|Advances in gas engine technology have been essential to cogeneration’s progress|
Cogeneration with new gas engine technology
As a widening variety of cogeneration applications continues to move into the mainstream, the time is right for large power users and producers to explore the economic possibilities of cogeneration with today’s gas engine technology, argues Michael Devine.
Cogeneration today goes well beyond simultaneously generating electricity and hot water or steam. Today’s usable engine outputs can also include heated air, chilled water produced by absorption chillers, and carbon dioxide from purified exhaust. In other words, a single engine-generator can produce two, three or four useful outputs at once. With today’s generating technologies, electrical efficiencies up to 45% and total resource efficiencies upwards of 90% are achievable. And cogeneration systems do not necessarily need to operate full-time at full load to be cost-effective – low-cost and low-intensity configurations can bring attractive returns in many settings.
Modern lean-burn gas-fueled reciprocating engines are rich sources of heat. A great deal of heat otherwise wasted can be extracted for productive uses, depending on the user’s heat requirements (see Table 1).
Engine exhaust provides by far the highest temperatures and the greatest heat output. Typical exhaust temperature is about 460°C. Exhaust heat can generate intermediate-pressure steam for boiler feedwater heating, and low-pressure steam for sterilisation, pasteurisation, space heating, tank heating and humidification. Supplemental firing with natural gas can increase exhaust temperatures and heat output to produce steam at greater volumes and pressures, creating even more possibilities.
Heat also can be extracted from the engine jacket water, oil cooler and aftercooler to produce warm or hot water for space heating and a broad variety of industrial processes.
Deploying heat energy
Hot water and steam are the classic engine outputs in cogeneration systems, but they are not the only ones. They can be converted to other forms to suit additional purposes:
Heated Air: Steam or hot water can be passed through heat exchangers to create hot air to feed equipment such as kilns and dryers. The heated air is mixed with fresh outside air to enlarge the volume and enable precise temperature control.
Chilled Water: Steam, hot water or exhaust can be passed through absorption chillers to produce cold water for space or process cooling. Absorption chillers use heat instead of electricity as the energy source. Their efficiency is measured by coefficient of performance (COP).
Simple, relatively low-cost single-effect absorption chillers are activated at temperatures as low as 93°C; COP typically ranges from 0.7–0.9. More complex double-effect units, activated at 175°C, deliver higher COP (1.05–1.4), although at greater up-front cost.
Heat recovery systems can be configured to deploy some heat for water and steam production and the balance to absorption chillers (trigeneration). Alternatively, systems can produce space heat in winter and air conditioning in summer.
In São Paulo, Brazil, energy services company Ecogen operates an energy plant serving the 8000 m2 Rochavera commercial office complex and several office buildings. The energy plant uses a total of four Cat G3520C gas generators sets with jacket water and exhaust gas heat exchangers that capture the engine’s thermal energy and transfer it to a common water circuit that is fed to four 540-tonne-rated (TR) hot water absorption chillers. These operate in parallel with two 340-tonne electric chillers, three 450-tonne electric chillers, and a 320-tonne natural gas-fired chiller to maintain the facility’s cooling needs. All electrical power from the generator sets is then fed through paralleling switchgear and distributed to the campus.
Ecogen also employs two Cat G3512B diesel generator sets, each standby rated at 1500 ekW to provide emergency or peaking power to the facility.
More Electricity: Where a site requires continuous prime power and has little or no heat load, engine exhaust heat can be used to boost electrical output through the organic Rankine cycle. Here the exhaust, typically from multiple engines, feeds a boiler that converts a working fluid to vapour, which in turn passes through a turbine. This configuration, similar to combined-cycle electric power plants, can boost generating capacity by roughly 10% and improve electrical efficiency by five to six percentage points.
Shanxi Jincheng Anthracite Coal Mine Mining Company operates one set of facilities in Jincheng, Shanxi, China. The company collects coal gas from underground coal seams to power four separate power generation plants, each housing fifteen Cat G3520C high voltage generator sets that are integrated via paralleling switchgear and controls. Each powerhouse is designed with a combined-cycle system that recovers waste exhaust heat to power a 3000 kW steam turbine. The result of this heat recovery scheme is an additional 12 MW of electrical power to the local grid, which is equivalent to 10% of the power plants’ 120 MW total electrical output.
|Industrial heat pumps installed alongside a Cat G3520C gas generator set|
More Heat: Heat pump technology can extract useful heat from lower-quality heat sources: the engine aftercooler circuit, residual heat from the exhaust downstream from the exhaust heat recovery boiler, and even radiant heat from the engine block. This heat can be used to preheat the engine jacket water heat recovery circuit, or for low-temperature space or process heating. Such heat pump installations actually can raise overall system thermal efficiency to slightly greater than 100% (based on the fuel low heating value).
Using exhaust CO2
Beyond heat recovery, carbon dioxide is a usable byproduct of power generation. Engine exhaust rich in CO2 can be cleaned in a catalyst reductor, cooled and fed to a process. In greenhouses, for example, CO2 improves yields by 10–20%. Exhaust can also provide a low-cost source of CO2 for industrial applications or even for carbonation in soft drink bottling. A single generator set can deliver electricity, space or process heating, space or process cooling, and CO2 – a concept known as quad-generation.
At the Eric van den Eynde Greenhouse in Kontich, Belgium, 95% of the electricity generated is sold to the local utility based on the premium paid for high-efficiency power. The jacket water, exhaust, first stage aftercooler, and oil cooler heat from two Cat G3516A (1070 ekW) generator sets and one Cat G3520E (2070 ekW) gas generator set are captured and stored in the form of hot water, which is used to stabilise the greenhouse temperatures. Water with a temperature of up to 95°C is stored in a 1200 m3 ank which acts as a thermal battery. The tank provides hot water at 45°C via metallic tubes in the greenhouse. The temperature is maintained between 19°C and 21°C throughout the year.
The exhaust gases from the generator sets are scrubbed of nitrogen oxides (NOx), carbon monoxide (CO) and unburned hydrocarbons (CnHm). Selective catalytic reduction (SCR) and oxidation catalyst systems convert these gases into pure CO2 before they are released to the atmosphere. However, 75% of the emitted CO2 is reintroduced as an organic fertiliser into the root structures of the growing vegetables via a network of tubes. The CO2 stream helps boost crop weight up to 20%. The simple financial payback of the complete system was around 3.5 years.
Cogeneration is not limited to highly engineered systems that maximise production of both electricity and heat. Almost any application that entails roughly 1000 or more annual operating hours offers potential for economical heat recovery. The only firm requirement is that the value of heat recovered outweighs the added cost of the heat recovery and control.
Heat recovery from the engine cooling circuit is simple. A shell-and-tube or plate-and-frame heat exchanger can produce water at 82–99°C, depending on the engine jacket water temperature. This water can serve purposes that include space or domestic water heating, light production process heating, and boiler condensate or make-up water preheating, as well as air conditioning, process cooling and desiccant dehumidification.
In each case, the recaptured heat displaces some costs for fuel or utility electricity. To the extent that the recovered heat supports energy needs during times of peak electric load, total demand – and thus demand charges – are also reduced. Examples of low-intensity, limited-duty cogeneration include:
Commercial Real Estate: Office buildings can cost-effectively operate generator sets during business hours, avoiding utilities’ highest time-of-use rates. If heat recovery from a jacket-water heat exchanger can partially offset the cost of fuel for space heating, water heating or dehumidification, return on investment improves.
Light Industry: A small or mid-sized manufacturer with an on-site genset could install a heat exchanger in the engine cooling system loop, with a thermostatically controlled diverter valve to regulate the flow to the in-plant load, cost-effectively satisfying a variable hot-water requirement.
Hospitality: Hotels can readily use heat recovery for domestic hot water, laundries, kitchens or swimming pool heaters. In summer, the recovered heat can power absorption chillers or desiccant dehumidifiers for air conditioning.
Food Processing: Food producers can recover exhaust and jacket water heat to create low-pressure steam for light process loads such as cooking or raising dough, or to produce hot water for cleaning and sanitising. Depending on the size and character of the heat load, such systems can be cost-effective in single- or multiple-shift service, even if heat demand is cyclical or seasonal.
Gas engines and CHP
Advances in gas engine technology have been essential to cogeneration’s progress. By nature, gas engines serve well as isolated or grid-connected on-site power sources. They offer high power density, low first cost per kW, and quick, simple installation. Fuel and operating costs are competitive, emissions are clean, and service is readily available worldwide.
The basic technology is highly reliable – uptime can approach 98%. Engines readily handle full or part loads, tolerate varied altitudes and ambient conditions, and can be brought online quickly. The latest engines develop high power output in footprints up to 50% smaller than traditional units, fitting easily in small engine rooms or containerised power plants.
Gas engines are well suited to operate in cogeneration service on refined natural gas and propane, as well as on fuels of variable heating value and purity. Landfill gas, agricultural biogas and wastewater treatment plant digester gas may produce exhaust containing corrosive compounds, requiring stainless steel surfaces in exhaust gas heat exchangers, but no special modifications are required for heat recovery from liquid cooling circuits. Other gases like coke gas and coal mine methane are also viable cogeneration fuels.
The latest engines use a variety of digital microprocessor-based monitoring and control technologies that include:
- Air/fuel ratio control based on charge air density, maintaining NOx emissions within the tightest available tolerance under all ambient and load conditions, irrespective of changes in air temperature and humidity;
- Air fuel ratio control based on Total Electronic Management (TEM) systems that optimise generator set performance, measuring each cylinder’s temperature and adjusting combustion to minimise fuel consumption and engine emissions and prevent detonation. Systems can control functions beyond the engine itself, such as radiator motors, electrical breakers and systems, and even a complete plant;
- Detonation sensing by individual cylinder with automated control to retard timing if detonation occurs.
The engines also have intake systems that enable efficient airflow and minimise heating of charge air, increasing the air/fuel charge to the cylinders for optimum performance. An open-chamber cylinder design and low-pressure fuel system (0.5–5 psi) eliminate the need for a fuel compression skid.
In recent years, an alternative approach to off-the-shelf engines has emerged in the form of individual generator sets customised to fit the application. Rather than purchase an engine and accessory package, users provide a sample of the fuel to be used, describe the ambient conditions and altitude, and specify the application and key operating objectives (for example, top fuel economy, lowest emissions or block loading capability). The manufacturer then custom designs a gas engine-generator system to fit those criteria.
The breadth of customisation is considerable. Advances include:
Optimised air and exhaust flow: Both the inlet and the exhaust system are tuned to enable highly efficient, laminar flow. A technology called Pulsed Energy Advanced Recovery Line (PEARL) uses flow-optimised exhaust pipes that convey a constant exhaust mass flow to the turbochargers. Exhaust flow is timed to keep the turbocharger spinning at the optimum speed over the engine’s entire operating load range. Precise ignition timing, automatically adjusted by cylinder for fuel quality changes, augments this process.
Miller cycle: This adjustment in the combustion cycle increases fuel efficiency by about 1%. The Miller cycle differs from the more traditional Otto cycle in that the intake valves close not when the piston reaches bottom dead center but at typically 10°–15° before bottom dead center (in the early inlet close version). As the piston continues down with the intake valves closed, the air/fuel mixture expands and cools, increasing the detonation margin. This enables a higher compression ratio of 14:1 or 15:1, versus 11:1 or 12:1 for the Otto cycle – translating to higher fuel efficiency.
High-Energy Ignition: Prechamber spark plug technology has been improved with optimised plug geometry. These spark plugs admit air and fuel through small orifices and, upon ignition, eject flame through those same orifices. Coupled with high-energy ignition, this enables the engine to operate on an extremely lean fuel mixture without risking lean misfire, sustaining high power output and low emissions over extremely long lifespans.
An economic decision
Every cogeneration project comes down to a question of economics. In general, the outlook is most favourable where:
- The utility electricity cost is relatively high;
- The fuel price is relatively low;
- The system will operate with a high electrical and heat load factor;
- Electric and thermal loads coincide during a typical day;
- The site requires high reliability and power quality;
- The cogeneration system can double as a standby power source.
A low-cost ‘opportunity fuel’, such as anaerobic digester or landfill gas, generally improves the economics. Digester gas-fueled cogeneration in particular is a key contributor in a growing quest for energy self-sufficient wastewater treatment plants.
In exploring project design alternatives, engine fuel efficiency is just one of many considerations. For example, capacity factor – the percentage of total theoretical output the generators actually achieve – may far outweigh fuel savings. Furthermore, if high efficiency comes at the cost of increased downtime from more frequent maintenance or engine sensitivity to fuel variability or quality, or if performance is degraded at higher ambient temperatures, then project economics are compromised. Other engine capabilities like low emissions or fast response to block loads may also be more important than fuel economy in some settings.
The time is right
Seldom, if ever, have market conditions been more favourable for on-site gas-fueled power cogeneration. The time is right for industrial and commercial facilities, institutions, utilities and other large power users and producers to explore the economic possibilities of cogeneration with today’s gas engine technology.
Michael Devine is gas product/marketing manager for Caterpillar’s Electric Power Division (www.cat.com)
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