Converting sugar obtained from cellulosic biomass such as corn stalks or wood chips is a proven path to creating biofuels and biochemicals. For decades, researchers have been focused on the use of enzymes to get sugars from biomass. While it works, the use of enzymes is expensive and so far has made the production of biofuels from cellulosic biomass economically unattractive.
Our research team at Iowa State University has recently discovered a cheaper and simpler alternative to obtain sugar from biomass. This “pyrolytic sugar,” as we call it, is produced by infusing biomass with small amounts of mineral acids and then subjecting it to the thermochemical process of fast pyrolysis. The sugar can then be fermented to ethanol or catalytically upgraded into biofuels or other bio-based products. (Figure 1) This thermochemical approach increases the prospects for economical production of advanced biofuels and commodity chemicals from cellulosic biomass.
Harnessing abundant cellulose
Cellulose is the most abundant biomass on the planet. For biofuels, common sources of cellulose include switchgrass, corn stover and wood. Chemically, cellulose is a structural polysaccharide consisting of a long chain of glucose molecules linked by glycosidic bonds. Breaking the links in the chain releases the glucose and makes it available for food or fuel production.
If cost-effective methods can be developed to depolymerize cellulose into monosaccharaides (simple sugars), cellulosic biomass could become the basis for renewable fuels and commodity chemicals. To date, much of the research and development effort in advanced biofuels has focused on biological approaches that employ enzymes to hydrolyze cellulose to monosaccharaides.
Enzymes, however, are relatively expensive to produce, currently estimated to cost as much as $1.60 per gallon of ethanol.1 This represents one of the major impediments to commercializing cellulosic ethanol.
Rethinking fast pyrolysis
In contrast to biochemical processing, thermochemical processing uses heat and catalysts to transform the chemical energy of biomass into fuels. One of the most promising thermochemical processes for converting biomass into biofuels and chemicals is fast pyrolysis, which is the rapid thermal decomposition of organic compounds in the absence of oxygen. Typical pyrolysis temperatures are 400°C to 500°C. Traditional fast pyrolysis of biomass produces liquid, gas and char. The liquid, commonly called bio-oil, is not generally suitable as a transportation fuel, but can be upgraded for this purpose.
Fast pyrolysis of biomass also can produce sugar. When pure cellulose is subjected to fast pyrolysis, the bonds between glucose rings are broken, yielding a sugar called levoglucosan and other anhydrosugars, which are attractive precursors for production of biofuels. Levoglucosan yields from the pyrolysis of pure cellulose can be as high as 59 percent of the weight of the initial biomass using pyrolysis.2
Most biomass, however, contains naturally occurring metals such as potassium and calcium. While these are important nutrients for plant growth, these alkali and alkaline earth metals (AAEM) can catalyze the pyrolitic decomposition of cellulose to yield less desirable smaller molecules. We believe this process occurs in competition with depolymerization, reducing the amount of sugar produced.
Our studies included both woody biomass (red oak and loblolly pine) and herbaceous biomass (switchgrass and cornstover), which provided a wide range of naturally occurring AAEM concentrations. Typically, woody biomass contains less than 500 parts per million (ppm) of total AAEM, while herbaceous biomass can contain more than 10x this amount.
If catalytic activity of AAEM in biomass can be substantially reduced or eliminated, then cellulose could be depolymerized to sugars by fast pyrolysis without the use of enzymes or strong acids, as currently envisioned for the production of cellulosic biofuels. Like the products of enzymatic or acid hydrolysis, these sugars can be fermented or catalytically upgraded.3