Most solid acids do not function effectively for cellulose hydrolysis because the surfaces of these solids do not have strong acid sites or the acid sites are hard to contact P-1,4-glucans. The type and amount of catalysts have a great impact on the conversion of cellulose. High catalytic acidity leads to higher reaction rate, so the most convenient method to improve hydrolysis efficiency is to load more catalyst to provide more acid sites access to |5-1,4-glucans in cellulose. The adequate catalyst loading is necessary to obtain the maximum conversion rate of cellulose to glucose. Table 15.1 summarizes hydrolysis results over some typical solid acid catalysts in water (except for FeCl3/silica). It can be seen that, catalyst weight is higher than that of cellulose in most of cases. Fe3O4-SBA-SO3H is the most active one, and that is related to its high amount of B acid sites. Catalyst amount is closely related to reac­tion time and hydrolysis yield. It was found that less catalyst requires longer reaction time to achieve high conversion rate [4]. The more catalyst, the higher concentra­tion of acidic sites, resulting in [H+] attacking on |5-1,4-glucans to produce more glucose.

On the other hand, as a good liquid fuel, bio-oil from pyrolysis of biomass, has to be homogenous and its properties should not change significantly during the storage [75]. As pyrolysis liquids contain a high number of compounds with var­ious chemical functionalities, the homogenity of the liquid highly depends on the

A: 0 for less than 10, 1 for less than 10-14, 2 for between 14 and 20, 3 for higher than 20MJ/kg. (Note that typically biomass has 12-14 MJ/kg), B: 1 if furthest from coal, 2 if near toward coal, 3 if very near towards coal, C: 0 for more than 50, 1 for between 35and 50, 2 for between 10 and 35,3 for less than 10 wt% moisture, D: 0 for more than 15,1 for between 6 and 14, 2 for between 3 and 6, 3 for less than 3 wt% ash, E: 0 for less than 10, 1 for between 10 and 25, 2 for between 25 and 50, 3 for more than 50 wt%. (Note that typically almost more than 60 wt% of biomass is volatiles, VM, and fixed carbon, FC)

Table 17.10 Characterization results of pyrolysis liquid from Malaysian empty palm fruit bunch [7]

complex solubility and reactivity of these chemical compounds. Typically, the pyrol­ysis liquids are single-phase liquids contaning varying amounts of solids (char). This char sediment gradually settles at the bottom of the barrel forming a thick sludge over time depending on the density difference between the liquid and particles [76]. Phase separation can occur if the total water content exceeds a certain threshold limit, making the liquid usage as fuel questionable unless it can be emulsified before use [75]. The properties of bio-oil such as moisture content, density, pH, and solid content from Malaysian EFB from palm industry was reported in reference [7] as 50-60 %, 1.2g/cm3, 3, and 0.02-2 %, respectively. The elemental compositions of the bio-oil are given in Table 17.10 [7].

Table 17.12 Yield of liquid fuel from Malaysian empty palm fruit bunch [77]

In this study, the stable single phase mixture of bio-oil contained water ranging from 40 to 60 %. The density of the liquid was 1.2g/cm3, which was higher than that of fuel oil, that is, at around 0.85 g/cm3, and significantly higher than that of the biomass which was at 1.1452 g/cm3 [8]. This indicated that the liquid had approximately 42 % of the energy content of fuel oil on a weight basis, but 61 % on a volumetric basis. This may impose implications on the design and specification of equipment to process and handle the bio-oil such as pumps and atomizers in boilers and engines. More importantly, the CV of the pyrolysis liquid as determined mathematically was 21.62 MJ/kg, compared to 42-44 MJ/kg for conventional fuel oils. Thus, the liquid produced needs to be upgraded to become the alternative substitute for the existing fuel oils.

For the upgrading/purification methods, the common processes are catalytic cracking, hydrogenation, and steam reforming [77-83]. There are two approaches for catalytic cracking of bio-oil: offline catalytic cracking that utilizes bio-oil as raw material and online catalytic cracking which utilizes pyrolysis vapors as raw material [77, 84-90].

Hew et al. reported an upgrading study on bio-oil from Malaysian EFB using an off line heterogeneous catalytic cracking process [77]. They applied Taguchi L9 method to identify optimum operating condition to upgrade empty fruit oil palm bunch-derived pyrolysis oil to liquid fuel, mainly gasoline or organic liquid vapor. The properties of the bio-gasoline obtained are given in Table 17.11.

The yields and optimal conditions for the upgrading process of the bio-oil into liquid fuels are reported in Table 17.12. The equations used to calculate the yield of
organic liquid product (OLP) and the yield of gasoline are as Eqs. 17.10 and 17.11, respectively [77];

YieldOLP = (WeightOLP/Weightbio-oil) x 100% (17.10)

Yieldgasoline = (Weightgasoline/ 12g) X 100% (17.11)

17.2 Summary

Pre-treatment process is essential for subsequent biomass conversion into bio-fuel and chemicals. The challenge lies in overcoming the resistivity of plant cells wall to deconstruct due to entanglement of highly crystalline structure of cellulose which is embedded in a matrix of polymer lignin and hemicelluloses.

The use of a catalyst for gasification is not essential, but it can increase gasification efficiency by reducing tar content or other unpleasant products, such as methane. The application of a catalyst promotes tar cracking at lower temperatures or promotes a steam-reforming reaction, which is a reaction between methane and steam in the temperature range of 700 °C-1,100 °C to produce syngas. The catalyst can be used directly in the gasifier or the secondary reactor downstream of the gasifier [47, 48]. There are different criteria for developing or choosing a proper catalyst, such as being inexpensive, effective, and resistant to attrition, carbon fouling, and sintering. Catalysts used in tar cracking can be classified into three main groups: alkali metals, non-metallic oxides, and supported metallic oxides.

The fruits and vegetables harvest and transformation processes yield many wastes: trimmings, hulls and shells. In 1995, the US Department of agriculture estimated that over 250 million dry tons of agricultural crops and residue were generated over a year in the country [23]. The chemical composition of agricultural crops and residues is very similar to that of perennial herbaceous plants (see Sect. 11.2.1.4). The interest in these feedstocks is reflected in the abundant literature found on agricultural crops and residues pyrolysis [24-26].

11.2.1.5 Animal Manures

Animal manures are used as fertilizers: their high urea, phosphorus and organic con­tents enrich soils dedicated to agriculture. Cattles are the main manure producers with production of over 200 million dry tons a year in the US (commercial broilers are showing comparable numbers) [23]. Because manure has a heterogeneous com­position, thermal decomposition has gained interest to recover that feedstock. Cattle manure is more difficult to collect than poultry manure [10]. Therefore, poultry ma­nure is considered as a good candidate for industrial pyrolysis, and the scientific literature has been mostly focused on this type of manure.

Development of the biofuel production processes had begun in the nineteenth century; in the beginning of the twentieth century interest to the biofuels had died out because of the rapid growth of cheaper fossil fuels usage; developments in this field have been resumed again due to the oil crisis in 1973 [12]. Today, the development of biofuels production and using technologies is driven by: increase in prices of energy resources, fossil fuels depletion, and also CO2 emission issue.

Energy crops are plants which have been cultivated as a source of energy. Basi­cally they are represented as herbaceous or woody fast-growing plants, for example switchgrass [13] and willow [14]. Algae are one of the most promising biofuels. Fertile lands are not required for their cultivation, and they grow in virtually any kind of water [15].

Brazil was the major producer of biofuels until 2000; however, by 2008 its world output has increased from ~20 billion liters a year to almost 75 billion, basically due to the rapid development of the bio-energy technologies in the USA and Canada [16].

One of the most promising sources of biomass is Panicum virgatum, well-known as switchgrass. Let us examine it as a characteristic representative of energy crops.

The economical conversion of lignocellulosic biomass to higher value products re­quires efficient recovery of cellulose, lignin, and hemicellulose components during the fractionation process. Organosolv pretreatment can meet the requirement due to the selective dissolving ability of the solvent employed in the process. In addition to the cellulose-rich materials, it produces organosolv lignin and hemicellulose sugars that is environmentally friendly.

The hemicellulose sugars recovered from the water-soluble stream can be con­centrated for fermentation using special organisms to convert the five-carbon aldose to ethanol or other products [33]. Organosolv lignin is a suitable feedstock for the production of phenolic resins/adhesives [73], antioxidant [74], bio-based polymer

3.04% mannose, 0.93 % galactose, 0.48 % arabinose)

composites [75], and even hydrocarbon products for blending with gasoline [76], owing to its unique high purity, low molecular weight, and abundance of reactive groups [21, 77].

However, the water-insoluble property of organosolv lignin may limit its appli­cations [78]. Therefore, conversion of the lignin dissolved in the organic solvents by catalyst or organic-solvent-stable enzymes may be a potential technological approach to resolve the problem [79-81].

16.3.1.1 Milling

Milling is a mechanical process of pretreatment that breaks down the structure of lig — nocellulosic materials and reduces the crystallinity of the cellulose [14]. During ball milling, biomass is grounded with the contact of the balls inside a cycle machine to get the uniform particles size [25]. This method can be considered environment-friendly due the absence of added chemicals in this process that produce toxic substances

[14] . A disadvantage of milling is the high power required by the machines and consequent high energy costs [14]. Buaban et al. [26] reported that the increase of the time of milling increased the amounts of the sugars (glucose, 89.2 ± 0.7 % and xylose, 77.2 ± 0.9 %) after 4 h of milling.

16.3.1.2 Irradiation

Gamma-rays-mediated pretreatment is irradiation pretreatment which allows the breakdown of beta-1, 4glycosidic linkages, thus enhancing the surface of area and crystallinity of cellulose [16]. It is a physical pretreatment which increase the surface area, consequently reducing the crystallinity. This method is expensive for large-scale operations with considerable environmental and safety concerns [16].

16.3.1.3 Microwave Irradiation

The use of high-energy radiation such as microwave causes one or more changes in the characteristics of cellulosic biomass, such as an increase in surface area, reduction in the degrees of polymerization, and crystallinity of cellulose and hemicelluloses, and the partial depolymerization of lignin [27]. However, irradiation pretreatment have the disadvantage of high energy consumption, and the methods are slow and expensive. Microwave irradiation process acts under the structural change in cellulose with the occurrence of the lignin and hemicellulose degradation, thus increasing the enzymatic accessibility [14]. To further improve the sugar yield after pretreatment, microwave radiation process was combined with chemicals. Binod et al. [28] tested different microwave pretreatment conditions for SB and reported highest reducing sugar yield (0.83 g/g dry biomass) in the microwave-alkali pretreatment followed by acid pretreatment.

Pretreatment of feedstocks with adsorbents such as magnesium silicates (such as Magnesol 600R, The Dallas Group, USA) were found to be very effective in remov­ing FFAs. D-SOL has been introduced successfully at commercial scales for food frying operations to remove FFAs from frying oil. At 2 % additive concentration, the Magnesol 600R reduced FFAs from 3.8 % to around 1.24 %. When a blend of chicken fat and vegetable oil with FFAs concentration of 1.45 % was tested with Magnesol, all concentrations of the 600R product reduced the FFAs to below 1 %. This means, the 600R is a low-cost solution for reducing FFAs levels of <4.0 % which works out to be about 5 % per gallon per 1% FFAs reduction [35].

Bentonite clay on the other hand, reduced calcium, magnesium, and phosphorus in feedstocks better than the Magnesol 600R. If a plant relies on a proprietary catalyst, the producer is tied-in to one manufacturer. It is possible that as the market matures different catalyst manufactures will offer drop-in alternatives [36].

18.6.1 Immobilized Enzyme-Catalyzed Reduction of FFAs

Enzyme is a new biocatalyst to the biodiesel industry. Lipases belonging to the enzyme group of hydrolases are capable of converting FFAs in an esterification reaction with methanol to biodiesel and water byproduct. If used properly, the use

Table 18.2 Advantages of the enzymatic process over the chemical process Chemical Enzymatic

of lipases is cost effective and environment friendly. Lipases can be easily used for lowering the FFAs in different feedstock through esterification with methanol to form FFAs and water [37]. The use of such type of biocatalyst would provide an elegant solution for reducing the environmental impact of yellow grease collected from restaurants, brown grease (>90 % FFAs) and fat collected in municipal and industrial waste-water treatment plants [38, 39].

Most recently TransBiodiesel, Israel has developed and commercialized unique immobilized biocatalysts for the conversion of crude and low-grade feedstocks to biodiesel. The developed biocatalysts are capable of converting any grade of veg­etable oil and animal fat to biodiesel with minimal waste products [40, 41]. The biocatalysts would act on any oil feedstock with any level of FFA-containing oil including crude vegetable oils, vegetable oil distillates, yellow and brown greases, and virgin oil, and to reduce their FFAs content to lower than 1 %. These feedstocks with high FFAs levels are much cheaper feedstocks than virgin plant oils (40-60 % cheaper). It is estimated that 20-40 % of the operational costs alone can be saved when dealing with the enzymes developed by TransBiodiesel (www. transbiodiesel. com).

The proposed enzyme technology offers biodiesel manufacturers flexibility in their choice for feedstocks which might contain FFAs in the range of 0-100 %. It allows biodiesel manufacturers to expand their feedstocks selection from expensive virgin oil (approx $1,100/t) to yellow grease ($700/ton) to inexpensive brown-grease feedstock obtained from waste-water treatment plants ($300/t). The major advantages of the enzymatic process over the chemical processes are summarized in Table 18.2.

It has been demonstrated that feedstocks need not be FFAs free in the enzymatic process, and de-hydrated feedstock is not a requirement as in the case of the chemical process. Operating at a relatively low temperature and with no need to neutralize acid, TransBiodiesel’s enzymatic process produces remarkably clear biodiesel and high-quality glycerol that needs little refining because enzymes are used at room temperatures (20-30°C) without any other acids or bases.

TransBiodiesel has two main enzymes TransZyme and EsterZyme. TransZyme is an immobilized lipase of high transesterification as well as esterification activity. TransZyme is capable of converting any type of feedstock, including virgin oils, crude plant oils, animal fats, waste-cooking oils, acid oils, and brown grease, regard­less of the FFAs content (0-100 %), to form biodiesel through transesterification and esterification processes simultaneously [40, 41]. TransZyme favors more transes­terification and esterification than hydrolysis even in the presence of 1-10 % water. TransZyme is also capable of transesterifying phospholipids and wax esters to form biodiesel and free alcohols allowing the use of crude unrefined vegetable oils. Due to the capability of the developed biocatalyst to transesterify phospholipids the overall yield of biodiesel production from crude plant oils would be increased by 1-3 %.

EsterZyme is an immobilized lipase of high esterification activity. It transforms free fatty acid in the presence of methanol (or other alcohols) and under reduced amount of water (preferably below 0.5 %) and glycerol into biodiesel and water byproduct [40, 41]. Furthermore, EsterZyme exhibits relatively high transesterifi­cation activity toward partial glycerides and wax esters and lower activity toward triglycerides. The biocatalyst can also be used for lowering the FFAs% in any type of feedstock down to 0-2 % starting from any type of feedstock containing FFAs from 3 % and up to 100 %.

Both enzymes developed by TransBiodiesel are suitable for use in batch and continuous reactors using stirred tank or packed column reactors (Fig. 18.6). While many plants using acid esterification and de-gum their feedstock, TransBiodiesel’s technology uses crude feedstock without resorting to de-gumming since gums don’t interfere with the enzymatic step.

Most of the leading chemical pretreatment technologies that have been described herein are effective on one or more factors that contribute to lignocellulosics recalci­trance. Despite much research that has been dedicated to understanding the chemistry and the plant cell wall structure changes during various pretreatment technologies, the insufficient knowledge of cell wall structure, ultra structure, and pretreatment effects still limits the economics and effectiveness of pretreatment. For instance, the biological and chemical properties of plants are very complex in terms of composi­tion, structure, and ultra-structure [162]. Although researchers have put significant

effort into optimizing the pretreatment effectiveness, the fundamental science behind these optimizations is still not fully understood. Furthermore, there has been a lack of mechanistic understanding of the ultrastructural and physicochemical changes oc­curring within the cell wall at the molecular level and the cellular/tissue scale during various pretreatment technologies. It is thus essential to understand the effects of pretreatment on plant cell walls at a more fundamental level, in order to develop a cost-effective pretreatment technology with maximum fermentable sugar recovery, minimum inhibitor production and energy input, low demand of post-pretreatment processes, and low capital costs for reactors, water, and chemicals. In addition, ad­vances in the analytical chemistry would provide useful tools to investigate the cell wall deconstruction and understand the recalcitrance during the pretreatment process [163, 164].

Acknowledgments The authors are grateful for the financial support from the US Department of Energy (DOE biorefinery project: DE-EE0003144) for these studies.

Carbon dioxide can be removed from syngas by chemical and physical absorption with a washing liquid or by adsorption with solid absorption. The choice for chemical or physical absorption (or a combination of both) depends on thepartial pressure in the gas. For chemical absorption in commercial processes substituted amines are used, while solvents, like methanol, polyethylene glycol, and dimethyl ether, are used for physical absorption. The CO2 concentration can be removed to approximately 0.1 vol% by these processes. When the syngas contains significant concentrations of other gases besides H2 and CO2, adsorption on solid materials, such as silica gel, active carbon, zeolites, and molecular sieves, is preferred.

As discussed earlier, the produced syngas often contains high amounts of prob­lematic impurities, such as sulfur, chlorine, and alkalis. The gasification process has several cleaning units and its total efficiency depends on the heat management of the various steps. One of the most important challenges for an efficient gas cleaning process is developing a hot gas cleaning technology, which works at or close to the gasifier temperature. These techniques include the development of novel particu­late removal techniques, an improved catalyst for tar cracking to produce a tar — and particulate-free product gas and a higher degree of process integration. For example,

one possible way to increase the efficiency of hot gas cleaning and also reduce the cost is to decrease the number of gas cleaning stages by combining different physical and chemical processes in the same equipment, such as catalytic tar cracking in a particular barrier filter. This was first proposed for combustion applications [62], but further applied to the gasification process by many research groups, such as the deposition of the Ni/MgO catalyst onto the pore walls in a-alumina in a candle [63] and a catalytically active fixed bed in a cylindrical catalytic filter element [64]. Still, there is continuous research and development being done to improve particulate fil­tration, various sorbents and associated equipment to achieve a high efficiency of gas cleaning, especially at high temperatures.