Parameters and Process Optimisation for Biogas
The metabolic activity involved in microbiological methanation is dependent on the following factors:
- Substrate temperature
- Available nutrients
- Retention time (flow-through time)
- pH level
- Nitrogen inhibition and C/N ratio
- Substrat solid content and agitation
- Inhibitory factors
- H2S Concentration
Each of the various types of bacteria responsible for the three stages of the methanogenesis is affected differently by the above parameters. Since interactive effects between the various determining factors exist, no precise quantitative data on gas production as a function of the above factors are available. Thus, discussion of the various factors is limited to their qualitative effects on the process of fermentation.
Temperature Range of Anaerobic Fermentation
Anaerobic fermentation is in principle possible between 3°C and approximately 70°C. Differentiation is generally made between three temperature ranges:
- The psychrophilic temperature range lies below 20°C,
- the mesophilic temperature range between 20°C and 40°C and
- the thermophilic temperature range above 40°C.
Minimal Average Temperature
The rate of bacteriological methane production increases with temperature. Since, however, the amount of free ammonia also increases with temperature, the bio-digestive performance could be inhibited or even reduced as a result. In general, unheated biogas plants perform satisfactory only where mean annual temperatures are around 20°C or above or where the average daily temperature is at least 18°C. Within the range of 20-28°C mean temperature, gas production increases over-proportionally. If the temperature of the bio-mass is below 15°C, gas production will be so low that the biogas plant is no longer economically feasible.
Changes in Temperature
The process of bio-methanation is very sensitive to changes in temperature. The degree of sensitivity, in turn, is dependent on the temperature range. Brief fluctuations not exceeding the following limits may be regarded as still un-inhibitory with respect to the process of fermentation:
- psychrophilic range: ± 2°C/h
- mesophilic range: ± 1°C/h
- thermophilic range: ± 0,5°C/h
The temperature fluctuations between day and night are no great problem for plants built underground, since the temperature of the earth below a depth of one meter is practically constant.
In order to grow, bacteria need more than just a supply of organic substances as a source of carbon and energy. They also require certain mineral nutrients. In addition to carbon, oxygen and hydrogen, the generation of bio-mass requires an adequate supply of nitrogen, sulfur, phosphorous, potassium, calcium, magnesium and a number of trace elements such as iron, manganese, molybdenum, zinc, cobalt, selenium, tungsten, nickel etc. "Normal" substrates such as agricultural residues or municipal sewage usually contain adequate amounts of the mentioned elements. Higher concentration of any individual substance usually has an inhibitory effect, so that analyses are recommended on a case-to-case basis to determine which amount of which nutrients, if any, still needs to be added.
Batch-type and Continuous Plants
The retention time can only be accurately defined in batch-type facilities. For continuous systems, the mean retention time is approximated by dividing the digester volume by the daily influent rate. Depending on the vessel geometry, the means of mixing, etc., the effective retention time may vary widely for the individual substrate constituents. Selection of a suitable retention time thus depends not only on the process temperature, but also on the type of substrate used.
Optimizing the process parameters retention time - process temperature - substrate quality - volumetric load determine, among others, the cost efficiency of the biological processes. But as each m3 digester volume has its price, heating equipment can be costly and high quality substrates may have alternative uses, the cost-benefit optimum in biogas production is almost always below the biological optimum.
For liquid manure undergoing fermentation in the mesophilic temperature range, the following approximate values apply:
- liquid cow manure: 20-30 days
- liquid pig manure: 15-25 days
- liquid chicken manure: 20-40 days
- animal manure mixed with plant material: 50-80 days
If the retention time is too short, the bacteria in the digester are "washed out" faster than they can reproduce, so that the fermentation practically comes to a standstill. This problem rarely occurs in agricultural biogas systems.
The methane-producing bacteria live best under neutral to slightly alkaline conditions. Once the process of fermentation has stabilized under anaerobic conditions, the pH will normally take on a value of between 7 and 8.5. Due to the buffer effect of carbon dioxide-bicarbonate (CO2 - HCO3-) and ammonia-ammonium (NH3 - NH4+), the pH level is rarely taken as a measure of substrate acids and/or potential biogas yield. A digester containing a high volatile-acid concentration requires a somewhat higher-than-normal pH value. If the pH value drops below 6.2, the medium will have a toxic effect on the methanogenic bacteria.
Nitrogen Inhibition and C/N Ratio
All substrates contain nitrogen. Table B2 lists the nitrogen content of various organic substances and the C/N ratio. For higher pH values, even a relatively low nitrogen concentration may inhibit the process of fermentation. Noticeable inhibition occurs at a nitrogen concentration of roughly 1700 mg ammonium-nitrogen (NH4-N) per liter substrate. Nonetheless, given enough time, the methanogens are capable of adapting to NH4-N concentrations in the range of 5000-7000 mg/l substrate, the main prerequisite being that the ammonia level (NH3) does not exceed 200-300 mg NH3-N per liter substrate. The rate of ammonia dissociation in water depends on the process temperature and ph value of the substrate slurry.
Microorganisms need both nitrogen and carbon for assimilation into their cell structures. Various experiments have shown that the metabolic activity of methanogenic bacteria can be optimized at a C/N ratio of approximately 8-20, whereby the optimum point varies from case to case, depending on the nature of the substrate.
Substrate Solids Content and Agitation
Substrate Solids Content
The mobility of the methanogens within the substrate is gradually impaired by an increasing solids content, and the biogas yield may suffer as a result. However, reports of relatively high biogas yields from landfill material with a high solids content may be found in recent literature. No generally valid guidelines can be offered with regard to specific biogas production for any particular solids percentage.
Many substrates and various modes of fermentation require some sort of substrate agitation or mixing in order to maintain process stability within the digester. The most important objectives of agitation are:
- removal of the metabolites produced by the methanogens (gas)
- mixing of fresh substrate and bacterial population (inoculation)
- preclusion of scum formation and sedimentation
- avoidance of pronounced temperature gradients within the digester
- provision of a uniform bacterial population density
- prevention of the formation of dead spaces that would reduce the effective digester volume.
In selecting or designing a suitable means of agitation, the following points should be considered:
- The process involves a symbiotic relationship between various strains of bacteria, i.e. the metabolite from one species can serve as nutrient for the next species, etc. Whenever the bacterial community is disrupted, the process of fermentation will remain more or less unproductive until an equivalent new community is formed. Consequently, excessive or too frequent mixing is usually detrimental to the process. Slow stirring is better than rapid agitation.
- A thin layer of scum must not necessarily have an adverse effect on the process. For systems in which the digester is completely filled with substrate, so that any scum always remains sufficiently wet, there is little or no danger that the extraction of gas could be impeded by the scum.
- Some types of biogas systems can function well without any mechanical agitation at all. Such systems are usually operated either on substrates with such a high solid content, that no stratification occurs, or on substrates consisting primarily of solute substances.
Since the results of agitation and mixing are highly dependent on the substrate in use, it is not possible to achieve a sufficiently uniform comparative evaluation of various mixing systems and/or intensity levels. Thus, each such system can only be designed on the basis of empirical data.
The presence of heavy metals, antibiotics (Bacitracin, Flavomycin, Lasalocid, Monensin, Spiramycin, etc.) and detergents used in livestock husbandry can have an inhibitory effect on the process of bio-methanation. The following table lists the limit concentrations (mg/l) for various inhibitors.
|Sulfide (as Sulfur):||200|
Hydrogen Sulphide results from the transformation of sulphur containing proteins in the plant or animal residues. For the cases where the biogas plants use a Combined Heat and Power Engine (CHP) for producing heat and electricity, H2S can combine with water to form Sulphuric Acid and can hence damage the engine. Further, high concentrations of H2S can poison the cell activity thereby reducing the methanation process. Therefore a low concentration (<500 ppm) of the gas is desirable.
Following are the measures that can be taken to reduce the concentration of H2S in the biogas being sent to the CHP Engine:
- Adding little O2 to facilitate breaking down of H2S by the small amount of aerobic bacteria residing in the upper crust of digestate mixture.
- Adding Iron salts (hydroxides and chlorides) to the silage. The iron salt-rich silage needs to be fed in the fermenter on a daily basis so as to enhance the H2S minimizing effect. Chlorides are corrosive and cause problems in the mixing tank. Therefore their use should be limited.
- Passing the gas through a scrubber that strips the gas of H2S and brings down the H2S level to almost zero ppm. However, this method may involve high initial investment cost for buying scrubber.
Conditioning of Biogas
Sometimes the biogas must be treated/conditioned before utilization.
The predominant forms of treatment aim at removing either water, hydrogen sulfide or carbon dioxide from the raw gas:
Reduction of the Moisture Content
The biogas is usually fully saturated with water vapor. This involves cooling the gas, e.g. by routing it through an underground pipe, so that the excess water vapor condenses at the lower temperature. When the gas warms up again, its relative vapor content decreases. The "drying" of biogas is especially useful in connection with the use of dry gas meters, which otherwise would eventually fill up with condensed water.
Reduction of the Hydrogen-sulfide Content
The hydrogen sulfide in the biogas combines with condensing water and forms corrosive acids. Water-heating appliances, engines and refrigerators are particularly at risk. The reduction of the hydrogen sulfide content may be necessary if the biogas contains an excessive amount, i.e. more than 2% H2S. Since most biogas contains less than 1% H2S, de-sulfurization is normally not necessary.
For small- to mid-size systems, de-sulfurization can be effected by absorption onto ferric hydrate (Fe(OH)3), also referred to as bog iron, a porous form of limonite. The porous, granular purifying mass can be regenerated by exposure to air.
The absorptive capacity of the purifying mass depends on its iron-hydrate content: bog iron, containing 5-10% Fe(OH)3, can absorb about 15 g sulfur per kg without being regenerated and approximately 150 g/kg through repetitive regeneration. It is noteworthy that many types of tropical soils (laterite) are naturally ferriferous and suitable for use as purifying mass.
Another de-sulfurization process showing good results has been developed in Ivory Coast and is applied successfully since 1987. Air is pumped into the gas store at a ratio of 2% to 5 % of the biogas production. The minimum air intake for complete de-sulfurization has to be established by trials. Aquarium pumps are cheap and reliable implements for pumping air against the gas pressure into the gas holder. The oxygen of the air leads to a bio-catalytic, stabilized separation of the sulfur on the surface of the sludge. This simple method works best, where the gas holder is above the slurry, as the necessary bacteria require moisture, warmth (opt. 37 C) and nutrients.
In industrialized countries and for large plants, this process has meanwhile reached satisfactory standard. For small scale plants in developing countries, however, using an electric pump becomes problematic due to missing or unreliable electricity supply. Pumping in air with a bicycle pump works in principle, but is a cumbersome method that will be abandoned sooner or later.
Avoiding de-sulfurization altogether is possible, if only stainless steel appliances are used. But even if they are available, their costs are prohibitive for small scale users.
Reduction of the Carbon-dioxide Content
The reduction of the carbon-dioxide content is complicated and expensive. In principle, carbon-dioxide can be removed by absorption onto lime milk, but that practice produces "seas" of lime paste and must therefore be ruled out, particularly in connection with large-scale plants, for which only high-tech processes like micro-screening are worthy of consideration. CO2 "scrubbing" is rarely advisable, except in order to increase the individual bottling capacity for high-pressure storage.