The table 11 shows the determined rates of admixture for LPG to attain the appropriate target calorific value range. With the LPG quantities shown, the respective initial concentrations of methane, the entire calorific value range of the respective base gas quality is covered, with some restrictions.

For the practical implementation of the listed quantities of the LPG admixtures, limits as given in Table 12 are to be observed, in accordance with the requirements presented on the need for the use and applicability of SGERG88 and AGA8 procedures and the resulting maximum admixture quantities according to Table 12. Due to these limits, defined in DVGW G 486-B2, it will not be possible in every case to reach the upper calorific value range at higher pressures. In addition, the availability of appropriate measuring technology for higher liquid gas fractions is limited. At the very high degrees of methane processing, there are limitations on attaining the lower calorific value range, since when processing to 99.5 Vol -% methane, an H gas with a calorific value of 11.009 kWh / m3 results.

Table 13 shows the maximum possible, compliant LPG admixture, a propane / butane mixture of 95 / 5 Mass.-%. As a result, it is clear that processing to a maximum methane content of 99.5% vol. with the maximum permissible LPG admixtures, a calorific value of maximum 11,361 kWh/ m3 (NTP) is possible at pressures above 100 bar, and a maximum of 12,075 kWh/m3 (NTP) at pressures below 100 bar.

Figure 14 shows the admixture required to achieve the corresponding H gas properties. The limits on the maximum concentration of propane are also shown according to DVGW regulations G 486 supplementary sheet 2 a ppendix B. Admixtures to achieve the properties of North Sea I / North Sea II H gas are, based upon all levels of methane processing, above the limits. A compliant mixture is not possible in this case, or needs to be tested on an individual basis.

96 98

Methankonzentration [Vol.-%]

Fig. 14. LPG quantities necessary to achieve the target calorific value

In order to achieve higher calorific values, alternative conditioning measures can be employed. For example, ready-made mixtures with customized propane / butane ratios can be used for conditioning. This can increase the calorific value, but technical and physical effects, such as condensation behaviour, methane number and k-number deviations need to be considered.

As indicated in Table 3, there are some digesters have been installed in a number of sub­Saharan Africa. These have mainly been pilot or demonstration projects aimed at testing the technical viability of small-scale biogas technology at a limited scale (Hivos, 2009a). These pilot projects have mostly been funded by non-governmental organizations and built for health clinics, schools, and small-scale farmers. While the small-scale biogas plants are located throughout Africa, only a few of them are operational (Parawira, 2009). There is also limited documentation on whether the existing biogas digesters have been successful in achieving the benefits highlighted in section 3.1. Some country specific examples is Tanzania, Ivory Cost and Burundi, which have produced biogas from animal and human waste using the Chinese fixed-dome digester and the Indian floating-cover digester (Omer and Fadalla, 2003). These have not been reliable and in many cases, poor performance has been reported (Omer and Fadalla, 2003). Thus, the plants have only operated for a short period due to poor technical quality (Mshandate and Parawira, 2009).

Currently, a number of different organizations are establishing biogas initiatives in Africa, particularly in rural areas, in order to supply cleaner burning energy solutions. These initiatives are at different stages of development such as: prefeasibility, feasibility, design and implementation to a limited extent. For instance, Burkard (2009) reports on five biogas case studies in Kenya which were to utilize agricultural leaves, residues from floriculture, and residues from vegetable production and canning. In 2010, it was reported that the Dutch government was to spend 200 million Kenyan Shilling to set up 8000 biogas digesters throughout the country. The initiative was targeting farmers practising zero grazing (Daily Nation, 2010). Similar projects are being implemented in Ethiopia, Uganda, Senegal, Burkina Faso, and Tanzania. There are also some other initiatives such as biogas for better life, which is at various stages of biogas development in Ethiopia, Kenya, Uganda, Sudan, Zambia, Malawi, South Africa, Lesotho, Swaziland, Mali, Senegal, and Ghana1. The Netherland Development Organization (SNV) has been supporting the development of National Biogas programmes in East Africa (Ethiopia, Kenya, Uganda, Tanzania, Rwanda) and West Africa (Senegal and Burkina Faso)[3] [4]. While there are few documented successful small-scale biogas plants in the rural areas of Africa, this section will present some selected country specific biogas projects.

1.1.2 Rwanda

Rwanda has a population of 10.2 million people of which 81% of this population reside in the rural areas in 2010 (United Nations, 2007). One of the famous biogas programmes is the Kigali Institute of Science and Technology (KIST) large-scale biogas plants developed and installed in prisons. The aim of these plants was to treat toilet wastes and generate biogas for cooking. The first plant prison which was operational in 2001, and by 2011, KIST has managed to build and operationalize biogas plants in 10 prisons. Each prison is supplied with a linked series of underground biogas digesters, in which the waste decomposes to produce biogas. After this treatment, the bio-effluent is safe to be used as fertiliser for production of crops and fuel wood. The project was funded by Red Cross and the plant consists of five interlocking chambers. KIST’s project saves 50% of wood for cooking and it won Ashden Award in 2006. The projects construction is managed by KIST, who also provides training to both civilians and prisoners.

Another biogas programme is the National Biogas Programme which is promoted by the Rwanda Ministry of Infrastructure, through the support by the Netherlands development organization. The programme aims at reducing firewood use by the households. The Ministry of Infrastructure estimates that 441 units have been installed to date, and approximately 15 000 households will be using biogas by end of 2011 for cooking and lighting[5]. The Ministry of Infrastructure of Rwanda is also collaborating with other ministries (e. g. Ministry of Education) in order to develop biogas plants in schools, clinics and community institutions.

(Fannin, 1987). In the natural environments, the optimum temperature for the growth of methane forming archaea is 5-25 °C for psychrophilic, 30-35 °C, for mesophilic, 50-60 °C, for thermophilic and >65 °C for hyprethermophilic (Tchobanoglous and Burton, 1996).

It is generally understood that higher temperature could produce higher rate of reaction and thus promoting higher application of organic loading rate (OLR) without affecting the organic removal efficiency (Chae et al., 2007; Choorit and Wisarnwan, 2007; Poh and Chong, 2009). Using palm oil mill effluent as the substrate, Choorit and Wisarnwan (2007) demonstrated that when the digester was operated at thermophilic temperature (55 °C), showed higher OLR application than the that of mesophilic (17.01 against 12.25 g COD/ m3-d) and the methane productivity was also higher (4.66 against 3.73 L/L/d) (Choorit and Wisarnwan, 2007). A similarly study by Chae et al (2007), indicated that the higher temperature of 35 °C led to the highest methane yield as compared to 30 °C and 25 °C although the methane contents only changed slightly.

Using cheese whey, poultry waste and cattle dung as substrates, Desai et al. (1994) showed that when the temperature was increased from 20, 40 and 60 °C, the biogas production and methane percentage increased as well. The digestion rate temperature dependence can be expressed using Arrhenius expression:

rt = r30(1.11)(£_30) (7)

where t is temperature in °C, and rt, r30 are digestion rates at temperature t and 30°C, respectively. Based in Eq. 7, the decrease in digestion rate for each 1 °C decreased in temperature below the optimum range is 11%. Similarly, the calculated rate at 25 °C y 5 °C are 59 and 7% respectively, relative to the rate at 30 °C (Dasai et al., 1994).

Although the thermophilic anaerobic process could increase the rate of reaction, the yield of methane that could be achieved over the specified organic amount is the same regardless of the mesophilic or thermophilic conditions. That value is 0.25 kg CH4/kg COD removed or 0.35 m3 CH4/kg COD removed (0 °C, 1 atm) which is derived by balancing the following equation (Eq. 8), taking into account the different operating conditions worked, can be explained that the values obtained for methane production is different in many scientific reports:

CH4+2O2 ^ CO2+2H2O (8)

Although thermophilic condition could result in higher application of organic loading rates and better destruction of pathogens, at the same time it is more sensitive to toxicants and temperature control is more difficult (Gerardi, 2003; Choorit and Wisarnwan, 2007). Furthermore, biomass washout that could lead to volatile fatty acids accumulation and methanogenesis inhibition could also occur if the thermophilic temperature could not be controlled (Poh and Chong, 2009). As a result, in tropical regions mesophilic temperatures are the preferred choice for anaerobic treatment (Yacob et al., 2005, Sulaiman et al., 2009).

Transport networks are designed to transport gas over longer distance. They are equipped with remote control which transmits all relevant data to the control centre. From the point of view of modeling these networks have an excellent information base for a moderate number of pipes and nodes (measurement points) making modeling straightforward and easy. The network structure of a transport system tends to be sparsely intermeshed.

2.4 Distribution networks

Distribution networks are designed to transport gas over shorter distance, e. g. within a city. They are equipped also with remote control, but only important data is transmitted to the control center. The amount of data handled may be subject to changes in the future when for each customer Smart Metering and on upper level Smart Grid will be introduced. From the point of view of modeling the distribution networks have an acceptable information base for all pipes but moderate number of measurement points making modeling an intensive work. The network structure of a distribution system tends to be strongly intermeshed. Distribution networks may have also a smaller trunk transportation system at a higher pressure level (e. g. 25, 16, 10 or 4 bar) while most of the pipes in the final distribution area are operated at 0.022 to 0.8 bar depending on the required flows.

Operating a 500 kWel CHP goes along with reduced feed-in tariffs of 20 €/MWh according to Austrian’s Eco-Electricity Act. The positive effect of lower investment and operating costs for larger capacities is therefore narrowed by less revenue for produced electricity. If is forbidden to use two CHPs with same capacity at one location in the maximum structure to gain higher feed-in tariffs the next larger CHP capacity has to be taken although this would

district heating

Fig. 5. PNS optimum structure with a central 500 kWel CHP

go along with shortened revenue. With this precondition the optimization of the maximum structure presented in Figure 2 but with only one central 500 kWel CHP unit whereas the rest of the optimum structure (Figure 3) stays the same.

The revenue is narrowed but not as much as it was in scenario 1. To use a 500 kWel central CHP would cause a revenue reduction of yearly 50,000 € within a payout period of 15 years.

Biogas residue contain 24 kinds of amino acid, many kinds of trace elements, B vitamins and other nutrients, biogas residue can be used to feed pigs, 50kg fodder can be saved and one month of fattening period can be shortened when fattening one pig. Feeding fish with biogas residue can not only improve fish yield and quality, but also reduce the occurrence of fish diseases, breeding earthworms with biogas residue can provide fodder of high-quality protein for livestock, while improve the utilization value of biogas residue.

1. Biogas residue fertilizer

Biogas residue is a high quality fertilizer, it can effectively improve soil physical and chemical property, increase soil organic matter and nutrient content, improve soil porosity, bulk density and water retention (Xu Shiwen, 1987).

2. Biogas residue adsorbent

The research which was made by C. Namasivayam (1995) showed that biogas residue can absorb heavy metal Cr6 in wastewater better at pH was 1.5; biogas residue can absorb the "Direct red 12 B" stain in industry wastewater better at pH 2.7; biogas residue can absorb Pb 2 in wastewater better, and adsorption capacity can reach 28mg/g.

3. Biogas residue brewing

Using artificial culture and old kiln mud as bacteria, together with anaerobic biogas residue, the yield rate of wine increased 10.5% than without biogas residue (Lu Baoqing, 1997).

4. Biogas residue compost

Mixing biogas residue and straw with a 1:1 ratio was used for mushroom matrix. The biggest biogas industry was built in Nyirbator of Hungary (S. Ranik, 2004).

sweet corn, silage maize) groups. The sensitive plants can be treated by digestate only in their certain life stages, for example, young alfalfa is very sensitive after sowing while old alfalfa is very sensitive before cutting. In the case of sensitive plants the burning effect of digestate can be observed but it follows a strong and quick recovering process. For the non­sensitive plants the digestate can be used in any developmental stage. It is favourable, because for example, in rainy period the digestate technically could not be applied (Makadi et al., 2008).

The right application rate of liquid or solid digestate depends on the plant nitrogen demand. It should be applied when plant N demand arises. This time for non-legume scpecies is the late winter and spring (Stinner et al., 2008). Similarly, Wulf et al. (2006) used 70% of the digestate in spring and 30% in autumn, while Makadi et al. (2008) and Nyord et al. (2008) split into two and three the applied rate through the vegetation period.

Because of its high available nutrient content, digestate application resulted in significantly higher aboveground biomass yields in the case of winter wheat and spring wheat than the farmyard manure and undigested slurry treatment. The effectiveness of a digestate depends on the composition of co-digestied material, the treated plant species and the treatment methodology. Co-digestion of different organic materials results in more effective digestate. (Moller et al., 2008; Stinner et al., 2008).

After the burning effect of digestate the soybean plants recovered and grew more, but lower sprouts. These sprouts were very productive, the number of pods was also higher in the treated samples, therefore the yield and thousand seed weight were also higher (Table 7, Makadi et al., 2006)

These yield parameters are close correlations with some soil parameters changing after digestate amendment. Increasing in important nutrient contents contribute to the better development of plants (Makadi et al., 2008b, Table 8).

Comparing the effect of liquid digestate and the equal quantity of water to the yield of sweet corn and silage maize, significantly higher yields were found in the digestate treatment. In this case the applied digestate on the bases of plants N demand was split into two parts (Makadi et al., 2006). That means that the favourable effects of digestate are caused by its soluble macro — and micronutrient content.

Comparing the effect of digestate and a bacterial manure (Phylazonit MC, the experimental conditions can be found in the section 4.5). The Phylazonit MC treatment increased the green weight of silage maize by 47.18% while the digestate by 142.34%, comparing to the control. The results obtained can be seen in Table 9 (Makadi et al., 2007).

The positive effect of Phylazonit MC treatment was the result of its microbes, plant growth promoters and microelement content, while the favourable effect of digestate treatment was caused by its macro — and microelement and high water content and the increase of soluble macroelement content of soil because of the increased microbial activity.

This is the process of lining the digester by mortar or using sheets of foam as in Figure 17. This is one of the most important construction steps and should be carefully and accurately achieved. In case of lining, the process is performed using mortar containing 1 % silica. After the completion of the lining, the digester is painted using the petroleum Albumen. In other designs, the walls are heat insulated with a clad with non-corroding and weather-proof aluminum trapezoidal panels. On the other hand, the rural digesters are coated with layers of dry dirt and asbestos.

10.1 Global availability of grasses and other wild plants

The grass family (gramineae or poaceae) is perhaps the most successful taxonomic group in the plant kingdom. Members of this group number about 9000 species distributed in about 635 genera and they grow in all ecosystems and agroclimatic zones. From economic and ecological standpoints, they are the most important species in the plant kingdom. The pea family (leguminosae or fabaceae) is the largest family of flowering plants and also contains a large number of species found flourishing in many ecosystems and agroclimatic zones. Both families of plants contain domesticated crops and wild plants which are being researched for their potentials as reliable sources of biofuels. These plants certainly have a significant role to play in an anticipated global scenario which is 100% dependent on bioenergy in the near future.

Thixotropic fluids are generally dispersions, which when they are at rest construct an intermolecular system of forces and turn the fluid into a solid, thus, increasing the viscosity. In order to overcome these forces and make the fluid turn into a liquid and which may flow, an external energy strong enough to break the binding forces is needed. Thus, as above a yield stress is needed. Once the structures are broken, the viscosity is reduced when stirred until it receives its lowest possible value for a constant shear rate (Schramm, 2000). In opposite to pseudoplastic and dilatant fluids, the viscosity of thixotrpic fluids is time dependent: once the stirring has ended and the fluid is at rest, the structure will be rebuilt. This will inform about the fluid possibilities of being reconstructed. Wastewater and sewage sludge can be examples of fluids with thixotropic behaviour (Seyssieq & Ferasse, 2003) as well as paints and soap.

1.3 Rheological mathematical models

There are several rheological mathematical models applied on rheograms in order to transform them to information on fluid rheological behaviour. For non-Newtonian fluids the three models presented below are mostly applied (Seyssiecq & Ferasse, 2003).