6.1 Computer-aided design

A software was developed by Samer (2010) to plan and design biogas plants, specify the dimensions of the different tanks (raw slurry tank, liquid organic matter tank, digester tank, secondary digester tank, and residue storage tank), and compute the amounts of construction materials (iron rods, cement, sand, and gravel) required to build the concrete constructions. Furthermore, the software is able to calculate the capital investment and the fixed costs, the variable costs, and the total costs. Figure 18 shows the user interfaces of the input and output data windows.

(b) User interface for input data of digester tank

A study by Sidibe and Hashimoto (1990) documented the fact that grass straw can be fermented to methane and the yield can be relatively high. This laboratory experiment showed that the ultimate methane yield of rye grass straw (341+ 5ml/g VS) and fescue grass straw (356+ml/g VS) are not significantly different but both grass straws had significantly higher yield (p < 0.01) than dairy cattle manure (288 + 3 ml/g VS). The paper noted that nitrogen does not appear to be a limiting nutrient in the fermentation of grass straw to methane; the length of time between inocula feeding does not affect the ultimate methane yield of the straw, and longer acclimation may increase the ultimate methane yield of grass straw. Among plants themselves, differences exist regarding their potentials as feedstock for biogas production. For instance, De-Renzo (1997) reviewed anaerobic digestion of plant materials and concluded that aquatic plants such as algae and moss can be much better digested than terrestrial plants because of their toughness. Ordinarily, more digestion results in more biogas production. Akinbami et al (2001) noted that in the tropics, the identified feedstock substrates for an economically feasible biogas programme include water lettuce, water hyacinth, dung, cassava peelings, cassava leaves, urban refuse, solids (including industrial waste), agricultural residue and sewage.

Uzodinma and Ofoefule (2009) investigated the production of biogas from equal blending of field grass (F-G) with some animal wastes which include cow dung (G-C), poultry dung (G­P), swine dung (G-S) and rabbit dung (G-R). The wastes were fed into prototype metallic biodigesters of 50 L working volume on a batch basis for 30 days. They were operated at ambient temperature range of 26 to 32.8oC and prevailing atmospheric pressure conditions. Digester performance indicated that mean flammable biogas yield from the grass alone system was 2.46+2.28 L/total mass of slurry while the grass blended with rabbit dung, cow dung, swine dung and poultry dung gave average yield of 7.73+2.86, 7.53+3.84, 5.66+3.77 and 5.07+3.45 L/total mass of slurry of gas, respectively. The flash point of each of the systems took place at different times. The field grass alone became flammable after 21 days. The grass-swine (G-S) blend started producing flammable biogas on the 10th day, grass-cow (GC) and grass-poultry (G-P) blends after seven (7) days whereas grass-rabbit (G-R) blend sparked on the 6th day of the digestion period. The gross results showed fastest onset of gas flammability from the G-R followed by the G-C blends, while the highest average volume of gas production from G-R blend was 3 times higher than that of F-G alone. Overall, the results indicated that the biogas yield and onset of gas flammability of field grass can be significantly enhanced when combined with rabbit and cow dung.

Ofuefule et al., (2009) reported a comparative study of the effect of different pre-treatment methods on the biogas yield from Water Hyacinth (WH). The WH charged into metallic prototype digesters of 121 L capacity were pre-treated as: dried and chopped alone (WH-A), dried and treated with KOH (WH-T), dried and combined with cow dung (WH-C), while the fresh water Hyacinth (WH-F) served as control. They were all subjected to anaerobic digestion to produce biogas for a 32 day retention period within a mesophilic temperature range of 25 to 36°C. The results of the study showed highest cumulative biogas yield from the WH-C with yield of 356.3 L/Total mass of slurry (TMS) while the WH-T had the shortest onset of gas flammability of 6 days. The mean biogas yield of the fresh Water Hyacinth (WH-F) was 8.48 + 3.77 L/TMS. When the water Hyacinth was dried and chopped alone (WH-A), dried and treated with KOH (50% w/v) (WH-T) and dried and combined with cow dung (WH-C), the mean biogas yield increased to 9.75 + 3.40 L/TMS, 9.51 + 5.01 L/TMS and 11.88 + L/TMS respectively. Flammable biogas was produced by the WH-F from the 10th day of the digestion period whereas the WH-A, WH-T and WH-C commenced flammable gas production from the 9th, 6th and 11th day respectively. Gas analysis from WH-F shows

Methane (65.0%), CO2 (34.94%). WH-A contained Methane (60.0%), CO2 (39.94%). WH-T contained Methane (71.0%), CO2 (28.94%), while WH-C had Methane (64.0%) CO2 (35.94%). The other gases were found in the same levels and in trace amounts in all the systems. The overall results showed that treating water Hyacinth with KOH did not have a significant improvement on the biogas yield. It also indicated that water Hyacinth is a very good biogas producer and the yield can be improved by drying and combining it with cow dung.

The Herschel Bulkley model is applied on fluids with a non linear behaviour and yield stress. It is considered as a precise model since its equation has three adjustable parameters, providing data (Pevere & Guibaud, 2006). The Herschel Bulkley model is expressed in equation 5, where to represents the yield stress.

T = t0 + К * y" (5)

The consistency index parameter (К) gives an idea of the viscosity of the fluid. However, to be able to compare К-values for different fluids they should have similar flow behaviour index (n). When the flow behaviour index is close to 1 the fluid’s behaviour tends to pass from a shear thinning to a shear thickening fluid. When n is above 1, the fluid acts as a shear thickening fluid. According to Seyssiecq and Ferasse (2003) equation 5 gives fluid behaviour information as follows:

To = 0 & n = 1 ^ Newtonian behaviour To > 0 & n = 1 ^ Bingham plastic behaviour T0 = 0 & n < 1 ^ Pseudoplastic behaviour T0 = 0 & n > 1 ^ Dilatant behaviour

1.3.1 Ostwald model

The Ostwald model (Eq. 6), also known as the Power Law model, is applied to shear thinning fluids which do not present a yield stress (Pevere et al., 2006). The n-value in equation 6 gives fluid behaviour information according to:

T = К * y(n_1) (6)

n < 1 ^ Pseudoplastic behaviour n = 1 ^ Newtonian behaviour n > 1 ^ Dilatant behaviour

1.3.2 Bingham model

The Bingham model (Eq. 7) describes the flow curve of a material with a yield stress and a constant viscosity at stresses above the yield stress (i. e. a pseudo-Newtonian fluid behaviour; Seyssiecq & Ferasse, 2003). The yield stress (t0) is the shear stress (t) at shear rate (y) zero and the viscosity (л) is the slope of the curve at stresses above the yield stress.

t = T0 + л * y (7)

T0 = 0 ^ Newtonian behaviour T0 > 1 ^ Bingham plastic behaviour

A lava rock biofilter was used to evaluate the degradation of H2S from the AD gas stream. The experimental setup for the biofilter used in this study was previously described (Ramirez-Saenz et al 2009). The gas stream was humidified and fed in the top of the biofilter using a mass flow controller. Sample ports were located in the output and input of the gas stream. For H2S degradation experiments, the biofiltration system was fed at the top with an air-diluted gas stream originated from the ADR, as previously reported (Ramirez-Saenz et al., 2009). Periodic water additions (once a week) were used to control moisture loss and to avoid SO4-2 accumulation. Recirculation was provided at a flux of 0.5 L/min over 1 h. All experiments were conducted at room temperature (20-25°C).

The factors affecting the production of biogas are mainly based on the operating conditions of the digester, such as pH and temperature which influence directly the micro-organisms. The perturbations in effluent (including the concentration of substrate and its composition in toxic compounds and inhibitors) can also affect the volume and the quality of the produced biogas. Sometimes, the toxic compounds are not present at the beginning in the effluent waste, but they are produced inside the digester starting from degradation of substrate (example: VFA and ammonia).

2.1 Substrate

The type and the composition of the substrate determine directly the quality of the produced biogas. In anaerobic process the substrate is often measured in term of chemical oxygen demand (COD) or of total volatile solids (TVS). It is significant to distinguish between the degradable and the inert fraction, because a considerable fraction of the COD in effluent is inert (Nielsen, 2006). The waste which contains a high percentage of water has a weak methane yield by COD or VS.

Organic waste contains various compounds: mainly saccharides (which are divided into two fractions, easily and slowly degradables), lipids (easily degradable), proteins (easily degradable), VFA (easily degradable), as well as others compounds (Moosbrugger & al., 1993). The production of methane is generally in the range from 100 to 400 L CH4 / kg VS.

On the other hand, the majority of organic waste contain a high fraction of the substrate easily degradable, which gives a high production of methane and VFA. It is thus significant to control the organic and hydraulic loading according to the capacity of the digester when the process functions are at low charge that gives also a low production rate of biogas. Although this can prevent the rupture of the process, it is not very ecomical because the capacity of the process is not completly used. The increase in the charge gives more biogas but also there is the risk of the overload, with, as a consequence, the accumulation of the VFA. The high concentration of VFA decreases the pH and making them more toxic for methanogens bacteria.

with others like sulphur, the phosphorus, the potassium, the calcium, the magnesium and the iron which are required (McMahon & al., 2001). The majority of the nutriments can be inhibiting if they are present at high concentrations.

In order to estimate an optimal fermentation time under ultrasonic exposure in this study, parameters such as ethanol concentration, ethanol volumetric productivity, ethanol yield and lactose consumption were investigated.

The maximum values of ethanol concentration and lactose consumption were achieved when the HRT was 36 h. Under the HRT of 36 h in the ultrasound-assisted fermentation, the average ethanol concentration of 26.30 g L-1, ethanol yield of 0.532 g g-1 lactose and lactose consumption of 98,9% were obtained (Fig. 9-11). Using S. cerevisiae without ultrasound exposure gave the results as 23,60 g L-1, 0.511 g g-1, 92,4%, respectively and the differences were statistically significant (p<0.05). Shortening the HRT to 24 h allowed remaining high ethanol yield of 0.520 g g-1 with sonicated S. cerevisiae, but in the control fermentation unit it was as low as 0.487 g g-1 (p<0.05). When the HRT was 12 h the ethanol yields were 0.318 and 0.365 g g-1 depending on using ultrasounds device (Fig. 11). From the economic viewpoint, shortening the fermentation time (HRT) could reduce costs of industrial ethanol production. The study showed that there is no need to extend the HRT over 36 h or more, because most of the lactose was converted into ethanol during 24 h (95.6% in the ultrasound-assisted fermentation. Nikolic et al. (2010) stated that optimal fermentation time for free and immobilized S. cerevisiae was 38 h. Ozmihci & Kargi (2008) studied ethanol production from cheese whey powder (CWP) solution containing 50 g sugar L-1 at six different HRTs varying between 17.6 and 64.4 h by Kluyveromyces marxianus strains. Percent sugar utilization,

12 24 36

HRT (h)

9. Effects of HRT and ultrasound irradiation on the ethanol concentration

HRT (h)

effluent ethanol concentration and ethanol yield increased with increasing HRT from 17.6 to 50 h. Further increasing in HRT to 64.4 h resulted in decrease of the analyzed parameters. Moreover, the time for fermentation decreased at higher initial substrate concentration (Guimaraes et al., 2008a; Nikolic et al., 2010; Ozmihci & Kargi, 2008). According to Guimaraes et al. (2008a) the fermentations with 50-150 g lactose L-1 reached completion in about the same time of 27 h but the maximum ethanol concentration increased linearly with increasing initial lactose concentration from 6.5 g ethanol L-1 with 20 g lactose L-1 to 57 g L-1 with 200 g L-1. They also stated that increasing lactose concentration led to incomplete fermentation and impair the fermentation due to nutrient limitation.

Interestingly, the volumetric productivities of ethanol decreased at longer HRT (Table 3). Maximum productivity of ethanol of 1.060 g L-1 h-1 was observed under the HRT of 12 h when the culture has been sonicated and 0.908 g L-1 h-1 under the HRT of 24 h in the fermentation process without ultrasound irradiation (p<0.05). The volumetric ethanol productivity in the ultrasound-assisted fermentation obtained in this work was higher than that reported for batch or fed-batch fermentations with S. cerevisiae strains: 0.3 g L-1 h-1 (Rubio-Texeira et al., 1998), 0.46 g L-1 h-1 (Guimaraes et al., 2008b), 0.14 — 0.6 g L-1 h-1 (Ramakrishnan & Hartley, 1993), 1 g L-1 h-1 (Compagno et al., 1995). Ozmihci & Kargi (2007) using Kluyveromyces marxianus to ferment concentrated cheese whey powder solution obtained higher volumetric ethanol productivity over 2 g L-1 h-1, but after 120 h fermentation.

1.2.6 Effect of beating degree and rosin on wet tensile strength

strength decreased with the increase of rosin; when the beating degree was higher than 0 level, wet tensile strength increased with the increase of beating degree and rosin, the maximum value occurred when beating degree was held at 30SR°, and rosin was held at 0.4%. This is because added rosin impacted adsorption effect of fibre to wet strength agents, wet tensile strength decreased, but with the continuing increase of beating degree, the fibre sub-wire broom degree further enhanced, the adsorption effect of fibre on wet strength agent was over than the rosin.

3.4.2 Effect of grammage and wet strength agent on wet tensile strength

of the grammage and wet strength agent; the maximum occurred when wet strength agent was held at 3%, and grammage was held at 110 g/ m2, this is because the number of fibre increased and bonding effect of fibre enhanced, when wet tensile strength increased with the increase of grammage, at the same time, wet strength agent provided cationic charge, fibre strongly adsorbed wet strength agent added because of pulp fibre with anionic charge, so that the wet tensile strength of film increased.

In a multiple response question, respondents were asked to mention the constraining factors towards biogas use adoption. The main factors mentioned were that the installation cost was too high (95.8%) and lack of credit facility (95%). Other reasons were lack of expertise (91.7%) and inadequate water (60%) to run the plants. Only a small proportion of 3.3% out of the 120 respondents said they do not need the facility. This may suggest that if the access to biogas is facilitated either through subsidy or access to credits many households in the district could adopt the technology. A comparison across the categories suggest that

respondents from the poor and less poor were more in demand for credits and also mentioned the facility to be too costly (Table 6), suggesting a different kind of approach to induce them adopt the technology.

The term liquid gas (Liquefied Petroleum Gas[22] it refers to C3 and C4 hydrocarbons or mixtures thereof. It is generated as a by-product in petroleum refining and as an associated gas from the extraction of oil and natural gas. LPG is gaseous at room temperature under atmospheric conditions, but can be liquefied at low pressures. In liquid form, its specific volume is about 260 times smaller than in the gaseous state. Therefore, large amounts of energy can be transported and stored in relatively small containers.

The transportation of LPG is carried out worldwide by tanker ships, barges, pipelines, by rail tank cars, road tankers or in liquefied gas cylinders. LPG is stored in stationary tank facilities or in gas cylinders. Up to a tank size of 2.9 t capacity, the above-ground installation does not require a permit. From a tank capacity of 2.9 tonnes, the federal emission regulations need to be considered when granting a permit. The technical conditions for setting up tank installations are defined in TRB 801 No.25 "LPG storage tank facilities".

Commercial LPG consists of at least 95 percent by mass of propane and propene, whereby the propane content must predominate. The remainder may consist of ethane (C2H6), ethene (C2H4), butane (C4H10) and butene (C4H8) isomers. The classification for commercial propene, butane and butene is equivalent. Note also the degree of purity according to DIN 51 622 [DIN 1985]: Data on sulphur or sulphur compounds are listed here.

In DIN 51624 "automotive fuels — natural gas requirements and test methods" [8-15]upper limits for the propane/butane mole fractions in natural gas of 6% / 2% in the total mixture and a methane number > 70 are required. EASEE-gas CBP (EASEE, 2005) specifies a hydrocarbon dew point of -2 ° C at 1-70 bar. For the calculations shown below, a typical LPG composition of propane / butane, 95 / 5 is used.

The political barriers that exist are mainly in the area of sovereignty rights and the will to initiate national biogas technology programmes. Another problem is the high number of armed conflicts and political instability in the continent which together with the region’s debt burden have reduced the region’s credibility. Hence, providing capital even for modest investments will prove difficult. African governments need to commit themselves to renewable energy programmes. Government constant commitments to the development and promotion of renewable energy sources have been instrumental in promoting an ambitious alcohol fuel in Brazil, biogas programmes in Europe, China and India. It could be helpful to learn from the experiences gained in the developed world but adapted to the needs and situation in developing countries. However, in some African countries, the hostile social climate and political instability prevent opportunities of international collaboration and support.