Atsushi Tsutsumi and Yasuki Kansha

Collaborative Research Centre for Energy Engineering, Institute of Industrial Science The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo

Japan

1. Introduction

Recently, biomass usage for fuel has attracted increased interest in many countries to suppress global warming caused mainly by the consumption of fossil fuels. (Mousdale, 2010). In particular, many researchers expect that bioethanol may be a substitute for petroleum. In fact, bioethanol loses less energy and exergy potential during chemical reactions, saccharification and fermentation for ethanol production, because it is produced merely through energy conversion by chemical reactions (Cardona et al. 2010). However, after fermentation, the product contains a large amount of water, which prevents maximizing the heat value of the product. Therefore, separation of the ethanol-water mixture is required to obtain pure ethanol for fuel (Zamboni et al. 2009a, 2009b, Huang et al. 2008). In practice, distillation is widely used for the separation of this mixture (Fair 2008). However, conventional distillation is well-known to be an energy-consuming process, and also pure ethanol fuel cannot be produced directly from a distillation column, because ethanol and water form an azeotropic mixture. To separate pure ethanol from ethanol-water mixtures by distillation, it is necessary to use an entrainer (azeotrope breaking agent), because the azeotropic mixture is one that vaporizes without any change in composition. Benzene, cyclohexane, or isopropyl alcohol can be used as entrainers for the ethanol-water mixture. Therefore, at least two separation units are required to produce pure ethanol, leading to further increases in energy consumption (Doherty& Knapp 2008). In fact, it is believed that about half of the heat value of bioethanol is required to distill the ethanol from the mixture. To reduce energy consumption during bioethanol production, many researchers have proposed membrane separations (Baker 2008, Wynn 2008) or pressure swing adsorption (PSA) (Modla & Lang, 2008) as alternatives to azeotropic distillation, often successfully developing appropriate membranes or sorbents to achieve an efficient separation. However, in many cases, they have paid little attention to the overall process scheme or have developed heat integration processes based on conventional heat recovery technologies, such as the well known heat cascading utilization. As a result, the minimum energy requirement of the overall process has not been reduced, because changes to the condition of the process stream are constrained in conventional heat recovery technologies (Hallale 2008, Kemp 2007). Moreover, most cost minimization analyses for bioethanol plants have been conducted based on these conventional processes and technologies. Thus, the price of product bioethanol still remains high compared to fossil fuels.

Nowadays, by reconsidering the energy and production system from an improvement of energy conversion efficiency and energy saving point of view, the concept of co-production of energy and products has been developed. However, to realize co-production, it is

necessary to analyze and optimize the heat and power required for production in each process. Therefore, the authors have developed self-heat recuperation technology based on exergy recuperation (Kansha et al. 2009) and applied it to several chemical processes for co­production (Fushimi et al. 2011, Kansha et al. 2010a, 2010b, 2010c, 2011, Matsuda et al.2010). In this chapter, self-heat recuperation technology is introduced and applied to the separation processes in bioethanol production for co-production. Moreover, the feasibility and energy balance for co-production of bioethanol and power using biomass gasification based on self-heat recuperation is discussed.

The proposed reaction pathway for catalytic hydrotreatment of pyrolysis oil (Figure 10) implies that the rate of the hydrogenation route should be much higher than the rate of the repolymerisation route to obtain good quality upgraded pyrolysis oil (low molecular weight, low viscosity, low coking tendency). An obvious solution is the development of highly active hydrogenation catalysts. These studies will be reported in the next paragraph of this paper. However, a smart selection of process conditions and reactor configurations may also be considered, particularly to enhance the rate of the hydrogenation/hydrodeoxygenation pathway compared to the repolymerisation pathway. In this respect, it is highly relevant to gain some qualitative insights in the factors that determine the rate of the individual pathways (hydrogenation versus repolymerisation).

A schematic plot is presented in Figure 11, where an envisaged reaction rate (arbitrary values, in mole reactant/ min) is presented versus the actual reaction temperature. The lines drawn are taken in case (i) gas-to-liquid mass transfer determines the overall reaction rate (ii) the catalytic hydrotreatment reactions dominate, and (iii) polymerisation reactions prevail. Figure 11 is derived on basis of simplified kinetics for the glucose hydrogenation — polymerisation reactions, but a detailed outline of all assumptions made is beyond the scope of the presentation here. For this reason the exact values on the x — and y-axes are omitted. The following relations are taken into account to derive Figure 11:

• The conversion rate due to the hydroprocessing reactions RH (mol/m3r. s) can be simplified as a product of the intrinsic kinetic rate expression kR and the surface area

available per reactor volume. Being a catalytic reaction, the influence of temperature can be rather high.

• The overall gas-to-solid mass transfer rate of hydrogen depends on reactor geometry and operating variables. In case of stirred tank reactors (including batch-wise operated autoclaves), the actual stirring rate is important, while in packed bed the catalyst particle wetness is relevant. In both, the concentration of hydrogen (thus hydrogen pressure) is important, together with catalyst particle size, and, to a limited extent, temperature.

• The rate of polymerisation, Rp, will depend largely on the temperature, and, being a reaction with order in reactant(s) > 1 (and probably up to 2 or 3), on the concentration of the reactant.

Temperature (K)

A number of options may be envisaged to promote the hydrogenation pathway:

• Increase the hydroprocessing reaction rate, for instance by a higher catalyst intake or by an increase in the effective hydrogen concentration in the liquid (pressure, application of a solvent with a high hydrogen solubility).

• Reducing the polymerisation reaction, a. o. by performing the initial stabilisation step at a low temperature (< 100°C) and reduction of the concentration of the reactants (a. o. by dilution).

• Increase the overall gas-to-liquid mass transfer rate in case the reaction is performed in the gas-liquid mass transfer limited regime. This may be possible by increasing the mass transfer surface area in the reactor, higher mass transfer coefficient and / or increasing the concentration difference between the gas and the liquid.

Pilot-scale densification of biomass is required to demonstrate the feasibility of production of pellets by application of various variables studied during single-pellet experiments. A pilot-scale pellet mill such as CPM CL-5 pellet mill (Figure 3) (California Pellet Mill Co., Crawfordsville, IN) can be used for processing of agricultural straw grinds into pellets. The pellet mill usually consists of a corrugated roller and ring die assembly, which compacts and extrudes the biomass grinds from the inside of a ring-shaped die by pressure applied by rolls where either the die or the roll suspension is rotating. Rolls are mounted close to the die surface, but still leaving room for a compacted feed layer to enter the roll gap. Friction between feed layer and rolls makes the rolls rotate (Larsson et al., 2008). In addition to variables indicated in the single-pellet testing, the quality of pellets also depends on machine variables such as the ring die size (radius), length (thickness, l), ring hole diameter (d), l/d ratio, and the rotational speed of the pellet mill (Adapa et al., 2004; Hill and Pulkinen, 1988; Tabil and Sokhansanj, 1996). A monitoring study of commercial pellets was done by Hill and Pulkinen (1988), on variables such as die geometry, conditioning temperatures, natural moisture of the grind, forage quality, bulk density of the grinds, and the use of binding agents. Similarly, Larsson et al. (2008) studied the effect of raw material moisture content, steam addition, raw material bulk density, and die temperature on production of high quality pellets. Also, Serrano et al. (2011), determined the effect of grind size, moisture content and customization of barley straw by adding pine dust to the mixture (blended pellets).

The feed rate of ground biomass to the pellet mill can be controlled using a vibratory feeder (Figure 3). The feed rate should be optimized according to the pellet mill capacity, which will directly affect the throughput. The pilot-scale pelleting test should be performed for a predefined period and the manufactured pellets should be collected and weighed to determine the pellet mill throughput (kg/h). In addition, the pellet mill energy consumption (kWh) should be recorded in real time using a data logger connected to a computer and should be used to calculate the specific energy (MJ/t) required to manufacture pellets from ground agricultural biomass.

Raw materials causing uneven pellet production have low bulk density compared to other milled biofuel pellet raw materials. Low raw material bulk density will put higher demands on the die feeding system of the pelletizer with greater volume throughput for maintained production level. Larsson et al. (2008) investigated the pre-compaction of reed canary grass as an alternative to avoid low and intermittent production of biofuel pellets. They have observed that the process of pre-compaction can increase the bulk density of raw material from 150 kg/ m3 to 270 kg/ m3, which resulted in the continuous production of pellets at a moisture content of 13.8% (w. b.). Pressurized steam conditioners are used in the feed pellet industry to decrease raw material porosity and to improve pellet hardness/ durability (Thomas et al., 1997). Adapa et al. (2010b) were unable to produce any pellets due to the low bulk density of both non-treated and pre-treated agricultural straw grinds at 10% moisture content (w. b.). Therefore, they have added moisture and oil to increase bulk density of grinds to a level of 17.5% (w. b.) and 10% (by weight), respectively, which resulted in production of pellets. Similar observation was made by Serrano et al. (2011) where they have to increase the grind moisture content in the range of 19-23% (w. b.) to produce pellets in a pellet mill. However, addition of pine sawdust to barley straw resulted in high quality pellets at a lower moisture content of 12% (w. b.).

Testing and, if required improving the durability of pellets is important for the industry to evaluate pellet quality and minimize losses during handling and transportation. The concept is not to add any external binders to enhance pellet quality, but rather activate the natural binders in the agricultural biomass by application of various variables, pre­processing techniques and pre-treatments. Biomass pellets can be customized based on proximate analysis data to make them suitable for direct combustion and thermo-chemical conversion applications. Customization can be achieved by forming composites of different straws to control important variables such as energy and ash content of pellets. Similarly, addition of biomass having good binding characteristics to straw with less cohesive characteristics may enhance particle bonding resulting in durable pellets.

Adapa et al. (2010b) reported pellet mill tests on both non-treated and steam exploded agricultural biomass at different hammer mill screen sizes. They have successfully produced pellets from ground non-treated barley, canola, oat and wheat straw at hammer mill screen sizes of 0.8 and 1.6 mm having moisture content of 17.5% (wb) and flax seed oil of 10% by weight. The non-treated ground straw at 3.2 and 6.4 mm screen size did not produce pellets. Similar pelleting process was followed for ground steam exploded straw. Due to very low bulk density and poor flowability, the steam exploded grinds did not produce pellets at any of the hammer mill screen sizes used in the investigation. However, the customized barley,
canola, oat and wheat straw having 25% steam exploded material by weight at 0.8 mm screen size successfully produced pellets. Addition of higher percentage of steam exploded straw and customization at screen sizes of 1.6, 3.2, and 6.4 mm did not produce pellets, which could be due to the fact that adding steam exploded (having very low bulk density) to non-treated straw (having relatively higher bulk density) decreased the overall bulk density and flowability of the grinds, thus hindering the production of pellets in the pilot scale mill. The pilot scale pellet mill in this test is constrained with a small motor (3.7 kW (5 hp)) running it, whereas in a commercial pellet mill, the motors are much bigger and more tolerant to changes in feed bulk density. Shaw et al. (2007) reported similar trends where the quality of wheat straw pellets increased with an increase in moisture content to 15.9% (wb). Figure 4 shows the photograph of pellets manufactured from barley, canola, oat and wheat straw from non-treated grinds at 0.8 and 1.6 mm screen sizes, and customized straw grinds at 0.8 mm having 25% steam exploded straw by weight (Adapa et al., 2010b).

1.

Feed Hopper

2. Vibrating Tray

3. Paddle Conveyor

4. Paddle Conveyor

5. Feed Hopper to Pellet Mill

6. Screw Conveyor

7. Pellet Collection Bucket

8. Steam Control Valve

9. Steam Pressure Gauge

10. Double Chamber Steam Chest

11. Double Chamber Steam Chest

12. V-belt Drive for Pellet Mill

Fig. 3. Schematic diagram of CPM CL-5 pellet mill

Hammer Mill Screen
Sizes

1.6 mm

0.8 mm

The correlation between development and energy consumption is well known. This is quite reasonable as we can consider the gross energy consumption from a society, as a way to amplify the human effort. Likewise, technological change allows for great development with a modest increased consumption of primary energy.

Energy access and its use strongly affect and are affected by population growth, urbanization, or development possibilities and poverty alleviation of a great part of the population. For example, energy consumption patterns of a third of humanity that use biomass as the sole source of energy, tend to reinforce their extreme poverty situation. Hundreds of millions of people, especially children and women, spend several hours a day looking for firewood or carrying water from considerable distances, this causes them to have fewer opportunities for education or more productive activities.

Current development and consumption model along with increased energy waste from rich people as well as consumption patterns from the most disadvantaged creates pollution and destruction that leads to poverty, and this poverty at the same time pollutes and destroys. This is the vicious circle: consumption — pollution — poverty. This is a complex relationship network, not always obvious, in which certain phenomena are cause and effect at the same time and no element can be considered separate or isolated. One of the most important challenges humanity has to face is to find how to produce and use energy in ways that in long term human development, in all its dimensions, is promoted: social, economic and environmental (Perez, 2002).

It is expected that in the course of this century, the use of oil for producing electricity will be replaced by gas, clean burning coal, energy from renewable sources, and nuclear energy. However, the largest contributors to oil consumption and increased pollution is the transport sector, where in the medium term there is not yet a replacement, since current petroleum products are characterized by their high calorific value, they are also easy to store, carry and use.

The creation of liquid biofuels has been one of the ways that science has developed for replacing gasoline and diesel and preserving the environment. Modern bioenergy technologies are renewable energy sources that produce transportation fuels and are advancing very fast, mainly towards ethanol made from maize or sugar cane, which is blended with gasoline in order to reduce both oil consumption and pollution. To use ethanol as fuel by blending it with gasoline, it is necessary to remove water for purity close to 99.9%, which requires special distillation methods.

Bioenergy competitiveness is associated with oil price, if price keeps current trends, there will be options for those trends. It must be considered if benefits and efficiency of these new fuels could survive without stimuli (subsidies) that now favor them. In a realistic framework it is necessary to avoid ambiguous positions that require a choice between biofuels or food production. It is important to combine both processes and add technology that improves production. But food safety issues cannot be jeopardized.

Security of supply is synonymous with the availability of all the energy needed, at an affordable price and for a long time, in fact indefinitely so it can be sustainable. If supply security is considered from a national perspective; dependence on external resources and the uncertainty of this non native supply becomes an important aspect.

"Opportunities for developing countries to take advantage of biofuel demand would be greatly advanced by the removal of the agricultural and biofuel subsidies and trade barriers that create an artificial market and currently benefit producers in OECD countries at the expense of producers in developing countries "said the director general of FAO, Jacques Diouf.

In the coming decades several challenges must be faced because of the energy and environmental problems arisen from it:

Energy efficiency: It will be necessary to radically increase the energy efficiency of processes and systems.

New technologies: It is necessary to develop and incorporate new technologies that achieve the above goal.

Diversify energy sources: At the moment we strongly depend on hydrocarbons as a primary energy source. It is necessary to incorporate new energy sources: natural gas, biomass, wind power, micro-hydro, solar energy and others.

Cogeneration: cogeneration as a means for energy efficiency and savings is not new. But incorporating it into the system on a grand scale may have a very large positive impact.

Then it is important to recognize that biofuels will not end industrialized countries’ dependency on oil — not even Colombia-, because there will not be enough land and water to meet the high energy demand. In spite of this, the development of a national biofuel industry is an opportunity for the country. There are a number of technological, regulatory, economic and environmental restrictions or challenges that may affect critical links in the chain of biofuel production, and if they are not superseded they could lead to failure.

In order to investigate the way to use a smart phone in each user, we executed the questionnaire between February 17 and February 24, 2011. 200 respondents in Japan
participated in this research. Also, target respondents are the users who use a smart phone, with their ages between 15 and 65 years, respectively. In the questionnaire content, the duration time of a talking, a SMS, music (MP3), a game, a web-site (internet), an e-mail checking, and an idle time were estimated for each age category. Fig. 7 shows the result of the duration time of each function. The checking time of internet would be larger in both weekdays and holyday (Dowaki et al., 2011b).

Oil samples obtained in conformity with chosen methodology were subjected to laboratory tests, which comprised of following analyses: determination of peroxide number (PN), acid number (AN) and composition of fatty acids. Determination of peroxide number (PN) in conformity with [ISO 3960] was based on titration of iodine released from potassium iodide by peroxides present in the sample, calculated per their weight unit. Results of analyses were expressed in millimoles of oxygen per weight unit of the sample.

Determination of acid number (AN) in conformity with [PN-ISO 660] was conducted by means of titration and evaluation of acidity of a sample, and expressed in numeric form in millilitres of 0,1M solution of sodium hydroxide, calculated per weight unit of analysed oil. Determination of fatty acids composition was conducted by means of method based on utilization of gas chromatography [Krelowska — Kulas, 1993]. Sample of fat was subjected to alkaline hydrolysis in anhydrous environment with utilization of methanol solution of sodium hydroxide. As a result of this reaction, fatty acids of investigated oil were transformed into a mixture of sodium soaps, which than were subjected of reaction of esterification with anhydrous solution of hydrogen chloride in methanol, yielding mixture of fatty acids methyl esters.

Obtained methyl esters were separated in a chromatographic column and than their participation in a sum of fatty acids was determined [Krelowska — Kulas, 1993]. Chromatographic separation was conducted by means of gas chromatograph with nitrogen as carrier gas, packed column (2,5 m with stationary phase PEGA — polyethylene glycol adipate on carrier GAZ-ChROM-Q) and flame ionization detector.

The Brazilian and American standards combine water content and sediment in a single parameter, whereas the European standard treats water as a separate parameter with the sediment being treated by the Total Contamination property. Water is introduced into biodiesel during the final washing step of the production process and has to be reduced by drying. However, even very low water contents achieved directly after production do not guarantee that biodiesel fuels will still meet the specifications during combustion. As biodiesel is hygroscopic, it can absorb water in a concentration of up to 1000 ppm during storage. Once the solubility limit is exceeded (at about 1500 ppm of water in fuels containing 0.2% of methanol), water separates inside the storage tank and collects at the bottom (Mittelbach 1996). Free water promotes biological growth, so that sludge and slime formation thus induced may cause blockage of fuel filters and fuel lines. Moreover, high water contents are also associated with hydrolysis reactions, partly converting biodiesel to free fatty acids, also linked to fuel filter blocking. Finally, corrosion of chromium and zinc parts within the engine and injection systems have been reported (Kosmehl and Heinrich, 1997). Lower water concentrations, which pose no difficulties in pure biodiesel fuels, may become problematic in blends with fossil diesel, as here phase separation is likely to occur. For these reasons, maximum water content is contained in the standard specifications.

The cultivation macro — and microalgae is a well-established practice, providing ample biomass for human nutrition, commercially important biopolymers, and specialty chemicals, that dates back nearly 2,000 years (Spolaore et al., 2006). As an example, growing the gelatinous cyanobacteria Nostoc in rice patties enabled much of the Chinese population to survive famine in 200 AD (Qiu et al., 2002). Since that time, the mass cultivation of microalgae has been commercialized for the production of either whole-cell algal nutritional supplements or nutraceutical extracts, such as P-carotene, astaxanthin, and polyunsaturated fatty acids (e. g. DHA, omega-3). In the international market, China, Japan, Australia, India, Israel, and the United States are leaders in algal production.

1.1.1 Constraints on photoautotrophic algal biomass production

In addition to certain biological limitations, several obstacles related to cultivation must be overcome to allow economical industrial scale-up of algal biofuel production. The conversion efficiency of solar energy to biomass by microalgae is governed, in part, by the inherent biological efficiency of photosynthesis, and largely by the effectiveness of light — transfer in liquid cultures. Some species of algae grown heterotrophically (i. e. supplemented with carbon sources other than CO2, such as sugars) can accumulate a greater amount of lipids (Wu et al., 2006); however, the costs associated with such cultivation may limit its applicability to biofuel production. The approach of heterotrophic algal biofuel production is the model for a number of algal biofuels start-up companies.

On the other hand, generating algal biomass for biofuels with energy directly from the sun rather than a chemical intermediate has its advantages. Microalgae essentially act as biological solar panels directly connected to biorefineries. Photoautotrophic cultivation has the added benefit of CO2 sequestration from a point source. Although current commercial raceway ponds operate with areal productivities of 2-20 g m-2 d-1, there remains much speculation regarding the maximum achievable algal biomass productivity. While heterotrophic modes of cultivation can yield very dense algal cell cultures, photoautotrophic cultures are not expected to exceed 60 g m-2 d-1. Figure 3 shows a compilation of realistic areal productivities and theoretical projections for photoautotrophic algal cultivation in open ponds (Chisti, 2007; Weyer et al., 2008; Schenk et al., 2008; Wallace, 2008).

While a wide range of predictions has been made for the maximum attainable productivity of algal cultures, a straightforward analysis of energy transfer in algal biomass production reveals a few key bottlenecks. By following a single photon from its origin at the sun to the desired end product of algal oil, there are a number of unavoidable losses imposed on this conversion of sunlight by the inherent bioenergetics of cellular processes. Additional diversions of solar energy can be attributed to the algal growth system and can be minimized with proper design parameters.

The first impediment that solar radiation faces when traveling to the Earth’s surface is the local weather. As we know from our daily observations, cloudy skies can dramatically reduce the amount of light that reaches the ground. Additionally, while the equator receives high-intensity light year-round, solar irradiance diminishes as one travels away from the equator in latitude, thus near-equatorial zones are ideal for algal biomass production. Accordingly, Asia, Australia, and the United States are common sites for algal growth facilities. Figure 4 presents a map of solar data collected from 1990-2004 where black dots represent locations for which detailed weather analysis is available for algal production facilities (Weyer et al., 2008). In geographies that receive more exposure to sunlight, and accompanying high temperatures, evaporative water loss and cooling mechanisms become more important considerations.

Since there is little one can do to change the weather beyond choosing an adequate site for algal cultivation, the next constraint on solar energy collection comes from the limited spectrum of light that plants have adapted to utilize, deemed photosynthetically active radiation (PAR: A = 400-700 nm), which accounts for only 45% of the total energy in the visible light spectrum (Weyer et al., 2008).

In conventional raceway ponds and photobioreactors, incident sunlight encounters billions of algal cells as it travels through the liquid culture — each cell absorbing some of the available energy. Thus, the transmission of light is severely inhibited by cell shading in these dense solutions. For example, the leaves of a tree have evolved to be essentially two­dimensional structures with only millimeter thicknesses; an algal culture volume can be thought of in a similar manner. In open ponds, only the cells on the surface are exposed to maximum sunlight, and those on the bottom of the 10-30 cm deep trough receive very little of this incoming energy. The advantage, though, of a liquid culture is that the shaded cells in the submerged regions can be recirculated to the surface periodically so that a large volume of biomass can be maintained. Additionally, advanced photobioreactor design can encourage optimal mixing patterns (Tredici and Zittelli, 1998).

The next hurdle that usable photons must surmount is absorption by the molecular photosynthetic apparatus. As microalgae have adapted to survive in conditions of low light, they have evolved biochemical mechanisms that are incredibly adept at collecting light energy.

Light-harvesting complexes, which surround the photosystems in the chloroplast membrane, act as antennae to gather and shuttle photons to the photosynthetic reaction center. One limitation to complete utilization of incident PAR lies in the maximum capacity of energy that each photosystem can handle. In fact, high-intensity light can quickly inundate the photosynthetic machinery, leading to the generation of free oxygen radicals, in a process called photoinhibition (Long et al., 1994). This obviously has a negative effect on the productivity of algal cultures by leading to cellular damage and premature population demise. Unlike the natural environmental conditions to which microalgae are accustomed, industrial cultivation strives for maximum collection of solar energy. In this case, algae can be exposed to high- intensity light for only a short period of time, on the scale of microseconds, allowing each photosystem to absorb light before becoming overwhelmed. Also at the biochemical level, there are inherent limitations to photosynthesis related to electron transfer. Estimates for the efficiency of photosynthesis correlate to roughly 25% energy conversion (Weyer et al., 2008). Optimization of photosynthesis is an ambitious genetic engineering goal and is likely to remain an intrinsic parameter of algal biomass production processes.

After the available sunlight has been harnessed by photosynthesis, there exist various levels of inefficiency related to the biological conversion of this energy to biomass. Principally, nearly 40% of the total energy is required to sustain basic cellular function and growth, leaving an estimated 60% of the total photosynthetically captured energy for biomass accumulation (Weyer et al., 2008). According recent analysis utilizing actual solar irradiance and weather data from various global locations (Figure 4), and taking each of the aforementioned assumptions into account, the projected range of algal biomass production is between 38 and 47 g m-2 d-1 (Weyer et al., 2008), which is in agreement other predictions (Figure 3). Current values for commercial biomass production in open ponds are typically 2­20 g m-2 d-1, which provide sufficient profit margins for high-value products such as P — carotene, but are anticipated to meet the demands of cost-effective biofuel production in the near future. A new partnership between Seambiotic and the Israeli Electric Company has plans to produce algal biomass inexpensively for use as biofuel, with operating costs and profit margins listed in Table 1.

Annual Expenses (USD yr-1)

10-ha Raceway Farm Manpower Electricity Nutrients Land

Carbon Dioxide Sea Water Fresh Water

Miscellaneous Expenditures Total Revenue

Biomass Production (yr-1) 70 tons (at 2 g m-2 d-1) 700 tons (at 20 g m-2 d-1)

Biomass Cost (USD kg-1) $17.00 $0.34

Market Price (USD kg-1) $4,000 (D. salina P-carotene) < $0.50 (Potential Biofuels)

Table 1. Cost analysis of microalgal biomass production facilities in Israel (Reprinted with permission from Dr. Ami Ben-Amotz).

3.1 Thermodynamic consideration

The two major components of synthesis gas, hydrogen and carbon monoxide are the building blocks of what is often known as C1 chemistry. Conversion of syngas to liquid fuels as well as conversion rates is directly related to the composition of the catalyst. Syngas can be efficiently converted to different products as alcohols and aldehyde (Figure 2).

Although many routes are available, the most promising route at industrial level is the
production of methanol since the synthesis yields are the highest. The conversion to
synthetic liquids as methanol is strongly influenced by thermodynamic factors (Rostrup-

Nielsen, 2000).

For methanol synthesis, a stoichiometric ratio defined as (H2-CO2)/(CO+CO2), of about 2 is preferred, which implies that there should be just the stoichiometric amount of hydrogen needed for methanol synthesis. For kinetic reasons and in order to control by-products formation, a value slightly above 2 is normally preferred (Dybkjr and Christensen, 2001).

Moreover, methanol synthesis is subjected to a thermodynamic equilibrium that limits the process to low conversion per pass and therefore implies a large recycle of unconverted gas. The reaction is strongly exothermic and consequently requires significant cooling duty. Different phenomena exist at high and low pressure conditions. As example, when the pressure is relatively low, increasing temperature, CO conversion is not monotonic, and the trend is that of an increase followed by a decrease with the maximum conversion appearing near 250°C. This phenomenon is in agreement with many works in the literature (Li and Inui, 1996, Liaw and Chen, 2001, Wang et al., 2002). As the reaction temperature increases, the reaction rate gets higher and leads to the increase of CO conversion. However, methanol synthesis is an exothermic reaction and low temperature is more beneficial considering equilibrium. The conversion does not continue to increase due to the thermodynamic limitation and a decrease trend will even appear. When the system pressure is relative high, because of the relatively low CO conversion, the system is far from the thermodynamic equilibrium and under the control of reaction kinetics. In this case, CO conversion increases monotonically with an increase in temperature. From a mechanistic point of view for the methanol synthesis, two main reactions need to be in line with real world situation: CO hydrogenation and CO2 hydrogenation.

Different chemicals such as acids, alkalis, oxidizing agents and ozone have been used for chemical pre-treatment of lignocellulosic materials. Depending on the type of chemical used, pre-treatment could have different effects on structural components. Alkaline pre-treatment, ozonolysis, peroxide and wet oxidation pre-treatments were reportedly more effective in lignin removal, whereas dilute acid pre-treatment was more efficient in hemicellulose solubilization (Galbe and Zacchi, 2002; Sanchez and Cardona, 2008; Tomas-Pejo et al., 2008). Acid Hydrolysis: Inorganic acids such as H2SO4 and HCl have been used for pre-treatment of lignocellulosic materials and have been used on a wide range of feedstocks ranging from hardwoods to grasses and agricultural residues. Acid hydrolysis can be classified as concentrated or dilute-acid hydrolysis based on the dose of acid used in the process. In the first case, the biomass is treated with high concentration of acids at ambient temperatures, which results in high conversion of lignocellulosic materials. Although concentrated acids are powerful agents for cellulose hydrolysis, they are toxic, corrosive, hazardous, and thus require reactors that are resistant to corrosion, making the pre-treatment process very expensive. In addition, the concentrated acid must be recovered after hydrolysis to make the process economically feasible (Galbe and Zacchi, 2002; Sun and Cheng, 2002).

Dilute-acid hydrolysis has been successfully developed for pre-treatment of lignocellulosic materials. Sulfuric acid at concentrations usually below 4% (wt) has been of the most interest in such studies as it is inexpensive and effective. Dilute H2SO4 pre-treatment can achieve high reaction rates and significantly improve cellulose hydrolysis (Esteghlalian et al., 1997). High temperature is favorable to attain acceptable rates of cellulose conversion. Despite low acid concentration and short reaction time, the use of high temperatures in dilute-acid hydrolysis accelerates the rate of hemicellulose sugar decomposition and increases equipment corrosion (Galbe and Zacchi, 2002; Taherzadeh and Karimi, 2007).

Alkali hydrolysis: Dilute alkali such as sodium, potassium, calcium, and ammonium hydroxides have been used for pre-treatment of lignocellulosic materials in alkali hydrolysis. The effectiveness of these agents depends on the lignin content of the materials. Temperature and pressure are lower in alkali pre-treatment compared with other pre­treatment methods (Mosier et al., 2005). Alkali pre-treatment can be conducted at ambient conditions, but process time is longer (hours or days instead of minutes or seconds). Compared with acid process, alkaline process causes less sugar degradation, and many of the caustic salts can be recovered and/or regenerated.

Sodium hydroxide has been studied more than other agents (Soto et al., 1994; Fox et al., 1989; MacDonald et al., 1983). Treatment of lignocellulosic materials using dilute NaOH results in swelling, leading to an increase in internal surface area, a decrease in the degree of polymerization, a decrease in crystallinity, separation of structural linkages between lignin and carbohydrates, and disruption of the lignin structure. However, calcium hydroxide (lime) is the least expensive hydroxide and has been shown to be an effective pre-treatment agent. The process of lime pre-treatment involves slurrying the lime with water, spraying it onto the biomass material, and storing the material in a pile for a period of hours to weeks. The particle size of the biomass is typically 10 mm or less. Elevated temperatures reduce contact time.

Oxidizing agents: In this pre-treatment, an oxidizing compound such as hydrogen peroxide (H2O2) or peracetic acid (CH3CO3H) is used to treat lignocellulosic materials and sometimes is applied in combination of an alkaline solution (e. g. NaOH) to improve effectiveness. This pre-treatment is usually carried out under mild temperature. This pre-treatment is more effective to increase crop residue digestibility compared with NaOH pre-treatment alone. Gould (1984) delignified agricultural residues using 1% H2O2 at 25°C for 18-24 h. Under this condition, more than half of the lignin and most of hemicellulose were solubilized. The pre­treatment of cane bagasse with H2O2 greatly enhanced its susceptibility to further hydrolysis. About 50% of the lignin and most of the hemicellulose were solubilized by 2% H2O2 at 30°C within 8 h, and a 95% efficiency of glucose production from cellulose was achieved in the subsequent saccharification by cellulase at 45°C for 24 h (Azzam, 1989). Ozonolysis: In this process, ozone is used to change the structure of lignocellulosic materials and has been used for different materials such as wheat straw (Ben-Ghedalia and Miron, 1981), bagasse, green hay, peanut, pine ( Neely, 1984), cotton straw (Ben-Ghedalia and Shefet, 1983) and poplar sawdust (Vidal and Molinier, 1988). Ozonolysis is carried out at room temperature and normal pressure. It can effectively remove the lignin without producing any toxic residues. In this process, hemicellulose is slightly affected, but no change in cellulose has been reported. The main restriction of this process is the large amount of ozone utilization that makes the process expensive (Sun and Cheng, 2002). Binder et al. (1980) reported 60% removal of the lignin from wheat straw using ozone pre­treatment. Enzymatic hydrolysis yield increased from 0% to 57% as the percentage of lignin decreased from 29% to 8% after ozonolysis pre-treatment of poplar sawdust (Vidal and Molinier, 1988). Garcia-Cubero et al. (2009) studied the ozonolysis pre-treatment of wheat straw in a fixed bed reactor at room conditions and concluded that enzymatic hydrolysis yield of up to 88.6% compared to 29% in non-ozonated sample.