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CHP system powers Electrabel Gas Distribution Center. (Heat & Power System).

In a novel installation of a combined heat and power (CHP) system, the Electrabel Gas Distribution Center in Brussels employs natural gas reciprocating generator sets from Cummins Power Generation to generate electricity and thermal energy for it gas distribution system.

The system generates power that is directly supplied to the national power grid, while waste heat from the engines is used to pre-heat high-pressure pipeline natural gas prior to pressure reduction and distribution. Electrabel is Belgium's main electric generating company and that nation's largest distributor of natural gas and electricity.

The Electrabel facility is designed to reduce natural gas pressure from as much as 14 bar to about 1.7 bar in preparation for delivery into the final gas distribution network. This network supplies approximately 300,000 domestic and industrial natural gas customers in north Brussels and seven surrounding municipalities in the Flanders region.

The CHP system is powered by two Cummins Power Generation lean-burn gas engine generating sets that produce a total of 2.7 MW for the electric grid and 3.5 MW of thermal energy. Most of that thermal energy is used to pre-heat pipeline gas entering a pressure reducing turboexpander.

In traditional gas transmission and distribution systems, the reduction or regulation of pressure in gas transmission and distribution systems is usually controlled with a special valve. However, the energy lost in this process can be a substantial proportion of all the energy put into the original compression of the gas system.

An alternative approach uses a turboexpander in place of valve to provide controlled reduction of gas pressure, and at the same time produce useful work by turning a turbine. The output from the turbine can be used to generate electricity and thus contribute to overall energy efficiency and reduce impact on the environment.

Since the use of a turboexpander is relatively new in a natural gas distribution network, it may be helpful to understand how it works. As the gas passes through the turboexpander, its pressure is reduced and energy is extracted by the turbine blades to turn a separate 2.6 MW alternator. However, as the gas is allowed to expand across the turbine, its temperature drops, causing any moisture in it to freeze.

To prevent this from happening, the incoming gas is heated with 2.7 MW of waste heat from the Cummins lean-burn gas engine generator sets. Most of that heat is produced by the exhaust system, and the remainder is recovered from the engine cooling water jacket and the oil sump.

The Cummins lean-burn engine design is unique in that it generates a much higher exhaust temperature than other reciprocating gas engines while also being highly fuel efficient and clean burning. This makes the lean-burn engine generators particularly useful in CHP installations where high quality heat is required in addition to electricity. Excess waste heat from the engines not used for heating the pipeline is used for building space heating during the winter. During the summer, this excess heat is dispersed using roof-mounted radiators.

The combined output of the CHP plant's lean-burn gas engine generator sets and turboexpander-driven alternator is 5.3 MW of electric power that is fed directly into the grid. The electrical system is controlled with the Cummins PowerCommand Digital Master Controller, which facilitates synchronization of the generators with the grid, and allows remote monitoring and control of the generator sets.

"We chose the cogeneration option for pre-heating the gas rather than boiling because it maximizes the plant's electrical power output. It is the most rational use of energy," said Pierre Gonze, Electrabel's Exploitation and Gas Distribution Manager. "Continuous operation at a nominal gas flow rate of 75,000 cubic meters per hour makes the system much simpler because, we don't need to modulate the engines. This has optimized plant efficiency at about 86%, reduced our initial investment, and has made the system more reliable."

Production of electricity in the gas distribution network is a relatively new concept for Belgium. Electrabel has some smaller cogeneration installations with gas engines, but the Brussels installation is the first public gas distribution network to feature the more efficient gas turboexpander for gas expansion and power generation.

In addition to being efficient, the system is clean and quiet. The Cummins lean-burn gas engine generators only produce about 0.85 grams per brake horsepower hour of NOx, and comply with all emissions regulations. The noise level at the property boundary is just 45 dB(a).

Eurodiesel, the Belgian distributor for Cummins Power Generation, was the main contractor for the CHP project.

What Is "Lean-Burn" Gas Engine Technology?

Engine combustion is considered "lean" when an excessive amount of air is introduced along with the fuel. The additional air introduced into the combustion chamber along with the gas produces two positive effects. First, the excess air reduces the temperature of the combustion process, and this reduces the amount of NOx produced by nearly half. Second, since there is also excess oxygen available, combustion is more efficient and more power is produced from the same amount of fuel.

This lean-burn technology is featured in the gas engine generator sets from Cummins Power Generation used in the Electrabel CHP installation. Compact in size for their power output, the lean-burn gas engine generator sets have proven to be fuel efficient and clean burning in numerous installations worldwide.

The Combustion Process

Any air/fuel reaction requires an energy source to initiate combustion. In natural gas engines, the spark plug performs this function. Using a pre-chamber principle, the combustion process is enhanced by pre-mixing the air and fuel upstream of the turbocharger before introduction into the cylinder. This creates a more concentrated mixture in the combustion chamber and reduces the occurrence of "knocking" or detonation.

To prevent either knocking or misfiring, the combustion process must be controlled within a narrow operating window. Charge air temperatures and volume, together with air to fuel ratio and compression ratio, are constantly monitored. The microprocessor-based engine controller regulates the fuel flow and air/gas mixture and ignition timing.

The lean-burn engines from Cummins are designed to operate at an air/gas ratio of Lambda = 1.7. On the chart that plots Break Mean Effective Pressure (BMEP) against Air Excess (Lambda), the operating window is a very narrow band where efficiency peaks and where NOx is near its minimum. A richer mixture would produce knocking and higher. NOx production; a leaner mixture may not combust reliably and cause misfiring. Full-authority electronic engines, sensors and microprocessors are critical for maintaining combustion within these boundaries.
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Title Annotation:combined heat and power
Publication:Pipeline & Gas Journal
Geographic Code:4EUBL
Date:Oct 1, 2002
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