Electricity can be produced at natural gas utility letdown stations, offering an opportunity to generate electric power without combustion. Natural gas pipelines transport gas at pressures in the range of 700 to 1,000 psi. Utility letdown stations reduce the pressure to 160 to 180 psi for further distribution to consumers. There are literally thousands of such so-called “gate stations” in the United States.
 A hybrid fuel cell-based power plant could help natural gas pipeline owners solve a common distribution problem. |
At a typical letdown station, a pressure regulator adjusts the high pressure gas to the desired lower pressure level. In the process, energy is lost; energy that, if captured, could be used for other purposes. Reducing the gas pressure with a throttling valve also results in severe cooling of the gas due to expansion. Unless the gas temperature is raised prior to reducing the pressure, the cold gas can create frost, heave roads and result in other operational concerns. The traditional solution is to pre-heat the high pressure gas with a combustion boiler and heat exchanger installed in the line upstream of the valve. This consumes gas to fire the boiler and releases pollutants to the atmosphere during the burning process.
Pipeline companies have looked at using turbines and reciprocating engines in place of the throttling valve, with mixed results. Electricity can be generated, but the percentage of captured energy is small. In the case of reciprocating engines, some degree of pollution still results. With turbines, pipeline operators have found that to maximize the electricity generated, the boilers need to be fired to a higher temperature to compensate for higher cooling in the turbine. This consumes still more gas.
Using a hybrid power plant consisting of a Direct FuelCell power plant and turboexpander-generator, titled the Direct FuelCell-Energy Recovery Generation (DFC-ERG) system, the high pressure gas is used to generate power, while at the same time reducing downstream pressure delivery via the network. Electricity is generated (essentially as a byproduct) using the energy that otherwise would be lost in reducing the line pressure with a throttling valve.
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The system design and integration is the result of a joint development effort by Enbridge Inc. of Alberta, Canada and FuelCell Energy Inc., based in Danbury, Conn. Enbridge is a gas distributor that also operates and maintains liquid and gas pipelines. FuelCell Energy develops stationary fuel cells for commercial and industrial applications.
The turboexpander works on a principle similar to that of a steam turbine. Rather than expanding steam for power generation, however, the ERG extracts inherent energy from the high pressure gas supply. In expanding through the turbine, the recovered pressure energy, combined with the flow through the turbine, drives the expansion turbine. The expansion turbine, in turn, powers the induction generator. As the gas rotates the turbine, the pressure is reduced and the exhaust gas flows into the low pressure line for downstream distribution.
Fuel cells are an integral part of the process. The fuel cell stack consists of a stationary molten carbonate Direct FuelCell (DFC). A small portion of the natural gas that passes through the expansion turbine is internally reformed within the fuel cell to produce hydrogen. Electricity results from the electrochemical process and the best-in-class efficiencies from the DFC-ERG reduces greenhouse gases. The entire power plant operates without combustion, so emissions of pollutants such as NOX and SOX are negligible. In addition, waste heat from the fuel cell is used to offset boiler fuel, so the entire combination can achieve system efficiencies of up to 80 percent.
The DFC power plant serves two purposes. First, the fuel cells produce DC power, which is then converted into AC by an inverter housed in the power conditioner. Second, waste heat from the fuel cells is supplied to the heat exchanger for pre-heating the high pressure gas passing through the pressure regulator valve, as well as the turboexpander.
The first DFC-ERG installation is at an Enbridge facility in Toronto, Ontario, and is scheduled to be operational by the end of 2007. Though modest in output - a 1.0 MW Cryostar turboexpander is being combined with a 1.2 MW Direct FuelCell - the installation serves as a field trial for further advancing the DFC-ERG concept.
A larger project, known as the DFC-ERG Milford Project, is currently underway. It is a combined fuel cell/turboexpander power plant planned for installation at a utility letdown station owned by Southern Connecticut Gas Co. The plant, when completed and operational, will generate 9 MW of power supplied to the grid through a 15 kV interconnection with United Illuminating Co. The DFC-ERG achieves electrical generation efficiencies of over 60 percent, a result that is virtually unmatched by any other combination of a turboexpander and combustion-based prime mover. This hybrid fuel cell is unique in that the entire power plant operates without a combustion process.
Distributed Generation Provides Reactive Power
By Drew Robb
Distributed generation offers the opportunity to deliver the power where it is needed without adding to the transmission network load. When coupled with cogeneration, it also boosts energy efficiency and cuts emissions.
 Because inadequate reactive power can result, voltage collapse reactive power sources must be placed near power loads. |
Cogeneration plants have a much higher efficiency than building separate plants to provide steam and electricity, said one manager of system capacity. Cogeneration has an efficiency of around 80 percent, whereas the best combined cycle electricity generation plants operate at about 50 percent efficiency.
Distributed generation also offers the potential of stabilizing voltage by providing the necessary reactive power near loads. But doing this effectively requires that the equipment be set up properly. “Entities interested in providing reactive power need to make sure that their equipment is properly functioning so that it can pass ISO/RTO (independent system operator/regional transmission organization) tests that verify their generators can deliver the amount of reactive power they first declared to the ISO/RTO,” said Ndeye K. Fall, and electrical engineer at Energetics Inc. of Columbia, Md.
The Need for Reactive Power
In the late 1800s, Thomas Edison and George Westinghouse faced off over the design of electrical transmission systems. Edison preferred DC transmission, which he considered safer. Westinghouse won the contest, however, because of AC’s ability to transmit power over longer distances. It is now common for power plants to be located hundreds of miles from the power loads. For example, much of Europe draws power from France’s nuclear reactors; Australia and South Africa rely on plants located near their coal mines, rather than their near cities; and Mexico and Arizona ship power to Los Angeles.
While AC can travel long distances, the reactive power required for voltage support does not. Inadequate reactive power can result in the type of voltage collapse that blacked out the Northeastern United States in August 2003. While not as dramatic, a reactive power shortage also limits how much power can be transmitted over a transmission line. This means reactive power sources must be placed near the power loads.
“Reactive power is very much a local issue and there is a real shortage of it in some areas,” says John D. Kueck, an engineer at the U.S. Department of Energy’s Oak Ridge National Laboratory in Oak Ridge, Tenn. “Unfortunately it does not travel well, so it is nearly impossible to transfer it from one location to another.”
Rather than install capacitor banks and other devices to correct power factor, a simpler approach is to use the rotating equipment that is already installed on the grid at or near the load. This can be done either using variable speed motors or by converting a generator into a synchronous condenser.
“It is important to note that capacitors are not always a good answer and they can lead to an excess amount of reactive capacity at night when loads are lower,” said Brian Marchionini, Program Manager for Energetics. “Distributed reactive power can fill a void by providing access to positive and negative reactive power as the need arises.”
Cogen Sources
While the current potential installed capacity for distributed reactive power in the United States is estimated at over 10,000 MVAR, energy and environmental policies may significantly boost that level through encouraging the building of more cogeneration facilities. The U.S. Environmental Protection Agency’s combined heat and power (CHP) partnership has targeted three industries for increased use of cogeneration: hotels and casinos, wastewater treatment plants and dry mill ethanol plants. The furthest along of these is the ethanol industry, which the CHP partnership started working on in 2003. Hotels, casinos and wastewater treatment facilities were added in 2005.
“Dry mill ethanol plants have large, constant and coincident electric and thermal loads, resulting in a strong technical fit for CHP to efficiently provide both steam and power for these facilities,” said Kim Crossman, Team Leader for the CHP partnership. “With CHP, ethanol production facilities can realize a reduction of approximately 15 percent in the energy intensity of ethanol production and these efficiency gains translate directly into greenhouse gas emissions reductions.”
According to Crossman, at least a dozen CHP ethanol plants currently are on line with another 14 planned or under construction. Given the exploding demand for ethanol not only in the UnitedStates but worldwide due to environmental and energy independence concerns, ethanol production may well continue its rapid rise. In the United States alone, according to the Renewable Fuels Association, ethanol production capacity hit 4.86 billion gallons in 2006, a nearly fourfold increase over 1999, with another 6 billion gallons of capacity under construction. Furthering this growth will be the development of plants that convert the cellulose from plants, not just the starch as is currently used. Not only will this allow non-food items to be used to produce ethanol, but the process has lignin as a residue, which can be burned to fire boilers. The federal government plans to boost the production of biofuels to 35 billion gallons over the next decade.
“Given the new construction activity in this sector, the timing is right to integrate CHP into new and expanding dry mill ethanol facilities and to ensure that CHP is part of the base design for the cellulosic biorefineries which will be constructed over the next 20 years,” says Crossman.
Generators to Synchronous Condensers
Distributed power sources provide a unique opportunity to provide dynamic reactive power as they are near the power load. According to the report “A Preliminary Analysis of the Economics of Using Distributed Energy as a Source of Reactive Power Supply” issued last year by they U.S. Department of Energy, “Distributed energy based reactive supply can provide dynamic support capabilities that static devices like capacitors cannot match. Furthermore, industry experts believe that supplying reactive power from synchronized distributed energy sources can be two to three times more effective than providing reactive support in bulk from longer distances at the transmission or generation levels.”
This can be accomplished in two ways. One is for the local generators to be excited as needed to ensure unity power factor to stabilize the grid at their point of connection whenever they are running and synchronized to the grid. But if they are not being used continuously, they can still be converted into synchronous condensers to provide needed reactive power. Even when the generator is part of a cogen plant (depending on market conditions and fuel costs) it may still be cheaper to buy electricity from a coal-fired plant than to run the gas turbine. For example, the city of Russell, Kan., installed a CHP facility at a local ethanol plant, but also sometimes buys from a coal plant in the region instead of running its own generator due to recent high natural gas prices.
However, these generators still may be used to generate reactive power. For example, the City of Perth in Western Australia is now getting most of its electricity from more than 100 miles away and has idled or decommissioned some of its metropolitan area generators. This has created a loss of reactive power. As Western Power described the situation in its 2006 Transmission and Distribution Annual Planning Report, “Most usually this displaced generation is located near to the Perth metropolitan area and due to its proximity to the load, it would normally provide dynamic reactive power support to stabilize the network during faults. When this generation is displaced, the reactive power support normally provided by it needs to be sourced from elsewhere.”
To address this issue, last year the utility installed clutches from SSS Clutch Co. of New Castle, Del. on three GE Frame 6 gas turbines at a peaking plant near the city. The clutches are designed so that when the turbine tries to rotate faster than the generator, the clutch automatically engages and the turbine drives the generator. Once the generator is up to speed and synchronized to the grid, the turbine shuts down and the clutch disconnects the turbine from the generator. The generator then continues spinning using power from the grid and providing dynamic reactive power. By not having also to keep the turbine spinning, this cuts down power and maintenance costs, as well as lowers emissions into the atmosphere.
This can also be done with non-utility distributed generators, according to the DOE report cited earlier: “Thus, there may be a real opportunity to increase their utilization and benefit the grid by enabling dual operation of the generator as a real and reactive power producing technology. A key design requirement for these units to double as sources of reactive power supply is the ability to operate at leading and lagging power factors, which is an off-design condition for most generators installed to provide only real power. Technology is available, however, allowing many types of generators to be converted into synchronous condensers, i.e., sources of reactive power using a clutch.”
Market Issues
While using distributed generators has great potential for stabilizing the grid, not all power markets include reactive power.
“There is a market for selling reactive power to the grid, for example in New York generators are paid for supplying reactive power as well as for having the capacity to supply it,” says Kueck. “In other areas, such as California and Texas, it is part of the rules that, if you connect to the grid, you have to be able to provide leading or lagging reactive power within set limits.”
Since the need for reactive power is highly localized, he recommends that markets pay for reactive power at a fixed rate, rather than on a real-time price basis.
Author:
Drew Robb is a freelance writer and frequent contributor to Power Engineering magazine
Mama Mia! Pasta Maker’s CHP System Serves Up Big Savings
Pastas Doria, a Colombian manufacturer of pasta products, was experiencing lost production time due to frequent utility voltage instability and power failures. The food processing giant also had high energy costs for electricity and fuel oil. To keep the plant’s production line up and running and to save money on energy, Pastas Doria installed a combined heat and power (CHP) system from Cummins Power Generation Inc. to generate more reliable electricity and produce heat for pasta drying.
Pastas Doria has been producing pasta for over 50 years and makes more than 50,000 metric tons annually - about 40 percent of all the pasta consumed by Colombians. By getting both electricity and heat from natural gas, the company benefits from savings on both forms of energy. As a result, Pastas Doria buys less fossil fuel and electricity than before it installed the CHP system, while it also addresses its voltage stability problems and reduces total emissions. The company estimates it has cut electricity purchases by 60 percent and fossil fuel purchases by 70 percent, resulting in a savings of approximately $50,000 a month on its energy bills.
The CHP system consists of a natural-gas-powered reciprocating engine generator set, an exhaust gas heat exchanger, switchgear and controls. The CHP system includes a lean-burn 1,750 kW natural gas generator set. The generator set operates 24 hours a day in parallel with the local utility to stabilize the voltage of the power coming into the facility and to replace a portion of the power the company buys. Should the utility fail, the CHP system would still operate, providing up to 1,750 kW of electricity to run plant operations.
Waste heat from the generator set’s exhaust provides 3.4 million Btu/hr of heat energy to the plant’s boilers, pasta-drying operations and space heating, which offsets fuel oil purchases. The 91-liter, 18-cylinder, spark-ignited lean-burn natural gas engine has proven to be ideal in CHP applications due to the high specific heat content of the engine’s exhaust. The heat exchanger can be designed to produce low-pressure steam, hot water or hot air as the application demands.
Since both the production facility and the CHP system run 24 hours a day, Pastas Doria has been able to schedule generator set maintenance at the same time the plant is shut down for routine maintenance. Once a month, the generator is taken offline for about four hours. During that time, plant personnel inspect the generator set and do routine cleaning and maintenance. After every 1,500 hours of operation, or two months, they adjust the valves, change the oil and replace the spark plugs, oil filters and air filters.
Materials Market to Top $3.8 Billion by 2011
U.S. demand for materials used to produce fuel cells and batteries will rise 4.4 percent a year to $3.8 billion in 2011, according to “Battery & Fuel Cell Materials,” a new study from The Freedonia Group, Inc., a Cleveland-based industry market research firm. Gains will be driven by increasing production of high-performance battery products, especially lithium and nickel metal hydride (Ni-MH) types, due to the enormous popularity of high-drain portable electronic devices.
Demand for fuel cell materials will rise nearly fivefold from a small base, the study forecast, as fuel cell products begin to see commercial success. Gains for battery and fuel cell materials will be limited by a less favorable outlook for more mature and outdated battery types.
Metals will continue to be the leading material type in batteries and fuel cells through 2011. Metal prices spiked between 2001 to 2006, spurring strong value gains for metals in batteries and fuel cells. Advances through 2011 will be restrained by an expected moderation in metal prices and sluggish demand in the sizeable lead metal market. More rapid growth will be seen for polymers and carbon/graphite materials.
Among functional categories for battery and fuel cell materials, the most rapid gains will be for performance additive and catalyst materials, which are expected to nearly double through 2011. Growth will be driven by the ongoing need to improve battery performance and durability, as well as by surging demand for fuel cells, which use expensive platinum catalysts. Active materials and electrodes are the leading outlet for materials, constituting over half of demand in 2006, but will experience belowaverage rises in demand.
The report said “stellar growth” is expected for fuel cell materials, which will rise from less than 2 percent of the market in 2006 to over 7 percent in 2011, as products such as proton-exchange membrane fuel cells and solid oxide fuel cells achieve more commercial acceptance.