Gas engines
Inhaltsverzeichnis
Introduction for CMM engine systems
Internal-combustion (IC) engines are able to accommodate to gas streams with a wide range of methane concentrations. Some CMM sources are mixed with air or with inert components like N2 and CO2, especially in gob gas. The size and generation capacity of an IC engine varies in a quite large range. Typical CMM powered engines have a capacity between 250 kW and 5000 kW. The gensets smaller than 2000 kW per unit are mostly installed in containers. In principal a genset consists of the combustion engine, a generator, the coupling, a base frame and an elastic mountings arrangement. The engine and generator are rigidly mounted on the base frame. This unit is used as a combined heat and power (CHP) to generate electricity and heat. The specifications for individual engines vary depending on the gas components and on the construction. Standard IC engines can run on gas with a methane concentration down to 40% methane in the entrance gas. If the content of methane is less than 40% specific engines have to be used, which can operate with methane concentrations down to 25%. It is important to adjust the correct ratio of O2, CO2 and N2 in order to gain a useable gas mixture. CO2 and N2 act like a fire extinguishant. On the other hand oxygen is essential for the combustion but utilisation of this gas mixture with high oxygen levels is very difficult and exceptional.
The mixture-system, as a crucial subsystem of the genset, prepares an air-methane mixture with a methane content of about 7 % in the mixture, which is able to be burned in the cylinder afterward. It has to work on quick response, so that the changes in the gas composition can be considered, due to avoid damage of the engine. The mode of operation depends on the local conditions and for CMM fuelled gas engines especially on the available gas volume. An efficient operation of the gen set is possible with loads higher than 80. The minimum of 50% is desired due to avoid of damages.
Components of CMM Gensets
The main part of a CHP module is the engine supported by the following additional components:
- cooling water heat exchanger
The explosions in the cylinder cause a drastic increase of pressure and temperature. The pressure is used to turn the crankshaft and the heat has to be discharged to ensure acceptable material temperatures. The waste heat is transported by cooling water from the engine to the air cooled heat exchanger. As the heat is in range 70-90°C it is also usable for e.g. district heating.
- exhaust gas silencer
The exhaust gas causes big noise emission and in many cases, depending on the local conditions, installation of an exhaust gas silencer might be necessary.
- exhaust gas cleaning system
Exhaust gas cleaning system in CHP systems are necessary to reduce CO emissions. As CMM normally doesn’t contain catalyst poison, catalytic systems are applied. In rarely cases e.g. abandoned mines H2S can be in the gas, but its concentration is low and do not harm the catalyst.
- fuel tank or gas supply
Whilst gas – diesel – engines use a fuel tank for gasoil, the gas (CMM) is supplied from the gas compressor station, which should deliver CMM with a constant pressure. To ensure a proper operation of the engine, liquid water has to be removed from the gas. Moreover appropriate measures have to be taken to avoid condensate formation.
- lubricating oil supply
Lubricating oil has to be changed several times per year. Therefore tanks for new and for used oil are installed on the site and connected to the system, due to make an automatically changes possible.
- control & monitoring system
A computer based control and monitoring system is responsible for the optimised operation of the genset inclusive all subsystem and components, as air mixer and pressure stabilizer. The frame condition is set by the operating staff.
- optional exhaust gas heat exchanger
The heat of the exhaust gas can be used by means of a heat exchanger. Its installation is not necessary for the normal engine operation and only useful if those energy amounts can really be utilised.
- generator
The generator, linked to the engine produces electric power with voltage harmonised with the electric grid. For systems up to 2000 kW the generator mostly delivers 400 V, which can be converted to higher voltages of the grid by transformers. If all CPH units of the same operator have the same voltage easy changes between installation places is possible.
- container
Many CMM fired engines are installed in a semi-mobile frame, which is often a container and can be moved/relocated with lorries. Only the gas and power connection has to be done on site.
Each of the components has a function in the system and is jointly responsible for the correct operation of the gen set.
Gas - Air - Mixture
The composition of the gas – air – mixture is at every time essential for a high operation efficiency and a long life time of the engine. In modern gas engines the control system checks many parameters (e.g. cylinder and exhaust gas temperature) continuously, as they are indicators for the correct adjustment of the gas – air – mixture. CMM compared to natural or sewage gas and biogas as fuel makes different demands on the control unit. The reason is its changing composition and the necessity to adjust the gas-air-mixture´s quality. CMM from active mines can contain methane – air – mixtures of about 30% methane in air. In consequence the dimensions of pipes, valves and accessories has to be adjusted for the different volume of fuel gas while the oxygen content of the CMM has to be considered in fixing the CMM – air – mixture. The mixing system has to work for the whole power range with possible big fluctuations of fuel gas volume in time.
Utilisation of engine waste heat
During operation gas engines produce waste heat which has to be dissipated. There are four main heat sources:
1 Radiant heat from the engine surface. This heat has to be dissipated by room ventilation.
2 Heat from the compressed fuel gas behind the turbo charger. In most cases this heat is transferred to a special cooling circuit. Due to its low temperature level of the cooling circuit (30 °C - 45 °C) an use for heat supply is not possible.
3 Heat from the motor cooling circuit. This circuit usually works at a temperature level from 80 °C to 95°C and comprises 25 % to 50 % of the fuel energy. Temperatures up to 105 °C are possible in some special system configurations. However, they are unusual and might cause technical problems, which could reduce the life time of the engine.
4 Heat of the exhaust gas The heat of the exhaust gas with up to 500°C can be captured by a special exhaust heat exchanger, designed individually for the local specification. In times when no heat is needed by the user, the heat has to be cooled down or the heat exchanger has to be protected against the hot exhaust gas.
Depending on the local conditions, the total efficiency of the engine system can reach more than 85%. An engine without heat production has usually an efficiency of about 37 % in the year average. The utilisation of the waste heat can contribute to more than 50% utilisation of the fuel energy. However, in most cases only a small heat portion is used, as the CMM capacity is much higher than the needed heat amount.
Econonics of gas engines for CMM utilisation
Two factors are essential for the economic of gas engines:
- power price
- granting of emission rights
Further information
ATEC “Technical equipment for coal mine suction and utilisation, Combined power and heat systems”
GE’s Energy; „GE Jenbacher Gas Engines“
