Canadian – based energy transportation and distribution company Enbridge Inc. has teamed up with fuel cell developer and manufacturer FuelCell Energy to create a system that combines their expertise in gas pipeline operations and fuel cell electric power generation , respectively. The result is the hybrid, multi-megawatt Direct FuelCell-Energy Recovery Generation (DFC-ERG) system for producing electric power at pipeline letdown stations.

Long – distance pipelines transport natural gas at very high pressures of up to 69 bar, which are needed to maintain high flow volumes. However, for safety reasons and to accommodate end-use equipment, the pressure must typically be reduced to 3to10 bar at local utility letdown stations before distribution to homes and businesses.

Simple,DFCERG_system

This schematic shows the layout of the DFCERG System, the result of collaboration between Enbridge Inc. and FuelCell Energy that is designed to produce electric power at pressure letdown stations along gas pipelines.

When the pressure is reduced , the gas naturally cools due to the refrigerant effect of expanding gas . The gas must be heated to keep the natural gas systems and pipelines from creating frost so municipal roads , which the pipelines run underneath , are not damaged. The energy produced during expansion is traditionally lost, and fuel must be used in gas-fired boilers to heat the as. Additionally,combustion in the boilers can produce polluting emissions, as well as carbon dioxide.

The DFC-ERG system combines a Fuel-Cell Energy Direct FuelCell (DFC) power plant with an unfired gas expansion turbine (turboexpander). The DFC-ERG generates ultra-clean electricity while recovering energy normally lost during natural gas pipeline distribution.The DFC-ERG produces electricity that can be supplied to the electric grid. Besides generating electricity with 60 to 65% efficiency, the DFC – ERG is quiet and produces negligible emissions of pollutants like nitrogen oxides (NOx)and sulfur oxides (SOx), and reduced CO2 greenhouse gas emissions.

The high electric efficiency numbers result from effective utilization of the waste pressure, which is combined with the fuel cell heat to generate additional electric power. The DFC is sized to produce the exact amount of zero-emission heat required for the specific letdown station’s needs, resulting in increased electrical production and efficiency.

turboexpander

The impeller from a turboexpander used in the DFC-ERG System.

The initial DFC-ERG system is being installed by Enbridge Gas Distribution at a gas pressure letdown station in Toronto, Ontario, Canada. There, a 1.2 MW DFC1500 fuel cell power plant is being used in combination with a 1 MW Cryostar ERG turboexpander to produce 2.2 MW of power. The Southern Connecticut Gas Co. plans to be the host site for a second DFC-ERG system in Milford, Connecticut, U.S.A. At that location, a 1.8 MW Cryostar ERG turboexpander will be combined with three 2.4 MW FuelCell Energy DFC3000 fuel cell units to produce 9 MW of ultra-clean power for delivery to the grid.

Energy normally lost when natural gas expands is harnessed by the turbo expander to drive an electric generator, and the DFC fuel cells — operating on pipeline gas — produce additional electric power. Internal reformation within the DFCs converts natural gas into hydrogen needed to operate the molten carbonate fuel cells (MCFCs).The MCFCs convert the hydrogen into dc electrical power and heat energy without combustion.

Electricity is produced in the fuel cell by an electrochemical reaction,and a power conditioner converts the d.c. electricity to a.c. power and combines it with the a.c. output from the turboexpander generator before delivering it to the electric grid. Waste heat from the fuel cell provides the heat required for gas expansion, replacing fossil fuels normally used in gas-fired boilers. The DFC-ERG power plants can be located almost anywhere a gas utility operates large pressure-reducing stations,including unmanned, remote locations.

The DFC-ERG system retains much of the simplicity of FCE’s fuel cell technology. The integrated reformer means separate fuel processing equipment is not needed. Being indirectly heated with fuel cell waste heat, normally at about 343°C,results in low NOx generation and yields the highest efficiency, since all primary fuel consumption is done in the fuel cell,the most efficient part of the system. The hybrid system has lower emissions per kilowatt-hour compared to the already best-in-class simple-cycle fuel cell plants,due to the higher hybrid efficiency.

Enbridge has identified 40to60 MW of opportunities for the DFC-ERG system in just the areas it operates. The remaining North American market represents another 200 to 300 MW, consisting of a half-dozen U.S. states currently seeking to add fuel cells’ environmental attributes to their Renewable Portfolio Standards (RPS).Another big advantage is that gas pipeline letdown stations are typically located in urban areas, close to the demand for new, ultra-clean electrical supplies, thereby allowing the DFC-ERG to provide this electricity where it will be of the highest benefit.
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