12.2.1 Solid waste

Solid waste is a general term that includes garbage; rubbish; refuse; sludge from a wastewater treatment plant, water supply treatment plant, or air pollution control facility; sewage sludge; and other discarded material, including solid, liquid, semisolid, or contained gaseous material resulting from industrial, municipal, commercial, mining, and agricultural operations, as well as from community and institutional activities. Whether natural or of human origin, oil, dirt, rock, sand, and other inert solid materials used to fill land are not classified as waste if the objective of the fill is to make the land suitable for the construction of surface improvements. Solid waste does not include waste materials that result from activities associated with the exploration, development, or production of oil, gas, geothermal resources, or other substances or materials regulated by local or federal governments.

Solid waste that is typically excluded from gasification feedstocks includes uncontaminated solid waste resulting from the construction, remodeling, repair, and demolition of utilities, structures, and roads, as well as uncontaminated solid waste resulting from land clearing. Such waste includes, but is not limited to, bricks, concrete, other masonry materials, soil, rock, wood (including painted, treated, and coated wood and wood products), land clearing debris, wall coverings, plaster, drywall, plumbing fixtures, nonasbestos insulation, roofing shingles, other roof coverings, asphaltic pavement, glass, plastics that are not sealed in a manner that conceals other wastes, empty buckets (10 gallons or less in size and having no more than 1 inch of residue remaining on the bottom), electrical wiring and components containing no hazardous liquids, and pipe and metals that are incidental to any of the above.

In summary, as used in this text, the term solid waste refers to any unwanted or discarded carbonaceous (containing carbon) or hydrocarbonaceous (containing carbon and hydrogen) material that originates from a variety of sources and is not a liquid or a gas. Furthermore, the disposal of a wide variety of wastes has become an important problem because the traditional means of disposal in a landfill has become much less acceptable in recent years due to concerns about the environmental impacts of the practice. Newer and stricter regulation of the conventional disposal methods has made waste processing for resource recovery much more favorable economically.

However, before moving on to the various aspects of the gasification process, it is worthwhile to describe in more detail the types of waste that arise from human activities and which might be suitable for gasification.

12.2.2 Municipal solid waste

MSW is solid waste resulting from, or incidental to, municipal, community, commercial, institutional, and recreational activities, and it includes garbage, rubbish, ashes, street cleanings, dead animals, medical waste, and all other nonindustrial solid waste.

MSW is generated by households, offices, hotels, shops, schools, and other institutions. The major components of MSW are food waste, paper, plastic, rags, metal, and glass, although demolition and construction debris is often included in collected waste, as are small quantities of hazardous waste, such as electric light bulbs, batteries, automotive parts, and discarded medicines and chemicals.

MSW is a negatively priced, abundant, and essentially renewable feedstock. The composition of MSW (Table 12.1) can vary from one community to the next, but the overall differences are not substantial. In fact, there are several types of waste that might also be classified within the MSW umbrella (Table 12.2).

The heat content of raw MSW depends on the concentration of combustible organic materials in the waste and its moisture content. Typically, raw MSW has a heating value of approximately half that of bituminous coal (Speight, 2013a). The moisture content of raw MSW is usually 20% w/w.

12.2.3 Industrial solid waste

Industrial solid waste is solid waste resulting from or incidental to any process of industry, manufacturing, mining, or agricultural operations. Industrial solid waste is classified as either hazardous or nonhazardous. Hazardous industrial waste includes any industrial solid waste or combination of industrial solid wastes identified or listed as a hazardous waste. Nonhazardous industrial waste is an industrial solid waste that is not identified or listed as a hazardous waste.

Industrial solid waste encompasses a wide range of materials of varying environmental toxicity. Typically, this range includes paper, packaging materials, waste from food processing, oils, solvents, resins, paints and sludge, glass, ceramics, stones, metals, plastics, rubber, leather, wood, cloth, straw, and abrasives. As with MSW, the absence of a regularly updated and systematic database on industrial solid waste ensures that the exact rates of generation are largely unknown.

A generator of industrial solid waste must classify the waste as:

• Class 1 waste: The class of waste includes any industrial solid waste or mixture of industrial solid wastes with a concentration level or physical or chemical characteristics that make it toxic; corrosive; flammable; a strong sensitizer or irritant; or a generator of sudden pressure by decomposition, heat, or other means, Class 1 wastes may also pose a substantial present or potential danger to human health or the environment when improperly processed, stored, transported, or disposed of or otherwise.

• Class 2 waste: This class of waste consists of any individual industrial solid waste or combination of industrial solid wastes that are not described as hazardous, Class 1, or Class 3.

• Class 3 waste: The class of waste consists of inert and essentially insoluble industrial solid waste, usually including, but not limited to, materials such as rock, brick, glass, dirt, and certain plastics and rubbers that are not readily decomposable.

12.2.4 Biosolids

Biosolids include livestock waste, agricultural crop residues, and agroindustrial by-products. In most traditional, sedentary agricultural systems, farmers use the land application of raw or composted agricultural wastes as a means of returning valuable nutrients and organics back into the soil, and this practice remains the most widespread means of disposal. Similarly, fish farming communities commonly integrate fish rearing with agricultural activities such as livestock husbandry, vegetable and paddy cultivation, and fruit farming.

Many countries with agriculture-based economies use agricultural wastes to produce biogas through anaerobic digestion (Speight, 2008, 2011a). The biogas (approximately 60% v/v methane) is primarily used directly for cooking, heating, and lighting, while the slurry from the anaerobic digesters is used as liquid fertilizer, feed supplements for cattle and pigs, and a medium for soaking seeds.

12.2.5 Biomedical waste

Biomedical waste refers to the waste materials produced by hospitals and health care institutions, which have been increasing over the past four decades to meet the medical and health care requirements of the growing world population. Until recent years, little attention was paid to biomedical wastes, which are potentially hazardous to human health and the environment. In fact, serious concern has arisen regarding the potential for spreading pathogens, as well as causing environmental contamination due to the improper handling and management of clinical and biomedical waste.

Regulated medical waste (RMW) is a waste stream that contains potentially infectious material, also called red bag waste or biohazardous waste. RMW is regulated on a state-by-state basis, but it also falls under the Bloodborne Pathogen Standard as defined by the US Office of Safety and Health Administration. Such wastes are subject to state and federal regulations, may not be suitable as gasification feedstock, and require higher processing temperatures to assure complete disposal of the constituents.

12.3.1 Feedstock properties

The individual feedstock constituents typically have their own hazards, including fire, dust explosion, and toxic gas formation, but when constituents are used in combination, handling the combined feedstocks may require extra precautions to ensure safety. For example, where feed materials such as biomass wood are stored in large piles, there is potential for selfheating or spontaneous ignition, which is always an issue when coal is stockpiled (Speight, 2013a, 2013b). Wood fuel is a source of nutrients for microbes, and in the presence of moisture, microbial activity can lead to the generation of heat in the wood over time, resulting in selfignition. Other feed safety considerations include hazards associated with dust, such as explosion hazards requiring protection through hot particle detection and explosion venting.

12.3.2 Plant safety

The gasification process produces a highly flammable gaseous mixture, including hydrogen and the extremely toxic carbon monoxide. In plant sections where pressure buildup exists, there is a risk of gas escaping into the atmosphere. Therefore, precautions are necessary to prevent such escape of toxic or environmentally destructive gases. The areas outside the equipment must be adequately ventilated to prevent buildup of an explosive atmosphere, but also to prevent poisoning due to carbon monoxide accumulation. Carbon monoxide detection equipment should be provided to detect possible leaks.

Thus, the gasification of waste introduces a series of safety issues that are, in fact, closely related to safety issues in chemical processing plants. These hazards are well understood in the chemical processing industries, where safety techniques, including hazard and operability, layers of protection analysis, and safety integrity level (SIL), have evolved to ensure the safe design and operation of plants. Crossindustry cooperation will result in quicker, safer implementation of new technology, greatly reducing the risk of a catastrophic incident.

Unlike conventional energy plants, which are numerous, waste gasification plants do not yet follow uniform design standards, and this variability makes the construction and operation of a waste gasification plant particularly challenging. Although guidance is now becoming available, the multitude of gasification technologies and available reactor configurations often defy the application of recommended practices. The chemical processing industry has a wealth of experience with the techniques required to ensure that plants meet the high standards of employee safety, however, and implementing these techniques to gasification is necessary to ensure safety and meet environmental standards.

12.4.1 Preprocessing

Gasification is a thermochemical process that generates a gaseous, fuel-rich product (Chapter 1), and regardless of how the gasifier is designed (Chapter 2), two processes must take place in order for the gasifier to produce a useable fuel gas. In the first stage, pyrolysis releases the volatile components of the fuel at temperatures below 600 °C (1110 °F). The by-product of pyrolysis that is not vaporized is char and consists mainly of fixed carbon and ash. In the second gasification stage, the char that remains after pyrolysis is either reacted with steam or hydrogen or combusted with air or pure oxygen. Gasification with air results in a nitrogen-rich, low-Btu fuel gas. Gasification with pure oxygen results in a higher quality mixture of carbon monoxide and hydrogen and virtually no nitrogen. Gasification with steam (steam reforming) (Chapter 6) also results in a synthesis gas that is rich in hydrogen and carbon dioxide, with only minor amounts of impurities (Richardson, Rogers, Thorsness, Wallman, & Leininger, 1995). Typically, the exothermic reaction between the feedstock carbon and oxygen provides the heat energy required to drive the pyrolysis and char gasification reactions.

MSW is not a homogenous waste stream. Given that inorganic materials (metals, glass, concrete, and rocks) do not enter into the thermal conversion reactions, part of the energy that could be used to gasify the feedstock is expended in heating the inorganic materials to the pyrolysis reactor temperature. Then the inorganic materials are then cooled in cleanup processes, and the heat energy is lost, reducing the overall efficiency of the system. To make the process more efficient, some preprocessing of the waste is typically required and includes the separation of thermally nondegradable material, such as metals, glass, and concrete debris. Preprocessing may include sorting, separation, size reduction, and densification (for reducing overall volume of feedstock being fed into the gasifier). Such preprocessing techniques are common in the waste recycling industry for recovery of paper, glass, and metals from the MSW streams.

Thus, the first function of the front-end (preprocessing) system is to accept solid waste directly from the collection vehicle and to separate the solid waste into two parts, combustible waste and noncombustible waste. The front-end separation produces the feedstock for the gasification process.

In order to enhance the process before gasification begins, feedstock pregasification systems (preprocessing systems) extract metals, glass, and inorganic materials, resulting in the increased recycling and utilization of materials. In addition, a wide range of plastics cannot be recycled as feedstocks for gasification. Thus, the main steps involved in preprocessing MSW are analogous to the preprocessing of coal (Speight, 2013a, 2013b) or biomass (Speight, 2008, 2011a). These steps include (1) manual or mechanical sorting, (2) shredding, (3) grinding, (4) blending with other materials, (5) drying, and (6) pelletization. The purpose of pre-processing is to produce a feed material with, as best as can be achieved, near-consistent physical characteristics and chemical properties. Preprocessing operations are also designed to produce a material that can be safely handled, transported, and stored prior to the gasification process. In addition, particle size or pellet size affects the product distribution (Luo et al., 2010).

If the MSW has a high moisture content, a dryer may be added to the preprocessing stage to lower the moisture content of the waste stream to 25% w/w, or lower (CH2MHill, 2009). Lowering the moisture content of the feedstock increases its heating value, and as a result, the system becomes more efficient. The waste heat or fuel produced by the system can be used to dry the incoming MSW.

In some cases, the preprocessing operation may be used for the production of a combustible fraction (a solid fuel) from MSW and from mixed waste, and its thermal conversion requires two basic and distinct subsystems, the front-end and the back-end. The combustible fraction recovered from mixed MSW has been given the name RDF. The composition of the recovered combustible fraction is a mixture that has higher concentrations of combustible materials, such as paper and plastics, than those present in the parent mixed MSW.

The main components or unit operations of a front-end subsystem are usually any combination of size reduction, screening, magnetic separation, and density separation (e.g., air classification). The types and configurations of unit operations selected for the front-end design depend on the types of secondary materials that will be recovered and on the desired quality of the recovered fuel fraction. The designer or supplier of the thermal conversion system must specify the fuel quality.

Typically, systems that recover a combustible fraction from mixed MSW utilize size reduction, screening, and magnetic separation. Some designs and facilities have used screening, followed by size reduction in the form of pretrommel screening (a trommel is a drum screen), as the fundamental foundation of the system design, while others have reversed the order of these two operations. A number of considerations enter into the determination and selection of the optimum order of screening and size reduction for a given application. Among others, the considerations include composition of the waste. The system design may also include other unit operations, such as manual sorting, magnetic separation, air classification, and pelletization (i.e., densification), as the need arises for recovery of other materials, such as aluminum, and for achieving the desired specification of the solid fuel product (Diaz & Savage, 1996).

12.4.2 Gasifier types

The gasifier is the core of the gasification system and is a vessel where the feedstock reacts with oxygen (or air) at high temperatures (Chapters 1 and 10) (E4Tech, 2009). In order to accommodate the different feedstocks and process requirements, there are several gasifier designs (Chapter 2), which are distinguished by (1) the use of wet or dry feedstock, (2) the use of air or oxygen, (3) the flow direction within the gasifier (upflow, downflow, or circulating flow), and (4) the cooling process for the synthesis gas and other gaseous products.

12.4.3 Process design

After being preprocessed into suitable particle-size pieces or fed directly (if a gas or liquid), the waste is injected into the gasifier, along with a controlled amount of air or oxygen. The high temperature conditions in the gasifier decompose the feedstock, eventually forming synthesis gas, which consists primarily of hydrogen, carbon monoxide, and, depending upon the specific gasification technology, smaller quantities of methane, carbon dioxide, hydrogen sulfide, and water vapor. Typically, 70-85% w/w of the carbon in the feedstock is converted into synthesis gas.

The ratio of carbon monoxide to hydrogen depends in part upon the hydrogen and carbon content of the feedstock and the type of gasifier used, but the ratio can be adjusted or shifted downstream of the gasifier through the use of catalysts. This ratio is important in determining the type of product to be manufactured (electricity, chemicals, fuels, hydrogen) (Chapters 1 and 6). For example, a refinery would use a synthesis gas consisting primarily of hydrogen, which is important in producing transportation fuels (Speight, 2011b, 2014). Conversely, a chemical plant will require synthesis gas with approximately equal proportions of hydrogen and carbon monoxide, both of which are basic building blocks for a broad range of products, including consumer and agricultural products such as fertilizers, plastics, and fine chemicals (i.e., complex, single, or pure chemical compounds). Thus, the inherent flexibility of the gasification process to adapt to feedstock requirements and the desired product slate can lead to the production of one or more products from the same process.

As a result of gasifier selection and the prerequisites for the use of the selected reactor, the correct design of the front-end system is obviously a necessity for the successful operation of a waste-to-energy facility. The key function of the preprocessing system is the segregation of the combustible components from the noncombustible components. In the production of a RDF, particular attention must be paid to the combustion unit in which the fuel is to be burned. For example, in order to facilitate handling, storage, and transportation, it may be necessary to produce a densified fuel (i.e., a pelletized fuel) that meets necessary specifications (Pellet Fuels Institute, 2011).

Processing MSWs for the production of a fuel is a seemingly straightforward process in terms of design and system operation. The performance and operation of the processing system is strongly and fundamentally determined by the feedstock, the type of equipment chosen, and the location of the equipment in the overall processing configuration. Although some of the equipment available for waste processing applications may be well suited to the processing tasks of other industries, such as mining, waste differs substantially from the raw materials that serve as feedstocks for other industries.

The failure to recognize and account for feedstock differences can result in operational problems at waste processing facilities, such as use of equipment that was improperly applied, the use of equipment that was improperly designed, or the use of equipment that was improperly operated. Plant operators and designers must now be aware of the need for a thorough understanding of the operating parameters of each piece of equipment as those parameters pertain specifically to waste preprocessing and gasification. This need for specialized knowledge extends to a detailed familiarization with the physical and chemical characteristics of the waste feedstocks (Savage, 1996).

In summary, gasification technology is selected on the basis of feedstock properties and quality, gasifier operation, the desired product slate, and product quality. The main reactors used for gasification of MSW are fixed-bed and fluidized-bed units. Larger capacity gasifiers are preferable for treatment of MSW because they allow for variable fuel feed, uniform process temperatures due to highly turbulent flow through the bed, good interaction between gases and solids, and high levels of carbon conversion (Chapter 2).

12.4.4 Plasma gasification

While the main types of gasifiers (Chapter 2) can be adapted for use with various waste feedstocks, plasma gasification is the object of much interest in connection with treatment of MSW.

Plasma is a high temperature, highly ionized (electrically charged) gas capable of conducting an electrical current. Plasma technology has evolved into a valuable processing option using very high temperatures (Ducharme, 2010; E4Tech, 2009; Fabry, Rehmet, Rohani, & Fulcheri, 2013; Gomez et al., 2009; Heberlein & Murphy, 2008; Kalinenko et al., 1993; Leal-Quirós, 2004; Lemmens et al., 2007; Messerle & Ustimenko, 2007; Moustakas et al., 2005). Plasma is formed by passing an electrical discharge through a gas such as air or oxygen (O₂), whereupon the interaction of the gas with the electric arc dissociates the gas into electrons and ions, causing the temperature to increase significantly. In theory, plasma temperatures often exceed 6000 °C (10,830 °F), but measurement of the temperature is not always possible, and the temperature range may be speculative.

There are two basic types of plasma torches, the transferred torch and the nontransferred torch. The transferred torch creates an electric arc between the tip of the torch and a metal bath or the conductive lining of the reactor wall. In the nontransferred torch, the arc is produced within the torch itself. The plasma gas is fed into the torch and heated, and it then exits through the tip of the torch.

In the plasma-based process, the gasifier is heated by a plasma torch system located near the bottom of the reactor vessel. In the gasifier, the feedstock is charged into a vertical reactor vessel (refractory lined or water-cooled) at atmospheric pressure. A superheated blast of air, which may be enriched with oxygen, is provided to the bottom of the gasifier, at the stoichiometric amount required for gasification. The amount of introduced air is controlled so that a low velocity of the upward flowing gas is maintained and the pulverized (small particle) feedstock can be fed directly into the reactor. Additional air and/or steam can be provided at different levels of the gasifier to assist with the pyrolysis and gasification components of the process. The temperature of the synthesis gas leaving the top of the gasifier is maintained above 1000 °C (1830 °F), and at this temperature, tar formation is eliminated.

The high operating temperatures in the plasma gasifier decompose the feedstock (and all hazardous and toxic components) and dramatically increase the kinetics of the various reactions occurring in the gasification zone, converting all organic materials into hydrogen (H₂) and carbon monoxide (CO). Any residual materials, from inorganic constituents and heavy metals, will be melted and produced as a vitrified slag, that is, highly resistant to leaching. Magmavication or vitrification is the result of the interaction between plasma and inorganic materials: in the presence of a coke bed or cokelike products in the cupola or reactor, a vitrified material is produced that can be used in the manufacture of architectural tiles and construction materials (Leal-Quirós, 2004).

Plasma gasification is increasingly considered for conversion of all types of waste, including MSW and hazardous waste, into electricity and other valuable products. The process produces the maximum amount of energy from waste, and different types of feedstocks, such as MSW and hazardous waste, can be mixed, avoiding the time-consuming and costly step of sorting the feedstock by type before it is fed into the gasifier. This makes plasma gasification an attractive option for managing different types of waste streams.

However, the main challenge facing plasma gasification is the skepticism among some observers regarding the process’s ability to fully convert MSW. Synthesis gas cleanup processes and oxygen separation methods could be improved to make the economics competitive, but public perception is the real deterrent to market penetration. More experience with operating the technology in the various countries, such as the United States, and using the process with MSW in particular would help the community recognize plasma gasification as a viable component of a waste management program.

12.5.1 Synthesis gas

As with many gasification processes, waste gasification is intended o produce a gas that can be used as fuel gas or used for hydrocarbons or chemicals production. In either case, the gas is synthesis gas, mixtures of carbon monoxide and hydrogen, and the yield and composition of the gas and related byproducts are dependent upon the properties and character of the feedstock, the gasifier type, and the conditions in the gasifier (Chapters 1 and 2) (Orr & Maxwell, 2000).

The raw synthesis gas produced in the gasifier contains trace levels of impurities that must be removed prior to its ultimate use. After the gas is cooled, virtually all the trace minerals, particulates, sulfur, mercury, and unconverted carbon are removed using commercially proven cleaning processes common to the gas processing, chemical, and refining industries (Gary, Handwerk, & Kaiser, 2007; Hsu & Robinson, 2006; Mokhatab, Poe, & Speight, 2006; Speight, 2007, 2014). For feedstocks containing mercury, more than 90% w/w of the mercury can be removed from the synthesis gas using relatively small and commercially available activated carbon beds.

12.5.2 Carbon dioxide

Carbon dioxide can also be removed during the synthesis gas cleanup stage using a number of commercial technologies (Mokhatab et al., 2006; Speight, 2007). In fact, carbon dioxide is routinely removed with a commercially proven process in gasification-based ammonia, hydrogen, and chemical manufacturing plants. Gasification-based plants for the production of ammonia are equipped to separate and capture approximately 90% v/v of their carbon dioxide, and gasification-based methanol plants separate and capture approximately 70% v/v of the produced carbon dioxide. In fact, the gasification process is considered to offer a cost-effective and efficient means of capturing carbon dioxide during the energy production process.

12.5.3 Tar

For the purposes of this text, tar is any condensable or noncondensable organic material in the product stream, and it is largely intractable and comprised of aromatic compounds.

When MSW is gasified, significant amounts of tar are produced, and if tar is allowed to condense (condensation temperatures range from 200 to 600 °C or 390-1110 °F), it can cause coke to form on fuel-reforming catalysts; deactivate sulfur removal systems; erode compressors, heat exchangers, and ceramic filters; and damage gas turbines and engines. Noncondensable tar can also cause problems for advanced power conversion devices, such as fuel cell catalysts, and complicate environmental emissions compliance.

The amount and composition of tars are dependent on the fuel, the operating conditions, and the secondary gas phase reactions, and tar can be divided into three categories based on the reaction temperature ranges in which it forms (Table 12.3). This categorization is important for assessing gasification processes, as the effectiveness of conversion and removal systems depends greatly on the specific tar composition and the concentration of tars in the fuel gas.

The primary tars are mixed oxygenates and are a product of pyrolysis. As gasification takes over at higher temperatures, the primary products thermally decompose to lesser amounts of secondary and tertiary products and a greater quantity of light gases. Tertiary products are the most stable and difficult to crack catalytically. Provided that there is adequate gas-mixing, primary and tertiary tars are mutually exclusive in the product gas. Both lignin and cellulose in the fuel result in the formation of tertiary tar compounds. However, lignin-rich fuels have been shown to form heavier tertiary aromatics more quickly.

Both physical and chemical treatment processes can reduce the presence of tar in the product gas. The physical processes are classified into wet and dry technologies, depending on whether water is used. Various forms of wet or wet/dry scrubbing processes are commercially available, and these are the most commonly practiced techniques for the physical removal of tar.

Wet physical processes involve tar condensation, droplet filtration, and gas/liquid mixture separation. Cyclones, cooling towers, venturi scrubbers, baghouses, electrostatic precipitators, and wet/dry scrubbers are the primary tools in this process. The main disadvantage of using wet physical processes is that the tar is transferred to wastewater, so the heating value is lost, and the water must be disposed of in an environmentally acceptable way. Wastewater that contains tar is classified as hazardous waste, and treatment and disposal of the wastewater can add significantly to the overall cost of the gasification plant.

Dry tar removal using ceramic, metallic, or fabric filters are alternatives to wet tar removal processes. However, at temperatures above 150 °C (300 °F), tars can become semisolid and adhesive, causing operational problems with such barriers. As a result, dry tar removal methods are rarely implemented. Injection of activated carbon into the product gas stream or in a granular bed may also reduce tars through adsorption and collection with a baghouse. The carbonaceous material containing the tars can then be recycled back to the gasifier to encourage further thermal and/or catalytic decomposition. On other words, the tar is recycled to extinction.

Chemical tar treatment processes are the most widely practiced in the gasification industry. They can be divided into four generic categories: thermal, steam, partially oxidative, and catalytic processes. Thermal destruction has been shown to break down aromatics at temperatures above 1000 °C (1830 °F). However, such high temperatures can have adverse effects on heat exchangers and refractory surfaces due to ash-sintering in the gasification vessel. The introduction of steam does encourage reformation of primary and some secondary oxygenated tar compounds, but it has a lesser effect on many nitrogen-containing organic compounds.

The presence of oxygen during gasification has been shown to accelerate both the destruction of primary tar products, and the formation of aromatic compounds from phenol cracking increases when the oxygen content of the gas is low (less than 10% v/v). Only above 10% v/v was a decrease in the amount of tertiary tars observed. A net increase in the carbon monoxide may also be observed as the product from the oxidative cracking of tar. Benzene levels are not usually affected by the presence of oxygen.

The most widely used and studied tar cracking catalyst is dolomite, which is a mixture of calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃). Dolomite has been shown to work more effectively when placed in a vessel downstream from the gasifier and in a low carbon monoxide environment. However, when used within the gasifier, catalytic materials often accumulate a layer of coke that causes rapid loss of catalytic efficiency.

The specific tar conversion and destruction processes chosen depend on the nature and composition of the tars present, as well as the intended end-use equipment. However, the advantages of recycling the tar product for further treatment include increased waste-to-energy efficiency, lower emissions, and lower effluent treatment costs. Although progress had been made in mitigating tar formation and increasing tar removal, the lack of affordable, effective tar removal processes continues to provide a barrier to the widespread commercialization of integrated gasification combined cycle power generation using MSW.

12.5.4 Particulate matter

The detrimental effect of particulate matter on the atmosphere has been of some concern for several decades. Chemicals ejected into the atmosphere and from fossil fuel combustion, such as mercury, selenium, and vanadium, are particularly harmful to the flora and fauna. There are many types of particulate collection devices in use, and they involve a number of different principles for the removal of particles from gasification product streams (Speight, 2013a, 2013b). However, the selection of an appropriate particle removal device must be based upon equipment performance as anticipated or predicted under the process conditions. To enter into a detailed description of the various devices available for particulate removal is well beyond the scope of this text, but the reader should be aware of the equipment available for particulate removal and the means by which this might be accomplished: (1) cyclones, which are particle collectors that have many potential applications in coal gasification systems; (2) electrostatic precipitators, which are efficient collectors of fine particulate matter and are capable of reducing the amount of submicron particles by 90% or more, while also collecting liquid mists and dust; (3) granular-bed filters, which comprise a class of filtration equipment that is distinguished by a bed of separate, closely packed granules that serve as the filter medium for collecting particulates at high temperatures and pressures; (4) wet scrubbers, which represent a simple method to clean exhaust air or exhaust gas, removing toxic or smelling compounds via close contact with fine water drops in a cocurrent or countercurrent flow of the gas stream.

12.5.5 Halogens/acid gases

The principal combustion products of halogen-containing organic waste are either hydrogen halides, such as hydrogen chloride (HCl) or hydrogen bromide (HBr), or metal halides, such as mercuric chloride (HgCl₂) or mercurous chloride (HgCl). Thee substances volatilize out of the reactor along with the other gases. In the gasification of pure MSW, which does not contain coal, biomass, or any other added feedstock, hydrogen chloride is the prevailing chlorine-containing product. Bromine constituents can accumulate to a greater extent in the bottom ash, but in the presence of hydrogen, bromine is transformed to hydrogen bromide (HBr), which is readily removed, along with the HCl, by scrubbing systems, thus causing no emission problems.

A significant advantage of gasification is that it takes place in a reducing atmosphere, which prevents sulfur and nitrogen compounds from oxidizing. As a result, most of the elemental nitrogen or sulfur in the waste stream ends up as hydrogen sulfide (H₂S), carbonyl sulfide (COS), nitrogen (N₂) or ammonia (NH₃), rather than sulfur oxides (SOx) or nitrogen oxides (NOx). The reduced sulfur species can then be recovered as elemental sulfur at efficiencies between 95% w/w and 99% w/w, or converted to a sulfuric acid by-product (Mokhatab et al., 2006; Speight, 2007).

The typical sulfur removal and recovery processes used to treat the raw synthesis gas are the same as commercially available methods used in other industrial applications, such as oil refining and natural gas recovery (Speight, 2007, 2008, 2014). One process commonly used to remove sulfur compounds is the selective-amine (olamine) technology, which extracts sulfur species from the synthesis gas using an amine-based solvent or related agent in an absorber tower. The reduced sulfur species removed in the solvent stripper are converted to elemental sulfur in a sulfur recovery process such as the Selectox/Claus process.

When MSW is gasified, nitrogen in the fuel is converted primarily to ammonia that, when fired in a turbine or other combustion engine, forms nitrogen oxide, a harmful pollutant. Removal of ammonia and other nitrogen compounds in the product gas prior to combustion can be accomplished with wet scrubbers or by catalytic destruction. Catalytic destruction of ammonia has been studied with dolomite and iron-based catalysts. This technique is of interest because tars are simultaneously decomposed (cracked) to lower weight gaseous compounds. Destruction of 99% v/v of the ammonia in the gas stream has been demonstrated with these catalysts.

If the product gas is first cooled, wet scrubbing with lime is also an effective ammonia removal technique. Gasification processes that use pure oxygen, steam, or hydrogen will only have nitrogen contents brought in through the fuel stream. Typical MSW has a nitrogen content of less than 1% w/w.

12.5.6 Heavy metals

Trace amounts of metals and other volatile materials are also present in MSW. These are typically toxic substances that pose ecological and human health risks when released into the environment.

Mercury found in the fly ash and flue gas is likely to be in the elemental form, but when oxidizing conditions are prevalent in the gasifier, the presence of hydrogen chloride (HCl) and chlorine (Cl₂) can cause some of the elemental mercury to form mercuric chloride (HgCl₂):

$\mathrm{Hg}+4\mathrm{HCl}+{\mathrm{O}}_2\leftrightarrow 2{\mathrm{HgCl}}_2+2{\mathrm{H}}_2\mathrm{O}$

$\mathrm{Hg}+{\mathrm{Cl}}_2\leftrightarrow {\mathrm{HgCl}}_2$

Volatilized heavy metals (or heavy metals that are entrained in the gas stream due to the high gas velocity) that are not collected in the gas cleanup system can bioaccumulate in the environment, where they can be carcinogenic and damage human nervous systems (Speight & Arjoon, 2012). For this reason, mercury must be removed from the product gas prior to combustion or further use. However, there has been extraordinary success removing heavy metals with activated carbon, baghouses, filters, and electrostatic precipitators (Mokhatab et al., 2006; Speight, 2007, 2013a).

12.5.7 Alkalis

The primary elements causing alkali slagging are potassium, sodium, chlorine, and silica. Sufficient volatile alkali content in a feedstock causes a reduction in the ash fusion temperature and promotes slagging and/or fouling. Alkali compounds in the ash from the gasification of MSW can cause serious slagging in the boiler or gasification vessel. Sintered or fused deposits can form agglomerates in fluidized beds and on grates. Potassium sulfate (K₂SO₄) and potassium chloride (KCl) have been found to mix with flue dust and deposit or condense on the upper walls of the gasifier.

Alkali deposit formation is a result of particle impaction, condensation, and chemical reaction. Unfortunately, most deposits occur subsequent to gasification and cannot always be predicted solely on the basis of analysis of the feedstock. There are two characteristic temperature intervals for alkali metal emission. A small fraction of the alkali content is released below 500 °C (930 °F) and is attributed to the decomposition of the organic structures. Another fraction of alkali compounds is released from the char residue at temperatures above 500 °C (930 °F).

Thus, the presence of alkali metals in gasification processes is known to cause several operational problems. Eutectic systems consisting of alkali salts are formed on the surfaces of fly ash particles or on the fluidized bed material. The eutectic system is a mixture of chemical compounds or elements that have a single chemical composition that solidifies at a lower temperature than any other composition made up of the same ingredients. The semisolid or adhesive particle surfaces can lead to the formation of bed material agglomerates, which must be replaced by fresh material. The deposition of fly ash particles and the condensation of vapor-phase alkali compounds on heat-exchanging surfaces lower the heat conductivity and may eventually require temporary plant shutdowns for the removal of deposits.

The challenges of removing alkali vapor and particulate matter are closely connected, since alkali metal compounds play an important role in the formation of new particles as well as the chemical degradation of ceramic barrier filters used in some hot gas cleaning systems. The most convenient method is to cool the gas and condense out the alkali compounds.

12.5.8 Slag

Most solid and liquid feed gasifiers produce a hard glasslike by-product (slag, also called vitreous frit) that is composed primarily of sand, rock, and any minerals (or thermal derivatives thereof) originally contained in the gasifier feedstock. Slag is the result of gasifier operation at temperatures above the fusion or melting temperature of the mineral matter. Under these conditions, nonvolatile metals are bound together in a molten form until it is cooled in a pool of water at the bottom of a quench gasifier or by natural heat loss at the bottom of an entrained bed gasifier. Volatile metals, such as mercury, if present in the feedstock, are typically not recovered in the slag, but are removed from the raw synthesis gas during cleanup. Depending upon the type of mineral matter in the feedstock, the slag is usually nonhazardous and can be used in roadbed construction, cement manufacturing, or in roofing materials.

Slag production is a function of the amount of mineral matter present in the gasifier feedstock, so materials such as MSW, as well as coal and biomass, produce much more slag than petroleum residua. Regardless of the character of the feedstock, as long as the operating temperature is above the fusion temperature of the ash (as in the modern gasification technologies under discussion), slag will be produced. Aside from being influenced by the waste feedstock, the physical structure of the slag is sensitive to changes in operating temperature and pressure, and, in some cases, physical examination of the appearance of the slag can provide a good indication of carbon conversion in the gasifier.

Furthermore, because the slag is in a fused vitrified state, it rarely fails the toxicity characteristic leaching procedure protocols for metals (Speight & Arjoon, 2012). Slag is not a good substrate for binding organic compounds, so it is usually found to be nonhazardous, exhibiting none of the characteristics of a hazardous waste. Consequently, it may be disposed of in a nonhazardous landfill, or sold as an ore for the recovery of metals concentrated within its structure. The hardness of slag also makes it suitable as an abrasive or roadbed material, as well as an aggregate in concrete formulations (Speight, 2013a, 2014).