Figure 2.2 lists some engines that can be operated with biogas. These have been improved during the recent years by following the development works inspired by the worldwide boom in biogas plants. The performance by some manufacturers even has already exceeded that of those given in this figures.
2.4.1 Four-stroke Gas Engine and Diesel Engine
Today’s four-stroke biogas engines were originally developed for natural gas and are therefore well adapted by the special features of biogas. Their electrical efficiencies normally do not exceed 34~ 40%, as the nitrogen oxide (NOx) output has to be kept below the prescribed values. The capacity of the engines ranges from 100 KW to 1 MW.
Four-stroke biogas engines often run in the lean-burn range (ignition window 1.3 < λ < 1.6, λ = air-fuel ratio/stoichiometric air-fuel ratio), where the efficiency is expected to drop. The efficiency of lean-burn engines with turbocharger is 33~ 39%. The NOx emissions can be reduced, however, by a factor of 4 in comparison to ignition (by compression) oil Diesel engines, and the limiting values can be met without further measures. Since the engines tend to knock with varying gas qualities, a methane content of at least 45% in biogas should be ensured.
In small agricultural plants, ignition oil Diesel engines are frequently installed. These engines are more economical and have a higher efficiency than four-stroke engines in the lower capacity range. However, higher NOx
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emissions are produced by Diesel engine. Their lifetimes usually are given as 35,000h of operation.
In general, Diesel engines burning gas fuel can be operated by direct injection because pre-chamber engines develop hot places, resulting in uncontrolled spark failures with biogas. Owing to the internal formation of gas mixtures, Diesel engines can be faster controlled. The ignition oil Diesel engine is operated ideally at λ < 1.9. The efficiency is then up to 15% better than that in a four-stroke engine.
2.4.2 Stirling Engine
An alternative to the commonly used four-stroke and the Diesel engines is the Stirling engine. The efficiency of the Stirling process is closest to that of the ideal cycle. The Stirling engine has been recommended for power generation for many years, but is seldom realized on an industrial scale because of technical problems in details. Power generated from biogas in Stirling engines is not known yet in industrial scale installations.
2.4.3 Gas Turbine
Biogas can be converted to current via gas turbines of medium and large capacity (20 MW and more) at a maximum temperature 1200 °C. The tendency is to go to even higher temperatures and pressures, whereby the electrical capacity and thus the efficiency can be increased. The main parts of a gas turbine are the compressor, combustion chamber, and turbine.
Ambient air is sucked and compressed in the compressor and transmitted to the combustion chamber, where biogas is introduced and burnt with the compressed air. The flue gas that is so formed is passed to a turbine, where it expands and transfers its energy to the turbine. The turbine propels the compressor on the one hand and the power generator on the other hand. The
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exhaust gas leaves the turbine at a temperature of approximately 400~600 °C.
The waste heat can be recovered by driving a steam turbine downstream for heating purposes or for preheating the air that is sucked in.
2.4.4 Micro Gas Turbine
Micro gas turbines are small high-speed gas turbines with low combustion chamber pressures and temperatures. They are designed to deliver up to 200 kW electrical powers. For normal operation, the compressor sucks in the combustion air. The fuel is normally supplied to meet the combustion air in the combustion chamber. When biogas with a low calorific value is used, it must be adjusted to a flammable mixture of biogas and air before it is supplied into the combustion chamber. gas-steam turbine seems already interesting and economical today because of its high electrical efficiency.
2.4.5 Fuel Cell
Comparing to combustion engines, the fuel cell converts the chemical energy of hydrogen and oxygen directly into current and heat. Water is formed as the reaction product.
In principle, a fuel cell works with a liquid or solid electrolyte held between two porous electrodes–anode and cathode. The electrolyte lets ions pass only and allow no free electrons from the anode to the cathode side. The electrolyte
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is thus ―electrically non-conductive.‖ It separates the reaction partners and thereby prevents direct chemical reaction. For some fuel cells, the electrolyte is also permeable to oxygen molecules. In this case the reaction occurs on the anode side. The electrodes are connected by an electrical wire.
Both reaction partners are continuously fed to the two electrodes, respectively. The molecules of the reactants are converted into ions by the catalytic effect of the electrodes. The ions pass through the electrolyte, while the electrons flow through the electric circuit from the anode to the cathode.
Taking into account all losses, the voltage per single cell is 0.6 ~ 0.9 V. The desired voltage can be reached by arranging several single cells in series, a so-called stack. In a stack, the voltages of the single cells are added.
Depending on the type of fuel cell, the biogas has to be purified to remove CO and H2S especially before feeding into the fuel cell. Only a small number of fuel cell plants, mostly pilot plants, are in operation for the generation of electricity from biogas.
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