology is one of three main (combustion and pyrolysis) thermochemical conversion pathways that can be used to recover energy from biomass materials. Although the gasification technology has been in existence for centuries, it has not been exploited to its full potential mainly because the fundamental principles underpinning its operation are still vague, particularly with regard to feedstock flexibility and the type of gasification system. Furthermore, due to the many types of gasification systems, the mechanisms involved in their feedstock conversion processes are still under debate and require further research to clearly establish the optimum conditions of performance of each type of gasifier. Therefore, this chapter presents an overview of the gasification technology and discusses the different types of gasification systems that are commonly used today for the recovery of energy. The limitations of each type of gasifier in relation to performance and feedstock conversion are also discussed, including research priority areas that will allow for system optimization in terms of efficiency.
Department of Engineering and Chemical Sciences, Karlstad University, Sweden
Anthony Anukam
Biochemical Process Engineering, Division of Chemical Engineering, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, Sweden
*Address all correspondence to: ali.mohammadi@kau.se
1. Introduction
Evidence suggests that conventional energy production has limited capacity to meet growing demand and that additional demands will have to be met by unorthodox sources. Since the world is now drifting toward sustainable development, renewable energy technologies are gaining traction. One of such renewable energy technologies that has received great attention in recent times include biomass gasification, which is one of three main (combustion and pyrolysis) thermochemical conversion pathways used to recover energy from biomass materials. Gasification produces energy from biomass and involves heating the biomass at elevated temperatures (above 1000°C) under a limited supply of oxygen to produce a mixture of gases (H2, CO, CO2) collectively referred to as syngas. However, the combustible constituents of the syngas are CO and H2 and can be used as fuel in gas engines for heat and electricity generation as well as for the production of chemicals (such as alcohols, organic acids, ammonia, and methanol) via the Fischer-Tropsch process [1]. The significance of gasification technology is such that it helps waste management, at the same time, produces energy and other valuable products needed for economic growth. Systems designed to gasify coal is assumed to be able to use biomass as well, however, differences in the characteristics of coal and biomass can have a significant impact on the sizing and design of the combustion chamber of the gasification system, as well as on the location of the gasifying agent [2]. A graphical representation of a gasification process, which depicts feedstock flexibility and the production of a wide range of products, is presented in Figure 1.
Figure 1.
A schematic representation of a gasification process depicting feedstock flexibility and the wide range of products that can be obtained from the process [3].
The gasification technology has existed for several decades and has, as of today, been commercialized in very few countries of the world like Sweden, Germany, Canada, the United States, India, and China. The use of this technology offers a number of ecological and economic advantages such as low emission of pollutants, reduction in the environmental effects of waste disposal, generation of non-hazardous by-products when biomass is used as the feedstock, and lower operating cost [4].
Gasification occurs in a gasifier under a series of chemical reactions that are mostly endothermic in nature; however, to provide the heat required for the reactions to proceed successfully, and the heat needed for drying and pyrolysis to occur, a certain amount of exothermic combustion is allowed in the gasifier [4, 5]. The gasification reactions are described in greater detail in subsequent sections. The gasifier and its configuration are key factors that affect the entire gasification process, including the reactions occurring and their products [6]. This is true because gasifiers are generally classified into three broad groups, namely: the fixed bed gasifiers, the fluidized bed gasifiers, and the entrained flow gasifiers. Table 1 shows the main characteristics of these three gasifiers.
Type of gasifier
Characteristics
The fixed beds
Small capacity gasifiers (typically from 0.01–10 MW)
Can handle large and coarse particles
Low product gas temperature (450–650°C)
High particulate content in the gas product stream
High gasification agent consumption
Ash is removed as slag or dry
May result in high tar content (0.01–150 g/Nm3)
The fluidized beds
Medium capacity (1–100 MW)
Uniform temperature distribution
Better gas-solid contact
High operating temperature (1000–1200°C)
Low particulate content in the gas stream
Suitable for feedstocks with low ash fusion temperature
Ash is removed as slag or dry
The entrained flows
Large capacity (60–1000 MW)
Needs finely divided feed material (0.1–0.4 mm)
Very high operating temperatures (1200°C)
Not suitable for biomass feedstocks
Very high oxygen demand
Short residence time
Ash is removed as slag
May result in low tar content (negligible)
Table 1.
The main characteristics of the three types of gasifiers commonly used for the recovery of heat and electricity from biomass [7].
Although the gasification technology may be considered as a useful technology for the recovery of energy from biomass materials, the technological choices with regards to the type of gasification system (fixed bed, fluidized beds, or entrained flow reactors) for the conversion of biomass are still faced with a host of technical barriers that have hindered the significant exploitation of the gasification technology and biomass energy as a whole. The quality of the syngas produced from the gasification process, the lack of feedstock flexibility and its mechanism of conversion are the main obstacles. This chapter, therefore, presents an overview of the gasification technology and discusses its main technical barriers with reference to the gasification systems commonly used today. The status of current research in gasification and future research focus are also presented.
2. Types of gasification systems and their configurations
There are different types of gasification systems but the most commonly used are the fixed-bed, fluidized-bed, and entrained-flow gasification systems. The main differences between these gasifiers are connected to their mechanism of heating and the way feedstock and gasifying agents are introduced in the gasification process, as well as by the location of syngas output [8, 9, 10]. However, the technological choices toward these gasifiers are guided by the nature and availability of biomass feedstocks. While the characteristics of biomass feedstocks intended for gasification are detailed in [11], the principles of operation of the types of gasifiers mentioned above and their merits and demerits are equally well described in [12, 13] and in [14]. These gasification systems may appear as simple devices but their successful operations are not so simple. The gasifiers are still faced with a host of technical issues that have hindered their broader market penetration. These technological barriers are described in Section 5. Nonetheless, in order to fully comprehend the technical barriers of each of these gasifiers, it is important to understand the differences between the gasifiers in terms of configuration, which also affects the thermodynamics of their operation. Therefore, a schematic diagram of each gasifier type is presented in Figure 2.
Figure 2.
Schematic representations of the gasification systems in use today: (a) fixed bed; (b) fluidized bed; (c) entrained flow. Reproduced with permission from [13, 15].
The key mechanism of the gasification technology involves the conversion of solid carbonaceous materials like biomass into flammable gas by partial oxidation. However, the chemistry involved in the process is quite complex and can be achieved via a series of physical and chemical transformation reactions that occur inside the gasification system [4, 16]. The major chemical reactions occurring are those that involve the degradation of large organic molecules into carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), water in the form of steam (H2O), and methane (CH4). These reactions take place in accordance with the chemical bonding theory and can be represented thus [3]:
The combustion reactions include:
C+½O2→CO−111MJ/kmolE1
CO+½O2→CO2−283MJ/kmolE2
H2+½O2→H2O−242MJ/kmolE3
Other key gasification reactions are:
C+H2O↔CO+H2+131MJ/kmolE4
C+CO2↔2CO+172MJ/kmolE5
C+2H2↔CH4−75MJ/kmolE6
The above reactions occur under standard operating conditions of gasification and are considered important reactions that form the major part of the syngas produced in the gasification process [4, 16]. While reaction (4) may be referred to as the “Water-Gas Reaction”, reactions (5) and (6) are termed the “Boudouard Reaction” and the “Methanation Reaction” respectively. Reactions (4) and (5) are the main reduction reactions. However, under high carbon conversion conditions, reactions (4)–(6), being heterogeneous in nature, are reduced to the following homogeneous gas-phase reactions [16]:
CO+H2O↔CO2+H2−41MJ/kmolE7
CH4+H2O↔CO2+3H2+206MJ/kmolE8
Reactions (7) and (8) are known respectively as the “Water-Gas-Shift Reaction” and the “Steam-Methane-Reforming Reaction”. These two reactions (7) and (8) play a key role in determining the final equilibrium of the composition of the syngas produced in the gasification process [3, 16]. Under a limited supply of oxygen to the gasifier, the sulfur composition of the feedstock is converted to hydrogen sulfide (H2S), with a minute amount forming carbonyl sulfide (COS). The nitrogen (N) chemically bound in the feedstock is converted to gaseous nitrogen (N2), ammonia (NH3), and traces of hydrogen cyanide (HCN). The chlorine in the feedstock is mainly converted to hydrogen chloride (HCl). It is important to however state that the concentrations of sulfur, nitrogen and chloride in the feedstock for gasification are sufficiently low that their effects in the gasification process are quite insignificant; trace elements (such as arsenic, mercury, and other heavy metals) that are associated with both the organic and inorganic components of the feedstock are mostly contained in the fractions of ash and slag formed during gasification, as well as in the gases emitted, and must be expunged from the syngas prior to further use [16].
3.1 The kinetics of gasification reactions
Temperature increases in a gasification process lead to dehydration, volatilization, and degradation of the biomass feedstock. The gasification process reactions are mostly reversible reactions. The order of the reactions and their conversion rates are often subject to the limitations of the reaction kinetics and thermodynamic equilibrium of the gasification process. For instance, reactions (1)–(3) presented in a previous section are combustion reactions that actually go to completion when equilibrium positions of the reactions shift to the right. However, not all reactants in a gasification process can be completely converted into products; as such, stoichiometric calculations may be required to determine the products of a completed reaction [5].
While the kinetics of a reaction can determine how fast products are formed and whether the reactions in the gasifier go to completion, the equilibrium state of the reaction determines to what extent the reaction can progress. The thermal efficiency of the gasification process and the composition of the syngas produced are strongly influenced by the thermodynamic equilibrium of the water-gas-shift reaction and the steam-methane-reforming reaction (reactions (7) and (8), Section 3) [3, 4]. A useful tool for evaluating important design parameters of a gasification technology is thermodynamic modeling. With this tool, process efficiency can be optimized at different operating conditions; the relative quantities of gasifying agents such as oxygen and steam can also be calculated including the composition of the product syngas.
4. Controlling factors to the stability of gasifier operation
An understanding of the technical challenges of gasification technology requires a basic understanding of the factors that control the stability of gasifier operation. A typical gasification process includes the following four key steps: drying, pyrolysis, oxidation, and reduction. There are no strict boundaries between these steps; they often overlap and a host of factors including the type of gasifier, feedstock type, and process parameters, such as temperature, determines the output of a gasification process involving the four key steps listed above [4, 5, 8]. The operating temperature of a gasification process is a function of the amount of oxygen fed to the gasification system (gasifier), which induces partial gasification. Temperature response will abruptly change at an equivalence ratio (ER) of about 0.25; depending on the source of oxygen, this change point is typical of gasifier temperatures in the range 600–800°C; some quantities of oil and tar are produced in the pyrolysis stage of the gasification process. These products of the pyrolysis stage are stable for about a second at temperatures lower than 600°C [13].
The fixed bed updraft gasifier operates at temperatures below 600°C and generates considerable amounts of tars that are often emitted with the syngas, while its counterpart, the downdraft gasifier (also of the fixed bed type) is self-regulating and produces far less tar relative to the updraft gasifier; the fluidized bed gasifier also has high tar production rate, in fact, its tar production rate is greater than other types of gasifiers like the fixed bed and the entrained flow gasifiers [13, 14, 17].
5. Technical challenges of the gasification technology
Although the gasification technology has experienced development over several decades and has been commercialized in a number of countries like those previously mentioned [14]; its successful operation is not as simple as can be imagined because of the thermodynamics of the operation of the technology are not well understood. Further exploitation of the technology still needs to overcome a considerable number of technical issues. A description of the technological barriers that are associated with each type of gasification technology is presented thus:
5.1 Limitations of the fixed bed gasifier
The fixed bed gasifiers (updraft, downdraft, and crossdraft) are the simplest of all the types of gasifiers and are mainly suitable for small-scale applications (<10 MWth) [5]. Although they (fixed bed gasifiers) are very advantageous in terms of their simplicity and ease of operation, they generally suffer from poor mixing and poor heat transfer within the gasifier, which makes it difficult to achieve even distribution of fuel and temperature across gasifier geometry hence scale-up of this type of gasifier is difficult. The fixed bed downdraft gasifier, which has not satisfactorily performed with feedstock capacity beyond 425 kg/h [18], is a typical example of the described technical issue. This is because air cannot travel up the center of the gasifier, which creates cold spots in and around the combustion zone of the gasifier during operation and results in reduced gasification efficiency. This limitation has been attributed to design characteristics in terms of gasifier geometry (throat angle and throat diameter) and air inlet velocity. In the case of the updraft gasifier however, its high tar production rate (5–20%) [19] remains a challenge to date and renders this type of gasifier unsuitable where a clean product gas (syngas) is desired. Due to its high tar production rate, the updraft gasifier is well-suited for the gasification of low-volatile feedstocks like charcoal [5].
5.2 Limitations of the fluidized bed gasifier
In terms of configuration, the fluidized bed gasifier (FBG) operates on the principle of fluidization where a gas stream is forced through a particle bed vessel that behaves like a fluid under certain conditions such as high particle flow velocity. The commonly used fluidization media include air, steam, or mixtures of steam/oxygen. The FBG is the most efficient of all types of gasifiers and its efficiency is mainly dependent upon the thermochemical and fluid behavior inside the gasifier; this type of gasifier is more appropriate for medium-scale units of about 5–100 MWth [5, 20, 21]. From a system performance and technical point of view, the operation of the FBG is quite complex because of the need to simultaneously control air supply, bed material, and feedstock during operation of the gasifier. As a result, the product gas obtained from the gasification process may be very high in particulates, which can circulate and cause equipment erosion. Although it may sway the gasification process, the FBG is operated at high-pressure conditions, which can result in low volumetric gas flow rates, condensation during compression, and other operational complications such as defluidization from particle agglomeration particularly when agricultural crops and wastes are used as feedstock in the gasification process. This is because agricultural crops and wastes contain an increased amount of ash/alkali and, the alkali content of ash (such as sodium and potassium alkali) can form low-melting eutectics with the silica in the sand, which is the regularly used bed material in FBG processes [22]. Under this condition, agglomeration and sintering will occur, triggering the formation of a thin sticky substance around the bed particles with an instant loss of bed fluidization (defluidization). Typical factors influencing agglomeration and the loss of fluidization in FBGs are presented in Table 2.
Parameter
Agglomeration and loss of fluidization (defluidization)
Temperature
The possibilities of agglomeration and defluidization are exacerbated by rising temperatures.
Steam
Agglomeration and defluidization can occur upon increase in steam during gasification due to the formation of molten sodium disilicate, which can occur via liquid-solid reaction under steam application conditions.
Alkalis, iron sulfides, and siderite
Increases the possibilities of the formation of sticky substances, which can, in turn, facilitate agglomeration and defluidization.
Fluidization velocity
The tendencies of agglomeration are lowered below the sintering temperature of ash when the velocity of fluidization is increased. The force of segregation also increases under this circumstance.
Particle size distribution
The possibilities of agglomeration and defluidization are high when bimodal or multimodal particle size distribution occurs.
Table 2.
The summary of the impact of operating parameters on agglomeration and loss of fluidization [23, 24].
Even if more sophisticated bed materials such as alumina and magnesite are used in the FBG process of feedstocks with high ash/alkali content, process cost will become an issue of concern. These types of technical issues call to question the feedstock flexibility of the FBG systems.
5.3 Limitations of the entrained flow gasifier
The entrained flow gasifier (EFG) is an old alternative energy production technology used on a large-scale (>50 MWth) [5] in the petroleum industry for the gasification of petroleum residues. This type of gasifier offers greater rates of collision between solid particles and is considered excellent in terms of performance because of vigorous mixing of feedstock and oxidizing agent as well as better feed conversion efficiencies in comparison to other types of gasifiers [25]. However, even though the EFG has been in existence for centuries, it has not been exploited to its full potential partly because the fundamental principles underpinning its operation are still vague, particularly with regards to the type of material suitable as feedstock. The mechanisms involved in the feedstock conversion process are still under debate. In addition, the EFG is operated at very high temperatures (1,200 – 2,000°C) and pressures, under these operating conditions, fuel-oxygen mixtures are turned into a turbulent flame of dust that ensures the production of liquid ash, which are deposited on gasifier walls. This constitutes a technical issue of concern, particularly when analyzing the ash melting behavior of the material used as feedstock in the gasification process. Due to this high operating pressure, numerical modeling and experimental validation of the EFG tend to be onerous. Furthermore, due to its operating conditions, only specific types of materials are used as feedstock.
Extensive studies have been undertaken on gasification technology over the last decade. Despite the numerous studies, however, there are still pending research-related issues (such as those described in preceding sections) that require further improvements. For example, Kaushal et al. [26] developed a one-dimensional steady-state model specific to the bubbling fluidized bed gasifier (BFBG). Gómez-Barea et al. [27] also reviewed the performance optimization of a small-scale FBG plant with the aim of maximizing char conversion rate and minimizing secondary gas treatments. The process performance of the downdraft gasifier was evaluated by Biagini et al. [28] in which the performance parameters such as syngas production, syngas heating value, cold gas efficiency, and the net efficiency of the gasifier were monitored using corn cobs as feedstock. Furthermore, the performance of a pilot-scale pressurized entrained-flow (EFG) plant using stem wood made from pine and spruce as feedstocks was assessed by Weiland et al. [29]. A combined system involving gasification, hydrothermal carbonization (HTC), and solid oxide fuel cell (SOFC) technologies was developed by Papa et al. [30] using commercial process simulation software (ASPEN Plus), where the focus was to investigate the efficiency of the system under various operating conditions. The challenges and opportunities of modeling the gasification technology using Aspen Plus were also detailed by Mutlu and Zeng who alluded to the issues of the gasification technology as hindering the widespread commercialization of the technology [31].
7. Research priority areas and solutions to the identified technical issues
The FBGs such as the downdraft gasifier is characterized by four distinct reaction zones including the drying, pyrolysis, combustion, and reduction zones respectively; the specific functions of each of these zones are described in [32]. Of these distinct reaction zones, the combustion zone, also known as the oxidation zone, is considered the most important zone because heat is generated in this zone. However, the presence of cold spots (a factor linked to uneven heat distribution in and around the combustion zone of the downdraft gasifier), is the main reason why these types of gasifiers are limited to small-scale applications [13]. There are basically two methods that can provide a solution to the problem of uneven heat distribution in fixed bed systems: one method is to decrease the cross-sectional area of the gasifier at a certain height. This means altering the design characteristics of the throat angle and throat diameter of the gasifier by way of size-reduction. The other method is to centralize the air inlet and its velocity using nozzles that are positioned in a way that allows the throat circumference of the gasifier to be captured.
In the case of the FBGs, although a well-established technology (in terms of design concept) for heat and power generation, bed defluidization, as indicated in a previous section, is considered the main technical issue, which as previously described, occurs due to agglomeration and pressure drops, particularly when gasifying feedstocks with high amounts of ash such as agricultural residues and wastes. Alkali silicates such as calcium, potassium, and sodium silicates present in ash can form low-melting eutectics with silica, which is often used as the bed material in FBGs [22]. A quick and easy solution to the defluidization problems in FBGs is to replace the commonly used bed material (silica) with more advanced artificial materials such as aluminum oxide or magnesium carbonate. However, the cost associated with the use of these materials may constitute a major drawback. Therefore, the hydrodynamics of the FBG needs to be further investigated and the hydrodynamic study must incorporate devolatilization kinetics, char gasification, and gas species in relation to particle agglomeration and sintering.
For the high-pressure EFG, the production of molten ash (which mostly originates from the ash constituents of the feedstock and forms deposits on the walls of the gasifier) is a commonly encountered technical problem. Depending on the operating conditions of the gasification process, the molten ash deposits often solidify, causing plugging and the blockage of critical parts of the gasifier thereby hindering process efficiency. Therefore, just like the FBG, a solution to the problem of molten ash formation in the EFG is to further investigate the feedstock conversion mechanism and gasifier hydrodynamics, particularly when more complex low-grade feedstocks such as agricultural residues and biomass-based chars are used in the gasification process under high-pressure conditions.
Studies [33, 34] have shown that modeling work has accelerated the research progress made in the field of biomass gasification since gasifier design and operating conditions can be optimized at minimal time and costs. However, modeling and simulation cannot replace good experimental investigations. In fact, studies [35] have determined that mathematical modeling and simulation of high temperature and pressure reaction systems involving gaseous, liquid, and solid phases is a major scientific challenge. Therefore, addressing the technical issues of the gasification technologies described in this chapter will not only require the development of a robust and sophisticated model that can be applied to a wider range of operating parameters of the gasifiers but also able to replicate actual operations of the gasification technologies with an acceptable level of anomaly.
Gasifier design and process optimization for complex biomass feedstocks, in general, are very challenging due to the lack of detailed understanding of the various thermochemical reaction steps governing the conversion of biomass feedstocks under high temperature and pressure conditions. The gasification technologies described in this chapter are multiphase systems that are characterized by complex operational steps. Therefore, in order to better comprehend the complex interactions between process steps during gasification and to address the technological issues earlier described, experimental studies under systematic variation of feed specification and process parameters are required. It is also necessary to ensure proper process mapping based on experimental data from lab- to pilot-scale in order to develop a comprehensive gasification process understanding and to provide a thorough data basis for the validation of numerical simulations. This implies the development of state-of-the-art experimental techniques that are applicable under the acrid conditions of gasification technologies.
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Written By
Ali Mohammadi and Anthony Anukam
Submitted: 24 December 2021Reviewed: 11 January 2022Published: 10 February 2022
Open access peer-reviewed chapter
