Ionised spray can boost output

UK company Power Generation Technology Ltd, which is based in Billingham in the north east of England, has developed a new variant on the inlet cooling theme for which startling claims have been made. It is apparently capable of improving turbine power output by 25%, fuel consumption by up to 15% and emissions performance by 40%.

Designated 'PowerPlus' it is a retro-fit overspray system that injects minute jets of water into the turbine by means of a high-pressure ionised spray, reducing the temperature of air into the suction side of the turbine by evaporative cooling.

Although the PowerPlus process is similar to other compressor inlet cooling systems, it has a unique feature that has been shown to give markedly improved results. The key to it is deceptively simple. Ionising the water spray has the effect of separating the single droplets of water for the entire time it takes them to travel through the gas turbine compressor. This keeps their size down to below 3 µ, which eliminates the threat of blade damage usually encountered with such systems, and allows complete evaporation of the droplets in the short dwell time available, which maximises the cooling effect.

PowerPlus is the brainchild of Power Generation Technology Ltd's chief executive Tony Archer. Currently, five global organisations have signalled their intentions to install this new patented technology, and a number of private investors have decided to back it, including a particularly prominent private investor, Sheikh Alli Said Al Harthy, a member of the Omani royal family.

The financial incentive for power stations to improve performance by this kind of margin is great. By way of example, a UK power station operating eight 200 MW gas turbines fitted with such a unit could save up to £150 million a year in fuel as well as generating extra revenue, up to £80 million, through increased MWh production.

PGT has agreed production and maintenance agreements with a number of key UK companies including NEL and specialist machine manufacturers Fairless. NEL's personnel will deliver power station survey, inspection and servicing provisions. Initially, around 50 locally based engineering specialists, manufacturers and equipment suppliers will be involved in the manufacture and maintenance of the units.

PGT expects to create 15 new jobs at once and is investing in a new head office building in Billingham. The company has also established offices and workshops in Oman and China, mainly because while the system is probably better suited to reducing fuel consumption in more temperate regions, it is expected to show great potential for increased power output in hot climates. Numerous plant owners in the Middle East and China are reported to have expressed great interest in installing the package.

How it works

Power augmentation by air cooling is of course well known and has been proven in use for over 50 years, mainly through inlet fogging devices, water evaporation and refrigerative chilling. Implementing it was based on the theoretical and observed relationship between ambient temperature and output shaft power.

Cooling with water also has the effect of introducing more mass, and therefore kinetic energy, to the gas being expanded through the turbine blades.

These systems are influenced by two factors, ambient air temperature and relative humidity. The systems operate up to, or very close to, 100 % saturation point, ie the wet bulb temperature reaches the dry bulb temperature. PowerPlus is an overspray system that injects the same quantity of water at the bell mouth regardless of the RH.

PGT's technological challenge has been to cool the air drawn into the turbine without causing damage. Water injection has a dramatic effects but due to the large size of the water droplets causes erosion of the turbine blades. In addition water separation causes corrosion. It is widely accepted that a water droplet of 10 µ or above causes erosion. A reasonable target would be 5µ. PGT's target was to get below three microns.

A further problem is that in water droplet systems the droplets impact with each other, collide and grow in size – in other words, they coalesce. Over a three metre distance a droplet will encounter seven collisions and grow in size from a 5 µ starting point to above 24 µ. This process is encountered throughout the range of existing fogging systems and hot and cold evaporation cooling systems.

Aerosol

What Power Technology is calling the PowerPlus effect is relatively simple; water is suitably agitated, rupturing the water film and creating billions of water nuclei all of 3 µ and below. These nuclei are less than one 1000th of the mass of a water droplet, which means that they are so small they behave like an aerosol. They are injected into the turbine intake, receiving the ionising charge, which means the droplets do not collide, maintaining a 3 µ maximum size droplet that evaporates in milliseconds.

This process reduces the air temperature in the compressor stage of the turbine, but with no adverse affects. The compressor discharge temperature is reduced, as is the work required of the compressor stage. As a result the turbine inlet temperature drops. By the addition of more fuel the inlet temperature is brought back to its original level.

Therefore, more power available in the compressor allows more air – more mass – to be pumped to the combustor per unit of gas. As a result, the compressor discharge pressure increases and extra power becomes available.

Emissions benefit

This technology also produces a NOx reduction effect, and from a similar cause. Combustion air at increased humidity owing to the evaporating water, combined with the lower compressor discharge temperature, reduces the flame temperature in the SAFT (synthetic aperture focussing technique) combustor.

Approximately one third of NOx reduction is caused by the lower compressor discharge temperature and two thirds by the higher water content in the combustion air.

Reduced compression loss

Compressor injection reduces the parasitic compressor work lost to the continuous inter-cooling effect of evaporating droplets, because cooler air requires less energy of compression.

This gain becomes available as net power to the shaft. Lower compression loss and higher turbine shaft power leads to higher rpm of the gas turbine for the same gas consumption.

Test results

To date eight gas turbines have been modified with the equipment in a series of field trials on a selection of turbines up to 150 MWe in capacity, with what PGT reports as “very positive” results leading to favourable initial interest from several potential users. The information gained from these early tests has been used to develop the first production skids and the process design documents necessary for commercial installations. PGT has assembled a group of companies to take early trialled units into full production.

At present these trials are bound by client confidentiality and are still subject to process development. However, PGT says that the test results are based on good test practices and procedures.

The tests confirmed that, if a gas saving option is chosen, a still greater gas burn reduction of up to 25 % can be experienced, which, even before the 'P+' effect benefits are added, gives an emission reduction of up to 25 % simply because less gas is being combusted. This figure is increased by adding the up to 40 % NOx reductions described above. Tests also corroborated projections of an up to 25 % power increase along with a potential 40 % extension of gas turbine life. Achievement of these benefits leads to a very short payback time for the package.

PGT is prepared to guarantee these improvements based on an agreed set of criteria that are certified by a series of pre- and post-installation tests on each gas turbine modified.