Research review paperWater-based woody biorefinery
Introduction
Food, energy and materials are the basic needs of humans and Fig. 1 illustrates the flow from various sources to meet these requirements. Compared with the available solar energy, our other primary energy sources—geothermal, nuclear, petroleum, coal, natural gas, and minerals—are limited. Biomass was historically the primary source for all human needs, but we have learned to utilize fossil energy sources, such as petroleum, coal and natural gas, to significantly increase our standard of living.
Fossil carbon has become the dominant energy and chemical source for mankind since the industrial revolution (Amidon et al., 2008). However, there is only a finite amount of available fossil energy. World Oil (EIA, 2006) puts the known recoverable reserves, as of January 1, 2006, at 1,119.615 billion barrels for crude oil; 6,226.555 trillion cubic feet for natural gas; and 997,748 million tons for coal. Klass (1998) estimated that world crude could last until 2027, based solely on the 1990s proven conventional reserve. His most optimistic estimate was that the oil could last until 2100, based on the current growth in energy consumption and assuming that other potential oil sources, such as new discoveries and oil sands, are developed. Using the longest time estimate, it would take less than 280 years to completely deplete all the oil that ever existed on Earth. We must therefore look for alternatives to petroleum, since the time scale required for petroleum to recycle or naturally replenish is, at best, in the order of 280 million years (Table 1). Fossil fuel replenishes when a carbonaceous age occurs on the Earth, resulting in the preservation of a large amount of organic matter, but the chances of a high carbon-storing event occurring while higher life forms survive is unlikely. Nevertheless, the amount of reserves is too small relative to the 280 million-year rotation, even assuming mankind could survive the fossil replenishment era. Thus, petroleum and fossil energy resources are considered to be effectively nonrenewable. Societal awareness of environmental impacts, as well as problems in stability and sustainability of energy supply, has made the development and implementation of bio-based chemicals/energy urgent. Domestic energy security and rural economies could both benefit from a plant-derived chemical/energy economic base.
Biomass has been an important energy source since the beginning of civilization. Ligno-cellulosic biomass is the most abundant organic source on earth, with an annual production of about 170 billion metric tons (Klass, 1998). Tapping into the chemical energy of biomass and restoring its historically important position in energy generation and transportation is imperative to the sustainability of the world economy. Forests cover about 9.5% of the Earth's surface, or about 32% of the land area, but account for 89.3% of the total standing biomass and 73 billion metric tons/year, or 42.9%, of the total annual biomass production. Savanna and grasslands come second, accounting for 11% of total biomass production. In energy terms, forests alone could produce 1030 quadrillion British Thermal Unit (Btu)/year (Klass, 1998), which is equivalent to more than double the world's total primary energy consumption of about 460 quadrillion Btu in 2005 (EIA, 2003). Research and development is required to boost the energy conversion efficiency from plant and woody biomass to meet industrial and residential energy and commodity chemical requirements. Plants synthesize chemicals from solar energy, as shown in Fig. 2. The chemical energy stored by the biomass can be converted into energy and chemicals that can then be utilized, also shown in Fig. 2. The use of dedicated or managed energy crops (agriforest and/or agricultural biomass) could further increase the available biomass (Keoleian and Volk, 2005, Ragauskas et al., 2006).
Renewable forest material is carbon neutral, i.e. utilizing forest material will not create a carbon imbalance over the life cycle of the forest, which is an extended 5–80 years for managed forests, as shown in Table 1. “Catch and release” is the key to biomass utilization. Carbon dioxide is drawn from the atmosphere to allow the plants to grow, while planting, management, the conversion of biomass into bio-products, and the utilization and decomposition of bio-products will all produce carbon dioxide. In an optimally-balanced operation, carbon dioxide is simply being recycled through plant growth and bio-products, as illustrated in Fig. 2 (Liu et al., 2006). The net effect is that solar energy and atmospheric carbon dioxide can be converted into energy and materials that can be utilized by humans.
Because the nonrenewable energy sources predominantly used today will cease to be available in the future, it is necessary to consider expanding the production and use of plant biomass, especially forest material, as a sustainable energy and chemical source (Amidon, 2002). The biorefinery is a manufacturing concept for converting plant biomass into energy and chemicals, in which plant biomass can be separated into a number of compounds and energy is recovered. Utilization of the heterogeneous, bonded composite structure is a key for the future supply of chemicals and energy. The use of renewable carbon eliminates the fossil-derived carbon dioxide burden to the environment and reduces the greenhouse gas (GHG)-driven global climate change. Managed forests thus have significant potential for reducing GHG emissions through conversion of the forest material into liquid fuels, electricity and other products that are currently derived from nonrenewable carbon.
Section snippets
SUNY College of Environmental Science and Forestry (ESF) biorefinery: water-based technology
In addition to its direct utilization as a building material, the major uses of wood today are for making paper and for generating energy by burning/combustion. Large volumes of wood, in the order of 1000 metric tons/day or more, are consumed at many industrial sites. In a pulp mill, residual wood chips and shavings from nearby lumber mills and/or wood chips from round wood are either chemically or mechanically disintegrated into fibers. In a chemical pulp (Kraft) mill, aqueous NaOH and Na2S
Hot-water extraction
Woody biomass is composed of four main components: extractives, hemicellulose, lignin and cellulose. Cellulose provides the structure and strength, while hemicellulose and lignin provide bonding to the structure. Extractives are extractable compounds of the woody biomass that can be readily dissolved with organic solvents or water at room temperature and under atmospheric conditions. Inorganic compounds are also present in the woody biomass. There are over 70 metal, earth elements and inorganic
Hydrolysis
The wood extracts after hot-water extraction consisted of monosaccharides, polysaccharides, acetic acid, degraded lignin, and other low-molecular weight extractable substances. Hydrolysis of oligomers to monosaccharides or sugars could be achieved by increasing the residence time for the wood extracts at relatively high temperatures under acidic conditions, or by enzymatic digestion.
Fig. 9, Fig. 10, Fig. 11 show the effects of pH on the hydrolysis or reducing-sugar end group formation as a
Separation/purification
Wood extracts after hot-water extraction consisted of monomeric and oligomeric sugars (hexoses and pentoses), acetic acid, degraded lignin (or aromatics), furfurals and other low-molecular weight extractable substances. Nano-filtration membranes have been used to analyze these. The separation of sugars and acetic acid is a key step in the biorefinery process, as acetic acid is a potential inhibitor of fermentation processes.
The choice of membrane depends on the physical and chemical properties
Conclusions
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Hot-water extraction of sugar maple wood chips at 160 °C for 2 h removed approximately 23% of the woody biomass. Cellulose and lignin mostly remained within the residual wood chips, but more than half of the wood acetyl groups were present in the extraction liquor. The extraction liquor pH decreased with increasing extraction time and leveled off 1 h after the desired extraction temperature was reached. The final pH was about 3.5. The total measured dissolved solids in the extract increased
Acknowledgments
The authors are thankful to NYSERDA and US DOE for financial support for this study. The authors are indebted to the Biorefinery Research Institute (BRI) for financial and research support.
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