3.1 Introduction

Effective utilization of various energy resources is currently a worldwide issue worthy of investigation. Coal, petroleum, and natural gas are the main fossil-based feedstocks for energy production and are responsible for about three-quarters of the world’s primary energy consumption, each corresponding to 33%, 24%, and 19%, respectively (Stocker, 2008). With increasing focus on global warming, CO₂ emission, secure energy supply, and less consumption of fossil-based fuels, use of renewable energy resources is essential. In addition to these resources, some of the end-of-life products such as MSW and biosolids could be effectively used for gasification. The so-called black liquor (BL) obtained as a waste product of the paper industry serves as an example that could be gasified. Additional sources of lignin are the sugar industry, where the fermentable sugars are converted into ethanol, and lignin is left over as residue. Today, most of this lignin is burnt, but it could be utilized by various other processes to produce electrical energy or various hydrocarbons. It is essential to move the world market dependence away from fossil-based energy resources to renewable alternatives, such as biomass, to make an important contribution toward the establishment of favorable conditions for climate and a sustainable economy (Ragauskas et al., 2006). Lignocellulosic biomass is composed of cellulose, hemicellulose, lignin, and other inorganic materials. Third-generation algal biofuel has a by-product cell mass that can be referred to as de-fatted algae.

There are numerous methods by which all these materials can be converted to various types of electrical/chemical energies. All the processes that occur in high temperature, high/low pressure, and in the presence or absence of catalysts come under the umbrella of thermo-chemical/catalytic methods of conversion. There are various types of processes such as combustion, gasification, pyrolysis, liquefaction, and carbonization under the thermo-chemical methods of conversion. Gasification takes place at high temperature in the presence of an oxidizing agent (also called a gasifying agent). Heat is supplied to the gasifier either directly or indirectly to reach the gasification temperature of 600–1000 °C. Oxidizing agents typically used are air, steam, nitrogen, carbon dioxide, oxygen, and a combination of these. In the presence of an oxidizing agent at high temperature, the large polymeric molecules of biomass decompose into lighter molecules and eventually to permanent gases (CO, H₂, CH₄, and lighter hydrocarbons), ash, char, tar, and minor contaminants. Char and tar are the result of incomplete conversion of biomass (Kumar, Jones, & Hanna, 2009). This process produces a low- to medium-Btu gas (4–10 MJ/m³) that can be used to run gas-powered devices for heat generation as well as internal combustion engines, gas turbines, and fuel cells. The gas derived from this process may contain up to 90% of the energy of the initial feedstock.

Gasification can be carried out using any carbonaceous material; currently, coal is the major substance used as feedstock. The biomass gasification process would be carbon neutral because it will not have a net affect on the existing greenhouse gas concentrations. The pre-treatment process for any feedstock constitutes the steps that must be imposed on the raw material in preparation for the use in a gasification reactor. Raw material typically requires an initial drying before pulverization and screening to the desired size (http://agronomyday.cropsci.illinois.edu/2010/tours/c3chips/). The pre-treatment process is an essential step to avoid any damage to the gasifier and to produce high-quality gas as a product.

There are several complexities in the process of feedstock pre-treatment for gasification due to the presence of non-convertible (inorganic) materials, which further increases when renewable resources are used. The preparation of both the categories (fossil and renewable based) of feedstocks is distinct and varies with the type of gasifier to be used in the next step. When low-density feedstocks are used, their logistics becomes difficult and uneconomical if certain handling processes are not applied. Hence, such feedstocks have to be densified into pellets, or briquettes, to ensure efficient and safe transportation.

All the preceding requirements and processes will be discussed in detail in the following sections. The various characteristics and properties of feedstocks such as coal, petroleum residue, BL, biomass, and MSWs are mentioned under the various sections that follow.

3.2 Feedstock types, properties, and characterization

There are various kinds of feedstocks under the two categories for gasification. The fossil-based feedstocks are coal and petroleum residue. The renewable feedstocks are the lignocellulosic biomass, MSW, biosolids, and BL.

Coal is a complex chemical latticework of carbon, hydrogen, and dozens of trace elements (Franco & Diaz, 2009). Coal preparation or cleaning is the removal of mineral matter from as-mined coal to produce clean coal. The primary purpose is to increase the quality and heating value (Btu/lb) of coal by lowering the level of sulfur and mineral constituents (ash). In the case of most eastern bituminous coals, roughly one-half to two-thirds of the sulfur exists in a form that can be liberated by crushing and separated by mechanical processing. Western coals typically contain much lower levels of sulfur, have lower heating values (LHVs), and are not readily amenable to physical cleaning methods for sulfur reduction. All coals contain mineral matter that can also be removed through physical cleaning. Coal preparation as currently practiced in the coal industry involves four generic steps: characterization, liberation, separation, and disposition. During characterization, the composition of the different-sized raw coal particles is identified. The composition of the raw coal and the required clean coal specifications dictate the type of equipment that must be used to remove the mineral matter. Crushing liberates mineral matter, and complete liberation can be achieved by reducing the mined coal to very fine sizes as particles containing both coal and mineral matter, called middlings, are produced during crushing. Separation involves partitioning the individual particles into their appropriate-sized groupings – coarse, intermediate, and fine fractions – and separating the mineral matter particles from the coal particles within each size fraction. Separation techniques for larger-sized raw coal particles generally depend on the relative density difference between the organic coal and inorganic mineral matter particles. In the case of fine raw coal particles, difference in the surface properties of the particles in water is utilized. Disposition is the dewatering and storage of the cleaned coal and the disposal of the mineral matter. Entrained-solid gasifiers are insensitive to most coal properties so long as the coal can be pulverized to about 80% below 200 mesh (44 μm) size (Longwell, Rubint, & Wilso, 1995).

Petroleum coke is the final by-product during the refining process in delay-coke equipment. With a continuous increase in the worldwide supply of heavy crude oil and the installation of more petroleum deep conversion process units, the output of petroleum coke is steadily increasing (Gary & Handwerk, 2001; Wang, Anthony, & Abanades, 2004). Gasification reactivity of petroleum coke is improved by adding coal liquefaction residue (CLR) as a catalyst (Zhou, Fang, & Cheng, 2006). There are plenty of alkali and alkaline earth metallic (AAEM) species and iron oxygen in the CLR. They are effective catalysts for combustion and gasification of carbonaceous materials (Liu, Zhou, Hu, Dai, & Wang, 2011). Petroleum residues refer to the heavy fractions generated in petroleum refining, including atmosphere residue, vacuum residue, and de-oiled asphalt. The newly exploited heavy crudes, such as natural bitumen and shale oil, also have properties similar to such petroleum residues. Thus, the terminology of heavy oil or heavy residue can also be used to indicate all such heavy petroleum oils (Zhang et al., 2012).

BL, a major waste from chemical pulp and paper production, contains, on a dry basis, about 40% of inorganic compounds and 60% of organic compounds (Naqvi, Yan, & Dahlquist, 2010; Sricharoenchaikul, 2009). The organic compounds are composed mainly of degraded lignin (alkali lignin), and the inorganic compounds are mostly recyclable pulping chemicals (alkali salts) (Pettersson & Harvey, 2010; Sánchez et al., 2004).

The degree of pre-treatment of biomass feedstock is dependent on the gasification technology used. High mineral matter could make gasification impossible. Fuel with moisture content above about 30% makes ignition difficult and reduces the calorific value (CV) of the product gas due to the need to evaporate the additional moisture before combustion/gasification can occur. High-moisture content reduces the temperature achieved in the oxidation zone, resulting in the incomplete cracking of the hydrocarbons released from the pyrolysis zone. Increased levels of moisture and the presence of CO produce H₂ by the water–gas shift reaction, and in turn the increased H₂ content of the gas, produces more CH₄ by direct hydrogenation. The gain in H₂ and CH₄ of the product gas does not compensate for the loss of energy due to the reduced CO content of the gas, thereby producing a product gas with lower CV.

The oxidation temperature is often above the melting point of the biomass ash, leading to clinkering/slagging problems in the hearth and subsequent feed blockages. Clinker is a problem for ash contents above 5%, especially if the ash is high in alkali oxides and salts that produce eutectic mixtures with low melting points. The gasifier has to be designed to destruct tars as well as the heavy hydrocarbons released during the pyrolysis stage of the gasification process. The particle size of the feedstock material depends on the hearth dimensions but is typically 10–20% of the hearth diameter. Larger particles could form bridges that would prevent the feed from moving down, whereas smaller particles would tend to clog the available air void, leading to a high pressure drop and subsequent shutdown of the gasifier (McKendry, 2002).

MSW composition does not always remain the same. It varies from site to site regionally as well as varies depending on developing or developed countries. The segregated waste is easier to be gasified than the non-segregated waste due to the presence of non-convertible matter, glass, water, and metals.

3.3 Feedstock suitability and utilization challenges

There are various challenges for gasification depending on the feedstock used. The feedstock has to be made suitable by employing a variety of techniques for gasification. Some of the critical issues are explained in this section.

The mechanical properties and the moisture content of the feedstock mainly govern the type and scope of pre-treatment such as storage, conveyance, crushing, drying, and feeding systems. The chemical analysis, the content of volatile matter, and the CV, which are interrelated in a certain manner, are decisive for the selection of the gasification process and its conditions. Gasification is comprised of two successive steps when, during pyrolysis, volatile matter is released and the remaining char, essentially consisting of fixed carbon and ash, is partially oxidized. Consequently, not only are the properties of the used feedstock important for the gasification process but also the behavior of the char. This especially applies to the reactivity normally attributed to the char. The coalification index (also termed rank) is an indication of the natural age of a fossil fuel. As the coalification index rises, the carbon content and the CV increase, whereas the oxygen content, in particular, and the portion of volatile matter decrease.

Wood has a coalific index of 0–0.18, a net CV of 17.5–20 MJ/Kg (moisture- and ash-free, MAF, basis), and a volatile matter 80–90 wt.% (MAF). Municipal waste has a coalific index of 0.03, net CV of 17 MJ/Kg (MAF) and volatile matter 85 wt.% (MAF). Peat has a coalific index of 0.18–0.36, a net CV of 22 MJ/Kg (MAF), and a volatile matter 61–73 wt.% (MAF). Brown coal has a coalific index of 0.45–0.48, a net CV of 25–27 MJ/Kg (MAF), and a volatile matter of 45–55 wt.% (MAF). Lignite has a coalific index of 0.52, a net CV of 28 MJ/Kg (MAF), and volatile matter of 40–50 wt.% (MAF). Sub-bituminous coal has a coalific index of 0.58–0.59, a net CV of 28.5–31.5 MJ/Kg (MAF), and a volatile matter of 30–35 wt.% (MAF). Bituminous coal (medium volatile) has a coalific index of 0.63, a net CV of 31 MJ/Kg (MAF), and a volatile matter of 25–30 wt.% (MAF). Anthracite has a net CV of 31–32 MJ/Kg (MAF) and volatile matter of 2–14 wt.% (MAF). Heavy residues have a coalific index of 0.32–0.65, a net CV of 35–38 MJ/Kg (MAF), and volatile matter of > 40 wt.% (MAF). All these factors (ash content and properties, sulfur, and chloride) must be taken into account when selecting and designing the process from both the technical and economic aspects. Intended utilization of the gas obtained and required gas treatment steps must be taken into consideration (Keller, 1990).

It is important to understand the properties and thermal behavior of feedstock to design a suitable gasifier. The properties of fuel that influence gasification are energy content, moisture content, particle size and distribution, form of the fuel, bulk density of the fuel, volatile matter content, ash content and composition, and reactivity of the fuel. Energy content of fuel is mostly obtained in an adiabatic, constant volume bomb calorimeter. The values obtained are higher heating values, which include the heat of condensation from water formed in the combustion of fuel and may be reported on moisture and ash basis. Fuel with higher energy content are preferred for gasification, and most biomass feedstocks (wood, straw) have heating value in the range of 10–16 MJ/kg, whereas liquid fuel (diesel, gasoline) has a higher heating value. Moisture content of the fuel is usually referred to inherent moisture plus surface moisture. Generally, a weight less than 15% is desirable for trouble-free and economical operation of the gasifier. In general, a wood gasifier works well on wood blocks and wood chips ranging from 80 × 40 × 40 mm to 10 × 5 × 5 mm. For a charcoal gasifier, charcoal with size ranging from 10 × 10 × 10 mm to 30 × 30 × 30 mm is quite suitable.

Bulk density is defined as the weight per unit volume of loosely tipped fuel; it varies significantly with moisture content and particle size of fuel. Volume occupied by stored fuel depends on the bulk density of fuel and the manner in which fuel is piled. Bulk density has considerable impact on gas quality, as it influences the fuel residence time in the fire box, fuel velocity, and gas flow rate. The form in which fuel is fed to a gasifier has an economical impact on gasification. Cupers and Pelletisers densify all kinds of biomass and municipal waste into “energy cubes.” These cubes are available in cylindrical or cubic form and have a high density of 600–1000 kg/m³. The specific volumetric content of cubes is much higher than the raw material from which they are made. Volatile matter and inherently bound water in the fuel are given up in the pyrolysis zone at the temperatures of 100–150 °C, forming a vapor consisting of water, tar, oils, and gases. Fuel with high volatile matter content produces more tar, causing problems to the internal combustion engine. Volatile matters in the fuel determine the design of the gasifier for removal of tar. Compared to other biomass materials (crop residue: 63–80%; wood: 72–78%; peat: 70%; coal: up to 40%), charcoal contains the least percentage of volatile matter (3–30%). The mineral content of fuel that remains in oxidized form after combustion of the fuel is called ash and also contains some unburned fuel. Ash content and ash composition have an impact on the smooth running of the gasifier. Melting and agglomeration of ashes in the reactor causes slagging and clinker formation. If no measures are taken, slagging or clinker formation leads to excessive tar formation or complete blocking of the reactor, and in general, no slagging occurs with fuel having ash content below 5%.

Wood chips contain 0.1% ash, whereas rice husk contains a high amount of ash (16–23%). Reactivity determines the rate of reduction of carbon dioxide to carbon monoxide in the gasifier and depends on the type of fuel. There is relationship between reactivity and the number of active places on the char surfaces. Reactivity of the char surface can be improved through various processes, including stream treatment (activated carbon) or treatment with lime and sodium carbonate. A number of elements act as catalysts that influence the gasification process, and small quantities of potassium, sodium, and zinc can have a large influence on reactivity of the fuel (http://cturare.tripod.com/fue.htm).

Li and colleages have carried out various studies to explain the char reactivity during gasification. It has been observed that the information about char reactivity is important for the effective utilization of coal, especially low-rank coals, in the low-temperature gasification processes (Li, Tay, Kajitan, & Zhang, 2013). The reactivity of Victorian brown coal char is affected by a few factors (Li, 2007). Victorian brown coal contained inherent alkali and AAEM species (Hayashi & Li, 2004). When the AAEM species are retained in the char during pyrolysis, they could act as catalysts for the gasification of char. Therefore, the concentration of the AAEM species in the coal/char has a direct influence on the char reactivity (Wu, Hayashi, Chiba, Takarada, & Li, 2004; Wu, Li, Hayashi, Chiba, & Li, 2005). Dispersion of the AAEM species in the char matrix also plays an important role in terms of char reactivity. This is because a catalyst could be active only for gasification if it is on the char (pore) surface and accessible to the gasifying agents (Li, 2007). The char structure could also affect the char reactivity, and these factors also influence each other. When the concentrations of large aromatic ring systems in char are increased, the dispersion of sodium in char appears to deteriorate to affect the char gasification reactivity (Li & Li, 2006).

Any coal can be gasified if properly pre-treated. High-moisture coals, for example, may require drying and some caking coals may require partial oxidation to simplify gasifier operation. Other pre-treatment operations include crushing, sizing, and briquetting of fines for feed to fixed-bed gasifiers. The coal feed is pulverized for fluid or entrained-bed gasifiers. Coal pre-treatment generally consists of coal pulverizing and drying. The dissolution of coal is best affected if the coal is dry and finely ground. The heater used to dry coal is typically coal fired, but it may also combust low-BTU-value product streams or may use waste heat from other sources (http://www.epa.gov/ttnchie1/ap42/ch11/final/c11s11.pdf). The chemical reactivity of the coal is potentially very important for underground coal gasification. The reported intrinsic reactivities of low-rank coals differ by up to four orders of magnitude when extrapolated to typical gasifier operating temperatures (Perkins & Sahajwalla, 2006). The intrinsic reactivity of coal has a big impact on the distributions in the gasifier and on the final product gas. In particular, high reactivity favors the production of methane via the char-H₂ reaction. Because this reaction is exothermic, the increased reactivity for this reaction can lead to big changes in the final product gas CV (Bhutto, Bazmi, & Zahedi, 2013). The heavy petroleum residues have the characteristics of high boiling point, high Conradson carbon residue, and high content of heavy metals (i.e., Ni and V), sulfur, and nitrogen (Zhang et al., 2012).

The performances of a waste-to-energy gasification-based process are necessarily affected by the specific properties of the MSW. The most important properties for gasification are elemental composition, LHV, ash content (and composition), moisture content, volatile matter content, other contaminants (such as N, S, Cl, alkalis, heavy metals, etc.), and bulk density and size (C-Tech, 2003; Heermann, Schwager, & Whiting, 2001; Zevenhoven-Onderwater, Backman, Skifvars, & Hupa, 2001). Some of these properties are so crucial that most current gasification technologies generally utilize pre-processed waste or refuse-derived fuel rather than the waste as it is. The pre-treatment adequately limits the highly heterogeneous nature of the waste and reduces its size as well as its ash and moisture content. Moreover, the composition of waste (in particular its heating value) and that of its ash (which in some cases could provide a catalytic action) could prompt an investigation regarding the possibility of using a co-gasification process – in other words, to feed into the gasifier a mixture of different fuels because the possible synergy between their products and intermediates could lead to maximizing the process performance, to reducing the carbon losses (in both particulate and tar fractions), and to increasing the energy content of syngas (Arena, 2012; Mastellone, Zaccariello, & Arena, 2010; Pinto, Lopes, André, Gulyurtlu, & Cabrita, 2007, 2008).

The advantage of the direct melting system process is that no pre-treatment of MSW is required, which differs from other gasification technologies such as a fluidized-bed gasifier. MSW is directly charged into a gasification and melting furnace from the top with coke and limestone, which function as a reducing agent and a viscosity regulator, respectively (Tanigaki, Manako, & Osada, 2012).

A fixed-bed gasifier is attractive for relatively large and dense fuels (wood chips or densified biomass/waste material) for small-scale application. Its main advantages are high ash content, feedstock acceptability, and high carbon conversion efficiency. However, several disadvantages – such as hot spots and channeling are possible in the fixed-bed as well as limited ability to handle fines – have to be considered. An updraft gasifier is more suitable for air as a gasifying agent due to LHV product gas with high levels of tars and a relative small feed rate. A downdraft gasifier favors the relatively dry biomass as feed, although the product gas has relatively low tar. Cross-flow is more suitable for feed such as charcoal with poor reactivity and low content of tar or ash because the temperature is around 2000 °C in the combustion zone. A fluidized-bed gasifier has high throughput capability and great fuel flexibility to handle low-density feedstocks such as undensified crop residues or sawdust. Cylindrical bubbling fluidized-bed systems are generally operated for industrial application at the current stage, which requires a narrow particle size distribution (PSD) for obtaining a better fluidization of particles within the bed. Unfortunately, biomass feed used in the gasification process is generally crushed by a mill, thereby exhibiting a wide PSD in nature. As a result, the small particles tend to be entrained out of the gasifier, while the large particles still remain above the distributor when operational gas velocity is fixed. This results in poor fluidization and unstable operation (Zhang et al., 2013). Biomass has volatile matter of 80–90% by weight and forms a very reactive char, which enables effective gasification in a fluidized bed at a moderate temperature. The ash-melting behavior of these feedstocks therefore is not critical. Moreover, the ash content of this group of feedstocks is usually very low (Keller, 1990).

3.4 Preparation techniques for onward processing

Preparation techniques are very much essential in the case of solid feedstocks for gasification. The onward processing steps depend on the individual feedstocks as well. Coal only needs to be pulverized, but biomass has be dried, powdered, and also compacted owing to its low bulk density. It also depends on the end-products to be formed from the process such as electrical energy or chemical energy in the form of hydrocarbons. In the planning of biorefineries and production of liquid biofuels for transport via synthesis gas route, several biomass materials, such as wood, forest residues, bark, straw, energy crops, peat, and agricultural residues, are used. In addition to conversion, the pre-treatment of feedstocks is important, including transfer, storage, chipping, crushing, and drying, and there are many different techniques with variable cost structures (Fagernäs, Brammer, Wilén, Lauer, & Verhoeff, 2010).

3.5 Advantages and limitations of feedstocks for gasification

The various processes described in the preceding sections and the various feedstocks that can be gasified have their own advantages and limitations. The feedstock and, in turn, the kind of preparation steps to be used for a particular type of gasifier is dependent on various factors. The process, economics, and the end-product requirement generally dictate the selection of the steps involved. Some of the challenges in the processes are mentioned here, along with their solutions wherever possible.

During chipping, care must be taken to remove any metal that may be mixed with the wood, as this can severely damage the knives; however, this problem is usually limited to waste wood residues and is not a large concern for dedicated feedstocks. The mobility of these grinders also allows on-site grinding, potentially reducing transportation costs of the feedstock. It is conceivable to envision a system of on-site tub-grinding at dedicated-feedstock wood farms, allowing further sizing to be performed with larger hammermills located within biomass power plants (Cummer & Brown, 2002).

Biological activity may cause slow self-heating in stock piles of wet biomass. Smoldering lumps of biomass constitute a significant ignition source to a violent dust explosion. Spontaneous ignition is another risk factor when handling or storing thermally dried fuels. Proper cooling of the biomass after drying is important to avoid self-ignition problems in intermediate storage bins (Fagernäs et al., 2010; Wilén et al., 1999).

Drying to low-moisture contents is problematic and has not been optimized for biomass conversion processes. The organic emissions during drying can be categorized into volatile organic compounds and condensable compounds. In addition, there are particulate emissions. At low-drying temperatures (under 100 °C) the compounds emitted consist mainly of monoterpenes and sesquiterpenes.

A dryer fire or explosion can arise from ignition of a dust cloud if substantial amounts of fines are present, or from ignition of combustible gases released from the drying material. Both causes of ignition require the presence of sufficient oxygen and either a sufficiently high temperature or some other source of ignition. Under conditions found in most dryers, the risk of fire or explosion becomes significant if the drying medium has an oxygen concentration over approximately 10% (vol.) (Fagernäs et al., 2010).

If a low-oxygen environment can be guaranteed, much higher inlet temperatures may be used, provided material temperatures do not become excessive; prevention of accidental air in-leakage can be difficult and expensive. The user is cautioned to maintain a sufficiently inert atmosphere in the dryer during operation and especially during startup and shutdown. A high-drying temperature creates a risk of spark development and carbon monoxide release through slow pyrolysis and smoldering. Evolution of combustible gases induces a risk of a gas explosion, which may trigger a chain of dust explosions. Proper ventilation and maintaining the inert atmosphere is required before restart. Carbon monoxide together with dust creates a risk of hybrid explosion, which is very violent. With carbon monoxide present in the atmosphere, the safe oxygen level is decreased substantially. The oxygen level has to be kept below 8%. During startup and shutdown of the drying processes, temporary high-oxygen content has to be considered as a risk factor. In superheated steam drying, the guaranteed absence of air and oxygen eliminates fire and explosion risks (Fagernäs et al., 2010; van Deventer, 2004).

Dust emissions from coal storage, handling, and crushing/sizing can be controlled with available techniques. Controlling air emissions from coal drying, briquetting, and partial oxidation processes is more difficult because of the volatile organics and possible trace metals liberated as the coal is heated. The coal gasification process itself appears to be the most serious potential source of air emissions. The feeding of coal and the withdrawal of ash release emissions of coal or ash dust and organic and inorganic gases are potentially toxic and carcinogenic. Because of their reduced production of tars and condensable organics, slagging gasifiers pose less severe emission problems at the coal inlet and ash outlet. Emissions from coal preparation include coal dust from the many handling operations and combustion products from the drying operation. The most significant pollutant from these operations is the coal dust from crushing, screening, and drying activities. Wetting down the surface of the coal, enclosing the operations, and venting effluents to a scrubber or fabric filter are effective means of particulate control (http://www.epa.gov/ttnchie1/ap42/ch11/final/c11s11.pdf). In spite of the several limitations, it must be acknowledged that gasification is the only process that completely utilizes the carbon to produce value-added hydrocarbons.