The gasification of any carbonaceous or hydrocarbonaceous material is, essentially, the conversion of the carbon constituents by any one of a variety of chemical processes to produce combustible gases. The process includes a series of reaction steps that convert the feedstock into synthesis gas (syngas, carbon monoxide, CO, plus hydrogen, H2) and other gaseous products. This conversion is generally accomplished by introducing a gasifying agent (air, oxygen, and/or steam) into a reactor vessel containing the feedstock where the temperature, pressure, and flow pattern (moving bed, fluidized, or entrained bed) are controlled. The gaseous products – other than carbon monoxide and hydrogen – and the proportions of these product gases (such as carbon dioxide, CO2, methane, CH4, water vapor, H2O, hydrogen sulfide, H2S, and sulfur dioxide, SO2) depends on the: (1) type of feedstock, (2) the chemical composition of the feedstock, (3) the gasifying agent or gasifying medium, as well as (4) the thermodynamics and chemistry of the gasification reactions as controlled by the process operating parameters. In addition, the kinetic rates and extents of conversion for the several chemical reactions that are a part of the gasification process are variable and are typically functions of: (1) temperature, (2) pressure, and (3) reactor configuration, and (4) the gas composition of the product gases and whether or not these gases influence the outcome of the reaction.

It is the purpose of this chapter to present descriptions of the various reactions involved in gasification of carbonaceous and hydrocarbonaceous feedstocks as well as the various thermodynamic aspects of these reactions which dictate the process parameters used to produce the various gases.

Keywords

Chemical concepts; pretreatment; reactions; primary gasification; secondary gasification; water gas shift reaction; carbon dioxide gasification; hydrogasification; methanation; products; effect of process parameters; thermodynamics; kinetics

1 Introduction

The gasification of any carbonaceous or hydrocarbonaceous material is, essentially, the conversion of the carbon constituents by any one of a variety of chemical processes to produce combustible gases (Higman and van der Burgt, 2008; Speight, 2008, 2013a). With the rapid increase in the use of gasification technology from the 19th century onwards it is not surprising the concept of producing a flammable gas for domestic heating, industrial heating, and power generation became common-place in the 19th and 20th centuries (Speight, 2013a, 2013b).

Gasification includes a series of reaction steps that convert the feedstock into synthesis gas (carbon monoxide, CO, plus hydrogen, H2) and other gaseous products. This conversion is generally accomplished by introducing a gasifying agent (air, oxygen, and/or steam) into a reactor vessel containing the feedstock where the temperature, pressure, and flow pattern (moving bed, fluidized, or entrained bed) are controlled. The gaseous products – other than carbon monoxide and hydrogen – and the proportions of these product gases (such as carbon dioxide, CO2, methane, CH4, water vapor, H2O, hydrogen sulfide, H2S, and sulfur dioxide, SO2) depend on: the (1) type of feedstock, (2) the chemical composition of the feedstock, (3) the gasifying agent or gasifying medium, as well as (4) the thermodynamics and chemistry of the gasification reactions as controlled by the process operating parameters (Singh et al., 1980; Pepiot et al., 2010; Shabbar and Janajreh, 2013; Speight, 2013a, 2013b). In addition, the kinetic rates and extents of conversion for the several chemical reactions that are a part of the gasification process are variable and are typically functions of: (1) temperature, (2) pressure, (3) reactor configuration, and (4) the gas composition of the product gases and whether or not these gases influence the outcome of the reaction (Johnson, 1979; Penner, 1987; Müller et al., 2003; Slavinskaya et al., 2009; Speight, 2013a, 2013b).

Generally, the reaction rate (i.e., the rate of feedstock conversion) is higher at higher temperatures, whereas reaction equilibrium may be favored at either higher or lower temperatures depending on the specific type of gasification reaction. The effect of pressure on the rate also depends on the specific reaction. Thermodynamically, some gasification reactions such as the carbon−hydrogen reaction to produce methane are favored at high pressures (>1030 psi) and relatively lower temperatures (760 to 930°C; 1400 to 1705°F), whereas low pressures and high temperatures favor the production of synthesis gas (i.e., carbon monoxide and hydrogen) via the steam or carbon dioxide gasification reaction.

Because of the overall complexity of the gasification process, it is necessary to present a description of the chemistry of the gasification reactions and it is the purpose of this chapter to present descriptions of the various reactions involved in gasification of carbonaceous and hydrocarbonaceous feedstocks as well as the various thermodynamic aspects of these reactions which dictate the process parameters used to produce the various gases.

2 Chemical Concepts

Chemically, gasification involves the thermal decomposition of the feedstock and the reaction of the feedstock carbon and other pyrolysis products with oxygen, water, and fuel gases such as methane (Table 2.1). In fact, gasification is often considered to involve two distinct chemical stages: (1) devolatilization of the feedstock to produce volatile matter and char, followed by (2) char gasification, which is complex and specific to the conditions of the reaction – both processes contribute to the complex kinetics of the gasification process (Sundaresan and Amundson, 1978).

Gasification of char in an atmosphere of carbon dioxide can be divided into two stages: (1) pyrolysis and (2) gasification of the pyrolytic char. In the first stage, pyrolysis (removal of moisture content and devolatilization) occurs at a comparatively lower temperature. In the second stage, gasification of the pyrolytic char is achieved by reaction with oxygen/carbon dioxide mixtures at high temperature. In nitrogen and carbon dioxide environments from room temperature to 1000°C (1830°F), the mass loss rate of pyrolysis in nitrogen may be significantly different (sometimes lower, depending on the feedstock) from mass loss rate in carbon dioxide, which may be due (in part) to the difference in properties of the bulk gases.

Using coal as an example, gasification in an atmosphere of oxygen/carbon dioxide is almost the same as gasification in an atmosphere of oxygen/nitrogen at the same oxygen concentration but this effect is slightly delayed at high temperature. This may be due to the lower rate of diffusion of oxygen through carbon dioxide and the higher specific heat capacity of carbon dioxide. However with an increase in the concentration of oxygen, the mass loss rate of coal also increases and, hence, shortens the burn out time of coal. The optimum value of oxygen/carbon dioxide for the reaction of oxygen with the functional groups that are present in the coal feedstock is on the order of 8% v/v.