Developments in the design and operation of gas, diesel and dual-fuel reciprocating engines continue (though slowly), with improvements to efficiency and lower atmospheric emissions being the main aims. James Hunt reports on engine developments and how these may impact on cogeneration plants.
Although gas turbines are highly suitable for large power plants and provide more exhaust heat (useful where a large amount of process heat is required), for the smaller power generation and typical cogeneration plant (CHP), the reciprocating engine is the prime mover of choice. This is because of its greater flexibility, in terms of starts-ups and cycling, and because it is more thermally efficient.
Gas, diesel and dual-fuel (DF) powered reciprocating engines are all used in cogeneration plants, but gas engines are usually preferred as they have considerably lower exhaust emissions, work well with CHP applications, and utilize the fuel highly efficiently. Gas engines also produce virtually no particulates, so in CHP applications, exhaust heat boilers and heat recovery steam generators are not sooted up. Sooting can compromise plant thermal efficiency and be a considerable maintenance problem when diesel (heavy fuel oil) engines are used.
Gas engines tend to be used in more technologically developed countries and in those with a good natural gas infrastructure. India, for example, is now developing an extensive natural gas infrastructure and a large liquefied natural gas (LNG) plant is being built near Mumbai. Those countries without a good supply of natural gas will usually find diesel/heavy fuel oil engines more suitable. Even so, many reciprocating engine manufacturers devote a significant part of their overall production to gas engines. For example, around 15% of Wärtsilä’s oil-driven power plant engines are for applications having a heat load requirement, and 45%–50% of the company’s gas engines go to CHP applications.
A typical MAN Diesel product – the company has recently been involved in the major HERCULES co-operative research project
A good modern example of a gas engine is GE Jenbacher’s new 4 MW J624 GS lean-burn, turbocharged, mixture-cooled machine – an expansion of the earlier Type 6 engine, but featuring many developments. The 24 cylinders provide a very smooth, almost vibration-free torque, so the engine is very reliable with low service costs. Genset and total plant reliability is also improved. Special features include combustion optimization and providing a higher mechanical efficiency through reduced losses. The end result is a very high electrical efficiency of over 45%. Another characteristic is the vibration-decoupled turbochargers, intercoolers and other auxiliaries; which further improve availability, life and maintenance. Customers benefit from a high power density, with low installation and operating costs, low specific fuel consumption and a high heat recovery rate. The latter means that this engine is ideal for cogeneration.
The CHP plant in Györ, Hungary, comprising three Wärtsilä 18V34SG engines, 19 MWe and 16.4 MWth
Natural gas is becoming valuable, so some engines, such as GE’s Jenbacher machines, are now being designed to run on a wide range of gases, including off-specification and stranded gases and biogas. Such gases may be more suitable for distributed power, on-lease use and/or sale to the grid. Savings can range from 20% to 80%, depending on price.
Diesel engine initial investment and operational costs are relatively low – and they are fuel-efficient. For example, Tognum GmbH’s MTU 20V 4000 engine uses only 192 gm/kWh of fuel, representing 44% thermal efficiency. Although diesel emissions are higher than gas engine emissions, they are improving. For example, Wärtsilä’s 23,000 kW 20-cylinder 20V46F heavy fuel oil engine is especially environmentally friendly, and has NOx emissions down to 710 ppm at 15% O2 – which complies with valid World Bank requirements.
Dual-fuel (DF) and multi-fuel engines are usually a compromise, but they may be ideal where heavy fuel oil is a back-up supply. Caterpillar’s DF engines, for example, burn natural gas and diesel fuel together, the latter being the combustion pilot; the start-up uses 100% diesel fuel, and after warm-up diesel substitution occurs. The cycle is effectively diesel.
HOW RECIPROCATING ENGINES CAN BE USED IN CHP
Reciprocating engines are highly successful in small- to medium-sized CHP installations, where the prime movers might typically be 5–6 MW machines. More power is obtainable using several engines, which also provides valuable redundancy.
Medium-speed diesel and gas reciprocating engines are very efficient in terms of electricity and power production – a gross electrical efficiency of up to 47% is achievable for engines larger than 3 MWe. This is very high, even compared with large power plants using gas turbines. The rest of the engine fuel input goes to heat in the exhaust gases and to engine cooling, both of which can be used for valuable CHP heat production. The utilization factor for a CHP plant might be only 50%–60% because exhaust heat might not always be required. Even so, the percentage of time that the plant performs its duty, compared with the time it should work, is very important – many plant owners require this to be 97%–99%, so good engine reliability and availability is crucial. Modern engines should show little or no decrease in efficiency over their lifetimes, and all with very high utilization.
Fuel efficiency is a very important factor. In some countries, the average fuel efficiency of all fossil fuelled electricity production is only around 34% (not including system losses), but generators driven by large reciprocating engines can attain a net fuel efficiency for electricity generation of about 44% (or more) simple cycle, and 80%–90% for CHP, depending on the temperature of the heat used (around 38% of the fuel energy should ideally be converted into useful heat for district heating or other processes).
GE Jenbacher’s new 24-cylinder 4MW J624 GS lean-burn, turbo-charged, mixture cooled gas engine, is ideal for cogeneration
Very large office/retail developments, such as shopping centres and malls, are not just confined to the developed world, they are booming in developing countries too. Such facilities use much electricity, which has led to increased power requirements, including CHP. Both electrical power and comfort cooling must be supplied in hot countries by compact plant with 24/7 reliability. Power and comfort cooling requirements often vary daily; varied fluctuation suits reciprocating engines far better than gas turbines.
Conventionally, the required power comes from grids, with diesel genset backup – but in some developing countries, the latter may be used for up to 30% of the day because of grid unreliability. This can be very expensive. One solution is for developers to install their own combined cooling, heating and power (CCHP or trigeneration) power plants. Such plant improve efficiencies by using heat from exhaust gases/hot coolant in absorption chillers (though reciprocating engines may not cope with the chilling requirement on their own, so electrical chilling may also be needed). Extra reciprocating engines as standby can provide redundancy. However, piped gas is not easily stored, so operating the plant using an alternative fuel such as diesel may be necessary, so, if the main engines run on natural gas, standby diesel gensets will be needed. The alternative is using dual-fuel engines, which might be the cheaper and more compact option.
Many modern reciprocating engines now burn biofuels, but growing crops to produce them reduces land available for food production. However, Jatropha seeds do not greatly affect food production, it is claimed, and in a new Belgian Greenpower-owned CHP project, a Wärtsilä engine will be the first to burn crude Jatropha oil. The heat produced dries digested biomass, and the carbon dioxide goes to tomato greenhouses. The CHP plant also produces hot water, plus electricity for 20,000 homes. The 9 MW Wärtsilä 20V32 engine with selective catalytic reduction (SCR) provides a gross electrical efficiency of 44.2% and an overall efficiency of more than 85%. Annual carbon dioxide savings will total over 36,000 tonnes.
LATEST TECHNICAL DEVELOPMENTS
The aim is to reduce exhaust emissions still further, whilst improving thermal and electrical efficiencies and delivering reduced maintenance requirements. For example, Waukesha, Caterpillar and Cummins have been working with the US Department of Energy (DOE) to improve all of these parameters.
So what are the latest reciprocating engine developments? Current research and development drives include: achieving better combustion and volumetric efficiencies, better combustion chamber optimization, more sophisticated turbochargers (having variable turbine geometries for optimum exhaust temperatures/compressed air volumes) and (for diesels) better high-pressure common rail fuel injection equipment (FIE). Improvements also derive from bore-cooled cylinder liners and higher compression ratios. Special lower carbon-cutting rings, such as in Rolls-Royce’s K-gas G4.2 gas engine, can reduce ‘dead space’ for the air/gas mixture around the piston’s top land – this increases the amount of mixture in combustion and improves thermal efficiency while reducing unburned hydrocarbon (UHC) emission. Further improvements will come about through special piston ring pack designs and better tribology/hydrodynamic design to reduce friction.
The latest electronics control systems provide ultimate engine optimization, control and monitoring. For example, Guascor’s SFGLD gas engines have speed, load and other parameters electronically controlled so they can operate at constant limited emissions. The engines adapt to different gas qualities and can switch from natural gas to biogas with no interruption. Spark plug condition is automatically reported and ignition advance can be adjusted remotely. A detonation detection and control system enables working under severe conditions without knock.
A significant area of development is providing individual ignition adjustments (by cylinder speed/load/emissions/energy requirement etc). Generally, automated load-sharing between cylinders allows higher loads and efficiencies, and adaptive load balancing enables wider engine operating regimes.
Note that having a high electrical efficiency is often considered more important than high thermal efficiency, and engine control systems are now often linked to plant control and management systems. These may be optimized to take as much energy out of the fuel as possible for efficient electricity generation. Exhaust heat is often a secondary consideration, but trade-offs are possible using the latest engine control and management techniques.
These developments have a bearing on CHP applications, because the engines’ exhaust gas and cooling water temperatures/flows – the heat output needed to obtain heat or steam for processes – are crucially important. However, as Anders Ahnger, general manager of combined technologies at Wärtsilä Power Plants, commented: ‘With the gas engine, we are now starting to reach temperature limits, so while I would always like a hotter running engine to supply heat for CHP plants, we cannot now do this without developing new high temperature materials – there is much R&D going on into this.’
LEGISLATIONS AFFECTING ENGINE DEVELOPMENT
The most important legislation that affects engine design and operation, at least in Europe, are the CHP and IPPC Directives. Looking first at the CHP Directive (2004/8/EC), this aims to promote the use of cogeneration to increase energy efficiencies and improve supply security. The intention is to create a framework for promotion and develop high efficiency cogeneration – but some EU member countries have been slow to implement the directive and others are introducing their own legislation.
In late 2007, the EC adopted a proposal for a directive on industrial emissions. This is aimed at incorporating seven existing industrial emissions-related directives into a single coherent legislative instrument – including, in particular, the IPPC Directive (2008/1/EC). This has been in place for over a decade and the EC has undertaken a two year review to examine how it can offer the highest environmental protection while simplifying existing legislation and cutting unnecessary administrative costs. For reciprocating engines, this covers NOx and carbon monoxide emissions – not carbon dioxide currently, though this will come.
Finally, there is the Promotion of Electricity from Renewable Energy Sources in the Internal Electricity Market Directive (2001/77/EC), also called the Renewables Directive. This requires each EU country to commit to specific targets for renewable energy. It is a challenge to convert these directives into national law.
Research continues. For example, a multinational team of over 40 European companies and research institutions has been involved in the major HERCULES co-operative research project (www.ip-hercules.com) into the technologies necessary to achieve greatly reduced gaseous and particulate emissions, yet with better fuel efficiency and reliability. Though this work was aimed at the marine power sector, many benefits will be applicable to stationary power.
In another important research project: the Internal Combustion Engine Laboratory of the Helsinki University of Technology, built the EVE medium-speed, 200 mm bore, single-cylinder research engine. This highly loaded machine features an electro-hydraulic valve actuating system – all four valves (two inlet, two exhaust) can be opened and closed independently of each other and of crank angle (as far as avoidance of mechanical disaster allows). The complex interrelated timings – overseen by a high-speed control system – allows for cycles to be achieved that were impossible previously. The Miller cycle could be improved with early intake valve openings to increase effective compression ratio and, therefore, cycle efficiency. Variable valve timings and independently electronically operated valves are being tested to help achieve this. The EVE target maximum cylinder pressure is 400 bar (50 bar indicated mean effective pressure), and it has been successfully shown that the electro-hydraulic valve mechanism works successfully. This is potentially very important for future reciprocating engine development.
More potentially important R&D work includes the High-Efficiency Dilute Gasoline Engine (HEDGE). Primarily aimed at heavy road vehicles, it could also be applicable to smaller generating engines. The theory is that diesel fuel costs more than gasoline, and that the diesel exhaust after-treatment is far more expensive than petrol engine catalysts. The US Government is aiming for 50% thermal efficiency and current diesels engines of this type have 43%–44% – but this is compromised by the emissions control equipment, so diesel have quite a long way to go to meet the requirements. Therefore, the thinking goes, gasoline engines are worth considering, even though they lack low-speed torque. Remedying this means increasing the compression ratio, but this risks severe detonation.
The EVE high-pressure research engine has an electro-hydraulic valve actuating system – the importance for further engine development could be profound
The HEDGE Consortium (see www.swri.org) aims to solve the detonation problem by using large amounts of exhaust gas recirculation (EGR). In HEDGE engines, the exhaust gas that provides the EGR is cooled from around 600ºC–700ºC down to 40ºC–50ºC (in small CHP applications, this heat could be used to good effect). This suppresses detonation, but the resulting mixture (with 50% exhaust gas) is hard to ignite. Therefore, some HEDGE gasoline engines use 1% diesel fuel pre-injection to initiate combustion. Spark ignition is also being tried as this is cheaper than diesel FIE. Special dual-coil inductive ignition systems fire off one coil to break down the mixture’s initial resistance, then immediately afterwards fires the second coil with high energy. Both approaches work and the high EGR, high compression ratio, high torque, low emissions gasoline engine is now a reality. HEDGE technology is potentially applicable to engines for CHP, although there is a bore size knock limitation that currently restricts power to around 300 kW per cylinder.
The development of engines and fuels continues, but the time frame for producing new engines for sale from an initial research and development stage can be very long.
James Hunt is a UK-based writer on energy and electro-technical issues.
Thanks are due especially to Anders Ahnger, general managercombined technologies, Wärtsilä Power Plants; Michael Wagner, marketing leader, GE Energy’s Jenbacher gas engines; and Jean-Pierre Pirault of PTL Powertrain Technology (www.ptlengines.com) for their help in the preparation of this article.