Most refineries have some form of onsite power generation. In fact, refineries offer an excellent opportunity for energy efficient power generation in the form of combined heat and power production (CHP).
Combined Heat and Power System (CHP) in Plants and Refineries
CHP provides the opportunity to use internally generated fuels for power production, allowing greater independence of grip operation and even export to the grid. This increases reliability of supply as well as the cost-effectiveness. The cost benefits of power export to the grid will depend on the regulation in the state where the refinery is located. Not all states allow wheeling of power (i.e., sales of power directly to another customer using the grid for transport) while the regulation may also differ with respect to the tariff structure for power sales to the grid operator.
Worldwide now significant portion of the steam used in gas plants and oil refineries is generated in cogeneration units. The petroleum refining industry is among the industries with the largest potential for increased application of CHP. In fact, an efficient refinery can be a net exporter of electricity. The potential for exporting electricity is even enlarged with new innovative technologies currently used commercially at selected petroleum refineries (discussed below). The potential for conventional cogeneration installations is estimated at an additional 6,700 MWe, of which most in medium to large-scale gas turbine based installations.
Where process heat, steam, or cooling and electricity are used, cogeneration plants are significantly more efficient than standard power plants because they take advantage of what are losses in conventional power plants by utilizing waste heat. In addition, transportation losses are minimized when CHP systems are located at or near the refinery.
Third parties have developed CHP for use by refineries. In this scenario, the third party company owns and operates the system for the refinery, which avoids the capital expenditures associated with CHP projects, but gains (part of) the benefits of a more energy efficient system of heat and electricity supply. In fact, about 60% of the cogeneration facilities operated within the refinery industry are operated by third party companies. For example, in 2001 BP’s Whiting refinery, Indiana (U.S.) installed a new 525 MW cogeneration unit with a total investment of $250 million, carried by Primary Energy, Inc. Many new cogeneration projects can be financed in this way (third party concept). Other opportunities consist of joint-ventures between the refinery and an energy generation or operator to construct a cogeneration facility.
Optimization of the operation strategy of CHP units and boilers is an area in which additional savings can be achieved. The development of a dispatch optimization program to meet steam and electricity demand demonstrates the potential energy and cost-savings.
For systems requiring cooling, absorption cooling can be combined with CHP to use waste heat to produce cooling power. In refineries, refrigeration and cooling consumes about 5-6% of all electricity. Cogeneration in combination with absorption cooling has been demonstrated for building sites and sites with refrigeration leads. The authors do not know of applications in the petroleum refinery industry.
Innovative gas turbine technologies can make CHP more attractive for sites with large variations in heat demand. Steam injected gas turbines (STIG or Cheng cycle) can absorb excess steam, e.g., due to seasonal reduced heating needs, to boost power production by injecting the steam in the turbine. The size of typical STIGs starts around 5 MWe, and is currently scaled up to sizes of 125 MW. STIGs have been installed at over 50 sites worldwide, and are found in various industries and applications, especially in Japan and Europe, as well as in the United States. Energy savings and payback period will depend on the local circumstances (e.g., energy patterns, power sales, conditions).
Some oil companies now considered the use of a STIG to upgrade an existing cogeneration system.
Steam turbines are often used as part of the CHP system in a refinery or as stand-alone systems for power generation. The efficiency of the steam turbine is determined by the inlet steam pressure and temperature as well as the outlet pressure. Each turbine is designed for a certain steam inlet pressure and temperature, and operators should make sure that the steam inlet temperature and pressure are optimal. An -7.8°C decrease in steam inlet temperature will reduce the efficiency of the steam turbine by 1.1%.
Similarly, maintaining exhaust vacuum of a condensing turbine or the outlet pressure of a backpressure turbine too high will result in efficiency losses.
One of the oldest cogeneration applications in a US refinery constructed a 34 MW cogeneration unit in 1990, using two gas turbines and two heat recovery steam generators (boilers). The system supplies all electricity for the refinery and occasionally allows export to the grid.
Even for small refineries, CHP is an attractive option. Recent energy audits identified CHP unit installation as the largest energy saving measure in small refinery too. A 6.5 MWe gas turbine CHP unit would result in annual energy savings of $3.8 million and has a payback period of about 2.5. In addition, the CHP unit would reduce the risk of power outages for the refinery.
Steam Expansion Turbines
Steam is generated at high pressures, but often the pressure is reduced to allow the steam to be used by different processes. For example, steam is generated at 120 to 150 psig. This steam then flows through the distribution system within the plant. The pressure is reduced to as low as 10-15 psig for use in different process. Once the heat has been extracted, the condensate is often returned to the steam generating plant. Typically, the pressure reduction is accomplished through a pressure reduction valve (PRV). These valves do not recover the energy embodied in the pressure drop. This energy could be recovered by using a micro scale backpressure steam turbine. Several manufactures produce these turbine sets, such as Turbo steam.
The potential for application will depend on the particular application and steam system used. Applications of this technology have been commercially demonstrated for campus facilities, pulp and paper, food, and lumber industries and so on.