Bio-oils are dark red-to-black liquids that are produced by biomass pyrolysis. Biomass is typically obtained from municipal wastes or from agricultural and forestry by-products (Demirbas, 2007). With an efficiency rate as high as ~70%, pyrolysis is among the most efficient processes for biomass conversion. The density of the liquid is approximately 1,200 kg/m3, which is higher than that of fuel oils and significantly higher than that of the original biomass. The gasification of bio-oil with pure oxygen and the further processing of syngas into synthetic fuel by the FT process, is being investigated; however, this process does not appear to be economically attractive (Demirbas & Balat, 2006).

In order to use rapeseed oil as fuel, some physical and chemical properties of the oil must be met. The description of these properties as well as its effect on the diesel engine should be taken into account. Thus, the German norm DIN 51605 is to be followed.

This norm establishes the maximum and minimum values for the parameters selected to accept a rapeseed vegetable oil as appropriate biofuel to substitute diesel in modified engines. The parameters include some intrinsic rapeseed oil properties and some which are variable and indicate if the oil has been correctly processed. Between these properties, acid value, iodine index and oxidation time are the ones which indicate the vegetable oil degradation.

PBRs can be operated in batch or continuous mode. There are several advantages when using them in continuous mode. Firstly, continuous culture provides a higher control than batch mode. Secondly, growth rates can be regulated and keep in a steady state for long periods, and the biomass concentration can be modulated by dilution rate control. In addition, results are more reliable and reproducible owing to the steady state of continuous reactors, and the system yields better quality production (Molina et al., 2001).

However, there are limitations that can make the continuous process unsuitable for some cases. One of these limitations is the difficulty in controlling the production of some non­growth-related products. For instance, the system often requires feed-batch culturing and continuous nutrient supply that can lead to wash-out. Filamentous organisms can be difficult to grow in continuous PBRs because of the viscosity and heterogeneity of the culture medium. Another problem is that the original strain can be lost if it is displaced by a faster-growing contaminant. The contamination risk and loss of reliability of the bioreactor becomes more relevant when long incubation periods are needed, so the potential initial investment in necessary better quality equipment could rise and hamper the economic viability of the production unit (Mata et al., 2010).

The possible coproduction of high value chemicals could lead to the solution of the above problems, but it implies taking multiple parameters and options into consideration. The microalgae production units will suffer drastic changes, both in the operational aspect (temperature, insolation, wind, microalgal and bacterial or fungical contaminations etc.) and in the commercial one (oscillations in value of by-products, improvements in centrifugation or extraction strategies or development of non-algal biofuels, etc). Taking into consideration all the above mentioned parameters, it can be ascertained that any microalgae-based biodiesel project is unique. Hence, such projects must be designed by thinking in terms of a flexible or even multipurpose and adaptable installation (Richmond, 2010).

Ladislav Kolar, Stanislav Kuzel, Jiri Peterka and Jana Borova-Batt

Agricultural Faculty of the University of South Bohemia in Ceske Budejovice

Czech Republic

1. Introduction

1.1 Is the waste from digesters (digestate) an excellent organic fertilizer?

A prevailing opinion of bio-power engineers as well as in literature is that wastes from digesters in biogas production are an excellent fertiliser and that anaerobic digestion is to some extent an improvement process in relation to the fertilising value of organic materials used for biogas production. These opinions are apparently based on the fact that in anaerobic stabilisation of sludge the ratio of organic to mineral matters in dry matter is approximately 2:1 and after methanisation it drops to 1:1. Because there is a loss of a part of organic dry matter of sludge in the process of anaerobic digestion, the weight of its original dry matter will decrease by 40%, which will increase the concentration of originally present nutrients. In reality, anaerobic digestion will significantly release only ammonium nitrogen from the original material, which will enrich mainly the liquid phase due to its solubility; the process will not factually influence the content of other nutrients (Straka 2006).

The opinion that waste from anaerobic digestion is an excellent fertiliser is also due to the observation of fertilised lands. The growths are rich green and juicy. They have a fresh appearance — this is a typical sign of mineral nitrogen, including larger quantities of water retention by plants due to the nitrogen. However, the content of dry matter is changed negligibly, which shows evidence that the fertilisation is inefficient.

If organic matter is to be designated as organic fertiliser, it has to satisfy the basic condition: it has to be easily degradable microbially so that it will release necessary energy for soil microorganisms.

The main alternative energy carriers considered for transportation are electricity and hydrogen. With interest in its practical applications dating back almost 200 years, hydrogen energy is hardly a novel idea. Iceland and Brazil are the only nations where renewable — energy feedstocks are envisioned as the major or sole future source of hydrogen (Solomon & Banerjee, 2006). Fuel-cell vehicles (FCVs) powered by hydrogen are seen by many analysts as an urgent need and as the only viable alternative for the future of transportation (Cropper et al., 2004).

Unlike crude oil or natural gas, reserves of molecular H2 do not exist on earth. Therefore, H2 must be considered more as an energy carrier (like electricity) than as an energy source (Song, 2006). H2 can be derived from existing fuels such as natural gas, methanol or gasoline; however, the best long-term solution is to produce H2 from water by (for example) using heat from solar sources and O2 from the atmosphere.

Today, hydrogen is mainly manufactured by decarbonizing fossil fuels, but in the future it will be possible to produce hydrogen by alternative methods such as water photolysis using semiconductors (Khaselev & Turner, 1998) or by ocean thermal-energy conversion (Avery,

2002) . Such methods are still in the research and development stage and are not yet ready for industrial application.

Hydrogen production from biomass requires multiple reaction steps. The reformation of fuels is followed by two steps in the water-gas shift reaction, a final carbon monoxide purification step and carbon dioxide removal.

Biomass can be thermally processed through gasification or pyrolysis. The main gaseous products resulting from the biomass are expressed by equations (6), (7) and (8) (Kikuchi, 2006).

pyrolysis of biomass — H2 + CO2 + CO + hydrocarbon gases (6)

catalytic steam reforming of biomass — H2 + CO2 + CO (7)

gasification of biomass — H2 + CO2 + CO + N2 (8)

Hydrogen from organic wastes has generally been produced through equations (9), (10) and (11).

In the long run, the methods used for hydrogen production are expected to be specific to the locality. They are expected to include steam reforming of methane and electrolysis when hydropower is available (such as in Brazil, Canada and Scandinavia) (Gummer & Head,

2003) . When hydrogen will become a very common energy source, it will likely be distributed through pipelines. Existing systems, such as the regional H2-distribution network that has been operated for more than 50 years in Germany and the intercontinental liquid-hydrogen transport chain, demonstrate that leak rates of <0.1% can be achieved in industrial applications (Schultz et al., 2003). However, a major threat associated with the hydrogen paradigm is the fact that it is the smallest atom and that leakage is apparently unavoidable. One has to face the possibility that a significant amount of H2 will be released into the stratosphere. Hydrogen is expected to react with ozone following the reaction H2+O3 ^ H2O+O2. This mechanism (reviewed by Kikuchi, 2006) is a potentially dangerous promoter of ozone depletion. Alternatively, hydrogen can be produced from another fuel (e. g., ethanol, biodiesel, gasoline, or synfuel) via onboard reformers (hydrogen fuel processors). This is probably the best solution because synfuel can be produced from local feedstocks through the Fischer-Tropsch process, transported and distributed through existing technologies and infrastructures (Agrawal et al., 2007; Takeshita & Yamaji, 2008). This consideration also applies to biofuels. In addition, the feasibility of cars with onboard reformers has already been proven. The importance of synfuel is expected to increase rapidly because growing reserves of natural gas (or "stranded" gas) are available in remote locations and are considered to be too small for liquefied natural gas (LNG) or pipeline projects.

The biological generation of hydrogen (or biohydrogen) provides a wide range of approaches for generating hydrogen, including direct biophotolysis, indirect biophotolysis, photo-fermentation and dark-fermentation (Lin et al., 2010). Biological hydrogen production processes are found to be more environmentally friendly and less energy intensive as compared to thermochemical and electrochemical processes. There are three types of microorganisms that produce hydrogen, namely cyanobacteria, anaerobic bacteria, and fermentative bacteria (Demirbas, 2008a).

Photosynthetic production of H2 from water is a biological process that can convert sunlight into useful, stored chemical energy. Hydrogen production is a property of many phototrophic organisms and the list of H2 producers includes several hundred species from different genera of both prokaryotes and eukaryotes. The enzyme-mediating H2 production seen in green algae is effected by a reversible hydrogenase that can catalyze ferredoxin oxidation in the absence of ATP (Beer et al., 2009). The enzyme is sensitive to oxidation; however, tolerant allozymes are being selected (Seibert et al., 2001). Hydrogen production has also been obtained from glucose using NADP+-dependent enzymes, glucose-6 phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH) and hydrogenase (Heyer & Woodward, 2001).

Carbon monoxide (CO) can be metabolized by a number of naturally occurring microorganisms along with water to produce H2 and CO2 following equation (12), which is the "water-gas shift" reaction, at ambient temperatures.

CO + H2O ^ CO2 + H2 (12)

The biological water-gas shift reaction has been used in the processing of syngas from biomass with the bacterium Rubrivivax gelatinosus (Wolfrum & Watt, 2001).

Nitrogenases can produce hydrogen but require relatively high energy consumption. However, the nitrogenase reaction is essentially irreversible, which allows for hydrogen pressurization. Rhodopseudomonas palustris can drive the nitrogenase reaction using light (Wall, 2004).

Delinking biofuels production from food crops is a necessary condition to expand the scale of the market penetration of biofuels globally. Among the challenges this strategy faces is the inherent resistance of cellulosic feedstocks to conversion to simpler sugars that can be fermented into ethanol. Here, the promise lies in nano-particles used as immobilizing beds for expensive enzymes that can be used over and over again to breakdown the long chain cellulose polymers into simpler fermentable sugars [16].

The Louisiana Tech University is one among many organizations worldwide engaged in this endeavour, through the work of Dr. James Palmer, in collaborating with fellow professors Dr. Yuri Lvov, Dr. Dale Snow, and Dr. Hisham Hegab [17]. The focus is on non-edible cellulosic biomass, such as wood, grass, stalks, etc, to be converted into ethanol. This approach to produce ethanol can reduce GHG emissions by some 86% over fossils fuels.

The broader field of nanotechnology research into converting biomass into biofuels is growing fast. For example, in 2007 the oil company BP has granted a research fund of $500 million to the University of California, at Berkeley, and the University of Illinois, to explore the conversion of corn, plant material, algae and switch grass into fuel [18].

In the past, Berkeley had used nanotechnology in research for cost-effective solar panels [19]. But, the new Energy Biosciences Institute — EBI created at Berkeley will focus on fuel production with minimum environmental impacts and carbon emissions. A three pronged approach is being employed that begins with technologies for better crop production, improved feedstocks processing and development of new biofuels. The application of this approach aims at developing better feedstocks, breaking down plant material into sugars and their conversion to ethanol. Success along this pathway is expected to lead EBI to investigate the use of nanotechnology to develop other alternative fuels, such as butanol and renewable hydrocarbon fuels.

Another relevant application of nanotechnology is the use of nano-catalysts for the trans­esterification of fatty esters from vegetable oils or animal fats into biodiesel and glycerol [20]. The nano-catalyst spheres replace the commonly used sodium methoxide. The spheres are loaded with acidic catalysts to react with the free fatty acids and basic catalysts to react with the oils. This approach eliminates several production steps of the conventional process, including acid neutralization, water washes and separations. All those steps dissolve the sodium methoxide catalyst so it can’t be used again. In contrast, the catalytic nanospheres can be recovered and recycled. The overall result is a cheaper, simpler and leaner process. In summary, the process claims to be economical, recyclable, to react at mild temperatures and pressures, with both low and high FFA (free fatty acid) feedstock, producing cleaner biodiesel and cleaner glycerol, greatly reducing water consumption and environmental contaminants, and can be used in existing facilities.

1.1 Biorefinery definition

A biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power, heat, and value-added chemicals from biomass. The biorefinery concept is analogous to today’s petroleum refinery, which produces multiple fuels and products from petroleum (Smith & Consultancy, 2007).

The IEA Bioenergy Task 42 on Biorefineries has defined biorefining as the "sustainable processing of biomass into a spectrum of bio-based products (food, feed, chemicals,

materials) and bioenergy (biofuels, power and/or heat)." The biorefinery is not a single or fixed technology. It is collection of processes that utilize renewable biological or bio-based sources, or feedstocks, to produce an end product, or products, in a manner that is a zero — waste producing, and whereby each component from the process is converted or utilized in a manner to add value, and hence sustainability to the plant. Several different routes from feedstocks to products are being developed and demonstrated, and it is likely that multiple biorefinery designs will emerge in the future.

By producing multiple products, a biorefinery takes advantage of the various components in biomass and their intermediates, thereby maximizing the value derived from the biomass feedstock. A biorefinery could, for example, produce one or several low-volume, but high — value chemical or nutraceutical products and a low-value, but high-volume liquid transportation fuel such as biodiesel or bioethanol (see also alcohol fuel), while also generating electricity and process heat through combined heat and power (CHP) technology for its own use, and perhaps enough for sale of electricity to the local utility. In this scenario, the high-value products increase profitability, the high-volume fuel helps meet energy needs, and the power production helps to lower energy costs and reduce greenhouse gas emissions, as compared to traditional power plant facilities. Although some facilities exist that can be called biorefineries, the technology is not commonplace. Future biorefineries may play a major role in producing chemicals and materials that are traditionally produced from petroleum.

If besides decomposing exothermic processes synthetic endothermic processes are also to take place in compost when high-molecular humus substances (fulvic acids, humic acids and humins) are formed, these conditions must be fulfilled: very favourable conditions for the microflora development must exist in compost, and minimum losses and the highest production of heat must be ensured. For this purpose it is necessary to use a high admixture of buffering additive (limestone) in the compost formula, sufficient amount of very labile organic matter, thermal insulation of the base of fermented material because the heat transfer coefficient does not have the highest value for transfer from the composted pile into the atmosphere but mainly into solid especially moist materials, i. e. into concrete, moist or frozen earth, clay, bricks, etc. At a sufficient amount of labile fractions of organic matter the maximum heat production can be achieved only by a sufficient supply of air oxygen. Beware of this! The ventilation through vertical and horizontal pipes provides sufficient air for aerobic processes in the fermented material but at the same time the ventilation is so efficient that a considerable portion of reaction heat is removed, the material is cooled down and the onset of synthetic reactions with the formation of humus substances does not occur at all.

When sufficiently frequently turning the fermented material, the safest method of compost aeration and ventilation is the addition of coarse-grained material while inert material such as wood chips, chaff and similar materials can be used. It is however problematic because inert material in the fermented blend naturally decreases the concentration of the labile fraction of organic matter, which slows down the reaction rate of aerobic biochemical reactions and also the depth of fermentation is reduced in this way. It mainly has an impact on the synthetic part of reactions and on the formation of humus substances while the influence on decomposing reactions is smaller.

It would be ideal if during compost fermentation in a microbially highly active environment the inert aeration material were able not only to allow the access of air oxygen into the fermented material but also to decompose itself at least partly and to provide additional energy to biochemical processes in the pile in this way.

These requirements are excellently met by the solid fraction of digestate from biogas plants. It aerates the compost and although it lost labile fractions of organic matter in biogas plant digesters, it is capable of further decomposition in a microbially active environment. It releases not only energy but also other mineral nutrients. So this waste is perfectly utilised in this way. The average microbial activity of even very fertile, microbially active soils is not efficient enough for the decomposition of this stable organic material when the solid phase of digestate is used as an organic fertiliser. The decomposition rate is slow, especially in subsequent years, and therefore the resultant effect of the solid fraction of digestate as an organic fertiliser is hardly noticeable. The combination of anaerobic decomposition in the biogas plant digester and aerobic decomposition in compost could seem paradoxical, and some agrochemists do think so. The preceding exposition has shown that it is not nonsense. Now let us answer the question: what dose of the solid fraction of digestate should be used in the compost formula? It depends on many factors: on the amount of the labile fraction of organic component and mainly on the degree of its lability (which can be determined in a reliable way by the above-mentioned method according to Rovira and Vallejo 2002, 2007, Shirato and Yokozawa 2006), on the aeration and porosity of materials used in the compost formula, on the number of turnings, on prevailing outdoor temperature, water content, degree of homogenisation and on other technological parameters.

In general: the higher the amount of the labile component of organic matter and the higher its lability (e. g. the content of saccharides and other easily degradable substances), the higher the portion of the solid fraction of digestate that can be used.

Now short evidence from authors own research is presented:

The basic compost blend was composed of 65% fresh clover-grass matter from mechanically mown lawns, 10% ground dolomite, 2% clay in the form of clay suspension, 20% solid phase of digestate (obtained by centrifugation with fugate separation) or 20% crushed wood chips and 3% PK fertilisers. The C : N ratio in the form of Chws : Nhws (hot-water-soluble forms) was 15 : 1, nitrogen was applied in NH4NO3 in sprinkling water that was used at the beginning of fermentation at an amount of 70% of the beforehand determined water — retention capacity of the bulk compost blend. Inoculation was done by a suspension of healthy topsoil in sprinkling water. Fermentation was run in a composter in the months of April — November, and the perfectly homogenised material was turned six times in total. Water loss was checked once a fortnight and water was replenished according to the increasing water-retention capacity to 60%. The formation, amount and quality of formed humus substances were determined not only by their isolation and measurement but also by their specific manifestation, which is the ion-exchange capacity of the material. The original particles of composted materials were not noticeable in either compost (with the solid part of digestate and with wood chips), in both cases the dark coloured loose material with pleasant earthy smell was produced. Tab. 6 shows the analyses of composted materials and composts. The digestate was from a biogas plant where a mix of cattle slurry, maize silage and grass haylage is processed as a substrate. The material in which the aeration additive was polystyrene beads was used as compost for comparison.

The results document that the ion-exchange capacity, and hence the capacity of retaining nutrients in soil and protecting them from elution after the application of such compost, increased very significantly only in the digestate-containing compost. The ion-exchange capacity of this compost corresponds to the ion-exchange capacity of heavier-textured humus soil, of very good quality from the aspect of soil sorption. The compost with wood chips produced in the same way does not practically differ from the compost with polystyrene but it does not have any humic acids and the ion-exchange capacity of these composts is on the level of light sandy soil with minimum sorption and ion-exchange properties. However, the total content of humus acids in the compost with the solid phase of digestate is very small and does not correspond to the reached value of the ion-exchange capacity of this compost. Obviously, precursors of humus acids that were formed during the fermentation of this compost already participate in the ion exchange. Humus acids would probably be formed from them in a subsequent longer time period of their microbial transformation. If only humus acids were present in composting products, at the detected low concentration of CFA + CHA the T value of the compost with the solid phase of digestate would be higher only by 1 — 1.2 mmol. kg-1 than in the compost with polystyrene or wood chips. Because it is more than a triple, other substances obviously participate in the ion exchange.

The experiments were performed on "model fuels" that differed greatly in their thermal and kinetic properties. Low-ash high-reactivity bio fuels of medium (wood) and high (dates seeds) density and products of their treatment (charcoal) were used to study the affect of pyrolysis kinetics, material density on formation of coke-ash residue reaction structure. Charcoal as oxydizing pyrolysis product entering the reaction zone of gas generators was used for investigation of gasification modes with different blow conditions. Fuels characteristics are given in Table 1.

Conditions of porous structure formation and porosity during preheating (devolatilization) were studied based on biomass particles with equivalent size dp « 10 mm. The particles were heated by two methods: fast heating by placing the particle in muffle furnace preheated up to preset temperature (100, 200, … 800 °С with accuracy +20 °С) and slow heating simultaneously with muffle heating under conditions of limited oxidizing agent supply. This allowed to simulate real conditions of thermal processes i. e. fast heating (for instance, particle pyrolysis in

fluid flow — or fluidized bed-type carbonizer) and slow heating (when the particle enters a cold fluidized bed and gets warmed gradually with the fluidized bed). After cooling the porosity was measured (mercury porometry: volume and sizing the pores with d > 5.7 nm) and specific surface area (nitrogen adsorption: surface area of pores with diameter d > 0.3 nm).

NA — not available.

Table 1. Model fuels characteristics

Kinetics of conversion in combustion mode was studied on individual particles with equivalent diameter dp = 3-75 mm. The range of diameters examined corresponds with values showed in (Tillman D. A., 2000) as allowed for individual and co-combustion (gasification) of biofuels. Test sample placed (centered) on thermocouple junction (Ch-A type) was brought into the muffle which was preheated up to preset temperature (100, 200, … 800 °С with accuracy +20 °С). The tests were performed with air flow rate 0-3.5 m3/h (upstream velocity of the flow is 0-0.5 m/s in normal conditions). Average effective burning velocity for coke-ash residue was calculated as loss of coke-ash residue estimated weight per surface unit of equivalent sphere (based on original size) during coke-ash residue burning out: j = AM / (Azcar — F). Coke-ash residue (CAR) burn-out time (Azcar) was estimated by thermograms (fig. 1.) as time interval between points C and D. The length of A"-B segment was not accounted for.

In some aspects, individual particle burning, combustion in fluidized bed and in flame, may be assessed on the same basis. Both in flame and in fluidized bed the fuel particles are spaced at quite a distance from each other and are usually considered as individual particles. The intensity of heat-mass-exchange of particles burning in FB inert medium is comparatively close to individual particle intensity. Application of experimental data for individual particle burning to calculation and assessment of thermo chemical pretreatment of large-size particles in furnaces with dense bed is justified by the fact that heat-mass — exchange processes in its large size elements are the same as for individual particle, within the statement of the problem. Therefore the experimental data on individual particle burning are usually used in calculations to assess thermo chemical pretreatment of large particles in furnaces of various types.

Experimental data was compared with other researches’ data on thermal pretreatment of low-grade fuel particles in the air showed in Table 2.

Kinetics of conversion in gasification conditions was studied at the plant consisting of quartz retort with inner diameter 37 mm, length 650 mm, located in cylinder-shape muffle

furnace (Nel = 2.5 kW, Tmax = 1250оС), air blower (Qmax = 6 m3/h, Hmax = 0.6 m), electric heater, steam generator, rotameter, a set of thermocouples, carbon dioxide cylinder and thermocouple polling and temperature recording system. Combustible gas components (СО, Н2, СН4) were determined by gas chromatograph, air flow coefficient was determined by effluent gas composition.

The experiments were performed in dense bed which provides for the most strict fulfillment of fuel thermochemical pretreatment as stratified process, stepwise and in compliance with temperature and concentration conditions, without flow disturbances and fluid mechanics problems. Gasification was based on downdraft process. Particles with initial diameter, varying from 3 to 20 mm in different experiments, were placed in retort having a tube welded to its bottom for gas release and sampling for analysis. Fuel bed was heated in muffle furnace up to 600-1000оС (the temperature depended on experiment). Gasifying agent (air, air and water vapor, water vapor, or carbon dioxide) was fed via furnace tuyere inside the bed to a different depth. Blown fluid was heated by electric heater up to 700- 750оС. The experiment was considered to be completed at the moment when СО and Н2 content in gas lowered by less than 1% of volume.

It can be said that the adsorbent material is the "heart" of the PSA unit. All the properties of the cycle (operating conditions and operating mode) depend on the initial choice of the adsorbent. As mentioned before, several materials can be employed in PSA technology. The material selected should at least satisfy one of two criteria:

i. have a higher selectivity to C02: this gas should be more "attached" to the surface of the material than CH4; in most solids C02 can create stronger bonds with surface groups than CH4. This kind of materials will be termed as equilibrium-based adsorbents since its main selectivity is due to differences of interaction forces between C02 and CH4 with and the surface.

ii. the pores of the adsorbent can be adjusted in such a way that C02 (kinetic diameter of 3.4 A) can easily penetrate into their structure while larger CH4 molecules (kinetic diameter of 3.8 A) have size limitations to diffuse through them. These materials will be termed as kinetic adsorbents since its main selectivity is due to diffusion constrains.

Carbon molecular sieves are one of the most employed materials for biogas upgrading. Adsorption equilibrium isotherms of C02 and CH4 in CMS-3K (Takeda Corp., Japan) are shown in Figure 2 (Cavenati et al., 2005). This material has a clear selectivity towards C02, but the most important property in CMS-3K is not its equilibrium selectivity, but the kinetic selectivity. In this material, C02 adsorbs much faster than CH4: adsorption equilibrium of CH4 is achieved only after two days of solid-gas contact. In fact, the pore mouth of CMS-3K is narrowed to dimension closer to the kinetic diameter of CH4 creating a specific resistance (mass transfer in the micropore mouth) (Srinivasan et al., 1995) that can be seen in the initial moments of CH4 uptake in Figure 2 (b). Another material that also presents strong resistance to CH4 diffusion is ETS-4 (titanosilicate-4) modified with alkali-earth metals (Kuznicki, 1990; Marathe et al., 2004; Cavenati et al., 2009). In this material, the pore diameter can be tuned with different heating temperatures resulting in a "molecular gate" effect that actually named the process commercialized by Guild Associates Inc. (USA).

0ther normally employed adsorbents are activated carbons and zeolites. In these materials, the diffusion of both gases can be very fast and actually what is exploited is the difference between loadings of C02 and CH4. An example of these equilibrium-based materials is given in Figure 3, where adsorption equilibrium of C02 and CH4 on zeolite 13X (CECA, France) is shown (Cavenati et al., 2004). Note that the loading of C02 is much higher than the loading of CH4 at given P, T conditions. Furthermore, recasting the conclusions taken from Figure 1, the cyclic C02 capacity at lower temperatures is smaller than at higher temperatures, which means that if zeolite 13X is employed at 323 K, it will be easier to regenerate than at 298 K.

Fig. 2. Adsorption of C02 and CH4 in carbon molecular sieve 3K (Takeda Corp, Japan) at 298 K: (a) adsorption equilibrium; (b) uptake rate curves (data from Cavenati et al., 2005).

Another topic that is important for the selection of materials for the PSA process for biogas upgrading, is the presence of contaminants. Apart from CH4 and CO2, other gases present in biogas are H2S and H2O. In almost all adsorbents, H2S is irreversibly adsorbed, reason why it has to be removed before the PSA process. When carbonaceous materials are employed it is possible to remove H2O in the same vessel as CO2. However, that is not possible using zeolites since water adsorption is also very steep, resulting in a very difficult desorption.