Figure 9.19 shows a characteristic molecular structure of coal, featuring an aromatic carbon backbone and a wide range of bond strengths. General plant matter (as distinct from the fruiting bodies that are often used as food and are primarily composed of starch and sugar) is known as lignocellulosic biomass and is composed of a more or less even mixture of cellulose, hemicellulose, and lignin. As shown in Figure 9.20, cellulose and hemicellulose are both composed of polymers of oxygen-containing ring compounds linked by relatively weak carbon–oxygen bonds. Lignin, in contrast, is composed of small aromatic units connected in weakly linked branched structures. As coal and biomass particles are heated, the internal structure of the carbonaceous material undergoes internal molecular rearrangements. Many weakly bonded moieties break their connecting bonds to the main structure and form gas molecules that aggregate within the solid and, after building sufficient pressure, burst forth from the particle with substantial force. At the same time, some weakly bonded structures and some structures with intermediate-strength bonds pivot about their connecting bond and are able to form stronger, cross-linking bonds with neighboring regions of the structure. Thus there is an inherent competition between solid decomposition and char-forming reactions, with different characteristic activation energies and reaction times. As a consequence, the quantity and chemical composition of the volatile matter that is emitted from these fuels is highly dependent on the nature of the original fuel structure, the rate at which the particles are heated, and the final temperature attained by the particles. For large particles, this also means that the devolatilization process differs as a function of the internal radius of the particle, because the local heating rate varies with the distance from the surface of the particle, where heat is being applied.

The effect of heating rate on evolution of volatiles is most clearly evidenced in the case of woody biomass, which has been shown to have a volatile yield of greater than 90% when small particles are rapidly heated to 1200 °C and to have a volatile yield of only 65% when large particles are slowly heated to 500 °C in the commercial charcoal-making process.

With the importance of the devolatilization process to solid particle combustion and the complexity of the chemical and physical processes involved in devolatilization, a wide variety of models have been developed to describe this process. The simplest models use a single or multiple Arrhenius rates to describe the rate of evolution of volatiles from coal. The single Arrhenius rate model assumes that the devolatilization rate is first order with respect to the volatile matter remaining in the char [40]:

Fits of Eqn (9.43) to experimental data typically yield an effective activation energy of about 230 kJ/mol, which is consistent with the activation energy for rupturing an ethylene bridge between aromatic rings [41].

The single-rate approach defined by Eqn (9.43) adequately captures the increasing rate of devolatilization at higher temperatures, but fails to account for the observed change in the volatiles yield as a function of temperature. To capture this, Kobayashi et al. [42] proposed the use of two competing reaction paths with different activation energies and different volatile yields (with the higher activation energy path having a higher volatile yield). This “Kobayashi Model” is expressed schematically as

Another approach is known as the distributed activation energy model. This model recognizes that devolatilization occurs through many simultaneous reactions. To express this process in a mathematically tractable manner, these reactions are all presumed to be first order and to be describable by a continuous distribution of kinetic rates with a common preexponential and a defined distribution function of activation energy [43]:

If a Gaussian distribution is used for the activation energy, the distribution function has the form

Advances in the understanding of coal structure and its evolution during devolatilization have led to the development of several coal network models that give predictions of volatile mass evolved as well as predictions of the chemical speciation of the released volatiles [44–46]. Detailed speciation of volatiles is important for understanding NO_x and soot formation as well as flame ignition and stability. Extensions of these models to cover biomass fuels have recently been developed, although in some cases the dispersed alkali metal content (primarily potassium) in biomass plays a significant role in catalyzing the reaction steps during devolatilization, making accurate predictions difficult.

The earliest network model came to be known as the functional group depolymerization, vaporization, and cross-linking (FG-DVC) model [44]. The FG portion of the model considers that certain functional groups in the coal lead to the formation of light gas species on heating. The DVC portion of the model describes the deconstruction of the original macromolecular network through bridge-breaking reactions that yield tar and cross-linking reactions that produce char. The FG-DVC model relies heavily on data from thermogravimetric analysis (mass loss during heating) coupled with Fourier transform infrared spectrometer analysis of the evolved gases and uses Monte Carlo simulation of lattice statistics.

A significantly different approach to modeling coal devolatilization is known as chemical percolation devolatilization (CPD) [45]. This approach relies on a detailed description of the chemical structure of the coal, as provided by solid-state nuclear magnetic resonance (NMR) spectroscopy data, which characterizes the fraction of carbon atoms with different types of bonds. The NMR data can be used to infer the average molecular weight of fused aromatic clusters and the number of branching sites for the cluster. In the CPD approach, the NMR data are coupled with percolation statistics to describe the decomposition of the original array of aromatic ring clusters into side chains and reactive intermediates. The side chains convert to light gases and the reactive intermediates undergo a competitive reaction scheme to either form tar or generate light gases and char.

The last of the three major network models is known as FLASHCHAIN [46]. This approach uses a simplified description of the original coal structure as a linear chain, which decomposes according to chain statistics into a broad distribution of fragment sizes. The fragments are partitioned into volatile or condensed species according to mixture-phase equilibrium (i.e., flash distillation). Char formation occurs in the condensed phase with concurrent evolution of fixed gases. Input parameters in the original formulation of this approach included the coal ultimate analysis (i.e., atomic composition), NMR data, and extract yield, but later studies demonstrated that good predictability is still achieved by only using the ultimate analysis and basing the values of the other input parameters on correlation functions.

The ambient pressure has a complex effect on the devolatilization process. For one thing, in most practical situations an increase in ambient pressure will tend to increase the coal particle heating rate for a given reactor temperature. This effect, in and of itself, tends to increase the yield of tar (and therefore of total volatiles), for the reason discussed earlier. However, increasing the ambient pressure also shifts the vapor–liquid equilibrium of the tar species to smaller tar species (with higher vapor pressures) and thus tends to diminish the overall release of tar. Wire-mesh experiments with well-controlled particle heating rates show a significant reduction in the yield of tar and total volatiles as the pressure is increased. The rate of devolatilization, however, is nearly insensitive to pressure, as would be expected for unimolecular reaction processes.

The extent to which a given reactant, such as oxygen, is able to utilize this additional surface area depends on the difficulty in diffusing through the particle to reach the pore surfaces and on the overall balance between diffusion control of the burning rate and kinetic control. To broadly characterize these competing effects, three zones of combustion of porous particles have been identified, as shown in Figure 9.21. In Zone I the combustion rate is fully controlled by the surface reaction rate (kinetically controlled), because the diffusion rate of oxygen is so fast (relatively) that the oxygen concentration throughout the particle and through the particle's boundary layer is essentially equal to the bulk gas oxygen content. Zone I combustion is favored by low temperatures (reducing the surface reaction rate) and small particle sizes (enhancing the surface-specific diffusive flux to the particle). In Zone II the combustion rate is determined by the combined effects of the surface reaction rate and the diffusion of oxygen (both to the particle and through the interior of the particle). The large internal surface area of most char particles coupled with a wide range of pore sizes and different degrees of access to the pores means that this zone is active over a wide range of combustion conditions. In Zone III, the surface burning rate is so fast that oxygen does not effectively penetrate the particle before being consumed. This is diffusion-limited combustion, as was described in Section 9.4. Zone III combustion is favored by high temperatures (increasing the surface burning rate) and large particles (reducing the surface-specific diffusive flux). In one sense, Zone I and Zone III modes of combustion are simply limiting extremes of the general condition of Zone II combustion.

Zone I combustion proceeds at an overall rate equal to the product of the intrinsic burning rate, evaluated at the ambient oxygen concentration, and the total internal surface area. The char diameter necessarily stays constant and the particle density continually decreases as particle mass is evenly removed throughout the particle on the pore surfaces (constant-diameter combustion).

In Zone III combustion, the burning rate is determined by the diffusive flux of oxygen through the particle boundary layer. The particle density remains constant throughout burnout and the particle size continually decreases as mass is removed solely from the particle surface (constant-density combustion).

Zone II combustion proceeds with partial penetration of oxygen, resulting in simultaneous variations in particle density and diameter as the pores closest to the particle surface undergo oxidation, in addition to the external surface of the particle. The ratio of the actual burning rate to the maximum possible rate if the entire particle was subject to the oxygen partial pressure at the external particle surface is known as the effectiveness factor.

A standard commercial pulverized coal grind usually is required to have 70 wt% of the particles with a size less than 200 mesh (74 μm) and less than 2% over a 50-mesh size (300 μm). With these small particle sizes and the high temperatures within pulverized coal boilers, the coal char particles generally burn in Zone II with combined control of diffusion and chemical reaction. Traditionally the combustion kinetics of pulverized coal have been described through a global particle kinetics approach which calculates the particle reaction rate based on the external surface area of the particle and the concentration of oxygen at the particle surface (thereby accounting for the diffusion profile of oxygen through the particle boundary layer). The dependence of the reaction rate on the partial pressure of oxygen is usually expressed as a power law, yielding a rate expression that is referred to as an “nth-order, Arrhenius” apparent kinetic expression:

Because of the variability in deduced reaction orders for different experiments and carbon types, a general expression for the kinetic rate that includes the oxygen dependence could not be determined.

In reality, it is believed that the oxidation of carbonaceous surfaces occurs through adsorption of oxygen, either immediately releasing a carbon monoxide or carbon dioxide molecule or forming a stable surface oxygen complex that may later desorb as CO or CO₂. Various multistep reaction schemes have been formulated to describe this process, but the experimental and theoretical information available to date has been insufficient to specify any surface oxidation mechanism and associated set of rate parameters with any degree of confidence. As an example, Mitchell et al. [50] has proposed the following surface reaction mechanism:

Analogous to solid fuel boilers, gasifiers can be operated either as fluidized bed devices with relatively large fuel particles, moderate temperatures, and long particle residence times, or as entrained flow devices with pulverized fuel particles, high temperatures, and short residence times. Either air or oxygen can be used as the primary oxidant gas, although steam is generally added to assist in char gasification, according to the following reaction:

In addition to char gasification from steam, CO₂ produced locally from consumption of oxygen in the gasifier reacts with char according to the Boudouard reaction (previously shown). The generation of CO and CO₂ from the oxygen consumed in the gasifier releases the energy required to devolatilize the raw fuel particles and gasify the resultant char through the endothermic reactions with steam and CO₂.

Air-blown gasifiers benefit from a low-cost (free) source of oxidant, but produce a gasifier product gas that has a very low heating value (approximately 5 MJ/m³ for air-blown gasification of coal, compared to 38 MJ/m³ for a typical natural gas) and which is too dilute to use for liquid fuel synthesis. Oxygen-blown gasifiers yield a moderate heating value product gas (approximately 9 MJ/m³) and can operate at very high temperatures, but suffer from the significant cost of producing oxygen with an air-separation unit.

Because CO₂ and H₂O react much slower with char than O₂ reacts with char, gasifiers usually operate at elevated pressure (to increase the collision rate of the gasifying species with the char). For IGCC systems, operating the gasifier at elevated pressure eliminates the need for additional compression before injection of the product gases into the gas turbine combustor. Biomass gasifiers are typically operated at atmospheric pressure because of the difficulty of feeding biomass particles into a pressurized gasifier and because the high biomass volatile content and high reactivity of biomass chars obviate some of the need for elevated pressures to obtain good carbon conversion.

An alternative technology to gasification for power production with carbon capture and sequestration is to burn the solid fuel in oxygen, instead of air. To moderate the combustion temperatures and avoid the formation of molten, slagging deposits of mineral matter on the boiler tubes, the oxygen is usually diluted with recycled flue gas before entering the combustion chamber. The very high concentration of carbon dioxide in the exhaust gas from this process allows fairly simple capture of CO₂ (the major additional financial cost and efficiency loss in this system result from the oxygen generation process). In addition, compression of the flue gas for CO₂ transport allows capture of pollutant species, making this a near-zero emission process. In contrast to the option of installing new gasification power plants, the oxy-fuel combustion technology is generally believed to be retrofitable to existing boiler systems, thereby taking advantage of the large capital investment that has already been made in these boilers.

A critical consideration in oxy-fuel combustion systems is the extent of flue gas recirculation that is desired. From a cost perspective, one would like to minimize the amount of recirculation, thereby increasing the oxygen concentration of the gases entering the boiler. However, the volatile flame temperature and char combustion temperature both increase substantially with higher oxygen content. Counteracting this influence of increasing oxygen content is the substitution of CO₂ for N₂ as the predominant gas in the boiler. Carbon dioxide has a molar specific heat that is 1.7 times larger than that of molecular nitrogen. Therefore, combustion in CO₂ environments results in lower flame temperatures than combustion in equivalent N₂ environments. Figure 9.23 shows the computed adiabatic flame temperature for stoichiometric combustion of methane in mixtures of oxygen with nitrogen and oxygen with carbon dioxide. As is evident in this figure, combustion of methane in 32% O₂ in CO₂ produces the same flame temperature as combustion in dry air (with 21% O₂ in N₂). A further, important consideration for boiler operation is the thermal radiation emitted by hot CO₂ in the furnace—in contrast to N₂, CO₂ emits thermal radiation in the infrared and can impact the balance between radiant and convective heat transfer in a boiler. In addition, the flue gas recycle substantially increases the water vapor content of the furnace gases, thereby enhancing thermal radiation from hot H₂O.

For application to combustion of solid fuels, the presence of high concentrations of CO₂ decreases the volatile flame temperature (for a given O₂ level) because of the higher specific heat of CO₂. Also, CO₂ decreases the volatile combustion rate because of the reduced diffusivity of O₂ through CO₂ atmospheres (the diffusivity is about 20% lower). For char combustion, high levels of CO₂ decrease the burning rate during Zone II and Zone III combustion because of the lower oxygen diffusivity. It is possible that CO₂ enhances the overall kinetic rate of carbon oxidation through contributions from the Boudouard reaction, but it is generally believed that this reaction is too slow to effectively compete with char oxidation from O₂, even at the very high particle temperatures that are sometimes experienced with the use of high oxygen concentrations. NO_x formation, both from oxidation of nitrogen in the volatile fuel components and from oxidation of the nitrogen in the char, is enhanced for the high volatile flame temperatures and elevated char combustion temperatures associated with enhanced oxygen levels. However, practical oxy-combustion systems recycle a substantial fraction (typically on the order of 70%) of the flue gas back to the combustion furnace, allowing significant reburning of the previously formed NO_x in the flame zone and on the burning char particles. Also, substitution of CO₂ for N₂ eliminates the contribution of thermal NO_x formation. As a result, the net NO_x production is reduced for oxy-fuel combustion processes, typically by a factor of three or more.

Another approach to oxy-fuel combustion for carbon capture and sequestration is chemical looping combustion [53,54]. Chemical looping combustion makes use of an oxygen carrier, such as a metal oxide, to transport oxygen to the fuel, thus preventing air from ever being in direct contact with the fuel during combustion. The process is conducted with two reactors, a reducer (or fuel reactor) and an oxidizer (or air reactor) as illustrated in Figure 9.24. In the reducer, coal (or fuel) reacts with particulate metal oxide to form CO₂, H₂O, and particulate metal (or a metal oxide with a lower oxidation state). Depending on the metal oxide, this process is overall endothermic or slightly exothermic. The water formed results from oxidation of volatiles originating from the coal (or from an enhancer gas introduced to assist the reaction). In the oxidizer (or air reactor), the metal particles are reacted with air to reform the metal oxide through an exothermic reaction that may be used to supply energy to a heat engine. Low grade heat rejected from the heat engine is returned to the fuel reactor as the distribution of heat release from the two reactors is such that the temperature of the air reactor is operated at a higher temperature than that of the fuel reactor. The particulate metal oxide and metal are transported back and forth between reactors. Carbon dioxide capture is possible because the gaseous products of the fuel reactor consist largely of CO₂ and H₂O, and the air reactor is devoid of carbon.

The reactions in the air reactor are those described earlier in this chapter for metal combustion and can involve the two overall steps

With coal as the fuel, these reactions may be initiated through devolatilization

At temperatures below the decomposition temperature of the metal oxide, direct solid–solid reactions have also been proposed [55].

For complete oxidation, the stoichiometry of the carbon reaction with oxygen and the metal oxide should be written to form CO₂. Table 9.9 lists the primary metals that have been considered, their oxygen capacity, and the theoretical heat release distribution between the two reactors assuming an overall process reaction of C + O₂ → CO₂. Many other factors are important to the choice of the oxygen carrier including particle kinetics, support materials, strength, attrition, environmental and health concerns, and cost [54].