The production of transportation fuels, primarily ethanol, from biomass continues to attract interest, due to the low cost and wide availability of biomass, and because ethanol produced from biomass (e.g., bioethanol) may be used to displace the use of fossil fuels. For example, ethanol used for a transportation fuel may be blended into gasoline at predetermined concentrations (e.g., 10%).
The production of bioethanol from first generation processes, wherein the biomass contains sugar that is readily fermented (e.g., sugar cane or sugar beets), or starch that is readily converted to sugar and then fermented (e.g., corn grain, barley, wheat, potatoes, cassava), is well known. In fact, the diversion of farmland or crops for first generation biofuel production has led to much debate about increased food prices and/or decreased food supplies associated therewith. In addition, there are concerns related to the energy and environmental impact of these production processes.
Second generation biofuels, also referred to as advanced biofuels, wherein the biomass contains lignocellulosic material and/or is obtained from agricultural residues or waste (e.g., corn cobs, corn stover (e.g., stocks and leaves), bagasse, wood chips, wood waste), may allay some of these concerns. For example, when bioethanol produced using second generation processes (i.e., also referred to as cellulosic ethanol) is derived from agricultural waste or residue, its production should not affect the food supply. In fact, tremendous effort is currently being expended to advance cellulosic ethanol production processes.
Lignocellulosic biomass typically contains cellulose, hemicellulose and lignin, each of which is present in plant cell walls. Cellulose (e.g., a type of glucan) is an unbranched chain polysaccharide including hexose (C6) sugar monomers (e.g., glucose). Hemicellulose is a branched chain polysaccharide derived from several sugars, which may include different pentose (C5) sugar monomers (e.g., xylose and arabinose) in addition to glucose. Lignin is a complex organic polymer, which typically includes cross-linked phenol polymers. Although generally insoluble in water at mild conditions, lignin may be soluble in varying degrees in dilute acid or base alkali. The ratio and/or structure of these components may vary depending on the source of the biomass.
The production of bioethanol from lignocellulosic biomass most often involves breaking down the cellulose and/or hemicellulose into the constituent sugars, which may then be fermented. Unfortunately, the cellulose, hemicellulose, and/or lignin found in lignocellulosic biomass is typically structured within the plant walls to resist degradation. For example, lignin, which may be the most recalcitrant component of lignocellulosic biomass, is believed to be tightly bound to the cellulose and/or hemicellulose.
In general, lignocellulosic biomass may be broken down into sugars in one or more stages, wherein at least one stage includes a chemical hydrolysis (e.g., which may include the addition of acid, base, and/or heat) and/or an enzymatic hydrolysis (e.g., which includes using enzyme(s)).
For example, one common approach to converting lignocellulosic biomass to sugar(s) includes (a) a pretreatment stage, followed by (b) an acid or enzymatic hydrolysis. In this approach, the goal of the pretreatment stage is to break down the lignin structure and/or disrupt the crystalline structure of the cellulose, so that the acids or enzymes used in the hydrolysis can easily access and hydrolyze the cellulose to sugar.
In general, pretreatment methods that improve the rate and/or yield of sugar in the subsequent hydrolysis (e.g., by liberating the cellulose from the lignin and/or by making the cellulose more accessible) may be used. Some examples of suitable pretreatments include acid pretreatment, alkali pretreatment, autohydrolysis (e.g., hot water extraction that does not require the addition of acid or base) steam explosion, and wet oxidation. For example, dilute acid pretreatment is believed to hydrolyze the hemicellulose component of the feedstock to yield xylose, glucose, galactose, mannose and/or arabinose, whereas alkali pretreatments are believed to cleave hydrolysable linkages in lignin and/or glycosidic bonds of polysaccharides (e.g., thus disrupting lignin structure and/or reducing crystallinity of cellulose). Accordingly, acid pretreatment, alkali pretreatment, and autohydrolysis may be considered forms of chemical hydrolysis.
Although treating lignocellulosic biomass with a mild acid pretreatment (e.g., a high temperature, short residence time) has been proven useful in terms of hydrolyzing the hemicellulose component to produce xylose, glucose, and/or arabinose, chemical hydrolysis of the cellulosic component typically requires relatively harsh conditions (e.g., dilute acid under high heat and high pressure, or more concentrated acid at lower temperatures and atmospheric pressure). Unfortunately, these relatively harsh conditions may produce toxic degradation products that can interfere with the fermentation process. Accordingly, it is advantageous if the hydrolysis following the pretreatment stage is enzymatic rather than solely chemical (e.g., acid).
In each case, the enzymatic and/or chemical hydrolysis is typically followed by a fermentation stage, which for example uses one or more yeasts or bacteria to convert the sugar(s) produced by the hydrolysis to an alcohol (e.g., ethanol). Yeast cells, in particular, have experienced wide-spread use in cellulosic ethanol processes because these naturally occurring or genetically modified microorganisms are particularly efficient at converting sugars such as glucose and xylose to ethanol. In fact, yeast cells have been used in biotechnology for hundreds of years to produce ethanol.
Despite the fact that sugars are natural intermediates in the biological and chemical conversion of lignocellulosic biomass, and that a properly selected combination of pretreatment and enzymes can enable high yields of sugar from both hemicellulose and cellulose, there may be some challenges to this type of approach. For example, much effort has focused on optimizing the pretreatment and/or hydrolysis stages of the process to make it cost competitive with corn-based ethanol. Another challenge is that a portion of the biomass will not be converted to ethanol. For example, it is well known that some lignin typically remains after pretreatment/hydrolysis (e.g., insoluble lignin and/or solubilized lignin). For example, in some cases the remaining lignin may be burned to provide on-site power, thus recovering some energy from the biomass.
Apart from the biochemical process discussed above, another second generation approach to producing biofuels is to use a thermal process. In particular, the lignocellulosic biomass may be heated at high temperature in the absence (i.e., pyrolysis) or presence (i.e., gasification) of oxygen, air and/or steam. Pyrolysis of biomass may be used to produce bio-oil, whereas gasification (i.e., which occurs without combustion) may be used to produce syngas. Syngas, which may include carbon monoxide (CO), hydrogen (H2) and/or carbon dioxide (CO2), may be converted to a biofuel (e.g., via a Fischer-Tropsch reaction) or used as a biofuel. For example, in one approach, syngas is converted to mixed alcohols using a catalyst. In another approach, syngas is subjected to a gas fermentation to provide ethanol from CO, CO2 and/or H2. Unfortunately, some of the energy stored in the sugar polymers may be lost in the thermal process (e.g., a portion of the biomass may not gasify). In addition, these thermal processes may be difficult to operate and/or may require a high energy investment (e.g., especially if using a Fischer-Tropsch process), which means that these processes may not yet be economical (e.g., relative to the biochemical approach discussed above) and/or do not reduce greenhouse gas emissions.