224
Water for Energy and Fuel Production
2%–5% air must be present in the digester, which converts H 2 S to elemental sulfur
and sulfurous acid [5]. These bacteria are often present in the digester or added in
the headspace of the digester. An efficient desulfurization requires a high contact
area for microorganisms’ fixation, which can be accomplished by an installation of
specific wood or fabric support at the top of the fermenter.
For biological desulfurization outside the fermenter, trickling filter installations
filled with plastic support materials on which the microorganisms can grow are used
[139]. Raw biogas and air are injected at the bottom of the column, and the aqueous solution of nutrients is circulated to wash out the acidic products and supply the
nutrients to microorganisms. The process is carried out at 35°C (mesophilic condition), and the support material is washed with air/water mixture at regular intervals
to prevent sulfur deposits on the filters.
Desulfurization can also be done by adding commercial ferrous solution to the
digester. In this expensive method, the production of hydrogen sulfide is prevented
because ferrous binds sulfur to produce compounds which are insoluble in the liquid
phase.
8.10 UtiliZatiOn OF BiOGas and diGestate
The purified biogas can be used to generate electricity with about 43% efficiency [5].
It can be used in microgas turbine or fuel cell. While it is used in microgas turbine
with a lower (25%–31%) efficiency, it gives good loading efficiency and long maintenance intervals for the turbines [5]. Furthermore, the exhaust heat from microgas
turbine can be used to generate the process heat. The use of clean biogas in various
fuel cells, which are operated at temperatures between 80°C and 800°C, gives higher
efficiency. The investment costs for such applications are, however, higher. In the
recent years, significant efforts are being made to upgrade the biogas and inject it
into the grid or utilize it as a vehicle fuel [5].
The injection of biogas into natural gas grid requires further removal of all contaminants and carbon dioxide such that the final product must contain at least 95% methane.
Both bacteria and molds must also be removed to make the use of biogas environmentally acceptable. The carbon dioxide is absorbed with the use of polyethylene glycol or
mono- or diethanolamines. Carbon dioxide can also be removed using cryogenic separation, pressure swing adsorption, or membrane separation technology [5].
The process of anaerobic digestion reduces 80% of odor of the feedstock. The
digestate generated from anaerobic digestion process possess valuable properties
as fertilizers. Both nitrogen (in the form of ammonia) and carbon are useful as
fertilizers. The nitrogen content in the digestate depends on the feedstock; it can
be increased by a factor of 3 when only energy crops as substrate are used [5,52].
The faster permeation of digestate with improved flow properties can reduce loss
of ammonia in air, thereby making “digestate fertilizer” more effective. While the
“digestate fertilizer” inactivates weed seeds, bacteria, viruses, fungi, and parasites,
their decay rates depend on the temperature, pH, treatment time, and VFA concentration. The best and faster results are obtained at higher temperature (>50°C) [5].
For certain wastes, while a separate pasteurization after digestion (at 70°C) is effective, digestate is prone to recontamination [5].
Water for Energy and Fuel Production
2%–5% air must be present in the digester, which converts H 2 S to elemental sulfur
and sulfurous acid [5]. These bacteria are often present in the digester or added in
the headspace of the digester. An efficient desulfurization requires a high contact
area for microorganisms’ fixation, which can be accomplished by an installation of
specific wood or fabric support at the top of the fermenter.
For biological desulfurization outside the fermenter, trickling filter installations
filled with plastic support materials on which the microorganisms can grow are used
[139]. Raw biogas and air are injected at the bottom of the column, and the aqueous solution of nutrients is circulated to wash out the acidic products and supply the
nutrients to microorganisms. The process is carried out at 35°C (mesophilic condition), and the support material is washed with air/water mixture at regular intervals
to prevent sulfur deposits on the filters.
Desulfurization can also be done by adding commercial ferrous solution to the
digester. In this expensive method, the production of hydrogen sulfide is prevented
because ferrous binds sulfur to produce compounds which are insoluble in the liquid
phase.
8.10 UtiliZatiOn OF BiOGas and diGestate
The purified biogas can be used to generate electricity with about 43% efficiency [5].
It can be used in microgas turbine or fuel cell. While it is used in microgas turbine
with a lower (25%–31%) efficiency, it gives good loading efficiency and long maintenance intervals for the turbines [5]. Furthermore, the exhaust heat from microgas
turbine can be used to generate the process heat. The use of clean biogas in various
fuel cells, which are operated at temperatures between 80°C and 800°C, gives higher
efficiency. The investment costs for such applications are, however, higher. In the
recent years, significant efforts are being made to upgrade the biogas and inject it
into the grid or utilize it as a vehicle fuel [5].
The injection of biogas into natural gas grid requires further removal of all contaminants and carbon dioxide such that the final product must contain at least 95% methane.
Both bacteria and molds must also be removed to make the use of biogas environmentally acceptable. The carbon dioxide is absorbed with the use of polyethylene glycol or
mono- or diethanolamines. Carbon dioxide can also be removed using cryogenic separation, pressure swing adsorption, or membrane separation technology [5].
The process of anaerobic digestion reduces 80% of odor of the feedstock. The
digestate generated from anaerobic digestion process possess valuable properties
as fertilizers. Both nitrogen (in the form of ammonia) and carbon are useful as
fertilizers. The nitrogen content in the digestate depends on the feedstock; it can
be increased by a factor of 3 when only energy crops as substrate are used [5,52].
The faster permeation of digestate with improved flow properties can reduce loss
of ammonia in air, thereby making “digestate fertilizer” more effective. While the
“digestate fertilizer” inactivates weed seeds, bacteria, viruses, fungi, and parasites,
their decay rates depend on the temperature, pH, treatment time, and VFA concentration. The best and faster results are obtained at higher temperature (>50°C) [5].
For certain wastes, while a separate pasteurization after digestion (at 70°C) is effective, digestate is prone to recontamination [5].