HomeSmart Grid T&DEnergy EfficiencyHow gas turbines and engines fit into the new energy mix

How gas turbines and engines fit into the new energy mix

Germany wants to increase overall energy efficiency by 27 per cent and decrease CO2 emissions by 40 per cent by 2020. To hit these targets, the country is putting its faith in power from renewables, backed up and complemented by the most efficient use of fossil fuels

Since 2000, Germany has sought a low-carbon energy environment. The country’s renewable energy contribution has doubled since 2010 and now accounts for nearly 33 per cent of electrical power generation.

In the same time frame, however, CO2 emissions have only decreased by 4 per cent in comparison to 1990 levels. The federal government is responding with new concepts, with one key strategy being an exit from coal power, which still accounts for 40 per cent of the German power mix.Older plants with lower efficiencies are to be taken off the grid and replaced by gas-fired combined heat and power (CHP) technologies that are low in CO2 emissions. Plans call for the share of CHP in German electrical power generation to increase to 18 per cent by 2020.

An MDT type 20V 35/44G spark-ignited gas engine
An MDT type 20V 35/44G spark-ignited gas engine under test at Augsburg.

Looking at available fuels and methods of using them to achieve the stated aims, both gas turbines and large piston engines burning natural gas and other gaseous fuels at high efficiency represent a major opportunity. With these prime movers, it is possible to increase overall energy efficiency, reduce emissions of both greenhouse gases and raw noxious emissions, and provide flexible backup for renewable energy sources that depend on the weather.

Moreover, as well as their high simple-cycle efficiency, both gas turbines and gas engines have established records of achieving extremely high levels of energy efficiency in utility, industrial and municipal CHP plants.

On the municipal side, in Germany – as in many other European countries – the potential for low-emission, high-efficiency CHP is increased by the by the reliance of thousands of households on district heating systems using fired boilers. Finally, looking at security of supply, forecasts predict the long-term availability of natural gas and the increasing availability and use of biogases and non-natural gases, including feeding these gases into the supply grid.

The MAN 20V 35/44 gas engine
The MAN 20V 35/44 gas engine

Engine or turbine?

Already addressing the market for CHP plants based on gas turbines and gas engines is MAN Diesel & Turbo, based in Augsburg, Germany. MAN Diesel produces both spark-ignited gas engines and dual-fuel engines at Augsburg and at Saint Nazaire in France, as well as gas turbines at its plant in Sterkrade, near Oberhausen, Germany.

The gas engines cover an electrical power generation range from 450 kW to 87 MW, while the gas turbines cover 6.6 to 12 MW. The gas engines can deliver thermal energy roughly equivalent to their electrical output and the gas turbines around 75-94 per cent.

“Both our gas turbines and gas engines can deliver electrical and thermal energy considerably more efficiently than coal,” says Tilman Tàƒ¼tken, vice-president of MAN Diesel & Turbo and European sales manager for the power plant division.

“In fact, we can reach overall efficiency factors of up to 95 per cent.” He adds that because “a high proportion of power generation now depends on meteorological conditions, the flexibility of power generation we can achieve with both prime movers is a major benefit”.

“Both gas turbines and gas engines can be started at short lead times, and this type of flexibility has gained importance against the background of electricity markets.

“Gas turbines are well established for ‘peak shaving’ applications and our gas engines can reach their full performance within only minutes, allowing an operator to take advantage in times when electricity prices are high.”

Flexibility is key when looking at the configuration of CHP plants with gas turbines and gas engines. Tàƒ¼tken notes that “MAN’s CHP power plants are built based on a modular system that is scalable from seven megawatts to any desired size. This modular method of configuration allows for the load-based addition or removal of individual power units and the high degree of standardization intrinsic to the concept enables short planning and delivery times.”

Operating contexts

CHP plants based on high- and medium-speed gas engines are a well-established technology in municipal markets in Europe, while CHP facilities powered by gas turbines tend to predominate in industrial applications. “The correct choice of prime mover depends on the purpose of the plant,” Tàƒ¼tken states.

“For example, in the case where process steam needs to be generated, turbines are almost always the better solution since the exhaust heat has a higher initial temperature. In contrast, gas engine plants are usually advantageous as a district heating solution or for municipal applications like providing process heat for sewage works and warm water for spas.”

Operating on intrinsically clean-burning natural gas, both gas turbines and gas engines benefit from that fuel’s extremely low levels of sulphur to achieve extremely low emissions of oxides of sulphur (SOx).

In terms of oxides of nitrogen, (NOx), modern gas turbines with pre-mixed combustors and spark-ignited gas engines and dual-fuel engines employing lean burn combustion technology also produce very low levels of raw NOx. In terms of greenhouse gases, also, the high efficiency of both prime mover types combines with the low ratio of carbon to hydrogen of methane, the main constituent of natural gas, to give low emissions of CO2 per MWh.

With regard to efficiency and power density, gas turbines can deliver very large outputs from modules with very compact external dimensions, while gas engines currently have the edge in single-cycle fuel efficiency – i.e., before waste heat recovery is taken into account. However, depending on the application, both gas turbines and gas engines can achieve very high energy utilization levels in the CHP applications to which they are best suited.

CHP in practice

Early in 2016, MAN Diesel supplied a CHP plant with four gas turbines of the MGT series for the SAIC Volkswagen Automotive Company (SVW) in Shanghai. Each of the four turbines delivers electrical energy and process steam from the waste heat boilers they feed, allowing the plant to reach an overall efficiency factor of over 80 per cent.

“The inauguration of this cogeneration system is a milestone for our automobile production in China,” explains Prof Jochem Heizmann, president of Volkswagen Group China.

“The gas turbine CHP plant delivers steam and electricity for our Car Plant 3 here at SAIC Volkswagen and replaces all coal-fired plants. That means an annual reduction in CO2 emissions of 59,000 metric tonnes.”

Typical of what is happening in Germany, comparable CO2 savings and considerable further ecological benefits are also set to be achieved by energy utility EnBW with its new CHP plant at Stuttgart-Gaisburg, where a gas engine CHP plant from MAN Diesel will be used to modernize a coal and gas-/oil-fired plant.

“We are investing in climate protection and security of supply,” said Diana van den Bergh, project manager at EnBW. “Heat and electricity for the Stuttgart area have been generated in Gaisburg for over 65 years and the new gas engine power plant in Gaisburg will contribute significantly to improving the air quality in Stuttgart.

“It will reduce both noxious and greenhouse gas emissions from the site, including as much as 60,000 tonnes of CO2 per year. With an increasing share of renewable energies in electricity generation, flexible gas engine power plants with CHP are the perfect solution to achieve security of supply and efficiency with low emissions.”

Employing the modular concept, MAN Diesel is contracted to supply three type 20V 35/44G spark-ignited gas engine generator sets and two waste heat economizer boilers per engine to turn the thermal output of the engine into usable energy.

“In fact, in a building erected by EnBW, MAN Diesel will supply not only engines but also complete plant technology on a turnkey basis, from the air intake to the exhaust stack,” explains Martin Domagk, project manager at MAN Diesel.

The three 20-cylinder, vee configuration MAN type 20V 35/44G spark-ignited gas engines operate on the Otto cycle and employ lean burn combustion technology. Each engine is rated 10.4 MW at 750 rpm, giving a total output of just over 31 MW and roughly equal thermal output of around 30 MW.

Their specific task is as the first source of thermal energy from the plant when input is required by the Middle Neckar district heating network. Further thermal capacity of up to 210 MW is provided by the fired boilers. To maximize the energy utilization benefits when heat is the priority, MAN Diesel has designed the three 20V 35/44 G engines with two distinct operating modes which optimize either electrical output or total energy utilization.

Emissions

Minimizing emissions from the Gaisburg plant is a central target for operator EnBW. Accordingly, the contract with MAN Diesel specifies NOx emissions of 100 mg/Nm3 at 5 per cent O2, which is only 50 per cent of Germany’s current 13. BImSchV clean air code for large scale firing plants.

Also significantly below the 13.BImSchV prescriptions are carbon monoxide (CO) emissions of less than 100 mg/Nm3 and emissions of formaldehyde of less than 20 mg/Nm3.

To achieve the very low NOx emissions specified in the contract, the low levels of raw NOx produced by lean burn combustion are complemented by an SCR system, while CO levels are dealt with by an oxidation catalyst.

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