Rentech HRSGs selected for Ford’s new CHP plant

A core element of the project is reliable power, and that element has now been completed. DTE Energy’s Dearborn Central Energy Plant (CEP) is a 100 000 square foot facility that was built over a two-year period and required 509 475 total trade hours. It provides electricity, chilled water, hot water, and steam to serve buildings at the Dearborn Research and Engineering Campus (REC).

The CEP replaced aging on-site boilers in the now retired Elm Street steam plant. It comprises several elements: a combined heat and power (CHP) plant; solar power arrays; a 16 000 ton chiller system; a 156 MMBtu/h hot water supply system; a 640 ton geothermal system; 40 000 ton-hours of chilled water storage; and an energy distribution system.

“This CHP facility increases the efficiency of the grid, lowers emissions, and improves the cost equation for both Ford and DTE Energy Services,” said Michael Larson, director business development, DTE Energy Services.

The CHP plant can generate 34 MW and produce up to 225 kilopounds per hour of 200 psig saturated steam. It consists of two 14.5 MW Solar Titan 130 gas turbines, a 5 MW condensing steam turbine from Siemens Energy, and two Rentech heat recovery steam generators (370 000 lb/h).

The facility offers a higher capacity factor and efficiency, as well as enough steam load for Ford, and optimised sizing to provide electricity for the grid. It consumes ten times less natural gas than a traditional boiler plant.

DTE considered HRSGs from Deltak, Cleaver- Brooks, and Victory before naming Rentech as the supplier.

The Dearborn HRSGs are high-fired waterwall, O-type HRSGs that include supplemental firing and fresh air firing capability, along with diverter and space for future SCR catalyst if ever required.

By raising the gas turbine exhaust temperature via supplemental firing, steam production is increased by approximately 300%, from an expected unfired rate of 63 000 pounds per hour (pph) to 185 000 pph per unit.

“Rentech HRSGs were selected over other OEMs based on lifecycle costs including unfired and fired steam output, capital cost, and O&M cost,” said Larson.

Fresh air firing means that a forced draft fan enables the HRSG to be operated even when the gas turbine is offline. This is accomplished via a fan which draws in ambient air. A duct burner provides the heat input needed to create steam. During periods of scheduled gas turbine maintenance, therefore, Ford can still generate 80 000 pph of steam per HRSG.

The firing temperature of the furnace is about 2000°F. To cope with the heat, a membrane wall composed of tubes connects to the steam and mud (lower) drum. In this way, naturally circulating water keeps the unit cool. The steam drum is situated directly over the mud drum and shares the same centreline.

A diverter, a large louvre-type valve, is installed between the HRSG and the turbine to facilitate steam/electricity switching. If there is no current steam demand, flue gas from the turbine can be diverted up a bypass stack and not through the HRSG to generate only electricity and not steam. Steam and hot water from the CHP plant is used mainly for heating and general building comfort. A small portion of the steam and hot water is used for processes within the Ford campus.

Power sharing

DTE Electric picked up the tab for CHP construction and operation. But it gains by being able to sell power to external customers. Ford negotiated attractive contracts to meet its steam and power needs. In the event of a grid event, it can fall back on this on-site generation asset.

“100% of the net power from the CHP facility is available to DTE Electric customers minus the auxiliary power consumed within the CHP facility of around 500 kW,” said Larson.

Author details: Archie Robb (a writer based in southern California specialising in business, energy, and technology)