Co-flred combustion involves using biomass to supplement the primary feed, typically coal or natural gas. Two methods of co-firing biomass exist. The first method, direct co­firing, involves biomass being co-fed directly into the boilers with the coal or natural gas. Direct co-fire technology allows up to 20% of the feed to be supplied by biomass without significant changes to the facility [13]. The second, indirect co-firing, involves first gasifying biomass then co-firing the resulting gases in a combustion system. Co-firing is an immediate, low-cost option for efficiently and cleanly converting biomass to electricity by adding biomass as a partial substitute fuel in high-efficiency coal boilers. There is little or no loss in total boiler efficiency after adjusting combustion output for the new mixture. Co-firing with biomass will also help reduce greenhouse gas emissions, primarily SO2, NOx and CO2.

Opportunities for biomass co-firing are great because large-scale coal-powered boilers represent 310 GW of generating capacity [1]. While direct co-firing usually requires less modification to a generation system, it restricts the amount and type of biomass that can be used. Indirect co-firing through gasification allows for removing alkali metals and chlorine, which can cause fouling, slagging and corrosion in a coal or natural gas boiler system, from the feedstock. Indirect co-firing also reduces the effects of feedstock variability. Biomass feedstocks, by their nature, have a high degree of variability in terms of moisture content, heating value and ash profile. By using gasification to convert these fuels to a gaseous form, much of this variability can be reduced or eliminated. This has the effect of making operation of the boiler more stable [14]. There are a lot of issues yet to be completely solved with co-firing biomass with coal; however, biomass co-firing has been a proven opportunity for coal facilities for more than a decade [13].

Energy density, often termed “heating value”, refers to the amount of energy released per unit fuel combusted, usually measured in terms of energy content per unit mass for solids (e. g. MJ/kg) and per unit volume for liquids (e. g. MJ/l). Energy density can be expressed in two forms, higher heating value (HHV) or lower heating value (LHV). HHV represents the total energy released when the fuel is combusted in air, including the latent heat contained in the resulting water vapor product — the maximum potentially recoverable energy from a given feedstock. The latent heat contained in the water vapor, however, typically cannot be used effectively. LHV, therefore, is the appropriate value to use when quantifying the energy available for subsequent use. As noted above, moisture content significantly affects biomass feedstock energy density. Freshly cut wood, for example, might have as much as 60% moisture and a relatively low energy content (e. g., 6 MJ/kg). In contrast, oven-dried wood with little moisture might have up to 18 MJ/kg. Representative LHV values for many of the biomass crops considered in subsequent chapters are listed in Table 1.1.

Alternatively, the transformation of organic compounds into all kind of industrial products can also be performed by biocatalysis. Biocatalysis can be defined as the use of biolo­gical systems (including whole cells or isolated components thereof, natural and modi­fied enzymes and catalytic antibodies) to perform chemical transformations on organic compounds [31-33]. Millions of years of evolution have created thousands of microorgan­isms containing enzymes known to catalyze almost every chemical reaction. Biocatalysis is increasingly used in the chemical industry and has developed into a main contributor for sustainable chemistry. Biocatalytic reactions are typically performed at normal tempe­ratures and pressures, whereby no dangerous intermediate products are needed, nor are dangerous waste products generated. Biocatalysts, substrates, intermediates, by-products and the product itself are biodegradable. Water is usually used as a solvent. The use of enzymes as biocatalysts can have significant performance benefits compared to conven­tional chemical technology; for example, a high reaction selectivity, higher reaction rate, increased conversion efficiency, improved product purity, lowered energy consumption and a significant decrease in chemical waste generation. There are also frequent disadvantages, however, including difficult enzyme recovery, low product concentration, low productivity due to substrate and/or product inhibition and, hence, high recovery costs [34]. An impor­tant route to improving the performance of enzymes in non-natural environments and their ability to work in continuous processes is to immobilize them by either adsorption, covalent attachment or by incorporation in hydrophobic organic-inorganic hybrid materials [35-38].

As they are typically very selective (contrary to most conventional chemical catalysts), enzymes are particularly useful to produce, for example, chiral molecules or enantiomeri — cally pure compounds [39]. Biocatalysts can be used to initiate major chemical reactions, such as the direct polymerization of phenols, the direct oxidation of propylene, or highly selective transformations with polyfunctional substrates, such as sugars [34]. A wide vari­ety of chemical substances are already industrially produced through the use of enzymes [40]. Numerous syntheses are conducted exclusively using enzymes (lipases, amylases, pro­teases, and, also, increasingly, cellulases [34]). As now all the molecular and biological tools to make enzymes more stable and even to discover more stable enzymes are available, more bulk chemical products from enzymatic processes can be expected in the coming years [41].

Repowering is the option of replacing existing equipment with new technology. The United States has roughly 1400 operating coal-fired generating units producing almost 2 billion MWh of electricity per year [1]. By 2015, more than 90% of those units will be over 30 years old. Furthermore, in addition to producing almost 50% of the United States’ electric power, these older units produce almost 35% of the total CO2 emissions in the United States and up to 40% of the ground level air pollutants such as SO2 and NOX [15,16]. As these systems age, steam production efficiency declines and the units struggle to meet emission compliance. An alternative to shutting down many of these facilities could be to “repower”.

Repowering can involve partial or total replacement of existing infrastructure. The extent of repowering depends on many factors including: (1) environmental discharge limits, (2) permitting requirements, (3) increased demand or generating load, (4) fuel cost, and (5) transmission requirements. For example, converting a coal burning facility to a biomass burning facility requires boiler modifications, addition of a gasifier, and addition of biomass handling facilities. These changes are essential because biomass has a lower heating value and more material will be required to produce the same amount of energy. Repowering, however, increases environmental performance, as the conversion from coal to biomass can reduce NOX emissions by 60%, SO2 by 80% and particulate matter by 80%. The down side is that biomass prices tend to fluctuate and competition for biomass is steadily increasing. High biomass feedstock prices will seriously impact the economics of repowered systems [3].

Fuel analysis that quantifies the amount of chemical energy stored as volatile matter (VM) and fixed carbon (FC) has been developed for solid fuels such as coal. The VM of a solid fuel is the portion released as gas (including moisture) by heating to 950°C in the absence of air for seven minutes; the FC is the mass remaining after the volatiles have been driven off, excluding the ash and moisture contents. Fuel analysis based upon VM content, ash, and moisture, with the FC determined by difference, is termed the proximate analysis of a fuel. Elemental analysis of a fuel, presented as C, N, H, O and S, together with the ash content, is termed the ultimate analysis of a fuel. The ratio of FC to VM provides an indication of the ease with which the solid fuel can be ignited and subsequently gasified, or oxidized, depending on how the fuel is to be converted. Representative proximate and ultimate analyses for many of the biomass crops considered in subsequent chapters are listed in Table 1.1.

The transformation of organic compounds into all kind of industrial products can also be done using chemical catalysts. Chemical catalysis uses an added — but not consumed — sub­stance to augment a chemical reaction. Catalytic conversion will be a primary tool for indus­try to produce valuable fuels, chemicals, and materials from biomass platform chemicals.

Catalytic conversion of biomass is best developed for producing synthesis gas, or syngas. In addition, research is being performed on the use of chemocatalysis for the production of biofuels out of lignocellulosic biomass. Heterogeneous catalysis offers potential to selec­tively convert lignocellulosic biomass into various useful chemicals; this methodology has progressed rapidly in the last several years [42]. Promising technologies are ‘Aqueous Phase Reforming’ for the production of liquid alkanes or hydrogen from biomass-derived sugars, as developed by Virent Energy Systems (www. virent. com). Different approaches and strate­gies are also available for catalytic lignin valorization. Generally, lignin reduction catalytic systems produce bulk chemicals with reduced functionality, whereas lignin oxidation cata­lytic systems produce fine chemicals with increased functionality [43]. Chemical catalysis further offers a large variety of possibilities to upgrade sugars: sugars can be hydrogenated to C5-C6 polyols (or sugar alcohols) such as xylitol, mannitol and sorbitol, hydrogenolysed to C2-C3 glycols, or further upgraded via oxidation or halogenation reactions [44-46]. Catalysts are also involved in liquefaction, fast pyrolysis and gasification to convert lignocellulosic biomass into value-added fine chemicals and biohydrocarbon fuels [47].

Direct hydrothermal liquefaction involves converting high-moisture biomass to an oily liquid. Depending on the biomass used, the resulting bio-oil can have a heating value com­parable to bunker crude oil (30-40 MJ/kg). The resulting oil can be burned in boilers or upgraded and refined into higher value fuel or chemical compounds. Direct hydrothermal liquefaction works by contacting biomass with water at elevated temperatures (300-350°C) and sufficient pressure to maintain the water in the liquid phase (12-20 MPa) [21]. Addi­tionally, alkali catalysts may be added to promote organic conversion. In the process, water acts as a necessary reaction medium, therefore eliminating the need to dry down biomass and, thus, reducing the total energy footprint. Hydrothermal liquefaction processes have the potential to become an important group of technologies for converting wet biomass or organic waste into bio-oil for fuel or other applications. The hydrothermal liquefaction process holds significant potential, particularly for producing specific fuels targeted for the heavy transport sector, combustion purposes, and as a raw material for further chemical processing [22].

The robust reaction conditions and aqueous environment make hydrothermal liquefaction well suited for the conversion of low-lipid, fast-growing algae that proliferate in wastewa­ter treatment facilities [23]. Integrating algae cultivation into a wastewater treatment plant offers the synergetic benefit of providing nutrient remediation because algae capture and use dissolved nitrogen and phosphorous present in wastewater to support growth [23]. These plentiful nutrients would otherwise be released into the environment, creating harmful eutrophication of natural systems. By converting nutrient waste into a resource, environ­mental pollution will be reduced, as energy is created and water resources are preserved [24].

Conversion of biomass feedstock, either thermochemically or biochemically, results in a solid residue. In themochemical processing via combustion in air, the residue consists solely of ash. For biochemical processing, it contains both ash and other unconverted material, especially lignin. The bioprocess residue can be further processed thermochemically to yield ash as the final solid residue. The ash content negatively affects the energy density of the feedstock. Ash can also pose operational problems in thermochemical processing, such as slagging in which the ash melts and fuses together. Relatively low-cost control measures, such as leaching the raw feedstock with water and using different mineral additives (e. g. kaolinite, clinochlore, ankerite), can be used to reduce negative effects [27]. Potential end uses of ash include mineral agricultural fertilizer [28] and construction material additive [29]. Representative ash content values for many of the biomass crops considered in subsequent chapters are listed in Table 1.1. As can be seen from the table, herbaceous feedstocks tend to have higher ash contents (e. g. >5%) than woody feedstocks (e. g. <2%).

Alternatively, biomass can be converted into fuels and chemicals indirectly (by gasification to syngas followed by catalytic conversion to liquid fuels or basic chemicals) or directly to a liquid product by thermochemical means such as pyrolysis or liquefaction. Thermochemical conversion processes use heat and pressure to convert biomass into liquid, bio-oil or gaseous intermediates. These intermediates, such as syngas and bio-oil, subsequently go through customized processing to produce biopower, biofuels or building blocks for biochemicals.

Thermochemical processes allow productive use of a wide spectrum of biomass resources. The relative high temperatures of thermochemical processes (300-1000°C) over­come the natural resistance of biomass to chemical or enzymatic conversion, thus expanding the range of feedstock that can be potentially used. Common thermochemical conversion pathways include gasification, pyrolysis and, to a lesser extent, liquefaction. The difference between the three processes is determined by three main parameters: the oxygen level (A.), pressure and temperature ([16,48]; Figure 2.3).

Anaerobic digestion is a biological process in which bacteria break down organic matter in the absence of oxygen. A biodigester or digester is an airtight chamber in which anaerobic digestion of manure, sewage, food waste, or other organic waste streams occurs [25].

Anaerobic digestion occurs in an aqueous environment, allowing high-moisture feedstock (less than 40% dry matter) to be used without any pretreatment [26].

Digesters have been used commercially for over 30 years and are currently found in the agricultural, wastewater treatment, and food waste management sectors. Anaerobic digestion produces commodities such as biogas (a blend of methane and carbon dioxide), biosolids (used as a soil amendment), animal bedding, and fertilizer. Biogas can be used as a fuel to generate electricity, as a boiler fuel for steam production, space or water heating, or upgraded to natural gas for pipeline injection or for vehicle fuel (compressed natural gas (CNG) and liquefied natural gas (LNG)) [27]. Regardless of the type of device, control of biogas emissions leads to significant reductions in greenhouse gas emissions. Additional benefits of anaerobic digestion include potential water pollution control opportunities, and additional revenue streams or financial savings. Finally, digester projects may be eligible to sell renewable energy credits (RECs) and/or carbon offsets, which can improve project economics. Despite their potential to address pressing environmental concerns and generate revenue, digester use is not widespread in the United States.