COGENERATION IMPROVES THERMAL EOR EFFICIENCY

Edmond R. Western
Oryx Energy Co.
Fellows, Calif.

Douglas W. Nass
Chas. T. Main Inc.
Pasadena, Calif.

The successful completion and operation of the Midway Sunset Cogeneration Co.'s (MSCC) cogeneration plant is a prime example of the multi faceted use of cogeneration.

Through high-efficiency operation, significant energy is saved by combining the two processes of steam and electrical production.

The 225-megawatt (mw) cogeneration plant provides 1.215 million lb/hr of steam for thermally enhanced oil recovery (TEOR) at the Midway-Sunset oil field in south central California.

Overall pollutant emissions as well as total electric and steam production costs have been reduced. The area's biological resources also have been protected. Therefore, cogeneration-powered TEOR is an excellent arrangement in which the users, operators, and environment can all benefit.

GOALS

Major goals established during the planning phase included:

• Improving the overall efficiency of Oryx Energy's TEOR operations by reducing steam cost and increasing system reliability

• Increasing thermal efficiency of the cogeneration operation as compared with producing each energy source separately

• Meeting new California clean-air requirements by reducing NOx, CO, SO2, and hydrocarbon emissions by replacing the 52 existing oil-fired steam generators and oil/water treating units with the more efficient cogeneration units.

MSCC, a partnership of Mission Energy Co. and Oryx Energy Co. (the steam host) owns and operates the new facility.

Prior to the start-up of the new 225-mw cogeneration plant in May 1989, Oryx Energy's thermally enhanced oil recovery operations in western Kern County near Fellows, Calif., consisted of 28 oil-fired steam generators and 24 oil fired, water/oil treating units.

These units burned about one eighth (3,500 b/d) of the extracted oil.

The process consisted of steam generators that produced 535 F. steam for injection into the oil wells to reduce oil viscosity.

To make the oil marketable and reduce hydrocarbons in the wastewater stream to less than 2 ppm, the oil/water emulsion was broken by reheating the produced fluid in the treating units to 200 F.

COGENERATION PLANT

The new cogeneration plant (Fig. 1) involves three 75-mw (nominal) gas-turbine generators (GTGs), each exhausting individually into a heat-recovery steam generator (HRSG) that produces about 405,000 lb/hr of 80% quality injection steam.

Compared with previous methods of steam generation, operation of this cogeneration plant will save the equivalent of 25 million bbl of oil over the next 20 years.

MSCC has contracted with Southern California Edison to provide 200 mw at slightly less than avoided cost and with Pacific Gas & Electric to supply 25 mw.

Fig. 2 details the major equipment in each GTG/HRSG unit.

The GTGs (Fig. 3) are axial-flow machines with 17 stage compressors, 3-stage turbines, and 10-can, annular, low emission, dual-fuel (natural gas or low-sulfur, distillate oil) "quiet" combustors.

The reason they are called quiet combustors is because lower dynamic pressure levels are obtained using multi element nozzles. Thereby more demineralized water can be injected to reduce NOx concentration levels to 25 ppm (by volume, dry, on natural gas at 15% O2) without reaching unacceptable noise levels.

The 25 ppm is a 40% improvement over the standard combustor, which is rated at 42 ppm. NOx emissions are controlled through an NOx feedback loop that controls water injection to the gas turbine combustors to maintain acceptable emission levels. The water used for NOx control is demineralized water feed from the plant's potable water system.

An independent emissions monitoring system samples NOx, CO, CO2, SO2, and O2 for each of the three gas turbines every 15 min. The sample data are combined with inlet-air mass flow and fuel gas flow in a microprocessor for instantaneous display to control room operators.

GTG design and operation output is shown comparatively in Table 1 for operation at 1,820 ft elevation, 65 F. average ambient temperature, and 50% relative humidity.

The once-through HRSGs (Fig. 4) each incorporates nine parallel passes to produce the 80% quality steam required. The primary source of feedwater for the HRSGs is recovered from the water which is produced along with oil from the oil wells.

This water carries many minerals and has a total dissolved solids content of 3,000 ppm. The generation of 80% quality steam, however, prevents solids deposition on the HRSGs' tube walls. This high-moisture content steam transports the solids through the system in a liquid state.

Steam quality is monitored by gamma-radiation, steam quality controllers at the discharge of each HRSG which measure the attenuation. The steam output from the three HRSGs (1,055 psig, 552 F.) is manifolded into a single main line which delivers the 80% quality steam to the oil field.

During start-ups, a dedicated steam line diverts steam to a steam separator to prevent slugging until 80% quality is achieved. HRSG design and field performance is shown comparatively in Table 2.

Since water conservation is essential in this relatively dry, semidesert area, water from the oil recovery process is treated to reduce calcium and magnesium hardness and then recycled as boiler feedwater. Currently, recycled water constitutes over 90% of the boiler feedwater used.

PLANT OPERATION

Summer operation, from early June to early October, is divided into three periods: on-peak, mid-peak, and off peak. Winter operation excludes on-peak operation and adds a super off-peak period.

Since start-up, overall availability of this cogeneration facility has exceeded 97.7%, and peak periods of availability exceeding 99% have been achieved. Availability is defined as the time the units are on-line and are available to deliver power to the grid.

Tables 1 and 2 compare design specifications to corresponding field-operation parameters. Note that on a recent test with one of the units, average electric generation has exceeded design guarantees by 3.4%, and HRSG steam output has also exceeded design guarantee.

The net result is that the energy required to sequentially produce the steam and electrical power in the plant is approximately one-third less than if the same power were produced in two separate plants.

ENVIRONMENTAL CONCERNS

Placing the 52 old oil-fired, steam-generation and treatment units in standby service and operating a new gas fired facility has resulted in significant overall air pollution reduction in the area. As an example, NOx levels permitted by the Kern County Air Pollution Control District and the U.S. Environmental Protection Agency (EPA) have been reduced from 330 to 255 lb/hr (for all three turbines on gas fire).

Combined with other pollutant reductions, an overall pollutant reduction of 12 million lb will be achieved by cogeneration over the next 20 years compared to precogeneration levels (as of 1985).

Field tests proved that the turbines operate well within permitted emission limits which are as shown in Table 3.

BIOLOGICAL ASSESSMENT

Before this cogeneration facility was built, a biological assessment was conducted in cooperation with the California Energy Commission, the California Department of Fish and Game, and the U.S. Fish and Wildlife Service to evaluate potential adverse effects on endangered wildlife species and wildlife habitat.

Included in the assessment were the local populations of the giant kangaroo rat, the blunt-nosed leopard lizard, the San Joaquin antelope ground squirrel, and the San Joaquin kit fox. All are considered endangered or threatened, and some exist nowhere else.

The assessment became the basis for a plan to protect endangered species by mitigating potentially significant impacts.

Some of the actions taken included:

• Relocating the plant site on the 80-acre tract to reduce disturbance to kit fox dens.

• Using gas-insulated switchgear instead of an air switchyard to reduce land use, thereby minimizing loss of habitat.

• Erecting exclusion zones around sensitive areas (dens and burrows) to prevent construction-related impacts.

• Establishing an education program for workers and plant visitors to alert them to the need for local environmental protection.

• Revegetating all land disturbed by construction after plant completion to restore foraging habitat.

As a result of these efforts, no endangered species were harmed during plant construction.

Biological monitoring performed each quarter verifies that endangered species continue to thrive in the vicinity, side by side with advanced technology.

Copyright 1990 Oil & Gas Journal. All Rights Reserved.