A biorefinery system starts with the contract harvesting of whole crops (grain and straw), which are then stored and fractionated (including drying as necessary) into products and byproduct for sale. A biorefinery is a factory that processes crops, such as wheat, barley, and oilseed rape, to produce various refined specialized fractions, such as flour, gluten, starch, oil, straw chips, etc. The concept of a biorefinery, com­pared with, for example, a flour mill, is that the use and value of all the fractions into which the input can be separated is maximized (Audsley and Annetts 2003).

The analysis of a biorefinery system can be considered in three parts. The first is the effect on the farm of selling products to a biorefinery, on the assumption that the biorefinery contracts to harvest the crop using a wholecrop forage harvester. The second part is the impact of the type of biorefinery system on the profitability of the processing required to produce the various products. The third part is the transport of crop to the biorefinery, which is a function of the distribution of farms around the biorefinery location (Audsley and Annetts 2003).

As biomass hydrolysis and sugar fermentation technologies approach commer­cial viability, advancements in product recovery technologies will be required. For cases in which fermentation products are more volatile than water, recovery by dis­tillation is often the technology of choice. Distillation technologies that will allow the economic recovery of dilute volatile products from streams containing a variety of impurities have been developed and commercially demonstrated. A distillation system separates the bioethanol from water in the liquid mixture.

The first step is to recover the bioethanol in a distillation or beer column, where most of the water remains with the solids part. The product (37% bioethanol) is then concentrated in a rectifying column to a concentration just below the azeotrope (95%). The remaining bottom product is fed to the stripping column to remove additional water, with the bioethanol distillate from stripping being recombined with the feed to the rectifier. The recovery of bioethanol in the distillation columns in the plant is fixed at 99.6% to reduce bioethanol losses.

After the first effect, solids are separated using a centrifuge and dried in a rotary dryer. A portion (25%) of the centrifuge effluent is recycled to fermentation and the rest is sent to the second and third evaporator effects. Most of the evaporator condensate is returned to the process as fairly clean condensate (a small portion, 10%, is split off to wastewater treatment to prevent buildup of low-boiling-point compounds) and the concentrated syrup contains 15 to 20% by weight total solids.

One of the advantages of alkane production from biomass by aqueous-phase de- hydration/hydrogenation is that the alkanes spontaneously separate from the aque­ous feed solution, whereas ethanol produced during fermentation processes must be removed from water by an energy-intensive distillation step.

Biomass-derived oxygenates can be converted into hydrogen and alkanes (rang­ing from C1 to C15) via aqueous-phase processing (Audsley and Annetts 2003). These aqueous-phase processes could be used in an integrated biorefinery to pro­duce a range of fuels, as shown in Figure 7.5. The first step in the biorefining process is conversion of biomass into an aqueous sugar solution. Production of hydrogen for biorefining processes is accomplished by aqueous-phase reforming. The biorefinery can also produce light alkanes ranging from C1 to C6 by aqueous — phase dehydration/hydrogenation (Audsley and Annetts 2003). The light alkanes could be used as synthetic natural gas, liquefied petroleum gas, and a light naptha stream. Aqueous-phase processing can also produce larger alkanes ranging from C7 to C15 by combining the dehydration/hydrogenation reactions with an aldol conden­sation step prior to the aqueous-phase dehydration/hydrogenation step (Huber et al. 2005).

Biomass has been traditionally converted into liquid fuels by either (a) fermen­tation or (b) pyrolysis methods. Modern improvements to these classical processes are many in number but do not essentially change the type of product resulting from these two vastly different sets of reaction conditions. While ethanol production by fermentation has become more efficient, it is still limited to a 67% yield due to the loss of one third (1/3) of the available carbon as carbon dioxide gas. Pyrolytic reactions also lose carbon as gases and char but may achieve about 80% carbon con­version. While most thermochemical processes usually require nearly dry feedstock, the hydrothermal upgrading (HTU) process requires a 3:1 ratio of water to biomass. However, HTU produces only 50% biocrude, which still contains 10 to 15% oxygen. Obviously, there remains a need for a variety of fuels from many sources, especially conventional liquid fuels for transportation purposes. To resolve this fuel problem

and to use a renewable resource, a strategy was selected to prepare valuable liquid hydrocarbons from biomass by a new chemical process.

Glycerol can be converted into higher-value products. The products are 1,3- propanediol, 1,2-propanediol, dihydroxyacetones, hydrogen, polyglycerols, suc­cinic acid, and polyesters. The main glycerol-based oxygenates are 1,3-propanediol, 1,2-propanediol, propanol, glycerol tertbutyl ethers, ethylene glycol, and propylene glycol. Glycerol has been pyrolyzed for the production of clean fuels such as H2 or a feedstock such as syngas for additional transportation fuel via FTS. The conver­sion of glycerol to H2 and CO takes place according to the following stoichiometric equation:

C3O3H8 ! 3CO C 4H2 (7.1)

The stoichiometry for conversion of glycerol into liquid alkanes, by the formation of synthesis gas coupled with FTS, is shown in Equation 7.2:

25C3O3H8 ! 7C8H18 C 19CO2 C 37H2O (7.2)

This overall reaction to produce liquid fuels from glycerol is slightly exothermic and the yield of liquid alkanes is 40% at 1.7 MPa pressure.

It is possible to produce light alkanes by aqueous-phase reforming of biomass — derived oxygenates such as sorbitol, which can be obtained from glucose by hy­drogenation (Huber et al. 2005; Metzger 2006). The production of alkanes from aqueous carbohydrate solutions would be advantageous because of the easy separa­
tion of the alkanes from water. Much hydrogen is needed to reduce biomass-derived oxygenates to alkanes as shown in Equation 7.3:

C6H14O6 C 6H2 ! C6H14 C 6H2O (7.3)

Production of ethanol (bioethanol) from biomass is one way to reduce both the con­sumption of crude oil and environmental pollution. Ethanol from lignocellulosic biomass has the potential to contribute substantially to bioethanol for transporta­tion. In the process evaluated, prehydrolysis with dilute sulfuric acid is employed to hydrolyze hemicellulose and make the cellulose more accessible to hydrolysis by enzymes. Residual biomass from hydrolysis and extraction of carbohydrates can be burned in a power plant to generate electricity and process steam. Figure 7.6 shows a flow diagram of pretreatment for fermentation of ethanol production from sugar crops and lignocellulosic feedstocks.

Carbohydrates (hemicelluloses and cellulose) in plant materials can be converted into sugars by hydrolysis. Fermentation is an anaerobic biological process in which sugars are converted into alcohol by the action of microorganisms, usually yeast. The resulting alcohol is bioethanol. The value of any particular type of biomass as feedstock for fermentation depends on the ease with which it can be converted into sugars. Bioethanol is a petrol additive/substitute. Bioethanol and the bioreflnery concept are closely linked. It is possible that wood, straw, and even household wastes may be economically converted into bioethanol. In 2004, 3.4 billion gal. of fuel

ethanol were produced from over 10% of the corn crop. Ethanol demand is expected to more than double in the next 10 years. For the supply to be available to meet this demand, new technologies must be moved from the laboratory to commercial reality. World ethanol production is about 60% from sugar-crop feedstock.

The corn-starch-to-fuel-ethanol industry has been developed over the past 30 years by bioethanol researchers. Most bioethanol researchers focus on the chal­lenge of producing bioethanol from lignocellulosic biomass instead of from corn starch. To this end, researchers have already developed effective technology to ther­mochemically pretreat biomass, to hydrolyze hemicellulose to break it down into its component sugars and open up the cellulose to treatment, to enzymatically hy­drolyze cellulose to break it down into sugars, and to ferment both 5-carbon sugars from hemicellulose and 6-carbon sugars from cellulose.

Cellulose is a remarkable pure organic polymer, consisting solely of units of an- hydroglucose held together in a giant straight-chain molecule. Cellulose must be hydrolyzed to glucose before fermentation into ethanol. Conversion efficiencies of cellulose to glucose may depend on the extent of chemical and mechanical pre­treatments to structurally and chemically alter the pulp and paper mill wastes. The method of pulping, the type of wood, and the use of recycled pulp and paper prod­ucts also could influence the accessibility of cellulose to cellulase enzymes.

Cellulose fraction of the structural components is insoluble in most solvents and has a low accessibility to acid and enzymatic hydrolysis. Hemicelluloses (arabino — glycuronoxylan and galactoglucomammans) are related to plant gums in composi­tion and occur in much shorter molecule chains than cellulose. The hemicelluloses, which are present in deciduous woods chiefly as pentosans and in coniferous woods almost entirely as hexosanes, undergo thermal decomposition very readily. Hemi — celluloses are derived mainly from chains of pentose sugars and act as the cement material holding together the cellulose micells and fiber. Hemicelluloses are largely soluble in alkali and as such are more easily hydrolyzed.