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close this bookBiogas Plants in Animal Husbandry (GTZ, 1989)
close this folder5. Biogas technique
View the document(introduction...)
View the document5.1 Fundamental principles, parameters, terms
View the document5.2 Design principles of simple biogas plants
View the document5.3 Biogas plants of simple design
View the document5.4 Design and construction of plant components
View the document5.5 Biogas utilization
View the document5.6 Measuring methods and devices for biogas plants

5.1 Fundamental principles, parameters, terms

Biochemical principles

The generation of biogas by organic conversion (anaerobic fermentation) is a natural biological process that occurs in swamps, in fermenting biomass and in intestinal tracts, particularly those of ruminants.

The symbiotic relationships existing between a wide variety of microorganisms leads, under air exclusion, to the degradation and mineralization of complex biomass in a sequence of intermeshing stages. The resultant biogas, consisting primarily of methane (CH4) and carbon dioxide (CO2) and the mineralized slurry constitute the ultimate catabolites of the participating bacteria and residual substances.

The process of anaerobic fermentation can be illustrated in the form of a three-stage model, as shown in figure 5.1.

Table 5.1: Basic criteria for acetobaeters (acid-forming bacteria) and methanobacters (methane-forming bacteria) (Source: OEKOTOP, compiled from various sources)

Criterion

Acetobacter

Methanobacter

Dominant microorganisms

facultative anaerobes

obligate anaerobes

Temperature range

3 °C - 70 °C

3 °C - 80 °C

Optimum temperature

approx. 30 °C

approx. 35 °C (sensitive to temperature fluctuations of 2-3 °C or more)

pH range

acidic (3.0) 5.0-6.5 relatively short duplication period, usually less than 24 hours

alkaline, 6.5-7.6 relatively long duplication period (20 - 10 days)

End metabolites

org. acids, H2, CO2

CO2, CH4

Mass transfer by . . .

intensive mixing

gentle circulation

Medium

aqueous (water content > 60%)


Sensitivity to cytotoxins

low

substantial

Requirements regarding nutrient composition

well-balanced supply of nutrients


Special features

viable with or without free oxygen

viable only in darkness and in absence of free oxygen


Table 5.2: Energy potential of organic compounds (Source: Kaltwasser 1980)

Material

biogas (I/kg)

CH4

CO2

Energy content



vol. fraction %

(Wh/g)

Carbohydrates

790

50

50

3.78

Organic fats

1270

68

32

8.58

Protein

704

71

29

4.96

Anaerobic fermentation converts the "volatile solids" (proteins, carbohydrates, fats). The "nonvolatile solids" are essential to the bacteria as "roughage" and minerals. Water serves simultaneously as the vital medium, solvent and transport vehicle.

Theoretical/laboratory data on maximum gas yields from various organic materials show that anaerobic fermentation is just as capable of achieving complete mineralization as is the process of aerobic fermentation. Note: The theoretical maximum biogas yield can be ascertained by way of the basic composition of the biomass.

Table 5.3: Energetical comparison of aerobic and anaerobic fermentation (Source: Inden 1978)

Metabolite

aerobic

anaerobic


energy fraction (%)

Cytogenesis

60%

10%

Heat

40%

-

Methane

-

90%

Characteristics that set anaerobic fermentation apart from aerobic fermentation (e.g. composting) include:

- fixation of biochemical energy in biogas
- little formation of new biomass
- low heat development
- fixation of minerals in the digested slurry.

It is important to know that anaerobic fermentation involves a steady-state flux of acetobacters and methanobacters, with the methanobacters, being more specialized and, hence, more sensitive, constituting the defining element. Any biogas plant can develop problems during the starting phase and in the case of overloading or uneven loading of the digester, and as a result of poisoning. This underlines the importance of cattle dung, which is rich in methanobacters and therefore serves as a good "starter" and "therapeutic instrument" in case of a disturbance.

With regard to technical exploitation, anaerobic fermentation must be regarded from a holistic point of view, since the "organism" is only capable of operating at optimum efficiency under a certain set of conditions. The process of anaerobic fermentation is quite variable and capable of stabilizing itself as long as a few basic parameters are adhered to.

Parameters and terminology of biomethanation

Feedstock/substrate:
As a rule, all watery types of biomass such as animal and human excrements, plants and organic wastewater are suitable for use in generating biogas. Wood and woody substances are generally unsuitable.

The two most important defining quantities of the biomethanation process are the substrate's solids content, i.e. total solids (TS, measured in kg TS/m³) and its total organic solids content, i.e. volatile solids (VS, measured in kg VS/m³ ). Both quantities are frequently stated as weight percentages.

The total-solids and water contents vary widely from substrate to substrate (cf. table 3.2 for empirical values). The most advantageous TS for the digester of a continuoustype biogas plant is 5-10%, compared to as much as 25% for a batch-operated plant. A TS of 15% or more tends to inhibit metabolism. Consequently, most substrates are diluted with water before being fed into the digester.

Substrate composition
All natural substrates may be assumed to have a nutritive composition that is adequately conducive to fermentation. Fresh green plants and agroindustrial wastewater, however, sometimes display a nutritive imbalance.

An important operating parameter is the ratio between carbon content (C) and nitrogen content (N), i.e. the C/N-ratio, which is considered favorable within the range 30 :1 to 10: 1. A C/N-ratio of less than 8: 1 inhibits bacterial activity due to an excessive ammonia content.

Fermentation/digester temperature
As in all other microbial processes, the rate of metabolism increases along with the temperature. The fermentation/digester temperature is of interest primarily in connection with the time required for complete fermentation, i.e. the retention time: the higher the temperature, the shorter the retention time. It has no effect on the absolute biogas yield, which is a constant that depends only on the type of biomass in the digester.

For reasons of operating economy, a somewhat shorter period of fermentation, the technical retention time (RT, t, measured in days) is selected such as to achieve an advantageous, temperature-dependent relative digestion rate (Dr, measured in Yo), also referred to as the yield ratio, since it defines the ratio between the actual biogas yield and the theoretical maximum. The average agricultural biogas system reaches a Dr-value of 30-60%..


Fig. 5.2: Gas yield as a function of temperature and retention time (fT,RT-curves). 1 fT,RT: relative gas yield, serving as a multiplier for the average gas yields, e.g. those listed in table 3.5, 2 retention time (RT), 3 digester temperature (T), measured in °C (Source: OEKOTOP)

Table 5.4: Temperature ranges for anaerobic fermentation (Source: OEKOTOP, compiled from various sources)

Fermentation

Minimum

Optimum

Maximum

Retention time

Psycrophilic

4-10 °C

15-18 °C

25-30 °c

over 100 days

Mesophilic

15 - 20 °C

28-33 °C

35-45 °C

30-60 days

Thermophilic

25-45 °C

50-60 °C

75-80 °C

10-16 days

Volumetric digester charge/digester load

The volumetric charge, i.e. how much substrate is added per unit of digester volume each day (Vc, measured in m³/m³ Vd x d), is given by the chosen (technical) retention time (RT).

The digester load (Ld, measured in kg digested TS (VS)/m³ Vd x day) serves as a measure of digester efficiency. The digester load is primarily dependent on four factors: substrate, temperature, volumetric burden and type of plant. For a typical agricultural biogas plant of simple design, the upper limit for Ld is situated at roughly 1.5 kg VS/m³ x day. Excessive digester loading can lead to plant disturbances, e.g. a lower pH. In practice, the amount of TS/VS being added is frequently equated to the digester load.

Specific biogas yields / specific biogas production

The specific gas yield (Gy, measured in m³ gas/kg TS (VS)) tells how much biogas can be drawn from a certain amount of biomass (cf. table 3.5 for empirical values). The rate of gas generation is naturally dependent on the digester temperature and retention time (cf. fig. 5.2).

The term specific gas production (Gp, measured in m³ gas/m³ Vd x day) supplements the above expression by defining the digester's biogas output.

pH/volatile acids

The pH is the central parameter of the biochemical bacterial environment.

As soon as the pH departs from the optimum range, bacterial activity is seriously impaired, resulting in lower gas yields, inferior gas composition (excessive CO2 content) and obnoxious odor (H2S - like rotten eggs).

Table 5.5: pH ranges for biomethanation (Source: OEKOTOP, compiled from various sources)

pH

7-7.2

optimum

pH

< 6.2

acid inhibition

pH

> 7.6

ammonia inhibition


Table 5.6: Substances with an inhibiting effect on biomethanation (Source: OEKOTOP, compiled from various sources)

Substance

Disruptive effects beginning (mg/l)

Copper

10-250

Calcium

8000

Magnesium

3000

Zinc

200-1000

Nickel

350-1000

Chromium

200-2000

Cyanocompounds

25

Chlorinated hydrocarbons

traces

Herbicides

traces

Insecticides

traces

Toxins
Even a slight concentration of cytotoxins suffices to disrupt bacterial activity, with a resultant shift in pH, lower gas yield, higher CO2 content and pronounced odor nuisance.