All heat engines produce power and heat as a by-product. As most are only 30%-40% fuel-efficient at best, a great deal of heat is produced – much of it the exhaust, but also given to the coolant. It is this heat that can be used in combined heat and power (CHP) applications. Engine-based CHP runs from small, factory-packaged systems of a few tens of kilowatts up to several megawatt-sized engines. Therefore, it is not only gas turbines that are suitable for CHP applications; diesel, gas and dual fuel reciprocating engines are all suitable.
Despite increasingly stringent exhaust emissions regulations putting diesel engines at a comparative disadvantage with gas reciprocating engines, this venerable type is still the power unit to use for a wide range of stationary power applications up to around 20 MW.
Heat engines are already a well developed technology, and they are still improving. Shown here is a modern gas engine (Rolls-Royce)
Diesel engine initial investment and operational costs are relatively low, and they are fuel-efficient. For example, MTU’s 20V 4000 engine uses only 192 grams/kWh of fuel, representing 44% thermal efficiency, excluding large CHP benefits (note that MTU is now a core brand of Tognum GmbH). This is important because, at current diesel fuel prices, fuel costs can make up well over 80% of the life-cycle costs in some plant. Obtaining maximum specific fuel consumption from diesel engines is therefore essential, as every 1% reduced fuel consumption results in (typically) 0.8% lower life-cycle costs.
Diesel engines are also operationally flexible, easy to maintain and are extremely reliable. Importantly, they are also reasonably fuel-tolerant. While diesel engine technology is mature, life-cycle costs are still being reduced through greater reliability and reduced maintenance requirements.
Diesel engines have a problem, however, in that their exhaust emissions are not as clean as other technologies. While modern diesels can give low exhaust emissions, typically 8 grams/kWh or better nitrous oxide (NOx), it has become ever more difficult to meet increasingly stringent exhaust emissions legislation. It should be borne in mind that in the US, on-highway heavy-duty engines have to achieve 0.27g/kWh tailpipe NOx at full load, as well as 0.013 grams/kWh tailpipe particulates at 2010 model year, with very low deterioration over 500,000 miles; similar figures can be expected for Euro V on-highway, and eventually stationary plants.
Recent technical advances have helped. These include common-rail fuel injection equipment (FIE), electronically controlled jerk pumps and unit injectors, and pilot and post injection. Electronic injection systems allow precise switching of the necessary high pressures while also delivering highly desirable variable timings throughout the operating range, although considerable mechanical muscle and work is required to generate the high pressures. Combustion systems are being examined using advanced computer modelling (and classical testing), including open chamber, pre-chamber and also dual/multi-fuel types.
Other advances have included turbocharger, engine control, electronic management and diagnostic system developments. All have helped to almost eliminate smoke and greatly reduce exhaust emissions, yet NOx, diesel particulate matter (DPM) – a suspected human carcinogen – and CO2 greenhouse gas emissions are still problems for diesel engine operators today. For practical sizes, cost-effectiveness and longevity, selective catalytic reduction (SCR) will reduce NOx emissions by up to 80% (to <2 grams/kWh). SCR comprises injection of a reducing agent (urea) into the exhaust of a catalyst that has absorbed NOx during the diesel engine’s oxygen-rich operating mode. Ammonia is released via a hydrolysis reaction, producing a reducing atmosphere which enables NOx conversion to nitrogen and water. The conversion efficiency can be as high as 90%.
However, excellent electronic engine management is necessary; otherwise, pungent ammonia emission can result, and even power loss. Precious metal-based catalysts are sensitive to poisoning from phosphorus-based compounds in the oil, and to fuel sulphur. Low sulphur fuels avoid catalyst poisoning and ‘bad eggs’ hydrogen sulphide (H2S) and also sulphur dioxide (SO2) generation.
Special injection and catalytic after-treatment equipment adds cost. Therefore, diesel power has, in recent years, lost out in many European countries to gas engines with their much cleaner exhaust. Gas engines can use relatively simple low-cost three-way catalyst systems, borrowed from light duty spark ignition vehicle engines. However, big strides are still being made in diesel engine emissions.
Longer-term solutions include successful electronic valve actuation (EVA – allowing cam timing independence), and alternative fuels for ultra lean-burn combustion. Modern ceramics have been tried for certain components. They allow higher operating temperatures, improving fuel efficiency and cutting costs. However, ceramics do not follow the usual probability curves and have often failed unpredictably and catastrophically.
So the diesel is still progressing, and is often the first and only choice where there is no natural gas supply, or where power is desperately needed, such as in many parts of Africa, and in countries with high growth, and where there are big gaps in power supply and demand. Even so, there is ever greater emphasis on emissions, and many authorities are now increasingly worried about NOx. Note that the World Bank sets emissions standards according to where power plant is situated in developing countries when it comes to lending.
As an example, Wärtsilä’s advanced diesel engines include the 201, Vasa 32LN, 32 and 462 machines. The company claims that its reciprocating engines are capable of 48% shaft efficiency (over 90 % in CHP mode).
Perkins’ modular 4012-46TAG3A/TWG3A Evolution Generation 2 diesel engines are available in up to 16 cylinders. These 1875 kVA (50 Hz) and 1500 kWe (60Hz) engines provide an increased power density and load acceptance with smaller package size, and an improved altitude/ambient capability. The standby rating represents a 13% increase on the earlier 4012TAG2A engine. The load acceptance is 65% prime (TAG2A 70%).
The D12 is one of a new series of compact 200-313 kW industrial engines from Volvo Penta. It allows certification for Stage 3A/Tier 3, ready to meet Stage 3B/Tier 4 emission levels
In another modern example, Volvo Penta’s 147-200 kW 6- and 7-litre machines use high-pressure common rail FIE, while the 200-313 kW 9- and 12-litre engines use high-pressure unit injectors. Volvo’s Advanced Combustion Technology (V-ACT) enhances performance and reliability, while reducing fuel consumption. It also allows certification for Stage 3A/Tier 3, ready to meet Stage 3B/Tier 4 emission levels. Higher pressure turbocharging and injection are the reasons for this higher performance, while NOx and DPMs are reduced through better-controlled injection, fuel atomization and exhaust gas recirculation.
Dual-fuel and multi-fuel engines, essentially based on diesel engines, are available in two types. One type can switch from gaseous to liquid fuels very quickly while running. This type is used to allow relative independence from fuel supply problems. Wärtsilä and Fairbanks Morse dual-fuel engines (for example) can be switched from one fuel to another virtually instantaneously.
The second type is essentially a gas engine that uses a small amount of injected diesel fuel to initiate combustion. This type typically uses in excess of 90% gas, with as little as 1%-5% diesel fuel for ignition, providing 70% or better NOx reduction because of the premixed flame front burning with very low excess oxygen, in contrast to the excess oxygen situation of diesel diffusion burning. However, some people will say that the fuel-efficiency of this type is 1%-2% lower than for pure gas engines. This may seem like little, but it is actually a very significant difference.
For those that can run on either diesel or gas alternatively, there is always a compromise. Such engines (while they have certain advantages for some applications) cannot burn both fuels with best efficiency. Either they are optimized in terms of compression ratio or heat balance, but not both. As a result, fuel efficiency suffers and these engines tend to be less efficient. As a result, dual-fuel engines, though once thought likely to become increasingly important, are relatively little used for CHP applications today.
A MAN B&W series high-speed four-stroke diesel engine
In addition to Wärtsilä and Fairbanks Morse, other manufacturers of dual-fuel engines include Caterpillar, MAN B&W, and Kybota.
Clean-burning lean-burn gas reciprocating engines have been making inroads into both gas turbine and diesel engine sectors because of their low exhaust emissions, combined with high thermal efficiency. Now however, natural gas price rises and supply difficulties are driving technical developments also towards improving fuel efficiencies. For example, engine manufacturers Waukesha, Caterpillar and Cummins are working with the US Department of Energy (DOE) to improve engine fuel efficiency and flexibility still further, lessen overseas fuel dependency, achieve ultra-low emissions goals, lower power costs and improve power system and grid reliability, as well as reducing maintenance costs. Other manufacturers are involved in similar programmes in Europe.
Gas engines, available up to around 6 MW output (50 MW or more multi-engine), are ideal for making best use of natural gas (NG). Recently, this technology has increasingly been viewed in Europe as being perfect for distributed power generation and CHP.
Gas engine emissions are lower than those of diesel engines. For an equivalent amount of heat, burning NG produces about 30% less CO2 than burning liquid fossil fuels. The lean (weak) mixture of gaseous fuel and air means that a typical 5 MW gas engine will have NOx emissions of around 1 gram/kWh (or better) – half that of diesel engines using SCR. Adding a catalytic converter enables gas engines to operate even in city environments. However, formaldehyde emission is now a topic of concern. For example, Danish authorities have recently been monitoring a gas engine prototype powering a CHP plant to establish future limits.
Where CHP plant is concerned, gas turbines have been the obvious choice, but gas reciprocating engine advances have increasingly made them alternatives, being highly efficient and also providing better flexibility for frequent starting, stopping and load changes. In these modes, gas engine electrical efficiencies are clearly higher, and they also generally produce lower emissions in relation to produced power. Burning NG, they can provide roughly 30% lower fuel costs, it is claimed, than turbines, though actual economy depends on application, heat recovery, and fuel price.
NG has been seen as best, combining availability, price, and good environmental performance. However, with this fuel now beginning to run out – at least long-term commercially – is this going to spoil the party such that gas engine developments take a different turn? The answer is probably ‘no’, although the rising price of NG has forced energy producers to think hard, and alternative energy sources can look more attractive. However, although gas will become more expensive, it will still be reasonably cheap energy, and even those countries that use NG as a political bargaining ploy will still need to sell it. Other forms of energy will also become more expensive, but because of the gas engine’s high efficiency (44%-45%, and up to 85% with cogeneration), it is likely that its popularity will increase, not decrease.
Over the next few years, therefore, gas engine manufacturers will be developing a new generation of advanced machines in order to still further improve performance. They will be aiming at a thermal efficiency of 49%-50%, with a 95% reduction in NOx emissions – all with major reductions in maintenance costs. Such performances will only be achieved using advanced fuel handling and processing systems, ignition and combustion systems, and materials (to allow elevated temperature running). Many gas engine performance improvements have come about through developments in the design of turbochargers (which increasingly feature variable turbine geometry for optimum exhaust temperatures/compressed air volumes), plus better fuelling, combustion systems and electronic engine management systems.
Individual ignition adjustments (by cylinder, by speed/load/emissions/energy requirement etc), and individual timings/cylinder are also significant areas of development. One innovative idea is adaptive load balancing. This enables wider operating fields for the engine, since the cylinders’ operating points can be adjusted automatically much closer to each other than with more conventional control systems. Generally, automated load sharing between cylinders allows higher loads, higher compression ratios and higher efficiencies. Wärtsilä has just such a system for large gas engines.
Further developments will come about through reducing friction (special piston ring pack designs and better tribology/hydrodynamic design), and improving the Miller Cycle with early intake valve openings to increase effective compression ratio and, therefore, cycle efficiency. However, the gas engine is now probably reaching the limits in terms of efficiency improvements, and 49%-50% will be the probable maximum.
CHP plant sizes are increasing and, whereas once engines were supplied up to 10 MW for a plant, now it’s getting towards 100-200 MW. This might be thought ideal for gas turbines, but there are still advantages in using gas engines. This is because more are needed for plants of this size, so there is the advantage of redundancy for greater reliability, and there is also greater operational flexibility. So plant of this size might now need 20-30 units each, and the future for gas engines in very large plant (single or combined cycle, or CHP) seems rosy, despite gas price increases. This trend is also likely to see the development of very large engine platforms. The optimum unit size for a 200 MW plant depends on the load profile, and on who you talk to, as some clients like to see a proportion of units at a plant stopped at any given time, for good redundancy.
Despite that fact that the era of cheap energy is coming to an end, there may be another boost for the gas engine. Off-specification (low-energy content, high inerts, or sour) and stranded (low volume or not close to pipelines) natural gases are becoming more valuable.
Such gases may be more suitable for distributed power, for on-lease use and/or sale to the grid. Savings can range from 20% to 80%, depending upon gas price. Another fuel is gaseous biomass. Biomass-fuelled gas engine plants are clean and efficient, and form a practical solution for the need for renewable energy supplies with the minimum environmental impact.
A good example of a modern gas engine is Rolls-Royce’s K-gas G4.2 gas engine. This is available in 12, 16 or 18 cylinders, and produces overall power from 2425 kWe to 3640 kWe, which is equivalent to 200 kW of electrical power per cylinder. It is claimed to have the highest electrical efficiency in its range and is ideal for CHP applications. It offers a 5.5% fuel saving, equivalent to £85,000 (E123,000) per year for an 18-cylinder engine operating for 7000 hours/year compared with the previous G4 model. Improvements derive from a bore-cooled cylinder liner with higher compression ratio.
Rolls-Royce’s new K-gas G4.2 gas engine is claimed to provide the highest electrical efficiency in its range
In addition, a lower carbon cutting ring has led to a reduction in ‘dead space’ for the air/gas mixture around the piston’s top land, so increasing the amount of mixture in combustion and improving thermal efficiency. There is reduced unburned hydrocarbon (UHC) emission too.
Additionally, Rolls-Royce is launching its ‘Integrated Power Solution’. This is a pre-engineered, pre-fabricated modularized power plant, with various engine sizes, some with heat recovery systems for CHP. The power ranges up to 8.5 MW.
Another recent gas engine example is the MTU (Tognum GmbH) long-stroke 4.77 litres/cylinder 12V 4000 L61. The lean-mixture process in an open combustion chamber ensures an efficiency of almost 43%, combined with low emissions and high power density. The exhaust heat can be easily recovered for use in CHP or trigeneration – very high overall efficiencies result (to 84%).
MTU’s (Tognum) lean-mixture 12V 4000 L61 gas engine provides an efficiency of almost 43%, combined with low emissions and high power density
Wärtsilä’s 18V50DF lean-burn gas engines have a dual-fuel capability (hence the ‘DF’ designation). These NG-fuelled engines can switch quickly to fuel oil as backup and are, therefore, also dual-fuel engines.
Wartsila’s 18V50DF dual fuel engine. Developed from a diesel engine, this machine burns mainly natural gas (but starts on light fuel oil)
Other suppliers of gas engines include Waukesha, GE Energy, Kybota, John Deere, Perkins and Lombardini.
Design for CHP
Most CHP plant is used to provide heat to make steam or process heat or heat for drying purposes, but paper and textile mills have power surges – another criteria entirely. So engines have to cope with these different aspects. Lean-burn gas engines are suitable for both types of applications – there isn’t really any difference between engines designed for one application or another. However, by changing certain engine parameters, such as the inlet valve timing, it is possible to optimize an engine for greatest thermal efficiency, or to produce more waste heat, if required. The same is generally true for diesels and dual-fuel engines.
Control and monitoring
Fast changing engine technologies have necessitated control, diagnostic and data reporting system developments. Electronic engine management systems – interfaced with the complete authority plant control system – supervise all start/stop/safety and control requirements. Such systems ensure lower fuel consumption and cleaner exhaust emissions, as well as better response at increased loads. These controls, often with high-speed Ethernet connectivity, also allow a more efficient dual-fuel capability. In Europe, CHP operators remotely monitor domestic, public service and industrial applications for engine units as low as 100 kW electrical power.
Modern engines are highly reliable. For example, MTU’s (Tognum GmbH) 3010 kW 20V 4000 diesel engine boasts 30,000 hours to first major overhaul, with oil changes every 1000 hours. Even so, faults do occur, so diagnostic monitoring, intervention plus predictive maintenance techniques are crucial. Modern, compact hand-held service tools allow engine testing, troubleshooting, diagnosis and adjustments.
Modern reciprocating engines for CHP applications, as for any other, are still being significantly improved in terms of reliability, reduced maintenance requirement, better power and torque characteristics, greater fuel-efficiency and still lower exhaust emissions. However, although progress will continue on all these aspects, in terms of fuel efficiency – which has a bearing on emissions – it is likely that the maximum possible results may be close to being achieved. Even so, exhaust emissions themselves still have significant potential for reductions, using a range of existing and developing technologies, both as part of the engines themselves and as ‘bolt-ons’.
Few engines are developed specifically for CHP applications. Standard engines are ideal for the application, needing just a few ‘tweaks’, usually in the electronic control systems, to provide, perhaps, a little more heat in the exhaust.
The lean-burn gas engine is, perhaps, the most suitable technology for CHP, especially as it is highly fuel-efficient and so clean in the first place. Even so, where fuel flexibility is essential, modern dual-fuel engines provide an effective option, and the highly reliable diesel engine is still ideal where gas infrastructures are poor or non-existent. Even so, the lean-burn gas engine almost certainly has the best future as reciprocating engine prime mover, and is increasingly challenging gas turbines, even for large CHP plant.
Further ahead, there is now an increasing effort (in the US, supported by European and Japanese OEMs), to further advance high-efficiency pressure-charged spark ignition engines using the ~99% efficient three-way catalyst technology with very high levels of cooled exhaust gas recirculation (EGR) to mitigate in-cylinder NOx and detonation. This so-called high-efficiency dilute gasoline engine (HEDGE) technology produces negligible NOx and particulates. It is now being developed for application to light/medium-duty engines but may be adaptable in the future to smaller CHP applications.
James Hunt is a UK-based writer on energy issues.
Reciprocating engine fuels around the world today
COSPP spoke with Bjørn Thorbjørnsen, Vice President Sales & Marketing for Rolls Royce’s Diesels Power Business, about fuels for reciprocating engines for CHP applications. Pointing out that Rolls-Royce has done much work on diesel engines, though most of what the company does today is on gas engines, he said: ‘Even so, 50% of the world’s market in our power range is liquid fuelled.
‘South America’, said Thorbjørnsen, ‘is forced to use liquid fuels, as are Pakistan, India and Indonesia because those countries’ gas infrastructures are not fully developed. In Bolivia, there is natural gas, and the Bolivian pipeline supplies Peru and some parts of Argentina. Even so, South America is a huge market for liquid fuel engines. Brazil, especially, is going towards biofuels, but this is still in the embryonic stage for power plants.
‘There are a few prototype bio-fuelled plants (in Italy, for example). Rolls-Royce is becoming involved in biofuelled plant on a global basis. Such fuels include palm, rape seed and soya bean oils. Some countries are working towards making 5% biofuel in fossil liquid fuels the norm. ‘In the Middle East, there’s a plentiful supply of oil and gas, and the market is mainly for water purification and desalination plant some CHP. The region uses both diesel and gas engines, depending on customer choice. ‘In the Far East, Rolls-Royce is working in India, Bangladesh and Pakistan, with significant opportunities in China and Indonesia for industrial power CHP, public utlities and also CHP for sewage sludge drying (using CHP heat recovery).
Europe is still the largest CHP market for Rolls-Royce, particularly in Italy, Spain and Holland, but we are actively growing into other European countries, as we do in the rest of the world, offering complete pre-engineered, prefabricated power solutions consisting of generator sets, auxiliary modules and building structures.
‘In terms of the global power market, in our power range sector (2-10MW), we estimate the addressable worldwide for all power plant except low-end standby units – is around 1000 units, averaging 3MW/unit. The reciprocating engines market is growing quite fast about 5- 6% annually.
Of this sector, around 50% is liquid fuel (diesel, residuals and biofuels), the rest being gas. For CHP plant only, the addressable worldwide market is, we think, around 500 units at least. We see the CHP sector as growing, and there are increasing numbers of enquiries, concluded Thorbjørnsen.