A variety of gasifier designs have been developed depending upon the nature of the gasification process involved, nature of feedstock used, scale of operation, and the product specifications required. These designs can be classified either on the basis of the manner in which the feedstock is handled in the gasifier or, on the basis of the manner in which heat is supplied to the gasifier. The first category of gasifiers are the fixed-bed gasifiers, fluidized-bed gasifiers, and the entrained flow gasifiers. The salient features of each of these are summarized in Table 1.6. The oxidation reactions taking place in the gasifier are generally exothermic reactions.

In most gasifiers the energy released as a result of these reactions is used to serve a dual purpose: first, to fuel the endothermic reactions taking place in the gasifier, and second, to maintain the high temperatures required in the gasifier. Such a gasification process, in which the heat released in one portion of the gasifier is partly or fully utilized to propel other endothermic gasification reactions taking place in the equipment, is a called direct gasification process. If no oxidizing agent is added, there will be no exothermic reactions taking place in the gasifier, and the heat required for the gasification processes will have to be supplied from some external source of heat. Such systems where the heat requirements for the gasification process are supplied externally are called indirect gasification systems or allothermal gasification. Figure 1.14 shows a schematic diagram of the direct and indirect gasification sys­tems. As the direct gasifiers use a part of their input stream to drive other reactions taking place in the system, the overall efficiency of such systems is reduced.

On the other hand, as the indirect gasifiers use an external source of energy for the purpose, such gasifiers are expected to be more energy efficient, especially if solar energy is used as the source. Use of sunlight to drive an endothermic gasi­fication reaction increases the calorific value of the initial biomass, with an added advantage of being a renewable source. An optimized design of such an indirect gasification system may even increase the energy content of the product stream beyond that of the feedstock. The gasification systems in which the heat producing processes or reactions are separated from the processes which consume heat are

Fluidized bed gasifier

• Uses inert material such as sand to

mix solid fuel with gas phase

• High operating temperatures

(1,000-1,200°C)

• Gasifier zones at microscopic levels

in individual particle

• Uniform temperature distribution

• Better solid-gas contact and heat

transfer rates

• Equipped with cyclone separators at

the top for removal of particulates from product

• Suitable for feedstocks with low ash

fusion temperature

• Ash removed as slag or dry

Updraft gasifier

• Can tolerate more moisture in feedstock

• Producer gas exits from top and at

lower temperature (130-150°C)

• Product contaminated with tars, oils and

particulate matter from incoming fuel

• Suitable for direct heating applications

only

Downdraft gasifier

• Radiant and conductive heat transfer

from lower pyrolysis and combustion zones provide heat for drying of biomass

• Properly designed and positioned

‘‘throat’’ increase velocity of gas and promote heat and mass transfer

• Gives least amount of tar

• Widely used for small-scale

applications
Cross-flow gasifier

• High temperature (>1,500°C) reached

in the combustion zone

• Reaction zone is small with low thermal

capacity

• Short start-up time and response time

• Tar production is low

• Generally used for gasification of

charcoal (with very low ash content)

• Suitable for small-scale biomass

gasification units Bubbling fluidized bed gasifier

• Exit gas temperatures usually

700-800°C

• Residence time is short

• Suitable for medium-sized units

(25MWth)

• Suitable for treated MSW biomass Circulating fluidized bed gasifier

• Provides long residence time

• Suitable for fuels with high volatiles

• Capacity of 60 MWth achieved

Table 1.6 (continued)

Entrained flow gasifier

• Operate at higher temperatures

(1,200-1,600°C) and higher pressures (2-8 MPa)

• High oxygen demand

• Require small and uniform particle

size distribution (<0.4 mm) in feedstock (not suitable for fibrous materials)

• High reactivities and high capacities

• Low higher hydrocarbons and low tar

formation

• Product low in methane content

hence, better suited for synthesis gas production

• High operating temperatures causes

ash to get converted into slag which may be corrosive

• Not suitable for high ash content

feedstocks

• Preferred for IGCC plants

Based on Direct gasifiers

method of • Use portion of product or input stream heat to drive gasification reactions

supply

Indirect gasifiers

• The combustion process is separated

from the gasification process

• Energy efficiency greater than direct

gasifiers

• Produces medium calorific value

product and flue gases

• Complete conversion of biomass is

possible

• High investment and maintenance

cost

Fig. 1.14 Schematic of direct and indirect gasification process

also categorized under indirect gasification systems. Such systems therefore con­sist of two reactors connected by an energy flow. Typically, the oxidation or combustion reactions which are exothermic in nature are separated from the pyrolysis and gasification reactions which require heat. The heat from the com­bustion reactor is provided to the gasification reactor by means of hot sand which is circulated between the two reactors. The different designs of gasifiers, along with their salient features, advantages and disadvantages, have been reviewed by a number of authors [10, 11, 17-19]. A summary of these is given in Table 1.6.

In addition to the gasifier designs summarized in Table 1.6, there are other gasifiers which are modifications of the existing designs. Transport gasifier, twin reactor system, and chemical looping gasifier designs are modifications of the circulating fluidized-bed gasifier. The transport gasifier is a hybrid of entrained flow and fluidized-bed gasifier systems. Its construction and process design attri­butes to it, a higher throughput, better mixing and consequently, higher heat and mass transfer rates. However, it is more suitable for gasification of coal; its suit­ability for gasification of biomass is yet to be proven [11]. The twin reactor system is a dual fluidized-bed gasifier where the combustion process in the gasification of biomass is separated from the gasification process using separate fluidized-bed reactors. Such a design prevents the dilution of the product mix by nitrogen which is released subsequent to air combustion in normal fluidized-bed gasification process. These are used for gasification of coal as well as biomass. Different industrial-scale units are described by Basu [11]. The chemical looping gasifier process is a relatively recent development in which a bubbling fluidized-bed gasifier is coupled with a circulating fluid bed regenerator to obtain a continuous stream of hydrogen from agricultural feedstock. In — situ sequestration of carbon dioxide, formed during the gasification process, is done by using calcium oxide which reacts with carbon dioxide to form calcium carbonate, which is then reconverted into calcium oxide in the circulating bed regenerator [20].

Plasma gasification technology is a newly developed self-sustaining technology which is used especially for conversion of municipal solid waste into electric power. It is relatively insensitive to the quality of feedstock which is the most desired feature for MSW processing. The garbage is converted into a finely shredded mass and is then fed into a plasma chamber which consists of a sealed stainless steel vessel which is filled with either ordinary air or nitrogen. A 650 V electric current is passed between two electrodes which tears off electrons from air to create plasma. The energy generated in the process is sufficient to disintegrate the garbage into its constituent elements, forming syngas along with other by-products depending on the nature of MSW used as feedstock. The syngas, which leaves the plasma chamber at a temperature of * 1,200°C is fed to a cooling system, where the heat transferred from the syngas to the cooling water generates steam, which can be used in a steam turbine to generate electricity. The gas, after appropriate clean-up, can be used either for the usual applications of syngas, or in a gas generator to generate electricity. Part of the electricity generated is used to generate plasma in the plasma chamber. Figure 1.15 shows a schematic of the process.

Hydrothermal gasification involves gasification of biomass in an aqueous medium using supercritical water, i. e., water at a temperature and pressure beyond the critical point of water (374.29°C and 22.089 MPa). The conventional thermal methods of biomass conversion are cost-effective only when the moisture content of biomass feedstock is low. However, certain biomass such as aquatic biomass and MSW may contain moisture even up to 90% weight basis. In such cases, either the biomass has to be dried separately or, process heat is used to dry the excess moisture, both of which reduce the efficiency of the process. Another alternative in such cases is the use of biochemical methods for biomass conversion. However, these methods suffer from a major disadvantage of being very slow, having a low efficiency and producing only methane and no hydrogen. In order to get hydrogen, steam reforming process is required to be carried out. In contrast, the hydrothermal gasification process is relatively rapid and can tolerate very high moisture contents without compromising on the efficiency of the process [21]. Supercritical water has some unique properties which make it suitable for biomass gasification:

— it causes rapid hydrolysis of biomass

— the intermediate reaction products, including gases have a high solubility

— single-phase reactions are possible, thus eliminating interphase barriers for mass transfer

— being non-polar, supercritical water is a good solvent for substances like lignin which show low solubility in ordinary water.

Fig. 1.15 Schematic of plasma gasification unit

Hydrothermal gasification causes splitting of the organic molecules present in the biomass by hydrolysis and oxidation reactions. The biomass gets broken down to methane, hydrogen, carbon monoxide, and carbon dioxide. The major advan­tages of hydrothermal gasification are that it is suitable for biomass with high moisture contents; the product formed is rich in hydrogen; char and tar formation is low; the tar that is formed gets cracked and dissolves in the supercritical water. In addition, automatic separation of the product gas from the liquid containing char and tar takes place which obviates the need for a separate gas cleaning process which is usually required for all the conventional gasification or, for that matter for all thermochemical conversion processes. Elements such as sulfur, nitrogen, hal­ogens, etc. leave the process along with the aqueous effluents. However, due to this, corrosion of the reactor is a major problem as the presence of water causes the elemental by-products to get converted into acids which can corrode the reactor. Products such as bio-oil, methanol, hydrogen and a range of chemicals including phenol can be obtained from hydrothermal gasification of biomass. Hydrothermal gasification is most suitable for processing of MSW, and other biomass having very high moisture content as the efficiency of this process is independent of moisture content.