Recovering Energy from Gas Distribution
Long-distance interstate natural gas pipelines typically operate at pressures between 700 and 1,000 psi to get the greatest efficiency and volume through a given pipe size. However, this pressure is not suitable for local distribution to individual homes and businesses. Gate stations take high-pressure gas from the transmission lines and reduce it to 160 to 180 psi for distribution, with additional reduction at latter stages and at individual consumer sites.
Traditionally, the way to reduce pressure in this application is with simple throttling valves. Since methane is a non-ideal gas, this reduction in pressure has a major cooling effect that can freeze the valve, distort piping and cause other cold-related problems. To solve this, gate stations typically have a boiler and heat exchanger to warm the gas just before the valve. Unfortunately, when this approach is used, much energy is wasted, not to mention consumed, in the form of gas required to fire the boiler.
With energy costs rising, pipeline companies have sought ways to harness that energy using either a turbine or reciprocating engine in place of a throttling valve. This generates electricity, although only a small fraction of what went into the gas at the compressor station, but any energy recovered is beneficial. However, gate stations that applied this approach in a way that captured the most energy possible found that the boilers had to be turned up to compensate for higher cooling in the turbine, so the free electricity gained was partially offset by increased gas use in the boiler. Nonetheless, the gain was normally positive, although the time to pay off such an investment was long enough that pipeline operators have not widely embraced the idea.
Adding a fuel cell
Fuel cells are known for their ability to convert hydrogen to electricity and heat very effectively, and when the two are combined, efficiency levels can reach 85%, placing the technology in the highest ranks of commercial generating methods. Moreover, fuel cells operate well on natural gas, creating more electricity per unit of fuel consumed and carbon emitted than any combustion technology. FuelCell Energy and gas pipeline operator Enbridge Inc., saw the potential to put these elements together to create a more efficient package to capture the maximum amount of energy available from the pipeline pressure reduction process.
The DFC-ERG (direct fuel cell-energy recovery generation) system uses a turbo expander operating in parallel with a fuel cell such that both generate electricity. Heat from the fuel cell is used in place of the traditional boiler to warm the gas prior to reaching the turbine. This technology package includes necessary control gear and power conditioning equipment to take ac from the generator and dc from the fuel cell and combine it into three-phase ac for distribution on the grid. FuelCell Energy and Enbridge own the technology in a partnership, and the latter supports marketing.
|The concept of using the turboexpander and fuel cell in tandem uses the unique advantages of both technologies to gain maximum efficiency|
In normal operation, the conventional pressure-reducing valves and boiler will not be used; however, they will be retained as backups for outages.
The first installation, at the Enbridge facility in Toronto, ON, is currently in the final stages of construction and should be operating before the end of 2007. This modest test plant will produce 2.2 MW using a combination of a turboexpander generating 1 MW combined with a fuel cell plant of 1.2 MW. The heat output of the fuel cell will be used in place of a separate boiler to pre-heat the gas before it reaches the turbine.
The next installation, which will scale up to commercial production levels, is slated for Milford, CT. It will use a larger fuel cell plant of 7.2. Given that gate stations have relatively low electrical consumption themselves, most of the energy generated at such facilities likely will be sold on the local grid.
The efficiency of such a plant is high by commercial generating standards. After all the tradeoffs of gas heating and pressure reduction are taken into account, efficiencies of 60 to 65% are expected based on electricity produced measured against the volume of gas consumed by the station. This is higher than any combustion technologies, which rarely exceed 45 to 50% at utility scale.
“The biggest challenge is engaging the pipeline companies and getting them interested,” says John Franceschina, vice president of business development for FuelCell Energy. “Since letdown stations are typically located near major metro areas where energy demand is high, supply is often limited, and emissions standards strict, the DFC-ERG is uniquely well-suited.”
A fuel cell’s efficiencies come with a price tag. While the situation is improving, the total cost per kilowatt hour for a molten carbonate fuel cell is still around $0.15 because of the high initial cost. While rising gas prices tilt in favor of high-efficiency technologies, fuel cells often need subsidies to get users past the initial purchase.
“In addition to the federal investment tax credit, energy technology subsidy programs are also available but vary by state,” says Andy Skok, executive director of strategic marketing for FuelCell Energy. “Natural gas distributors have to understand that we are taking a net negative where they are simply burning gas and creating a new level of delivery efficiency for the pipeline.”
|Peter Welander is process industries editor. Reach him at PWelander@cfemedia.com|
Connecticut mandates 100
In 2003, the state of Connecticut passed legislation that launched Project 100 to support renewable energy generation. Under the act, Connecticut’s two major utilities are required to enter into long-term power purchase agreements with developers to purchase not less than 100 MW of Class I renewable energy. The project was created because large renewable energy projects are difficult to finance without long-term contracts. In the implementation process, the Connecticut Clean Energy Fund (CCEF) is charged with issuing requests for proposals from developers and performing the initial screening and analysis to select projects that will benefit all Connecticut consumers. After the selection process, the best projects are forwarded to CL&P and UI for further review and purchase agreement negotiation. Those agreements approved by the Department of Public Utility Control also receive funding from CCEF.
Projects under the program include fuel cells, biomass, wind and other “green” generation technologies. Ultimately, the program hopes to provide the means to make alternative power sources commercially viable in the long term.