The efficiency of the anaerobic digestion (AD) process is influended by some critical operating data and parameters. The growth and activity of anaerobic microoragnisms is significantly influence by conditions such as exclusion of oxygen, constant temperature, pH-value, nutrient supply, stirring intensity as well as presence and amount of inhibitors.
13 Process temperature
The anaerobic digestion process can take place at different temperatures, divided into three temperature ranges: psychrophilic (below 25°C), mesophilic (25°C – 45°C), and thermophilic (45°C – 70°C). There is a direct relation between the process temperature and the hydraulic retention time (HRT) (Table 4).
Table 4: Thermal stage and typical retention times (Al Seadi, 2008) Thermal stage Process temperatures Minimum retention time
psychrophilic < 20 °C 70 to 80 days
mesophilic 30 to 42 °C 30 to 40 days
thermophilic 43 to 55 °C 15 to 20 days
Temperature stability is decisive for AD. In practice, the operation temperature is chosen with consideration to the feedstock used and it is usually provided by floor or wall heating systems inside the digester. It can also be provided by heating externally the feedstock. Figure 3 shows the rates of relative biogas yield depending on temperature and retention time.
Figure 3: Biogas yield in function of the temperature and retention time (Al-Seadi, 2008)
Many modern biogas plants operate at thermophilic temperatures as the thermophilic process provides many advantages, compared to mesophilic and psychrophilic processes:
effective destruction of pathogens
higher grow rate of methanogenic bacteria at higher temperature
reduced retention time, making the process faster and more efficient
improved digestibility and availability of substrates
better degradation of solid substrates and better substrate utilization
better possibility for separating liquid and solid fractions
%
Days
14 The thermophilic process has also some disadvantages:
larger degree of imbalance
larger energy demand due to high temperature
higher risk of ammonia inhibition
It is important to keep a constant temperature during the digestion process, as temperature changes or fluctuations will affect the biogas production negatively. Thermophilic bacteria are more sensitive to temperature fluctuations of ±1 °C and require longer time to adapt to a new temperature, in order to reach the maximum methane production. Mesophilic bacteria are less sensitive. Temperature fluctuations of ±3 °C are tolerated, without significant reduction in methane production.
pH values and optimum intervals
The pH value of the AD substrate influences the growth of methanogenic microorganisms and affects the dissociation of some compounds of importance for the AD process (ammonia, sulfide, organic acids). The optimum pH interval for mesophilic digestion is between 6.5 and 8.0, and the process is severely inhibited if the pH-value decreases below 6.0 or rises above 8.3. The solubility of carbon dioxide in water decreases at increasing temperature. The pH-value in thermophilic digesters is therefore higher than in mesophilic ones, as dissolved carbon dioxide forms carbonic acid by reaction with water.
The value of pH in anaerobic reactors is mainly controlled by the bicarbonate buffer system.
Therefore, the pH value inside digesters depends on the partial pressure of CO2 and on the concentration of alkaline and acid components in the liquid phase. The buffer capacity of the AD substrate can vary. Experience from Denmark shows that the buffer capacity of cattle manure varies with the season, possibly influenced by the composition of the cattle feed.
Macro- and micronutrients (trace elements) and toxic compounds
Microelements (trace elements) like iron, nickel, cobalt, selenium, molybdenum or tungsten are equally important for the growth and survival of the AD microorganisms as the macronutrients carbon, nitrogen, phosphor, and sulfur. The C/N ratio should be in the range between 15 and 30 (Weiland, 2010). Insufficient provision of nutrients and trace elements, as well as too high digestibility of the substrate can cause inhibition and disturbances in the AD process.
Another factor, influencing the activity of anaerobic microorganisms, is the presence of toxic compounds. They can be brought into the AD system together with the feedstock or can be generated during the process as the VFA (volatile fatty acids) and ammonia.
Dry matter content
For bacteria to be able to degrade the material, the dry matter content must not be higher than around 50%. In biogas plant, however, it should only be around 8 – 10%, if it is to remain liquid enough to be pumped. Higher levels can be tolerated in special reactor types with a direct feed line (Jørgensen, 2009).
15 Organic load
Obtaining the maximum biogas yield, by complete digestion of the substrate, would require a long retention time of the substrate inside the digester and a correspondingly large digester size. In practice, the choice of system design (digester size and type) or of applicable retention time is always based on a compromise between getting the highest possible biogas yield and having a justifiable plant economy. In this respect, the organic load is an important operational parameter, which indicates how much organic dry matter can be fed into the digester, per volume and time unit. The normal load for a CSTR reactor is 1 – 6 kg COD/m3 reactor volume/day (Jørgensen, 2009).
Hydraulic retention time (HRT)
HRT is the average time interval that the substrate is kept inside the digester tank. HRT is correlated to the digester volume and the volume of substrate per time unit. The retention time must be sufficiently long to ensure that the amount of microorganisms removed with the effluent is not higher than the amount of reproduced microorganisms. A short HRT provides a good substrate flow rate, but a lower gas yield. It is therefore important to adapt the HRT to the specific decomposition rate of the used substrates.
4 Biogas quality for energy uses
Most of the European biogas production is combusted in internal combustion engines to produce electric power. When possible the thermal energy from the engine exhaust and cooling systems is also used, but as the biogas plants are located mostly in rural areas the utilization of the thermal energy is often not satisfying. The presence of a district heating network near the biogas production unit obviously favors an external use of the produced heat. Instead of internal combustion engines turbines, micro-turbines and stirling engines can be as well utilized. Biogas is also commonly burned in boilers to produce hot water and steam.
Other possible alternative to conventional gas motors is the use of fuel cells. Fuel cells are an emerging technology that may improve the outlook for clean, efficient and economical energy use of biogas as they have much higher electrical conversion efficiency compared to motor engines, lower emissions of pollutants (NOx) and lower noise generation.
By removing carbon dioxide, moisture, hydrogen sulfide and other impurities biogas can be upgraded to biomethane, a product equivalent to natural gas, which typically contains more than 95% methane. The process can be controlled to produce biomethane that meets a predetermined standard of quality. In this way the full biogas range of conversion opportunities are open.
Biomethane can be used interchangeably with natural gas, whether for electrical generation, heating, cooling, pumping, or as a vehicle fuel. Biomethane can be pumped into the natural gas supply pipeline or store and transport as compressed biomethane (CBM), which is analogous to compressed natural gas (CNG), or as liquefied biomethane (LBM), which is analogous to liquefied natural gas (LNG). A report issued by the Swedish Gas Association shows the relation between transport distance and transported volumes for the different upgrading and distribution alternatives available on the market (Swensson, 2010). For short to medium distances and larger volumes,
16 local gas grids provide the best alternative. Considering road transport, CBMis the best option for all volumes up to distances of 200 km compared to LBM.
The methane content in the biomethane depends on the upgrading process, the quality of the biogas, and on the preconditioning of the biogas. For example the nitrogen is not separated from the methane by most of upgrading process; thus a desulfurization with air would lead to high nitrogen content in the biomethane.
Other potential high-grade fuels that can possibly be produced from biogas include liquid hydrocarbon replacements for gasoline and diesel fuels (created using the Fischer-Tropsch process), methanol, dimethyl ether, and hydrogen.
Figure 4 shows the main biogas use pathways.
Figure 4: Main biogas use pathways