The constant m in the Jones (1960) model can provided valuable information about the onset of plastic deformation of the ground agricultural biomass. It has been observed that ground particles obtained from larger hammer mill screen sizes has higher compressibility. In addition, application of pre-treatment also improves the compressibility of the agricultural biomass (Adapa et al., 2010).

The dimensionless coefficients, йг and й2 in Cooper and Eaton (1962) model represent the densification of powdered material by particle rearrangement and deformation, respectively. If the sum of coefficients (йг + й2) is less than unity, it is an indication that other process must become operative before complete compaction is achieved. For agricultural biomass grinds, the йг values were higher than й2 values, hence the material was primarily densified through the process of particle rearrangement. Occasionally, the sum of coefficients (йі + й2) for agricultural biomass was observed to be above unity. The phenomenon of having sum of coefficient more than unity was also observed by Adapa et al. (2002 and 2009a), and Shivanand and Sprockel (1992), which implies that the densification could not be fully attributed to the two mechanisms of compression as assumed by the Cooper and Eaton (1962) model (Adapa et al., 2010a).

In the Kawakita and Ludde (1971) model, constant й represents the initial porosity of the sample. It has been reported that the porosity and hammer mill screen sizes (corresponding geometric mean particle diameter) are positively correlated. In addition, porosity increases with application of pre-treatment since organized lignocellulosic structure of biomass disintegrates during this process. The parameter і/b in the Kawakita-Ludde model indicates the yield strength or failure stress of the compact. In general, the yield strength has negative correlation with hammer mill screen sizes. Also, application of pre-treatment lowers the yield strength of ground agricultural biomass. Statistically, the Kawakita and Ludde (1971) model has been observed to provide accurate representation of the compression and deformation characteristics of agricultural biomass (Adapa et al., 2010a).

Potential environmental and social benefits, among them; climate change mitigation and energy security contribution are mentioned as major support reasons from public sector for biofuel industries, where growth has been fast. Therefore, in recent years fuel production as an f alternative to fossil fuels, from renewable materials, has acquired in recent years, a global push because global production of biofules has doubled and in the medium term it is expected to have strong growth due to high demand. Motivations have been, among others, the inevitable decrease in fossil fuels, the regular oil crisis and the greenhouse effect caused by the CO2 accumulation in the atmosphere.

In particular, in our country, diesel is in higher demand when in comparison to the quantities supplied by the crude oil processed in the refinery. For that reason it must be partially imported already manufactured. In addition to high oil prices, the agriculture crisis and low international oil trade rates are some of the factors that have contributed to give an additional role to biodiesel.

Liquid biofuels also known as biofuels, which are derived from agricultural raw materials, are products that are being used as replacements for gasoline and diesel in vehicles, they can also be used as blends. At the moment many countries encourage the idea of planting their own fuel, to lessen dependency on imports or exhaustible reserves, while generating stable and top-quality jobs. Biofuels are alcohols, ethers, esters and other chemical products made from cellulosic biomass such as grasses and woods, agricultural and forest waste and most of the municipal and industrial waste; as wells as vegetable oils. The term Biofuel refers both to fuels for electricity or transportation. Unlike oil which is a nonrenewable resource, biofuels are renewable and represent an inexhaustible fuel source. Both commercial and noncommercial, these fuels should always be considered as valuable products which can meet the growing demand.

The most important biofuels are ethanol (from sugar cane, sugar beet and corn) and biodiesel (from oil palm, soybeans and other oilseeds). Biofuel promoters highlight the ecological character of these fuels: they are renewable and are apparently environmentally friendly and produce less greenhouse gas emissions (GHG) in comparison to petroleum fuels.

It has been extensively spoken about biofuel models for more than a decade, and the opportunities and challenges that these oil substituting fuels can offer. This potential is not only related to environmental improvement, but it also includes economical, cultural and social aspects (Cortes, 2007).

In many countries (e. g. Brazil, USA and some European), including Colombia, have implemented policies favoring biofuels. Now these actions are having an effect: more and more agricultural commodities are for biofuel production. For this reason demand grows and causes these agricultural commodities in world market have high prices.

Biofuels apparently help to reduce greenhouse gas emissions, but most of the time it is not considered that for biofuels production, fossil fuels are used (diesel for machinery and products transportation, inputs production, etc.) It is also left aside that the expansion of agricultural frontiers (caused by the increasing demand of agricultural commodities) does not reduce greenhouse gas emissions. In the contrary, the forest is a carbon reserve and turning it into cropland releases this carbon as CO2 (the most important GHG). However, with the production of biofuels there are side effects that disturb an apparent prosperity. There are several scientists who defend that for true CO2 reduction re-forestation is vital as opposed to de-forestation for farming.

We must also consider that in large scale certain that biofuel inputs are produced by an agro industrial agriculture. This type of agriculture is supported in large monocultures; abuse of agrochemicals and the soil fertility overexploitation provokes water pollution (with pesticides), soil erosion, air pollution and loss of biodiversity.

On a national level, within the context of energy replacement policies, a delay in exhaustion of reserves, prevention in the rise of import costs and reduction in the impact of gas emissions and particulate matter into the atmosphere, present a great opportunity for the biofuel industry due to in part to the rise in oil prices. This opportunity is supported by a regulatory and legal framework of agro-energy production, including Act 693, of 2001 that proposes an initial 5% replacement of gasoline with alcohol. Later increased to 10% by 2010 and 12% by 2012; with similar proportions for replacing diesel with biodiesel. The same for the use of suitable lands for energy crops such as: sugar cane, cassava, sugar beet, oil palm, castor oil plant and jatropha, all of them with studies and different productivity levels (ton/ha, l/ton). Without denying the possibility of cellulosic biofuels from different sources. In addition to the aforementioned, the National Government has promoted the development and search of new renewable energy sources, sustainable with the increasing pace of life, by partially replacing oil or its derivatives in different uses, especially within the transport sector. This promotion must also consider the implications when allocating millions of hectares for bioenergy production. This reality shows the urgent need for meeting food demands or allocating lands and feedstocks to meet the energy requirements of the automotive industry(Cortes, 2007). Within this context it is proposed that it will promote competition between the different biofuels, with criteria for financial sustainability and energy supply. For these purposes the feasibility and advisability of releasing biofuels prices and promoting removal of import duties on these products. Notwithstanding the aforementioned, the National Development Plan states that in any case the current pricing scheme based on opportunity costs of such energy, their replacements and raw materials used in its production must be considered. While simultaneously promoting strategies for prevention and control of air contamination, by promoting cleaner fuels, including those derived from crops with production potential for biodiesel and alcohol fuels.

The different types of biodiesel from the oil palm, castor oil plant, jatropha and sacha inchi, considered by the Ministerio de Agricultura y Desarrollo Rural (English: Ministry of Agriculture and Rural Development) as strategic for Colombia, have a wide range of chemical compositions and qualities. Oil palm biodiesel given its highly saturated chemical nature has excellent ignition and chemical stability qualities, but it has limitations as for its ease of flow at low temperatures. Oil palm biodiesel cloud point, i. e. the temperature at which crystals formation is visualized, is around 16°C. Crystals emerging and further agglomeration can clog fuel filters preventing the fuel to reach the engine. Castor oil biodiesel is characterized by high alkyl esters content from ricinoleic acid (about 90%), which are monounsaturated and have a hydroxyl group in its structure, giving the biodiesel a high viscosity. Jatropha biodiesel holds 80% of unsaturated alkyl esters from which 34.8% are di-unsaturated. Sacha inchi oil can reach an unsaturated level up to 94%, and it is the most unsaturated oil according to technical literature. Therefore, with jatropha biodiesel and, the oil obtained from sacha inchi, there could be problems in its chemical stability (Ministerio de Minas y Energia, 2007; Mesa, 2006).

Crops’ biggest problem is that they can be expensive as raw materials, which makes the final product price high; so the State must allocate considerable tax resources to make these energies competitive. Aside from being expensive, these raw materials are crops’ primary products, and only recently has the use of waste for biofuel production been taken advantage of. Faced with this adversity, many countries are researching and developing methods for producing ethanol from agricultural, forest and industrial waste, which are abundant and very cheap.

In this case, sugars shall be extracted from plant waste cellulose, such as banana or lumber industry. Today in Brazil there are technologies for the use of husks through fermentation processes. Are then biofuels a technically economic and environmental viable energy output for the nation, with sights to replace future fuel imports? Although after the laws regarding alcohol fuels and biological oils came to be, it can be surmised that employment rates in growing regions shall be positively influenced. It is necessary that the country not only encourage production of efficient biofuels and that from a cost perspective it can compete in the international market, but define programs that support need of new refineries for biomass production, so that the price of raw materials with dual purpose is not affected (for both, food and biofuel) (Cortes, 2007).

It is also necessary to define agricultural land management strategies in order to preserve forest areas and not turning them into biomass growing areas. According to previous observations, the country’s agricultural industry can be articulated with the energy industry, without affecting food industry through price increases of raw materials, as it has been with sugar, wheat and corn.

The goal of this study is to compare the CO2 emission of the conventional Li-ion cell phone and the PEFC cell phone. The functional unit is the specific CO2 emission per a life cycle (LC) of kg-CO2/LC. Fig. 6 shows the life cycle stage on the schematic design of system boundary, in which a pre-processing of raw materials, a manufacture, a transportation and distribution, an energy consumption of end users and a disposal process are included. Also, in this study, we referred to the duration time of each operation of cell phone (Dowaki et a!., 2010a).

Direct CO2 emission (Due to energy use)

Note:

* Estimated period=2.6 years

** This value is based on the wheel to tank, that is, the input energy for producing the fuel besides raw energy source (primary energy) is considered, too.

Fig. 6. System boundary of a cell phone analysis.

In the system boundary, as we described the prior section, we think about the availability of Bio-H2 through BT process. For the purpose, we executed the questionnaire on the way to use a smart phone firstly. Also, we executed the performance of a PEM cell which is based on a PEFC unit using the electric power measurement device.

The difference between a Li-ion and a PEFC cell phone is in electrical energy sources. The Li-ion cell phone is supplied by conventional electricity, whereas a PEFC cell phone is done by Bio-H2 as an energy input. The battery charge due to the conventional electricity emits CO2 of one of the greenhouse gases. On the other hand, since the Bio-H2 would be carbon neutral, the CO2 emission is equivalent to zero in a combustion process. However, the production process of a renewable fuel is accompanied with the conventional energy inputs (i. e. fossil fuels). Thus, it is extremely important to estimate the energy system based on LCA methodology.

1.1 Materials and methods

1.1.1 Preparation of samples for Investigation

In this research, comparison of influence of fried dishes assortment (potato chips and breadcrumbs coated fish fingers) on physicochemical properties and quality of post-frying plant oils to be utilized as raw materials for production of, used as a substitute of diesel fuel, fatty acids methyl esters, was conducted. Main focus of the research was on evaluation of effect of fried dishes assortment on quality of obtained post-frying oils (rapeseed, sunflower and soybean) with regard to their utilization as a substrate for production of engine biofuel. In model conditions of laboratory investigation, usability of post-frying waste oils as raw materials for production of fatty acids methyl esters was evaluated. Three most commonly used edible oils (rapeseed, sunflower and soybean) were used as material for this research. From total amount of each of raw oils, sample for laboratory analyses was taken. It was marked as "0" and was used as a reference sample. Remaining amount of each of oils was divided into three batches and poured into separate containers. Batch no. 1 was prepared by means of cyclic, five-time heating without fried product. Particular cycle within this batch comprised of heating whole amount of oil to temperature of approximately 180oC, and than maintaining it in such temperature for 10 min. Next, oil was left to cool down in room temperature and than a sample, to be used for laboratory analyses, was taken. The sample was marked as "heating I — without fried product". After 24 hours all described above actions were repeated yielding sample marked as "heating II — without fried product". Whole process of heating, cooling and sampling was repeated, yielding samples marked as "heating without fried product" bearing following, respective to number of cycle, labels: III, IV and V.

Preparation of oil from batch no. 2 was differed from previously presented in only one way. After heating it to 180oC, in each of three investigated oils, potato chips, prepared of purchased raw potatoes and cut to the size and shape of frozen potato chips found in trade, were fried.

After frying and separating chips, oil was cooled down to room temperature and than samples for research were taken. They were marked as "heating I — process of chips frying". Repeating whole process enabled obtaining samples marked following, respective to number of cycle, labels: III, IV and V.

Third part of oil (batch no. 3) was heated same way as batch no. 2 but in this case purchased breadcrumbs coated fish fingers were the fried product. After frying and separating breadcrumbs coated fish fingers, oil was cooled down to room temperature and than samples for research were taken. They were marked as "heating I — process of breadcrumbs coated fish fingers frying". Repeating whole process enabled obtaining samples marked following, respective to number of cycle, labels: III, IV and V.

The cetane number of a fuel describes its propensity to combust under certain conditions of pressure and temperature. High cetane number is associated with rapid engine starting and smooth combustion. Low cetane causes deterioration in this behaviour and causes higher exhaust gas emissions of hydrocarbons and particulate. In general, biodiesel has slightly higher cetane numbers than fossil diesel. Cetane number increases with increasing length of both fatty acid chain and ester groups, while it is inversely related to the number of double bonds. The cetane number of diesel fuel in the EU is regulated at >51. The cetane number of diesel fuel in the USA is specified at >40. The cetane number of diesel fuel in Brazil is regulated and specified at >42.

1.6 Sulfated ash

Ash content describes the amount of inorganic contaminants such as abrasive solids and catalyst residues, and the concentration of soluble metal soaps contained in the fuel. These compounds are oxidized during the combustion process to form ash, which is connected with engine deposits and filter plugging (Mittelbach, 1996). For these reasons sulfated ash is limited in the fuel specifications.

While microalgae are an abundant source of naturally oil-rich biomass, these cells can also act as biological factories designed to produce of a variety of promising biofuel precursors. By elucidating the complex metabolic networks involved in carbon utilization, there lies great opportunity for genetic and metabolic engineering of these organisms. Some of the major obstacles to metabolic engineering of algae stem from the lack of basic biological knowledge of these diverse creatures, including sparse genomic information and somewhat primitive methods of genetic transformation. As a result, the introduction of nuclear transgenes to microalgal cells relies on random chromosomal integration, which is highly susceptible to gene silencing; the subsequent recovery of stable transformants is limited to only a handful of species and is oftentimes irreproducible. Overcoming these biotechnological barriers, however, will present enormous opportunities to develop microalgae as versatile platform for biofuel production.

A number of improvements in the productivity of green algae and diatoms would significantly enhance their capabilities as biofuel producers. Photoautotrophic algal growth rates and cell densities at commercial are low compared to microbial fermentation. Enhancement of growth through metabolic engineering with control of cell cycle would be a breakthrough for algal biofuels. Increasing biofuel feedstock production by improving the synthesis of biofuel precursors is imperative. Metabolic engineering of secretion pathways or developing means to readily strip hydrocarbons would allow the organism to survive while producing biofuel metabolites on a continue basis. This would reduce the amount of

biomass that is produced per unit of biofuel, thus focusing resources on the primary product. Metabolic engineering might also increase the range of biofuel metabolites and other high-value added materials that can be synthesized. Furthermore, improved performance in a variety of photobioreactors and conditions is necessary. The development of organisms that can survive in environments that exclude invasive species and other contaminant microorganisms is another desirable attribute for open cultivation systems. Finally, the goal of designing suicide genes to prevent the unintended release of genetically modified organisms (GMOs) is an important consideration.

From a genetic engineering standpoint, there exist certain superior characteristics of eukaryotic algae as compared to other photosynthetic sources of biofuels. Focusing on green algae and diatoms will allow the transfer of well-established metabolic engineering
approaches used in other eukaryotic systems. With eukaryotic algae there is the ability to perform both chloroplast and nuclear transformation, possibly increasing the complexity and range of metabolic engineering. Because eukaryotes have evolved a membrane bound secretory pathway, it is conceivable that eukaryotic algae could be genetically engineered to secrete lipid bodies into the media (Benning, 2008). For example, the alga Botryococcus braunii has naturally evolved this mechanism for secreting hydrocarbons. If this biological feat could be achieved in other microalgal species, it would greatly simplify downstream processing by eliminating cell harvesting and lysis; thus, reducing the entire procedure to merely skimming the lipids from the top of the medium.

Many accomplishments have already been made in the field of microalgal bioengineering (Leon-Banares et al., 2004; Walker et al., 2005), the most relevant to biofuel production being increased photosynthetic efficiency and light penetration in C. reinhardtii (Mussgnug et al., 2007), but augmented lipid production through genetic alterations has yet to be achieved. Currently, C. reinhardtii remains the workhorse of algal genetic engineering for its history as a model photosynthetic organism (Harris, 2001). The recently completed genome sequence of Dunaliella salina may be a good starting point for genetic research of algal biofuel production; however, a single species cannot be expected to serve every application. With the limited availability of genomic data for microalgae (Hallmann, 2007), exploration of transgenic algae for bioenergy demands a genome project for a model biofuel production strain. Future efforts to probe the metabolic pathways of microalgae will likely employ technologies beyond genomic analysis, such as transcriptomics, metabolomics, proteomics, and lipidomics, to examine the broader biological landscape of algal metabolism (Jamers et al., 2009; Vemuri et al., 2005).

Gasification produces a synthetic gas or syngas mainly composed of hydrogen, methane and carbon monoxide (Marie-Rose et al., 2011, Villano et al., 2010). Gasification started in the late 1800’s and first applications were for town gas for street lights illumination and cooking purposes. Other applications were heating, production of raw material for chemical industry and also power generation. When the Second World War was raging and oil was limited, wood gasification was used for transportation and heat and power generation. The energy crisis of the 70’s (1973 and 1979 oil shocks) also provided motivation for improving gasification technologies (Higman and van der Burgt, 2008).

The basics behind gasification are to react the feedstock containing carbon with steam and oxygen (could be air, enriched air or pure oxygen). The following chemical reactions are predominant during gasification (Higman and van der Burgt, 2008)

Thermal decomposition (i. e. pyrolysis), which covers dehydration as well as cracking reactions leading to gases, intermediate vapours and carbon structures known as "char". Partial oxidation of the "char"which forms CO and CO2 generating heat for the otherwise endothermic reactions.

The partial oxidation

Steam-carbon, i. e. the water-gas reaction, that converts carbon structures into H2 and CO.

The water gas reaction

C + H2O = CO + H2 131 MJ/kmol (5)

Steam reforming of intermediates formed by thermal decomposition.

The Steam methane reforming reaction

CH4 + H2O = CO + ЗН2 206 MJ/kmol (6)

The steam reforming reaction

CxHy + XH2O = x CO + ( % y + x) H2 (7)

The dry reforming reaction

CxHy + XCO2 = 2x CO + ( % y + x) H2 (8)

Reactions involving CO2 and H2 with carbon and with intermediates are kinetically slower than the steam induced reactions at the conditions used in gasifiers.

The Boudouard reaction

C + CO2 = 2CO 172 MJ/kmol (9)

The methanation reaction

C + 2H2 = CH4 -75 MJ/kmol (10)

Water gas shift reactions that lead to a desired H2/CO ratio.

The Water gas shift reaction

CO + H2O = CO2 + H2 -41 MJ/kmol (11)

Steam is hence chemically involved in the gasification, but it also served as moderating agent to limit the temperature. Carbon dioxide could also be used to this purpose. Other elements in feedstocks such as sulphur, nitrogen and chlorine will lead to different molecular species contaminating the syngas such as H2S, COS, NH3, HCN and HCl.

Different types and configurations of reactors can be used to produce syngas: fixed bed, entrained flow gasifier and bubbling fluidized bed. Fixed bed gasifier (essentially slowly moving-beds) can be updraft or downdraft.

The Updraft fixed bed gasifier is probably the oldest technology. Biomass or coal are fed from the top and move slowly by gravity. A grate supports the material while oxidizing the gas in its way upwards. The synthesis gas is withdrawn at the top of the reactor and ash from the bottom. There is a combustion zone in the bottom where char combusts in contact of oxygen to form CO2. Char reacts with carbon dioxide to form two moles of carbon monoxide (equation 9). A zone of pyrolysis take place a little higher in the fixed bed because all oxygen has been consumed at this level. Finally, hot synthesis gas dries the feedstock at its entrance (Ciferno and Marono, 2002, Paes, 2005). This simple proven process has as drawback the production of a tar-rich syngas. Downdraft fixed bed gasifier employs co­current flow of oxidant and feedstock. This configuration has low tar formation as an outcome. Nevertheless, a part of the carbon remains unconverted (Ciferno and Marono, 2002).

Entrained flow gasifier utilizes a blast of oxidant in a co-current arrangement with the feed grounded to ensure carry-over. A high temperature is necessary because of the short residency time, counted in seconds. More oxygen is thus required to reach higher temperature (Higman and van der Burgt, 2008). With this type of gasifier, the particles entrained are passed to a cyclone and a riser to bring this material again through the bottom of the gasifier.

Bubbling fluidized bed gasifer is designed to keep the particles in suspension. This technology is based on a configuration where feedstock is added continuously to a bed of alumina, olivine (iron and magnesium orthosilicate) and/or other material serving as a heat carrier. This type of gasifier has high heat/mass transfer and conversion and is able to treat a large range of particles sizes and heterogeneous feedstock. Fluidized bed gasifier has the following advantages: flexibility and easiness for control and maintenance. Figure 1 illustrates an overview of the process developed by Enerkem Inc based on a bubbling fluidized reactor.

Feeding is assured by conveyor, lock hoppers, rotary valves and cooled feeding screw. Oxygen and steam are added through the bed in order to fluidize the medium. Low severity conditions are used in the bed, i. e. pressure lower than 4 atm and temperature lower than 750°C (Chornet et al., 2010). The reactor itself is made from carbon steel with insulation and refractory.

Ash is collected at the cyclones and larger inert material exits at the bottom of the gasifier. Ash can be composed of silica, alumina, potassium, phosphorus, sodium, magnesium, ferric oxide, and smaller amounts of titanium oxide and sulfur compounds (Higman and van der Burgt, 2008).

CaO/MgO is also introduced in the gasifier for neutralising a part of HCl, H2S and COS formed. Literature reports that the use of calcined dolomite and NiO-loaded calcined dolomite helps in the reduction of tar and char formation (Corujo et al., 2010). This dolomite contained 24,6 % of Ca, 19,7 % of Mg and 0,035 % of Fe on a weight basis. Calcination of dolomite produces magnesium and calcium oxides. Corujo et al. have stated that the use of NiO-loaded calcined dolomite catalysts increased the total product gas volume by 30% and decreased the rate of char and tar formation leading to a higher gas energy yield. In fact, bed materials are selected for two criteria: attrition resistance and possible catalytic activity in hydrocarbon and tar reforming. Olivine was compared to silica sand, dolomite, ash, magnetite and iron ore. With the use of Olivine, higher conversion of toluene was observed during steam gasification at 900°C (Rauch et al., 2004).

Comparison of Ni/olivine catalyst with olivine alone demonstrated a higher activity, a higher selectivity to hydrogen and carbon monoxide and a lower carbon deposition. These observations were explained by an enhanced steam adsorption on magnesium oxide facilitating surface carbon gasification rates and by nickel dilution effect, associated with Fe — Ni alloys, ensuring stable catalyst activity (Swierczynski et al., 2007)

Leaving the freeboard of the gasifier, the syngas is composed of: H2, CO, CO2, H2O N2, hydrocarbons, tar, fines, char and contaminants. Tar is a complex blend of condensable hydrocarbons, which includes single to five-rings aromatic compounds along with oxygen — containing hydrocarbons and complex PAH (Yan et al., 2006). The freeboard is sized appropriately for disengagement of alumina and/or other solid fluidized bed materials.

A reforming zone is used for converting hydrocarbons, tar and char to obtain more syngas (see equations 8, 9, 10 and 11). Since thermal decomposition of hydrocarbons requires activation and that reforming reactions are endothermic, heating the gas, by direct or indirect manner, is essential. For example, if oxygen is injected directly, it must be added carefully to avoid lowering the product gas heating value and cold gas efficiency. However, with just the right amount of the oxidizing gas, selective oxidation of methane, ethylene and propylene will occur (Villano et al., 2010). Target temperatures are in the 750°C to 1200°C range. Table 3 depicts general compositions of syngas produced out of some previously mentioned biomass.

Increasing temperature cause the decomposition of char and tar by thermal cracking and steam reforming reactions. Moreover, this has a direct impact on the production and composition of the gas as seen from table 3 (He et al., 2009)

Cyclones are used for entrained ash removal. The latter having typically a diameter greater than 10 microns. Efficiency of this cyclone system is from 90 to 95% and mostly recuperates char. Following the cyclone and recuperation of ash, the next unit operation is a heat exchanger. The latter cools the crude synthesis gas and allocates heat recovery for other application on the process using thermal oil or liquid water which turns to steam.

Note that the reforming step mentioned above could be by-passed directly to the cyclones if the feedstock contains inorganic materials that form eutectics with low melting point Indeed, high temperature in this situation would have as a consequence scaling and fouling of the walls of the system.

Quenching is required for further cooling and removal of condensable materials, tars and fines. In the following step, a wet venturi scrubber with alkaline water is used for neutralisation of acid gases as H2S and HCl. A coalescer/demister at the exit removes fines particles and mist.

A second scrubbing step involving neutral or slightly acidic water is used in order to capture ammonia, trace tars, residual fines and impurities such as chlorine and metals. After this cooling step, the cooled purified syngas is around 30 °C.

Water is recirculating after proper handling. The water loop includes knockout drums and separation of tars and fines particles with air flotation, decanters and/or centrifuges. The skimmed phase recovered (overflow), enclosing tars and fines particles, is emulsified and recycled to the gasifier. The underflow, containing heavier organics and particles, is also re­injected to the gasifier. A part of the recirculated water is withdrawn from the system to maintain the balance of water and contaminants. This purge is subjected to water treatment in the objective of meeting below the required environmental standards. Note that ammonia is separated from the water by steam stripping above 100°C and at about 1 to 3 atm. The ammonia is sent back to the gasifier producing hydrogen and nitrogen.

An additional reforming step is then possible for converting light hydrocarbons to more CO and H2 and final adjustment of the molar ratio of H2/CO.

The syngas is then passed through filters and adsorbers for dehumidification, elimination of traces of chlorine, sulphur and other contaminants such as metal carbonyls. An utlraclean syngas is obtained and compression follows to remove carbon dioxide assisted either by amine-based processes or chilled methanol.

Dielectric heating is an alternative method to conventional heating. Unlike conduction/ convection heating, which is based on superficial heat transfer, dielectric heating is based on volumetric and rapid heat transfer (de la Hoz et al., 2005). When lignocellulosic materials are placed in an electric field for dielectric heating pre-treatment, dipole molecules such as water or other dielectric materials, rotate vigorously to orient in the field. More polar components will absorb more energy, and thus, "hot spots" will be created in non-homogeneous materials. It is hypothesized that this unique heating feature results in an "explosion" effect in the particles and improves the disruption of the lignocellulosic structures. In addition, the non thermal effects of electromagnetic field accelerate the disintegration of the crystal structures (de la Hoz et al., 2005). Dielectric heating can be categorized as microwave or radio frequency depending on the wavelength used in the heating devices (Oberndorfer et al., 2000).

Microwave is electromagnetic waves between 300 MHz (wavelength 1 m) and 300 GHz (wavelength 1 mm). This range of spectrum lies between infrared and radio frequency radiation. Microwave irradiation has been extensively used in many processes because of its high heating efficiency and easy operation. Microwave energy can penetrate into materials and heat them quickly and uniformly. Microwave irradiation is considered to create thermal and non thermal effects. It has been applied as an efficient pre-treatment technique to enhance the hydrolysis of biomass materials. Some studies have demonstrated that
microwave irradiation can change the structure of lignocellulosic materials and degrade or reduce lignin content, reduce cellulose crystallinity, and increase porosity and surface area of the materials (Azuma, 1984; Zhu et al., 2006b; Kashaninejad and Tabil, 2011).

This pre-treatment involves microwave irradiation of immersed biomass in an aqueous environment. Different types of lignocellulosic materials have been pre-treated using microwave irradiation including wheat straw (Ooshima et al., 1984; Zhu et al., 2006b; 2006c; Kashaninejad and Tabil, 2011), barley straw (Kashaninejad and Tabil, 2011), rice straw (Ooshima et al., 1984; Zhu et al., 2005; 2006a), rice hulls (Magara and Azuma, 1989), sugarcane bagasse (Ooshima et al., 1984; Magara and Azuma, 1989; Kitchaiya et al., 2003), switchgrass (Hu and Wen, 2008; Keshwani et al., 2007) and woody materials (Azuma et al., 1984). These materials were subjected to microwave pre-treatment of 2450 MHz in the range of 250 to 1000 W. The temperature of operation ranged from 70 to 230°C, while heating time varied from 5 to 120 minutes. However, higher microwave power with short pre-treatment time and the lower microwave power with long pre-treatment time had almost the same effect on chemical composition of lignocellulosic materials (Zhu et al., 2005; 2006b; Kashaninejad and Tabil, 2011).

In order to increase the efficiency of microwave heating pre-treatment, some researchers have combined microwave treatment with alkaline treatment such as NaOH or Ca(OH)2. Some used alkaline solution during microwave heating treatment (Zhu et al., 2005; 2006a; 2006b; 2006c; Keshwani et al., 2007; Kashaninejad and Tabil, 2011) and some applied the alkaline solution before the lignocellulosic materials were subjected to microwave irradiation (Zhu et al., 2006a; Hu and Wen, 2008). Combination of microwave irradiation and alkali treatment improves the degradation of biomass by accelerating the reactions during the pre-treatment process compared with the conventional heating chemical pre­treatment process. Remarkable changes (Table 2) have been reported in the chemical composition of biomass samples after microwave-alkali pre-treatment, particularly in hemicellulose, lignin, and cellulose contents (Kashaninejad and Tabil, 2011). It has been reported that alkali treatment dissolves lignin and hemicellulose, and microwave irradiation facilitates dissolving these components in alkali solutions (Jackson, 1977; Kumar et al., 2009; Lesoing et al., 1980; Zhu et al., 2005). Biomass samples pretreated by microwave-alkali technique have lower lignin, hemicellulose, and cellulose than samples pretreated by microwave-distilled water or untreated samples. Moreover, degradation and depolymerisation of lignin to smaller phenolic components is another influence of microwave-alkali pre-treatment that could be considered as binder in densification process. The pellets made from microwave-chemical pre-treated biomass grinds have significantly higher density and tensile strength (Table 3) than the untreated or samples pre-treated by microwave alone (Kashaninejad and Tabil, 2011).

Radio frequency (RF) can penetrate more deeply into the materials compared with microwave heating because the radio frequency wavelength is up to 360 times greater than microwave (Marra et al., 2007). This unique characteristic is an advantage to treat large amount of material and it is easier to scale up the process. While radio frequency as a heating method has been widely applied in food-processing industries, there is not much report on application of radio frequency heating for lignocellulosic materials pre-treatment. Hu et al. (2008) used radio frequency heating in the NaOH pre-treatment of switchgrass to enhance its enzymatic digestibility. Because of the unique features of radio frequency heating (i. e., volumetric heat transfer, deep heat penetration of the samples, etc.), switchgrass could be treated on a large scale, at high solids content, and with a uniform temperature profile. At 20% solids content, radio frequency-assisted alkali pre-treatment (at 0.1 g of NaOH/ g of biomass loading and 90°C) resulted in a higher xylose yield than the conventional heating pre-treatment. The optimal particle size and alkali loading in the radio frequency pre-treatment were determined to be 0.25-0.50 mm and 0.25 g of NaOH/ g of biomass, respectively.

Generally, enzymes used to catalyze the reduction of oxygen into water are either laccase or bilirubin oxidase (BOD). The main property of these enzymes is their ability to directly reduce oxygen to water at potentials higher than what can be observed with platinum based electrodes (Soukharev et al., 2004). These two enzymes are classified in "multicopper oxidases" class and contain four Cu2+/Cu+ active centers which are commonly categorized in three types: Ti, T2 and T3. T1 site is responsible for the oxidation of the electron donor. The trinuclear center composed both of T2 center and two equivalent T3 centers is the place where oxygen reduction occurs (Palmer et al., 2001). The associated mechanism is proposed in Fig. 2.

Cu

Fig. 2. Oxygen reduction catalyzed by "multicopper oxidases"

In the next part the different properties and performances of both laccase and BOD electrodes will be discussed.

1.1.3 Reduction of oxygen catalyzed by laccase

Laccase is able to oxidize phenolic compounds and to simultaneously reduce oxygen into water. The microorganism from which it is extracted greatly determines the redox potential of the T1 site which can vary from 430 mV vs. NHE up to 780 mV vs. NHE (Palmore & Kim, 1999). Laccase from Trametes versicolor is the most attractive one since redox potential of its T1 site is ca. 780 mV vs. NHE (Shleev et al., 2005). Nowadays, the best performances with laccase electrodes are obtained with osmium based polymers as redox mediators (Mano et al., 2006). Actually these electrodes are able to deliver a current density of 860 pA cm-2 at only -70 mV vs. O2/H2O at pH 5. In the same conditions, the identical current density is obtained at -400 mV vs. O2/H2O with a platinum wire as catalyst. Nevertheless, performances of laccase (from Pleurotus Ostreatus) electrodes drop drastically in the presence of chloride ions (Barton et al., 2002) what constitutes both a major problem and a great challenge for its use in implantable glucose/O2 biofuel cells.

In this section we briefly describe conventional BFCs that have been miniaturized. The number of miniaturized BFCs mentioned in literature is mainly restricted to a few devices working from glucose and O2 that have mostly been designed by reducing the electrode size and cell volume. Different strategies have been used to miniature BFCs design.

Heller and co-workers have successfully demonstrated the efficiency of original miniature membraneless BFCs functioning under physiological conditions. They have developed the first handmade miniature device containing only two components, an anode and a cathode of 7-|im diameter and 2-cm long carbon fibers, placed in a polycarbonate support. The anode was modified by GOx and the cathode was modified by either laccase or BOD, within and mediated by redox osmium-based hydrogels (Mao et al., 2003). In 2001, they developed the first miniature membraneless BFC that delivered 140 pW cm-2 at 0.4V (Chen et al., 2001). This simple device suggested that the goal of a miniature autonomous sensor-transmitter system could be realistic (Bullen et al., 2006; Heller, 2004). After further developments based on the improved redox polymer connecting the reaction centers of enzymes to the electrodes, the devices delivered higher power densities of 431 pm cm-2 at 0.52 V (Mano et al., 2002) and 440 pm cm-2 at 0.52 V (Mano et al., 2003), in pH 7.2, 37 °C and 15 mM glucose. The high power density delivered by these devices comes from the cylindrical mass transport at the carbon fibers and the use of efficient redox polymers to transport electron. They also showed that this system produced a power density of 240 pm cm-2 at 0.52V when implanted in a living organism, near the skin of a grape. Later, by replacing carbon fibers by engineered porous microwires made of oriented carbon nanotubes, the most efficient glucose/O2 BFC ever designed was developed (Gao et al., 2010) and delivered high power density of 740 pW cm-2 at a cell voltage of 0.57 V. The success of the experiment probably results in the increase of the mass transfer of substrates.

Another miniature devices presently lower performance described stacked biofuel cell designs. One work described a stacking structure composed of six cells connected in series on a chip (Nishizawa et al., 2005), composed of GOx anode and polydimethylsiloxane- coated Pt cathode. The performance of the arrayed cells on the chip was 40 pW cm-2 in air — saturated buffer solution containing 5 mM glucose. Another work reported the development of a miniature BFC with a footprint of 1.4 cm2, by adopting the design of stackable proton exchange membrane (PEM) fuel cells (Fischback et al., 2006). This device consisted of an air­breathing cathode and an enzymatic anode composed of crosslinked GOx clusters on the surface of carbon nanotubes. This study demonstrated the important role of buffer solution in determining the performance and stability of miniature BFCs. In buffered fuel solution the initial performance was very high (371 pW cm-2), but quickly dropped due to a deactivation of the proton exchange membrane. However, in unbuffered solution, the initial performance was lower (117 pW cm-2) due to low pH condition, but its performance was very stable for 10 hours. This work suggested that the use of miniature system and unbuffered fuel solution will be a benefit to practical applications.

Currently, in such miniature devices, current density and delivered power output are mainly limited by the diffusion of fuel to the electrode surface. One interesting innovation to maximize the transport efficiency is to use hydrodynamic flow and to pump the fuel to the electrode.