Granite Ridge Natural Gas Power Plant, 8 Nov 2011

Ten of our Energy Safai class members travelled to the EcoPark south of the Manchester NH airport. There we visited the Granite Ridge Energy site of a natural gas combined cycle electric power generation plant, owned by NAES, a private company that owns 120 US power plants, including 61 other natural gas plants.

The power plant has two gas turbines that spin electric generators for power. The hot exhaust gasses are then used to power a steam turbine, hence the name combined cycle. The whole plant has a rated generation capacity of 660 MW in summer, and 798 MW in winter, when the ambient air is colder. Each of the Siemens 501G CT gas turbines is rated at 230 MW and the steam turbine is similar. This is the second largest power generation plant in New England, after the 1242 MW Seabrook nuclear power station. The heat rate is about 7000 BTU/kWh, corresponding to a thermal/electric power conversion efficiency of 49%. The best achievable efficiency, with the latest GE turbines, is 61%. Less expensive peaker turbines, without the steam generator, require about 10,000 BTU/kWh, or a 34% efficiency.


Natural gas flows from the 20 inch Tennessee Gas pipeline; this Granite Ridge plant uses about 15% of the pipeline capacity. Natural gas is not stored in tanks; up to two hours of fuel can be stored by increasing the pressure in the pipeline.  Arriving at the plant the gas is filtered and pressure raised to a standard 520 psi by this large pump if the pipeline pressure is too low.

Bill Vogel, the plant manager, graciously and patiently explained the workings of the power plant, starting with a conference room presentation and then a visit to the control room.

This is what the plant looked like -- pipes and pipes.

We expected to see something like a jet engine, but the Siemens industrial gas turbine is contained within a large box for soundproofing and heat insulation. The compressor stage increases the air pressure to about 350 psi at 750 F. In the subsequent combustion chamber, natural gas fuel is injected and burned, raising the temperature to about 2200 F. The hot gasses exit through the turbine, rotating it as the gas expands and cools to about 1100 F. From there the gas goes to the heat recovery steam generator (HSRG) a modern version of a boiler, to make steam for the steam turbine, which also spins a generator.

The turbines are taken apart for maintenance after each 8000 hours of service. There are boxes of spare parts in the plant. The single turbine blade in Bill Vogel's hand costs about $30,000. 

Here's a massive spare part -- the motor for a cooling water pump.

Starting up a gas turbine takes about 20 minutes before producing power. The steam generator takes longer. Normally the two gas turbines and the steam turbine are operated together. ISO-NE is Independent System Operator -- New England; it is "the grid". ISO-NE instructs Granite Ridge to increase or decrease power according to demand. The plant can increase or decrease power gradually at a rate of 5 MW per minute.

Each turbine gulps a lot of air. Here is the intake filter for one.

To limit emissions of nitrogen oxides formed in the high temperature combustion chamber of the gas turbine, ammonia is injected into the HSRG. The remaining gasses going up the stacks are mainly air, water, and CO2.

Air flowing through a rain of water in the cooling towers evaporates some water, cooling the flow over the (unseen) piping that cools the steam from the steam turbine, so the resulting water can be pumped back into the boiler and reused.

The ultimate product is electricity, flowing to National Grid's power lines. This plant built in 2003 was neat and clean throughout. Building a new one today would cost about $700 million, or $1/watt. We all enjoyed this last tour of our Energy Safai course.

Northeast Utilities Schiller Coal and Wood Power Plant, 1 Nov 2011 trip report

NextEra Energy Seabrook Nuclear Power Station, 25 Oct 2011 trip report

TransCanada Commerford Hydro Project, 18 Oct 2011 trip report

Life Sciences Center at Dartmouth, 11 Oct 2011 trip report

Springfield Power LLC, Springfield NH 4 Oct 2011 trip report

Iberdrola Wind Farm, 27 Sept 2011 trip report.

AllEarth Renewables Solar Farm, 20 Sept 2011 trip report.