4. Types and applications of gasification systems

Numerous researchers are looking into alternative ways to make use of low-grade biomass and wastes by avoiding the need even to produce bio-oils while still pyrolyzing and gasifying these materials [59–68]. One such system is the Viking gasifier [61] that is a two-stage process to crack the tars thermally. This system works by a screw pyrolysis system that produces hot vapors and char at a temperature of 500–600C at the top of the gasifier to be partially oxidized and tar fractions to be broken down into syngas. The char from the pyrolysis unit is transferred into fixed bed of the gasifier to act as a tar cracking unit where further tar cracking occurs. This system is reported to have a nominal tar content in syngas as low as 15 mg/m3 . Tar sampling has been performed in various stages of this process in another study [1, 59–68], which shows a progressive decrease in tar levels starting from the pyrolysis stage to partial oxidation to gasification stages and decreasing to 5 mg/m3 tar levels.

#### 4.1. Fluidized bed gasifiers

3.2. Trends in waste gasification

104 Gasification for Low-grade Feedstock

gasification within EU.

[45, 54, 59].

23 g/m3

As highlighted earlier, MSW has vast variations in material composition, particle size, density, calorific value, contamination and ash and moisture content composition. Due to these variations in MSW, waste feedstock after sorting valuable components such as metals, plastics, and paper is standardized into solid recovered fuel (SRF) or refuse-derived fuel (RDF). This standardization has led to waste being a commodity and is sold in the international market. Also, to reduce transportation costs, SRF and RDF are pelletized to increase energy density. The international movement of waste is widespread among the EU countries [52]. The huge size of installed incineration capacity within EU attracts waste movement on the mass scale. In continental Europe, cement kilns are regularly co-fired with RDF as these facilities have sophisticated emissions cleaning equipment. However, due to Waste Incineration Directive, there is an increasing shift towards advanced conversion treatment such as pyrolysis and

There is an increasing policy shift towards sustainable energy production at reduced carbon footprint. For example, the United Kingdom (UK) of Great Britain under its Climate Change Act 2008 committed to reducing its 80% of carbon emissions to 1990 level by 2050 [53]. The UK government has provided subsidies in the form of Renewables Obligation Certificates (ROC) for biomass waste utilization contained within the residual waste to support the low carbon energy production. This has created a big shift towards the UK becoming a market leader in WtE. There are currently 14 such WtE plants, mostly based on gasification technology, are

Tars are defined by Milne et al. [54] as "the organics produced under thermal or partialoxidation regimes (gasification) of any organic material are called "tars" and are assumed to be largely aromatic." Other researchers have described tars as a very complex mixture of aromatic and oxygenated hydrocarbons having a molecular weight higher than that of benzene of [55–57]. Benzene and other heavier molecular weight compounds are present in pyrolysis bio-oil, and their presence in syngas tends to cause problems. As previously said, intermediate pyrolysis is based around the concept of encouraging secondary reactions between the evolved vapors from biomass and resulting char. Some tars present in the biooil can have their molecular weight up to 500 Daltons [58]. The presence of these very high molecular weight tars in bio-oil and syngas lead to incomplete combustion when these fuels are used. High molecular weight tars act as promoters of high viscosity, and limit the atomization of the fuel, and cause blockages in fuel pipes and injector lines by condensation

Tar levels as reported by Milne et al. [54] exhibit a wide range of various gasification processes. For example, updraft gasifier tar content in the raw syngas is reported between 1 and 150 g/m<sup>3</sup>

Whereas, in a downdraft, it is 0.04–6 g nominally, and in the fluidized bed gasifiers it is 0.1–

syngas for multiple applications such as for engines, turbines, fuel cells, and compressors. It is

. Milne et al. [54] also reported various tar tolerance levels from various authors in the

operational in the UK alone, and much more are under construction [52].

3.3. Tars and their associated problems in gasification

Fluidized bed gasifiers are used for commercial applications >5 MW as they are only economical at large scale as compared to downdraft gasification that is economical up to 5 MW maximum. Such systems consist of a vessel with an air distributor nozzle assembly at the bottom of the vessel. Feedstock enters the bed and finely ground bed material is fluidized by air or oxidizing agent. The temperature of the bed in the gasifier is regulated by the air/feedstock ratio within 700–900C. Biomass is thermally broken down into gaseous compounds, and char is produced. The hot char and fluidizing bed material cause further reactions to break long-chain hydrocarbons or tars into syngas components. Thus, a syngas product with very low tar content is produced with tar content less than 3 g/Nm3 .

Advantages of fluidized bed gasifiers include uniform syngas product composition, uniform temperature distribution throughout the gasifier, rapid heat transfer between the feedstock, bed material, and oxidant. It is also possible to achieve high conversion efficiency and low tar content in the syngas. The effectiveness of tar removal can be further enhanced by using catalytic bed materials such as olivine, dolomite, and other industrial nickel-based catalysts. Disadvantages include problems with low ash melting point materials and large bubble size bypassing the bed [43]. Examples of these gasifiers include Royal Dahlman [69] bubbling fluidized bed gasifier and Gussing circulating fluidized bed gasifier [70].

#### 4.1.1. Type 1 gasifier

.

Two-stage close coupled gasification/combustion is used within the fluidized bed reactor with a multi-level injection of air. Primary air/oxygen injection in gasification level and secondary air injection is used in the combustion level for complete combustion of gases. Hot gases exiting the reactor are fed into a steam generator to produce steam which then drives a steam turbine to generate electricity. Minimal gas cleaning takes place in this type of gasifier configuration. This process layout suits steam turbines and usually have low overall efficiency up to 20%. Example include Energos process [71] where the objective is to reduce capital costs of downstream gas cleaning equipment.

gasification as most of the thermal energy is coming from external energy source rather than through exothermic reactions between the fuel and oxygen. Oxygen is only used to convert the fuel into syngas [75]. Commercial scale examples of operating plants are Advanced Plasma

Recent Trends in Gasification Based Waste-to-Energy http://dx.doi.org/10.5772/intechopen.74487 107

Although waste gasification plants are mostly feeding the syngas to turbine or engine for electricity, however, there is a significant interest in using the ultra-clean syngas in hightemperature solid oxide fuel cells (SOFC) [77]. This is because CO being the main constituent of the syngas does not poison the fuel cell electrodes as it happens to be in polymer electrolyte fuel cells. The interest is due to the high electricity conversion efficiency that can be achieved by using fuel cells. However, so far most of the development for fuel cell application is based on individual biomass feedstocks as opposed to mixed waste feedstocks. The sulfur poisoning of SOFC is one big hurdle that needs to be overcome for waste gasification. SOFC usually are tolerant to sulfur content in parts per billion (ppb) range. Nagel et al. [78] have studied the biomass integrated syngas fuel cell setup with an electrical power output of 1 MWe. In another development, Lobachyov and Richter [79] explored the integration of a biomass-fuelled gasifier to Molten Carbonate fuel cell (MCFC). With the trends of waste gasification leading to producing ultra-clean tar-free syngas, the quality of syngas permits to use it in high-temperature fuel cells. The need to clean the syngas with sulfur impurities down to ppb level is achievable through a separate sulfur removal process. Given the high electrical efficiency of fuel cells and excess heat available from such integrated system, it all lends to perfect combined heat and power (CHP) system through fuel

Separating oxygen from the air through pressure swing adsorption and cryogenic methods is all very well known for large-scale oxygen production. As the waste gasification is trending towards ultra-clean tar-free syngas with high calorific values, this demand can only be met by oxygen gasification rather than air. In all oxygen separation processes, the high costs of oxygen production are associated with high-pressure air, low-temperature cooling and or membranes. New techniques of oxygen separation from the air are emerging where oxygen can be separated using a ceramic ionic membrane separation at high temperatures [81]. Since gasification takes place at temperatures above 800C, this has attracted a lot of interest from industry to generate in-situ oxygen while gasifying the feedstocks. This mixed ionic-electronic conducting (MIEC) technology is based on dense ceramic membranes to separate the oxygen from air at temperatures around 800–900C [82]. Since these membranes are made up of ceramics, they can tolerate high temperatures and do not need electrodes for oxygen separation [83, 84]. These ion transport membranes (ITM) work on electronic conductivity principle that creates a short circuit that involves oxygen partial pressure gradient from high to low. Oxygen permeates from high partial pressure to low partial pressure side while the flux of electrons balances overall charge neutrality. Air

Power and Alter NRG (Westinghouse) [76].

cell application in urban areas [80].

4.4. In-situ oxygen separation from air for gasification

4.3. Waste gasification for fuel cell application

### 4.1.2. Type 2 gasifier

This kind of gasifier is usually better equipped as compared to type 1 gasifier. After waste feedstock entry into gasifier, air or oxygen is introduced to run the gasifier for converting waste into syngas. There is catalytic fluidized bed environment to crack tars, and the resulting syngas is then cooled to 400C and hot filtered and then combusted in the boiler to produce steam for electricity generation using a steam turbine. The excess heat is also recovered and used in district heating system. An example of this system is a Lahti gasification process [72].

## 4.1.3. Type 3 gasifier

Only this type of gasifier can deliver the future promise of meeting the need of syngas utilization for multi-modal products. This is due to the ultra-clean and tar-free syngas it can produce through various syngas tar cracking and polishing stages. In this type of gasifier setup, the waste feedstock is fed into the fluidized bed gasifier where oxygen or air/steam react with it in the presence of a catalyst bed such as dolomite or olivine to produce syngas. After removing solids through hot gas filtration, thermal tar cracking is performed by oxygen injection to raise the temperature (1200C) of the syngas. The higher calorific value of the syngas is maintained as nitrogen input through the air is kept minimal. The resulting syngas is then further cleaned and polished before being used either in the gas engine, gas turbine or in chemical synthesis. This syngas can be converted into Fischer Tropsch diesel, methanol or hydrogen. The tar free nature of syngas provides excellent future proofing potential for the product flexibility. The engine efficiency on this syngas can be as high as 35% [73], and with heat recovery or through methanation for synthetic natural gas (SNG) production, even higher efficiencies are easily achievable. An example of this system is Lurgi fluidized bed gasification system.

#### 4.2. Plasma gasification

Plasma gasification is preferred for mixed waste such as MSW or hazardous waste (asbestos and radioactive) where high temperatures are used to produce syngas and a melt arising from inorganic species of feedstock. A distinctive feature of plasma process resides in its ability to produce very high temperatures that are not achievable with conventional gasification and combustion; these high temperatures help to reduce tars and convert all the organic material into syngas. Tar content as reported by Refs. [54, 74] is shown to be 1000 times less than that of auto-thermal gasification processes. Arc discharges obtain thermal plasmas from DC or AC current or through radio frequency or microwaves. Mostly DC plasma technology is preferred for waste gasification plasma processes. Plasma is formed by high energy from AC or DC sources through the plasma torch close to the bottom of reactor and fuels are gasified through the plasma flames. The oxygen demand in this process is small as compared to conventional gasification as most of the thermal energy is coming from external energy source rather than through exothermic reactions between the fuel and oxygen. Oxygen is only used to convert the fuel into syngas [75]. Commercial scale examples of operating plants are Advanced Plasma Power and Alter NRG (Westinghouse) [76].

#### 4.3. Waste gasification for fuel cell application

exiting the reactor are fed into a steam generator to produce steam which then drives a steam turbine to generate electricity. Minimal gas cleaning takes place in this type of gasifier configuration. This process layout suits steam turbines and usually have low overall efficiency up to 20%. Example include Energos process [71] where the objective is to reduce capital costs of

This kind of gasifier is usually better equipped as compared to type 1 gasifier. After waste feedstock entry into gasifier, air or oxygen is introduced to run the gasifier for converting waste into syngas. There is catalytic fluidized bed environment to crack tars, and the resulting syngas is then cooled to 400C and hot filtered and then combusted in the boiler to produce steam for electricity generation using a steam turbine. The excess heat is also recovered and used in district heating system. An example of this system is a Lahti gasification process [72].

Only this type of gasifier can deliver the future promise of meeting the need of syngas utilization for multi-modal products. This is due to the ultra-clean and tar-free syngas it can produce through various syngas tar cracking and polishing stages. In this type of gasifier setup, the waste feedstock is fed into the fluidized bed gasifier where oxygen or air/steam react with it in the presence of a catalyst bed such as dolomite or olivine to produce syngas. After removing solids through hot gas filtration, thermal tar cracking is performed by oxygen injection to raise the temperature (1200C) of the syngas. The higher calorific value of the syngas is maintained as nitrogen input through the air is kept minimal. The resulting syngas is then further cleaned and polished before being used either in the gas engine, gas turbine or in chemical synthesis. This syngas can be converted into Fischer Tropsch diesel, methanol or hydrogen. The tar free nature of syngas provides excellent future proofing potential for the product flexibility. The engine efficiency on this syngas can be as high as 35% [73], and with heat recovery or through methanation for synthetic natural gas (SNG) production, even higher efficiencies are easily achievable.

Plasma gasification is preferred for mixed waste such as MSW or hazardous waste (asbestos and radioactive) where high temperatures are used to produce syngas and a melt arising from inorganic species of feedstock. A distinctive feature of plasma process resides in its ability to produce very high temperatures that are not achievable with conventional gasification and combustion; these high temperatures help to reduce tars and convert all the organic material into syngas. Tar content as reported by Refs. [54, 74] is shown to be 1000 times less than that of auto-thermal gasification processes. Arc discharges obtain thermal plasmas from DC or AC current or through radio frequency or microwaves. Mostly DC plasma technology is preferred for waste gasification plasma processes. Plasma is formed by high energy from AC or DC sources through the plasma torch close to the bottom of reactor and fuels are gasified through the plasma flames. The oxygen demand in this process is small as compared to conventional

An example of this system is Lurgi fluidized bed gasification system.

downstream gas cleaning equipment.

4.1.2. Type 2 gasifier

106 Gasification for Low-grade Feedstock

4.1.3. Type 3 gasifier

4.2. Plasma gasification

Although waste gasification plants are mostly feeding the syngas to turbine or engine for electricity, however, there is a significant interest in using the ultra-clean syngas in hightemperature solid oxide fuel cells (SOFC) [77]. This is because CO being the main constituent of the syngas does not poison the fuel cell electrodes as it happens to be in polymer electrolyte fuel cells. The interest is due to the high electricity conversion efficiency that can be achieved by using fuel cells. However, so far most of the development for fuel cell application is based on individual biomass feedstocks as opposed to mixed waste feedstocks. The sulfur poisoning of SOFC is one big hurdle that needs to be overcome for waste gasification. SOFC usually are tolerant to sulfur content in parts per billion (ppb) range. Nagel et al. [78] have studied the biomass integrated syngas fuel cell setup with an electrical power output of 1 MWe. In another development, Lobachyov and Richter [79] explored the integration of a biomass-fuelled gasifier to Molten Carbonate fuel cell (MCFC). With the trends of waste gasification leading to producing ultra-clean tar-free syngas, the quality of syngas permits to use it in high-temperature fuel cells. The need to clean the syngas with sulfur impurities down to ppb level is achievable through a separate sulfur removal process. Given the high electrical efficiency of fuel cells and excess heat available from such integrated system, it all lends to perfect combined heat and power (CHP) system through fuel cell application in urban areas [80].

#### 4.4. In-situ oxygen separation from air for gasification

Separating oxygen from the air through pressure swing adsorption and cryogenic methods is all very well known for large-scale oxygen production. As the waste gasification is trending towards ultra-clean tar-free syngas with high calorific values, this demand can only be met by oxygen gasification rather than air. In all oxygen separation processes, the high costs of oxygen production are associated with high-pressure air, low-temperature cooling and or membranes. New techniques of oxygen separation from the air are emerging where oxygen can be separated using a ceramic ionic membrane separation at high temperatures [81]. Since gasification takes place at temperatures above 800C, this has attracted a lot of interest from industry to generate in-situ oxygen while gasifying the feedstocks. This mixed ionic-electronic conducting (MIEC) technology is based on dense ceramic membranes to separate the oxygen from air at temperatures around 800–900C [82]. Since these membranes are made up of ceramics, they can tolerate high temperatures and do not need electrodes for oxygen separation [83, 84]. These ion transport membranes (ITM) work on electronic conductivity principle that creates a short circuit that involves oxygen partial pressure gradient from high to low. Oxygen permeates from high partial pressure to low partial pressure side while the flux of electrons balances overall charge neutrality. Air Products and Chemicals and Praxair have developed ionic transport based ceramic membranes driven by high process temperature. In an ideal environment, compressed air at 7– 20 Bar is heated in-situ to the gasifier where heat is applied from the gasifier to enable electronic conductivity and oxygen transport from high partial pressure atmosphere to lowpressure atmosphere. Oxygen production in this way will only require compressed air at moderate to low pressures, and the remaining energy is supplied from gasifier process heat generated by exothermic reactions.

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