Two types of sugars are present in biomass: hexoses (six-carbon sugars), of which glucose is the most common one, and pentoses (five-carbon sugars), of which xylose is the most com­mon one. Glucose and xylose can be easily hydrogenated to yield sorbitol [29] and xylitol [30] respectively. These two molecules can undergo C-C and C-O hydrogenolysis in the presence of hydrogenation catalysts, leading mainly to a mixture of ethyleneglycol, glycerol, and 1,2-propanediol. Other products such as butanediols, lactic acid, methanol, ethanol, and propanol can also be formed (Figure 6). Ni is known to show high hydrogenolysis activity towards C-C and C-O bond hydrogenolysis, this is the reason why, the use of Ni on differ­ent acid supports seems an interesting alternative for this process. For instance, Ni support­ed on NaY zeolite gave 68% sorbitol conversion with 75% combined selectivity to 1,2-PDO and glycerol at 220°C and 60 bar H2 pressure after 6 h [8]. The addition of Pt to the catalyst did not influence its activity and selectivity significantly. However, in the case of 20 wt% Ni/Al2O3 prepared by coprecipitation, the addition of 0.5 wt% of Ce significantly increased sorbitol conversion (from 41% to 91%) and the stability of the catalyst [31]. It seems that the addition of Ce considerably reduces Ni leaching, and hence improves the stability of the cat­alyst. Other catalytic systems have also been reported besides the Ni acid-support ones. For instance, Ru supported on carbon nanofiber and graphite felt composite catalysts gave 68% sorbitol conversion and 79% propylene glycol selectivity at 220°C and 8.0 MPa hydrogen pressure [32].

The quantity of wastes produced depend of factors as: the number of inhabitant in the city, urbanization rate, consumption habits, cultural practices to handle of wastes, the income, the application of technology and industrial development. According to the information reported by the "Superintendencia de Servicios Publicos Domiciliarios" by 2008, see [4], in Colombia were generated daily 25.079 tons of urban solids wastes, 10 million of tons/year, which 77% were domestics (19.310,8 ton); 15% Industrials (3.761,9) and 8% others (2.006,3 ton). In the
country, the management of wastes is focused to the final disposal in landfill; only 2,4% is dedicated to recycle and valorization [1].

In the country, the solid wastes are mainly composed of organic material (65%), followed by the plastics (14%), paper and cardboard (5%), glass (4%), other components with minor participation.

Table 2. Composition of solids wastes in Colombia

In Colombia the major quantity of solid wastes generated are collected and treated by municipal companies (waste from domestic activities, commercials and industrials); however in some regions the problem of wastes solids is very important as the final disposition is made with little control, generating environmental pollution. The production of wastes (kg/habitant/ day) is approximately 0,5 kg/habitant/day, oscillating between 1 kg/habitant/day for the big

cities until 0,2 kg/habitant/day in the small towns [15]. The "Superintendencia de Servicios

Publicos Domidlia/ios" pssbiishedby 2002 a /ludy absa^/ tha finaldisnosition of tirosolids wastes in t.086 aihes. Tlso tedmologieo тоаеОгарикпІаге^итрвіІе and oseb indnoration (52%), thentandfin (30 %/ a/sci Sinrsilyi t;lii<i ii.^e it:i <]o:^qt.::>is^t:i|^i. s|., incmeration and other. [1896:, ]b[.

Source: [6].

There are two options to solve the problems generated by urban solid wastes which can be applied simultaneously to reach an optimum result:

• The first option according to the National Politics of Solid Wastes is give priority to integral management of solids wastes, focusing the operations management in the reuse and valorization of different materials that integrate the urban solid wastes.

• The second option is to take the wastes and give them an adequate final disposal in landfill operated technically.

The biomass in Colombia has calorific values between 4,384 kcal / kg for stems of coffee and 1,800 kcal / kg for banana rachis [7]. These values are comparable with reports from other countries as China where biomass from agricultural and forest activities have values between 3,827 to 4,784 kcal / kg [8]. In Argentina the lignocellulose biomass has values between 3,000 — 3,500 Kcal / kg and the municipal wastes between 2,000 and 2,500 Kcal / kg [9].

2.1. Colombian normativity about solid wastes

The Colombian normativity related to management of organics solids wastes began with the code of renewable natural sources (decree 2811 of 1974) and were implemented the followed norms:

• Decree 2104 of 1983: management of solids wastes.

• Resolution 2309 of 1986: special solids wastes.

• Law 142 of 1994: Law of public services.

• Decree 605 of 1996: Indications for an adequate cleaning service, from the generation, storage, collection, transport, to final disposition.

• Committee Technical ICONTEC 000019 about environmental management of solids wastes.

• Decree 1716 of August 2002 of "Ministerio de Desarrollo Economico" (In English: Economic Development Ministry) by mean the law 142 of 1994, law 632 of 2000 and the law 689 of 2001, related to the cleaning public service, the law 2811 of 1974 and the law 99 of 1993. The article 8 related to the program for the integral management of solids wastes, which should be realized by the cities in a maximum time of 2 years [10].

Immobilization of microorganisms in the membrane or using membrane reactors is another option of butanol removal. The productivity can be enhanced obviously by this way. Huang et al. reported the continuous ABE fermentation by immobilized C. acetobutylicum cells with the fibrous as carrier and a productivity of 4.6 g/L/h was obtained (Huang et al., 2004). Qure­shi et al. studied the butanol fermentation by immobilized C. beijerinckii cells with different carriers such as clay brick, the reactor productivity was enhanced to 15.8 g/(lh) (Qureshi and Blaschek, 2005a). Although the butanol productivity increased by using immobilized cell fermentation, leakage of cells from the matrices is a frequent problem for the industrial ap­plication. There still some other problems such as poor mechanical strength and increase mass transfer resistance etc.

4.2. Butanol recovery by gas stripping

Gas stripping seems to be a promising technique that can be applied to butanol recovery com­bined with ABE fermentation. When the gas (ordinary N2 or CO2 ) are bubbled through the fer­mentation broth, it captures the solvents. The solvents then condensed in the condenser and are collected in a receiver. Ezeji applied gas stripping on the fed-batch fermentation, 500 g glu­cose was consumed and 233 g/l solvent was produced with the productivity of 1.16 g/(Lh) and the yield of 0.47 g/g. When combined with continuous fermentation with gas stripping, 460g/l solvent was obtained with 1163g glucose consuming (Ezeji et al., 2004a; Ezeji et al., 2004b).

The selection of organic or inorganic carbon feedstock for biofuel production has downstream ramifications on host selection, product yields, and process requirements. Clearly, the feedstock choice will determine whether a heterotrophic or autotrophic host is required, and in turn, this will influence the metabolic engineering strategy. In general, heterotrophic hosts have generated higher fuel titers than autotrophic hosts, with more than 10-fold higher concentrations of FFAs, FAEEs, fatty alcohols, and alkanes/alkenes (Table 1). This does not imply that heterotrophic production is more advantageous than autotrophic production, for the entire production process must be considered (Figure 1). The sugars from lignocellulosic biomass deconstruction (heterotrophic feedstock) have a higher energy content compared to inorganic carbon (autotrophic feedstock). The overall balances for obtaining one molecule of GAP from heterotrophic and autotrophic metabolisms provide evidence for this:

Heterotrophic: V Glc + ATP ® GAP + ADP

Autotrophic: 3 CO2 + 9 ATP + 6 NADPH + 5 H2O ® GAP + 9 ADP + 6 NADP+ + 8 Pi

While autotrophic GAP generation requires a significant investment of energy (9 ATP) and reducing equivalents (6 NADPH), heterotrophic GAP production only requires one energy equivalent. However, if a life cycle perspective is considered, the carbon from lignocellulosic feedstocks is ultimately derived from photosynthesis, requiring the same energy and reducing equivalent input as autotrophic microorganisms. Overlooking this fact will bias a direct comparison between heterotrophic and autotrophic fuel production.

One major difference between heterotrophic and autotrophic fuel production is the design considerations for the bioreactor. Heterotrophic microbes, such as E. coli and S. cerevisiae, are traditional industrial microorganisms with well-established, large-scale cultivation practices and bioreactors. On the other hand, autotrophic hosts like algae and cyanobacteria require light as the energy source to drive photosynthesis and inorganic carbon fixation. This can have a dramatic effect on bioreactor design. Transparent materials can be used with traditional bioreactor designs to allow for light penetration. Light availability, however, will ultimately limit the cell densities of photosynthetic microalgae, and the surface area of light exposure with traditional bioreactor designs is not optimal. Some have proposed to use fiber-optics within the liquid culture to improve light availability [114], but a costly solution such as this is not feasible for a low-value, commodity product like fuel. A wide-range of photobioreactor (PBR) designs have been proposed [115], yet generally, PBRs are characterized by the use of transparent materials, high surface area to volume ratios, and a relatively short pathlength for light. Other PBR design factors include a mechanism for air/CO2 delivery, dissipation of radiative heat, and removal of inhibitory O2 [115]. Due to the low value of fuel products, PBRs for fuel synthesis favor low-tech designs and inexpensive materials to reduce both capital and operating costs. In fact, NASA has proposed to float plastic bags of algal cultures in wastewater to allow for nutrient exchange [116]. Alternatively, open pond systems, traditionally a raceway configuration with a paddle-wheel for mixing, have proven successful for cultivating micro­algae at scale [117]. Unlike PBRs, ponds are open to the environment, allowing for evaporative water loss and pond crash due to contamination by predators and competitors. However, the low capital cost of an open pond system makes this design a contender for fuel production. Clearly, the large-scale cultivation techniques for autotrophic fuel production still require additional development and optimization compared to heterotrophic cultivation.

In the previous sections the significance that hydrogenolysis reactions have and will have in the future bio-refineries has been highlighted. In fact, they will be essential in fuel and chemical manufacturing. Hydrogenolysis involves chemical bond dissociation in an organic substrate and simultaneous addition of hydrogen. Therefore, hydrogen is required as reac­tant in all hydrogenolysis reactions. This is the reason why, most of the literature works re­ferred to hydrogenolysis report experiments conducted under molecular hydrogen (H2) atmosphere. Nevertheless, the use of molecular hydrogen has some important drawbacks:

i. Liquid phase processes are preferred to gas phase processes as they are more ener­gy efficient. However, H2 presents really low solubility on aqueous or organic solu­tions. As a consequence, when operating in liquid phase it is necessary to operate at elevated hydrogen pressures to obtain significant hydrogen concentrations near the catalysts. This, on one hand, notably increases the cost of design and building of the future plants, and on the other hand, increases the operating cost related to safety measures, as hydrogen is easily ignited and shows high diffusivity.

ii. Most of the nowadays available hydrogen gas is produced from fossil fuels by ener­gy intensive processes. Therefore, if sustainability is the goal it is a contradiction that the main reactant in most of the biorefinery processes is based on fossil resources.

iii. The low density and high diffusivity of hydrogen make problematic and expensive its transportation and storage. This problem is more relevant for small size biomass conversion facilities.

Hydrogen from non fossil origin will surely be a reality in the oncoming years, as reforming processes from various renewable compounds (like biomethane, glycerol or ethanol) and water splitting processes using solar light are being intensively developed. Nonetheless, the problems of transportation, storage and low solubility in liquid solutions will remain. One interesting option that could solve the problems associated to the use of molecular hydrogen is to directly generate the required hydrogen in the active sites of the catalyst.

3.1. Experimental set up

The experimental setting is shown in Fig. 16. The main part of the system is cylinder with height H = 10 mm, and radius R = 135 mm. Its lateral surface made of stainless steel with a thickness of 5 cm. This cylinder is filled with liquid for experimental operations. The electrodes are placed perpendicular to the cylinder axis. They have the diameter of 10 mm, made of brass, and their ends are shaped hemispheres with a radius of curvature of 5 mm. The discharge (2) is ignited between the rounded ends of the electrodes. At a distance of 40 mm from the lateral surface of the cylinder is piezo-ceramic pressure sensor (3), which records acoustic vibrations in the fluid, caused by electric discharge under water. The distance between the sensor head and the system axis = L.

The cylindrical system could be located in a horizontal position (Fig. 17a) or vertical one (Fig. 17b). The full volume (0.5 l) of system is fluid-filled. The fluid in the system can be processed
as in static mode (no flow), and dynamic one (with flow ~ 15 cm3/s). Additional supply of gas may be realized in the system also (airflow ~ 4 cm3/s), which is injected through a spray nozzle (source diameter 8 mm) located near the inner wall of the cylinder at a distance of 130 mm from the discharge gap (Fig. 16). The working fluids are: the tap water (with and without flow), distillate and ethanol (96%, no flow).

The main feature of electrical scheme for pulsed power feeding of discharge in a liquid is usage of two independent capacitors which are supplied two independent sources of power (1 kW). Pulsed discharge realized in two modes: single and double pulses. In the single pulse mode only one capacitor is discharged with a frequency of 0 — 100 Hz.

Double pulse mode is realized as follows: one capacitor discharges in the interelectrode gap through air spark gap; the clock signal from the Rogowski belt after first breakdown is applied to the thyratron circuit and second capacitor discharges through it. This set of events leads to the second breakdown of the discharge gap and second discharge appearance.

Delay of the second discharge ignition may be changed in range of 50 — 300 microsec­onds. The following parameters are measured: discharge current and the signal from the pressure sensor. The Rogowski belt has the sensitivity 125 A/V, and its signal is record­ed with an oscilloscope. Capacity for the first discharge (Q) = 0.105 |oF and it is charged to Uj = 15 kV (energy Ej = 12 J), capacity for the second discharge C2 = 0.105 |oF and it is charged to U2 = 18 kV (energy E2 = 17 J).

A distance between electrodes can be changed in the range of 0.25 — 1 mm. The second discharge can be ignited at the moment (according to the delay tuning) when the reflected acoustic wave, created by the first electric discharge in liquid, returns to the center of the system (the time of its collapse ~ 180 ms).

The composition of ethanol and bioethanol reforming products is studied with gas chromatography, in case of bioglycerol reforming — mass spectrometry and infrared spectrophotometry.

1.1. Bioreactor design

An optimum gas fermentation system requires efficient mass transfer of gaseous substrates to the culture medium (liquid phase) and microbial catalysts (solid phase). Gas-to-liquid mass transfer has been identified as the rate-limiting step and bottleneck for gas fermenta­tion because of the low aqueous solubility of CO and H2 respectively at only 77% and 68% of that of oxygen (on molar basis) at 35oC [185]. Hence, a bioreactor design that delivers suf­ficient gas-to-liquid mass transfer in an energy-efficient manner at commercial scale for gas fermentation represents a significant engineering challenge. A brief overview of reactor con­figurations reported in gas fermentation operations is given below.

3.1. Immobilization

Immobilization of C. Acetobutylicum strains prevents bacteria from existing in the ferment mash and is a very essential facility in a variety of integrated solvent recovery methods. Haeggstroem and Molin [87] concluded that immobilized vegetative cells of C. Acetobutyli — cum have a similar product formation pattern when incubated in a simple glucose-salts solution as ordinary growing cells. If vegetative cells of the organism are immobilized in the solvent production phase, solvents are continuously produced on extended incubation. By immobi1izing spores of the organism, the disturbance of the cells metabolic activity during the immobilization procedure was avoided. After the outgrowth of viable cells within the gel, the washed gel preparation retained at a high production capacity in the non-growth stage

and the results indicate that continuous production might be fully possible. The butanol productivity was also found to be higher with immobilized cells than in a normal batch process. Haeggstroem [88] used immobilized spores of Clostridium acetobutylicum in a calcium alginate gel. The productivity of the system was 67 g BuOH/L-day and with immobilized cells it was possible to achieve continuous BuOH production for 1000 h. Foerberg et al. [89] developed a technique for maintaining constant activity during continuous production with immobilized, non-growing cells. A single stage continuous system with alginate-immobilized C. Acetobutylicum, was mainly fed with a glucose medium that supported fermentation of acetone-BuOH but did not permit microbial growth. The inactivation that occurred during these conditions was prevented by pulse-wise addition of nutrients to the reactor. By using this technique, the ratio of biomass to BuOH was reduced to 2% compared to 34% in a traditional batch culture. At steady state conditions BuOH was the major end product with yield coefficients of 0.20 (g/g glucose). The productivity of BuOH was 16.8 g L-1 d-1 during these conditions. In a corresponding system with immobilized growing cells the ratio of biomass to BuOH was 52-76% and the formation of butyric and acetic acid increased thereby reducing the yield coefficients for BuOH to 0.11 (g g-1). With the intermittent nutrient dosing technique, const. activity from immobilized non-growing cells has been achieved for 8 weeks.

Several carriers have been tested for production of ABE solvents by immobilized local strain of C. acetobutylicum. Thus, both batch and continuous fermentations were performed by using sodium alginate, polyacrylamide, activated carbon, and silica gel carriers. Calcium alginate was found to be the most suitable with batch culture techniques where the total solvent production was 19.55 g L-1 after 4 days. On the other hand, higher solvent yields with continuous fermenta­tion was noticed with silica gel G-60 (0.063-0.2 mm) with 13.06 g L-1 solvent production. In all cases, the tested solid supports were of inferior effect for solvent production under the exptl. conditions used as compared with Ca-alginate [90]. High-strength carriers were also tested for C. acetobutylicum ATCC 824 in batch fermentation. Coke, kaolinite and montmorillonite clay appeared to have a beneficial effect on the fermentation, although the effectiveness appeared to be dependent on the medium used. One of the least expensive materials, coke, was suitable for use in continuous culture. Steady state conditions could be maintained for more than 30 days with total solvent productivity and a yield of 12 g L-1, 1.12 g L-1 h-1 and 0.3 g total solvent/g glu­cose used, respectively [91]. Entrapment of C. acetobutylicum AS 1.70 with PVA as the base and by means of absorption in the corncob as the carrier is recommended. Experiments have been done to produce acetone and butanol in a statical way in batches and by changing the corn as medium circulatingly [92]. The vegetative cells of C. acetobutylicum AS 1.70 were also immobi­lized onto CR (ceramic ring) carriers by adsorption. The continuous production of acetone- BuOH from 8% corn mash concentration was carried out for 90 days in a system of 3-stage packed column reactor (total vol. 5.18 L). The maximal concentration of solvent (acetone, BuOH, and EtOH) was 21.9 g L-1 and the productivity of the column was 24.73 g L-1 d-1. The resid­ual starch concentration was 0.43% and the conversion efficiency of starch was 40.5% [93]. ABE solvent production was also carried out with C. acetobutylicum DSM 792 (ATCC 824) in a two — stage stirred tank cascade using free and immobilized cells. The cells were immobilized by algi­nate, к-carrageenan or chitosan. The cell-containing pellets were dried or chemically treated to improve their long-term stability. Dried calcium alginate yielded the best matrix system. It re­mained stable after a fermentation time of 727 h in stirred tank reactors. The solvent (sum of ace­tone, butanol and ethanol) productivity of 1.93 g L-1 h-1 at a solvent concentration of 15.4 g L-1 with free cells was increased to 4.02 g L-1 h-1 at a solvent concentration of 4.0 g L-1 h-1 with calcium alginate-immobilized cells (25% cell loading, 12 g L-1 pellet concentration, 3 g L-1 wet cell mass concentration). With pellet diameter of 0.5 mm, the biocatalyst efficiency was <50% [94]. Immo­bilized cells of C. saccharoperbutylacetonicum N1-4 (ATCC 13564) were tested in an anaerobic batch culture system. Two different methods of immobilization, active immobilization in algi­nate and passive immobilization by employing stainless steel scrubber, nylon scrubber, polyur­ethane with uniform pore’s size, polyurethane with different pore’s size and palm oil empty fruit bunch fiber were studied. Immobilization in alginate was carried out on the effect of cell’s age, initial culture pH and temperature on the production of ABE. Immobilized solventogenic cells (18 h) produced the highest total solvents concentration as compared to other phases with productivity of 0.325 g L-1 h-1. The highest solvents production by active immobilization of cells was obtained at pH 6.0 with 30 °C with productivity of 0.336 g L-1 h-1. Polyurethane with differ­ent pore’s size is significantly better than other materials tested for solvents productivity and YP/S at 3.2 times and 1.9 times, respectively, compared to free cells after 24 h fermentation. We concluded that passive immobilization technique increases the productivity (215.12 %) and

YP/S (88.37 %) of solvents by C. saccharoperbutyl-acetonicum N1-4 [95]. C. beijerinckii was im­mobilized in calcium alginate to produce BuOH continuously from glucose. Two different algi­nate geometries (beads and coated wire-netting) were used for continuous experiments and two mathematical models (sphere and flat plate) were developed. Calculations revealed that no glucose limitation was present in both cases. Furthermore, the biomass build-up in the alginate was probably a surface process [96].

Cells of C. acetobutylicum immobilized on bonechar were used for the production of ABE sol­vents from whey permeate. When the process was performed in packed bed reactors operated in a vertical or inclined mode, solvent productivities up to 6 kg m-3 h-1 were obtained. However, the systems suffered from blockage due to excess biomass production and gas hold-up. These problems were less apparent when a partially-packed bed reactor was operated in horizontal mode. A fluidized bed reactor was the most stable of the systems investigated, and a productivi­ty of 4.8 kg m-3 h-1 was maintained for 2000 h of operation. The results demonstrate that this type of reactor may have a useful future role in the ABE fermentation [97]. Schoutens determined the optimal conditions necessary for the continuous BuOH production from whey permeate with C. beyerinckii LMD 27.6 immobilized in calcium alginate beads. The influence of three parame­ters on the BuOH production was investigated: fermentation temperature, dilution rate (during start-up and at steady state) and concentration of Ca2+ in the fermentation broth. Both a fermen­tation temperature of 30 °C and a dilution rate of <0.1 h-1 during the start-up phase are required to achieve continuous BuOH production from whey permeate. BuOH can be produced continu­ously from whey permeate in reactor productivities 16-fold higher than those found in batch cultures with free C. beyerinckii cells on whey media [98]. Fermentation of cane sugar molasses by immobilized C. acetobutylicum cells was greatly affected by inoculum size, calcium alginate concentration and molar ammonium nitrogen to molasses ratios. The pH value of the medium and incubation temperature both influenced the ABE production. The maximum total solvent content reached 22.54 g L-1 at inoculum size 6% (w/w), molasses concentration 140 g/l, sodium alginate amount 3 %, and molar ammonium nitrogen to molasses ratios 0.48, pH 5.5. Attempts to recyclize the fermentation process by using immobilized spores of C. acetobutylicum afford­ed total solvent contents of 22.54, 20.64, 19.31 g L-1 during the first 3 runs, respectively [99].

Pyrolysis oil is the product of fast pyrolysis of biomass, a process that allows the decomposi­tion of large organic compounds of biomass such as lignin at medium temperatures in the presence of oxygen. Pyrolysis, that is in essence thermal cracking of biomass, is a well estab­lished process for producing bio-oil, the quality of which however is far too poor for direct use as transportation fuel. The product yields and chemical composition of pyrolysis oils de­pend on the biomass type and size as well as on the operating parameters of the fast pyroly­sis. However, a major distinction between pyrolysis oils is based on whether catalyst is employed for the fast pyrolysis reactions or not. Non-catalytic pyrolysis oils have a higher water content than catalytic pyrolysis oils, rendering the downstream upgrading process a more challenging one for the case of the non-catalytic pyrolysis oils.

Untreated pyrolysis oil is a dark brown, free-flowing liquid with about 20-30% water that cannot be easily separated. It is a complex mixture of oxygenated compounds including wa­ter solubles (acids, alcohols, ethers) and water insolubles (n-hexane, di-chloor-methane), which is unstable in long-term storage and is not miscible with conventional hydrocarbon — based fuels. It should be noted that due to its nature pyrolysis oil can be employed for the production of a wide range of chemicals and solvents. However, if pyrolysis oil is to be used as a fuel for heating or transportation, it requires upgrading leading to its stabilization and conversion to a conventional hydrocarbon fuel by removing the oxygen through catalytic hydrotreating. For this reason, a lot of research effort is focused on catalytic hydrotreating of pyrolysis oil, as it is a process enabling oxygen removal and conversion of the highly corro­sive oxygen compounds into aromatic and paraffinic hydrocarbons.

For non-catalytic pyrolysis oils, the catalytic hydrotreating upgrading process involves con­tact of pyrolysis oil molecules with hydrogen under pressure and at moderate temperatures (<400°C) over fixed bed catalytic reactors. Single-stage hydrotreating has proved to be diffi­cult, producing a heavy, tar-like product. Dual-stage processing, where mild hydrotreating is followed by more severe hydrotreating has been found to overcome the reactivity of the bio-oil. Overall, the pyrolysis oil is almost completely deoxygenated by a combination of hy­dro deoxygenation and decarboxylation. In fact less than 2% oxygen remains in the treated, stable oil, while water and off-gas are also produced as byproducts. The water phase con­tains some dissolved organics, while the off-gas contains light hydrocarbons, excess hydro­gen, and carbon dioxide. Once the stabilized oil is produced it can be further processed into conventional fuels or sent to a refinery. Table 1 shows the properties of some common cata­lytic pyrolysis oils according to literature.

Catalytic pyrolysis oils have been reported to getting upgraded via single step hydropro­cessing, most of the times utilizing conventional CoMo and NiMo catalysts. During the sin­gle step hydroprocessing, the catalytic pyrolysis oil feedstock is pumped to high pressure, then mixed with compressed hydrogen and enters the hydroprocessing reactor. In Table 5 the typical operating parameters for single stage hydroprocessing and associated deoxyge­nation achievements are given according to literature [29;33-38].

However, in the case of non-catalytic pyrolysis oils or for achieving better quality products, multiple-stage hydroprocessing can be employed for upgrading pyrolysis oils. Multiple — stage hydroprocessing utilizes at least two different stages of hydroprocessing, which may

include hydrotreating or hydrotreating and hydrocracking reactions. In the first stage the catalytic hydrotreatment reactor stabilizes the pyrolysis oil by mild hydrotreatment over Co­Mo or NiMo hydrotreating catalyst [32;40-42]. The first stage product is then further proc­essed in the second-stage hydrotreater, which operates at higher temperatures and lower space velocities than the first stage hydrotreater, employing also CoMo or NiMo catalysts within the reactor. The 2nd stage product is separated into an organic-phase product, waste­water, and off-gas streams. In the literature [41], even a 3rdstage hydroprocessing has been used for the heavy fraction (which boils above 350°C) of the 2ndstage product, where hydro­cracking reactions take place for converting the heavy product molecules into gasoline and diesel blend components.

In commercial practice, cation-exchange resins (e. g. Bowex 50w, Amberlyst 15 (A15), Lewa — tit SPC 118 or Nacite) which are sulphonated copolymers of styrene and divinylbenzene (DVB), the cross-linking agent, are used as fixed-bed catalysts for etherification reaction [4].

Other catalysts such as HPA [6] and zeolites [7] are also being considered. Le Van Mao et al. [8] studied the synthesis of MTBE over triflic acid loaded Y-type zeolites. Ahmed et al. [9] developed MFI-type zeolites which were synthesized by the rapid crystallization method for production of MTBE. Collignon et al. [10] evaluated several acid zeolites, including H-Beta (ZB25, ZB75, ZBF, ZBSC), US-Y (CBV760) and ion-exchange resin (Amberlyst 15) for the liq­uid phase synthesis of MTBE. From all of the works, it was found that all zeolites appear to be as active as Amberlyst 15 but the zeolite catalysts produce less by-products and are more thermally stable than the resin catalyst.