15.2. Biomass feedstock for gasification
Biomass is the oldest source for energy production and it has been used for cooking and heating for centuries. It provides approximately 12% of the current world final energy consumption. However, half of this energy supply is produced from traditional nonefficient processes, mainly in developing countries; and, consequently, only 6% of the final energy consumption can be considered to proceed from renewable biomass (Demirbaş, 2001; REN21, 2014). Worldwide economic, political and environmental problems that have arisen from the dependency on fossil fuels as main energy sources have driven recent research on the modern use of biomass for bioenergy and biofuels production, including the efficient production of bio-syngas and hydrogen via gasification. Biomass is a promising feedstock for gasification due to its low sulfur content (Panwar et al., 2012); and virtually any biomass with moisture content less than 35% can be efficiently converted to fuel gas (Navarro et al., 2007; Panwar et al., 2012).
15.2.1. Biomass properties as gasification fuel
Biomass feedstock proceeds from diverse sources and exhibits variable chemical composition, which has an influence on their specific properties and applicability. In the case of biomass gasification, physical and thermochemical properties have an effect on the quantity and quality of the bio-syngas produced, and influence the selection of the technologies used for the entire conversion process (pre-treatment, gasification, and bio-syngas cleaning) (IEA, 2006a,b). Table 15.1 summarizes the main properties which define the potential of biomass as a gasification fuel. Moisture content is one of the main properties to consider when using biomass as feedstock for thermochemical processes. In general, “dry biomass” is more economically suited for thermochemical conversion; in particular, optimum moisture content for gasification ranges from 10 to 15 wt%. Other properties such as energy density, ash composition, or alkali and trace elements contents must be also taken into account. Apart from physical and thermal properties, general criteria have to be evaluated when choosing the most appropriate biomass feedstock, such as availability and sustainability of biomass supply, or cost of collection, transportation, and pretreatment.
15.2.2. Typical biomass feedstock for gasification
A large variety of biomass has been investigated as feedstock for bio-syngas and hydrogen production via gasification: dedicated energy crops; organic waste from agricultural farming and wood processing; and, domestic and industrial waste (Balat and Kirtay, 2010; Parthasarathy and Narayanan, 2014; Butterman and Castaldi, 2009; Xiao et al., 2010; Siedlecki and de Jong, 2011). Nevertheless, the main current feedstock for gasification is lignocellulosic biomass, namely wood and residues from forestry and agricultural activities (Balat and Kirtay, 2010; EC, 2005). According to IEA Task 33 (Thermal gasification of biomass) (IEA, a), around 80% of the commercial and operating biomass gasification plants run on woody biomass (mainly wood chips). The use of dedicated woody and herbaceous energy crops, and domestic and industrial wastes will be needed in order to supply the biomass required to meet the projected bioenergy production in the short and medium term (IEA, 2012).
15.2.2.1. Wood and woody biomass
Wood and woody biomass include dedicated forestry and forestry residues (stems, branches, foliage, barks, roots, wood blocks, wood chips from thinning, etc.), which usually constitute from 25% to 45% of the harvested wood. Dedicated short-rotation energy crops such as poplar, willow, or eucalyptus can provide complementary sources of woody biomass for energy (EC, 2005; IEA, 2002).
Table 15.1
Biomass properties with an effect on the gasification conversion process (Higman and Van der Burgt, 2011; IEA, 2015; Kirubakaran et al., 2009; Quaak et al., 1999; Stahl et al., 2004; ECN, 2015)
The consolidation of the use of wood as the main renewable feedstock for energy production, as an alternative to fossil fuels, will depend on the economically, environmentally and socially sustainable production of this biomass (IEA, 2002).
15.2.2.2. Herbaceous and agricultural biomass
Herbaceous and agricultural biomass includes both dedicated energy crops (miscanthus, switchgrass) and agricultural residues (straw, shells, husks, pits, seeds, coir, stalks, cobs, bagasse, food, fodder, pulps, etc.). This feedstock is expected to be the key for large expansion of biomass supply in the medium and long term.
Similar to the use of woody biomass, the development of bioenergy production from dedicated crops will be based on the positive impact on the farmed landscape and biodiversity. Bioenergy crops have to be cultivated to minimize soil erosion and compaction, nutrients leaching into ground and surface water, water abstraction, pesticide pollution and fire risk, and to enhance rural development (IEA, 2006).
15.2.2.3. Wastes
This category includes a very broad range of feedstocks; livestock manure, sewage sludge, municipal solid waste, demolition wood, food processing waste, or paper industry waste, among others.
Livestock waste (manure and other organic waste) has traditionally been used to produce biogas from anaerobic digestion. However, the development of new technologies, such as wet gasification, eliminates the need for drying and allows the direct use of wet manure as feedstock for waste-to-energy systems (Ro et al., 2007). A similar technological route is applicable to wet sewage sludge. The application of sewage sludge to produce energy via gasification presents the advantage of the supply assurance; the produced amount of sewage sludge is expected to be steady and will remain a permanent waste problem that requires an appropriate solution (EBA, 2013). In addition, high-temperature conversion of sewage sludge may contribute to reducing the environmental problems related to the toxics present in its composition, which are an issue in biochemical processes such as anaerobic digestion (Fytili and Zabaniotou, 2008).
Municipal solid waste (MSW) represents a significant resource for energy recovery operations. Energy from waste (EfW) conversion process is considered one of the most efficient commercially available technologies for the production of power, combined heat and power, and liquid biofuels via the Fischer–Tropsch reaction (Howes and Warren, 2013).
Black liquor is one of the industrial wastes with higher economical potential for power generation from gasification (Indufor, 2013; Consonni et al., 2009). Black liquor is a recycled byproduct from the papermaking industry, a combination of the lignin residue with water and the chemicals used for the extraction. Black liquor logistics are largely simplified compared to other biomass as its utilization takes place where it is produced, is already partially processed and is in a pumpable liquid form. The use of black liquor for biofuel production via gasification is particularly interesting in countries with strong pulp and paper industries. Around 1.8 tonnes of black liquor per tonne of pulp processed. Considering the usual capacity of the mills, the amount of black liquor produced is equivalent to a power of 250–500 MW (or MJ/s) per mill. The energy produced would depend on the selected time frame.
15.2.3. Improved biomass feedstock for gasification
The slow deployment of large-scale gasification technology for bio-syngas and hydrogen production is widely related to the biomass availability near to the conversion sites, and the physical and thermal properties of the solid biomass. Relatively long-distance transportation of low-energy-density solid biomass is expensive. Various pretreatment processes for improving biomass properties and increasing its energy density have been recently proposed and investigated (IEA, 2015; Wright et al., 2008; Svoboda et al., 2009; Sakaguchi et al., 2010):
• Torrefaction, which produces a char with improved friability;
• Flash pyrolysis to produce a pumpable bio-oil nearly free of ash or a pumpable bio-oil/char slurry;
• Addition of glycerol, ethylene glycol, phenolic oil, or a bioliquid for preparing a pumpable biomass slurry;
• Addition of a solvent to practically dissolve the biomass.
Torrefaction of lignocellulose biomass is a thermal conversion process which occurs at a slow heating rate, temperature range of 200–300°C and under inert atmosphere (van der Stelt et al., 2011; Kleinschmidt, 2011; Basu, 2010a,b). The main product is a solid material that presents improved physical and thermal properties compared to the original biomass. Torrefaction causes depolymerization of hemicellulose and removal of oxygen from biomass. The resulting torrefied biomass has lower O/C and H/C ratios and its energy density is increased by approximately a factor of 1.3 compare to original biomass (van der Stelt et al., 2011). Torrefied biomass also presents more hydrophobic behavior, higher friability, more uniform quality, and gives rise to less acid when it is heated at high temperatures. Consequently torrefied biomass is easier and less costly to transport, handle, mill, and store (van der Stelt et al., 2011; Basu, 2010a,b). In addition, the cost of feeding the torrefied biomass into gasifiers may be reduced (Svoboda et al., 2009). The first generation of torrefaction technology is currently in demonstration phase. The addition of a torrefaction step prior to the gasification stage represents an extra unit operation and an increase in the capital cost. However, the economic potential of the integrated scheme is still significant due to the decrease in operating costs. Despite the first steps toward commercialization, financing of the technology is still complicated due to the uncertain economic impact on the business. Thus, torrefaction technology presents challenges such as its limited applicability to woody biomass (eg, agricultural biomass tends to ignite or carbonize), the acidity of the torrefaction gas and the formation of primary tars, the optimization of operating conditions, and the product validation and technology (Kleinschmidt, 2011).
Fast pyrolysis of woody biomass, which occurs at moderate temperatures (≈500°C) and short hot vapor residence time (≈1 s), gives rise to approximately 75 wt% of bio-oil and 12 wt% of char (Bridgwater, 2012). The resultant bio-oil presents high energy density. As an example, the specific gravity of bio-oil produced from fast pyrolysis of switchgrass is approximately three times higher than that for the switchgrass pellets (Wright et al., 2008). Bio-oil is easier to handle and transport than the raw biomass, reducing transportation costs to a centralized large-scale gasification plant. The cost of the feeding system may also decrease since feeding bio-oil or slurries to high-pressure gasifiers is relatively simple (Svoboda et al., 2009). The distributed conversion of biomass to bio-oil or bio-oil/char slurry via fast pyrolysis as a potential alternative to centralized bio-syngas production via biomass gasification is attracting more and more attention. The produced bio-oil (or bio-oil/char slurry) would be transported to a centralized plant where it would be gasified and the resulting bio-syngas subsequently converted to other high-value products (Wright et al., 2008; Sakaguchi et al., 2010). Wright et al. (2008) demonstrated that the distributed fast pyrolysis of biomass followed by centralized gasification of the produced bio-oil and Fischer–Tropsch conversion of syngas to liquid biofuels presents significantly lower production cost than that of a centralized biomass gasification and FT synthesis process. One of the main challenges for the implementation of the distributed scheme is that the bio-oil degrades when stored due to its acidity and high oxygen content.
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