natural gas Pipelines
Natural gas compressor stations using compressors driven by gas turbines or internal combustion engines offer strong opportunities for waste heat recovery.
Transporting natural gas from producers to consumers requires an extensive and elaborate distribution system, which consists of a complex network of pipelines. Compression stations, usually placed at 40 to 100 mile intervals along the pipeline, are required to ensure proper pressurization of natural gas. The natural gas enters the compressor station, where it is compressed by a turbine, or engine. Compressor stations move on average about 700 million cubic feet (MMcf) of natural gas per day, with the largest moving upwards of 4.6 billion cubic feet (Bcf) per day.
Although natural gas compressor stations vary widely in size and layout, the basic compressor systems are comprised of two components – the “mechanical drive” that provides the shaft power that drives the compressor, and the “compressor” itself. The mechanical drive can be an internal combustion (IC) engine, gas turbine, or electric motor. The compressor itself can be a reciprocating, centrifugal, or screw compressor. IC engine and gas turbine drives burn natural gas from the pipeline. Electric motor drives can be used on any type of compressor and require reliable electrical power supply.
Facts and figures
According to U.S. DOE Natural Gas pipeline data, the U.S. features:
- More than 210 natural gas pipeline systems.
- 305,000 miles of interstate and intrastate transmission pipelines
- More than 1,400 compressor stations that maintain pressure on the natural gas pipeline network and assure continuous forward movement of supplies (see map).
- More than 11,000 delivery points, 5,000 receipt points, and 1,400 interconnection points that provide for the transfer of natural gas throughout the United States.
- 24 hubs or market centers that provide additional interconnections.
- 400 underground natural gas storage facilities.
- 49 locations where natural gas can be imported/exported via pipelines.
- 8 LNG (liquefied natural gas) import facilities and 100 LNG peaking facilities
Opportunity for waste heat recovery:
Compressor stations that use gas-fired mechanical drives are potentially very good candidates for waste heat recovery. Natural gas-fueled engines and turbines (mechanical drives) generate heat as a byproduct. Only about one third of the fuel energy consumed by an engine or turbine ends up as useful mechanical power, with the remaining two-thirds rejected as hot exhaust or in engine cooling systems.
Most of the waste heat associated with gas turbines is in the turbine exhaust. This coupled with the high exhaust temperature (850 to 1100°F) makes them particularly attractive for waste heat recovery. Waste heat recovery from IC engine exhaust (which ranges from 500 to 1200°F) is also practical, although the exhaust gases typically contain only about 40% of the waste heat. Most of the heat from IC engines is removed in the cooling water jacket, producing a low-quality heat source at about 140°F.
In either case, the excess heat produced by the compressor drive can be turned into renewable energy using the Rankine cycle, producing no excess pollution and requiring no additional fuel. More traditional waste heat recovery systems or CHP systems may also be practical if the site has hot water, steam or other thermal loads.
Some of the main issues affecting the financial viability of potential projects are these:
- Distance from the electric grid. Since natural gas compression stations that use gs-fired drives usually do not have a need for additional electric power onsite, the electric power must be exported to the grid. And, since natural gas pipelines often run through rural and unpopulated areas, the cost of extending the electric grid to the site can at times be prohibitive. A close grid connection means better economics.
- Capacity of the compressor station. Larger compressor stations are easier to get to “pencil.”
- Duty cycle of the compression station. Duty cycles can vary seasonally and annually. The economics are best at stations that run at a fairly constant rate throughout the year. Turbines running at least 5,250 hours per year over the previous 12 months (an annual load factor at or above 60%) are best.
- Availability of incentives. Some of the states that have a renewable portfolio standard include waste heat recovery as an eligible source, and as such, projects in these states may qualify for various financial incentives.
Case study:
- Dominion Transmission Compressor Stations - Dominion Transmission, which operates 7,800 miles of natural gas pipelines in the Eastern United States, completely disconnected its Ardell transmission station from the local utility in 2001. It found a more efficient and reliable power source in three 60kW CHP Capstone Micro Turbines® that provided electricity to the entire station while saving more than $1 million and reducing its emission of greenhouse gases and conventional pollutants.
Additional resources:
- Waste Energy Recovery Opportunities for Interstate Natural Gas Pipelines - ICF for Interstate Natural Gas Association of America (2009)
- Status of Waste Heat to Power Projects on Natural Gas Pipelines - ICF for Interstate Natural Gas Association of America (2009)
- The Transportation of Natural Gas - NaturalGas.org
- Natural Gas Compressor Stations on the Interstate Pipeline Network: Developments Since 1996 - EIA (Nov. 2007)
- Major Natural Gas Transportation Corridors - U.S. EIA