3.10.1.5 Micro Turbines
Micro gas turbines are small high-speed turbine, an established technology in sizes
above 500 kW. Micro turbines in biogas application, with range of 25–100 kW, have
been introduced. Micro turbines are high-speed, integrated power plants that include
a turbine of radial design, compressor, generator, heat exchanger, and power electronics to produce power. The compressor, the turbine, and the generator are fixed on
a single shaft. The biogas mixed with the air is supplied to the combustion air in the
combustion chamber, and the turbine sucks in the combustion air. Overall efficiency
of current generation micro turbines is still low (Deublein and Steinhauser 2008).
3.10.1.6 Fuel Cells
The fuel cell converts the chemical energy to electric current and heat. Biogaspowered fuel cells hold great potential to convert biogas directly into electricity with
zero emission. Major multinational companies and many residential buildings are
currently using biogas-powered fuel cells to generate energy for various purposes. It
simply utilizes chemical reactions to produce the energy. Fuel cells boost net output
of electricity by a minimum of 60%, with an average 0.6–0.9 V/single cell. Single
cells could be further arranged in a stack to get the desired voltage. However, biogas
has to be purified before using as a fuel in the fuel cell by mainly removing the CO
and H 2 S. Proton exchange membrane (PEM) and solid oxide electrolytes are the
currently used fuel cells (Chambers and Potter 2002).
3.10.1.7 Steam Turbine
High-pressure and superheated steam is injected into shaft power that drives the
generator to produce electric power in steam turbine. Generally steam turbine CHP
systems produce electricity as a by-product of steam generation. Biogas can be used
as a fuel to generate high-pressure steam from the boiler to power the turbine for
electricity generation. Low-pressure steam could be directly extracted from the
turbine and used as a thermal energy for other needs. The steam is condensed and
pumped back to the boiler, thus completing Rankine cycle (thermodynamic cycle).
Radial and axial flows are the two types of turbine in action. If the exhaust steam
contains >10% water, it can erode nozzle and blades. Special design in the turbine to
remove the moisture can be used when the superheated steam temperature is limited.
Reheating of the superheated steam after partial expansion will increase the cycle
efficiency. Three types of steam turbines are in operation: condensing steam turbine,
extraction turbines, and back pressure turbine. Steam turbines are one of the oldest
(100 years) technologies still in commercial production, with 50 kW–>100 MW
capacity (Ion and Popescu 2016).
3.10.1.8 Stirling Engines
A Stirling engine produces heat and electricity by external combustion. One of the
advantages of a Stirling engine is its external combustion; it can utilize a multitude of
fuels including biogas where other engines cannot. The engine uses nitrogen as a
“working fluid”. Heat exchanger from the combustion chamber is utilized to produce
thermal energy. Since the engine has external combustion chamber, it is not required
98
S. Elangovan et al.
Micro gas turbines are small high-speed turbine, an established technology in sizes
above 500 kW. Micro turbines in biogas application, with range of 25–100 kW, have
been introduced. Micro turbines are high-speed, integrated power plants that include
a turbine of radial design, compressor, generator, heat exchanger, and power electronics to produce power. The compressor, the turbine, and the generator are fixed on
a single shaft. The biogas mixed with the air is supplied to the combustion air in the
combustion chamber, and the turbine sucks in the combustion air. Overall efficiency
of current generation micro turbines is still low (Deublein and Steinhauser 2008).
3.10.1.6 Fuel Cells
The fuel cell converts the chemical energy to electric current and heat. Biogaspowered fuel cells hold great potential to convert biogas directly into electricity with
zero emission. Major multinational companies and many residential buildings are
currently using biogas-powered fuel cells to generate energy for various purposes. It
simply utilizes chemical reactions to produce the energy. Fuel cells boost net output
of electricity by a minimum of 60%, with an average 0.6–0.9 V/single cell. Single
cells could be further arranged in a stack to get the desired voltage. However, biogas
has to be purified before using as a fuel in the fuel cell by mainly removing the CO
and H 2 S. Proton exchange membrane (PEM) and solid oxide electrolytes are the
currently used fuel cells (Chambers and Potter 2002).
3.10.1.7 Steam Turbine
High-pressure and superheated steam is injected into shaft power that drives the
generator to produce electric power in steam turbine. Generally steam turbine CHP
systems produce electricity as a by-product of steam generation. Biogas can be used
as a fuel to generate high-pressure steam from the boiler to power the turbine for
electricity generation. Low-pressure steam could be directly extracted from the
turbine and used as a thermal energy for other needs. The steam is condensed and
pumped back to the boiler, thus completing Rankine cycle (thermodynamic cycle).
Radial and axial flows are the two types of turbine in action. If the exhaust steam
contains >10% water, it can erode nozzle and blades. Special design in the turbine to
remove the moisture can be used when the superheated steam temperature is limited.
Reheating of the superheated steam after partial expansion will increase the cycle
efficiency. Three types of steam turbines are in operation: condensing steam turbine,
extraction turbines, and back pressure turbine. Steam turbines are one of the oldest
(100 years) technologies still in commercial production, with 50 kW–>100 MW
capacity (Ion and Popescu 2016).
3.10.1.8 Stirling Engines
A Stirling engine produces heat and electricity by external combustion. One of the
advantages of a Stirling engine is its external combustion; it can utilize a multitude of
fuels including biogas where other engines cannot. The engine uses nitrogen as a
“working fluid”. Heat exchanger from the combustion chamber is utilized to produce
thermal energy. Since the engine has external combustion chamber, it is not required
98
S. Elangovan et al.