Heat capture on worksites can be used to generate electricity–and the added power can improve oil and gas revenues.
The development of new technologies has had an impressive impact on the oil and gas industry, especially in the past 30 years. Geoscience software, 3D seismic acquisition, shale fracking techniques, directional drilling, and reservoir modeling are just some of the innovations that have taken place. While the headlines have been made by breakthroughs in the realm of finding and producing oil and gas, other breakthroughs, in other realms, have been significant while not widely heralded.
Waste heat energy capture, while it has not won the same amount of acclaim as the hottest new happenings in the E&P arena, can significantly improve company revenue over time.
Waste heat energy is any heat energy produced as part of some other process that is vented or removed from the primary heat stream output. Thus, the designation of “waste heat” can be applied to any heat that is (A) stored in the walls of a vessel, (B) radiated through the walls to the environment, (C) purged as steam or a direct flame, or (D) released as part of an air or water cooling system. The Energy Information Administration (EIA) reported that in 2011, of the four energy use sectors (residential, commercial, industrial, transportation), the industrial sector accounts for more than 30 percent of all energy used. A U.S. DOE 2008 report on industrial waste heat recovery suggests that between 20 to 50 percent of energy consumed is ultimately lost as waste heat.
With so much energy being lost, the question arises: Can companies decrease this loss and reduce their operational costs in the process? Various companies—one of which is Gulf Coast Green Energy, known for its promotion of the ElectraTherm “Green Machine” (GM)—provide technology to capture this heat energy and convert it to electrical power for on-site usage. Three essential components are required for waste heat recovery: (A) an accessible source of waste heat, (B) a recovery technology, and (C) a planned use for the recovered energy.
Accessible Heat Stream
In the upstream to midstream oil and gas industry there exist at least four source areas where waste heat in low (≤ 450oF), medium (450 to 1,200oF), and high (≥ 1,200oF) temperature ranges can be successfully recovered. These include (A) coproduced hot liquid, (B) incineration and gas flares, (C) compressors (such as cooling compressed natural gas) and prime movers, and (D) amine gas treaters. “Coproduction” is a word that serves as a synonym for geothermal energy produced in the oil and gas industry. While most people think of geothermal power as power derived from near-surface hot rock features in areas such as Nevada and southern California, other sources exist. Specifically, there is a huge potential for geothermal energy production from deep wells drilled in sedimentary basins. The DOE, AAPG, and other organizations have conducted research and provided data showing where deep geothermal energy can be produced after oil and gas acquisition. However, another opportunity exists from produced hot water in various oil and gas operations. Companies such as Denbury Resources, Hilcorp Energy, and Continental Resources, Inc., have been or are presently involved with demonstration projects capturing waste heat from produced hot water at sites ranging from the Gulf Coast region north to the Williston Basin. Temperature for hot water production from these wells can range from 90 to 450oF, with temperatures generally above 200oF being appropriate for electrical power production in the 10’s of kW to the 1+ MW capacity.
Incineration and flaring of natural gas are often necessary when there is insufficient gas for market or if there is a lack of pipeline capacity for gas transport. This heat energy is presently lost to the atmosphere, with flare temperatures ranging from 1,000 to 2,000oF. Besides the heat energy loss, other drawbacks to flaring can include undesirable noise, smoke, heat radiation, and light, as well as pollutants that include SOx, NOx, and CO. Increased flaring worldwide has prompted several nations to place bans on natural gas flares, and some U.S. states are also restricting the durations permitted for flare activity. A mid-December 2012 workshop hosted by the Texas Railroad Commission looked at the potential for using excess natural gas that might otherwise be flared for the generation of electricity for on-site power generation. Depending on the size of the flare power, production in the 100’s of kW to several megawatts is possible.
A third area for waste energy capture is from heat generated by compressors and prime movers (engines or turbines). The cooling water from an internal combustion engine (ICE) can be in the range of 150 to 250oF while the heat exhaust from a gas turbine or reciprocal engine can be in the 600 to 1,100oF range. A natural gas compressor throws off heat from multiple sources as it operates. A CAT 3616 used at a three-stage compressor station can produce a derived output of as much as 270 kWe from the engine exhaust, engine jacket cooling water, multiple gas cooling stages, and reduced cooling load.
A fourth area of waste heat production is through amine gas treaters. Amine gas treatment removes H2S and CO2 from the natural gas and liquid hydrocarbons. During this process heat rejection occurs at multiple locations within the amine treatment process. Steam and acid gases separated from the rich amine are condensed and cooled, with heat rejected in the process. Later, hot, regenerated lean amine is cooled in a solvent aerial cooler which again releases heat. Temperatures in this process can reach 220oF, which is within the range for heat-to-power generation.
Heat Recovery Technology
There are numerous ways of capturing waste heat, but each method is dependent upon end-use planning. Methods for waste heat recovery include transferring heat between gases and/or liquids, transferring heat to the load entering a furnace, generating mechanical and/or electrical power, or using waste heat with a heat pump for heating or cooling facilities. Our focus here is on the generation of electrical power.
Electricity generation from waste heat involves capturing this heat to create mechanical motion that then drives an electric generator. These power cycles are well-developed, though research into more direct heat-to-electric production is underway but not yet economical for industrial applications.
Three primary methods presently exist for electricity generation. The traditional steam Rankin Cycle has worked well in the medium-to-high temperature range. Typical sources of this waste heat have been exhaust from gas turbines, reciprocating engines, incinerators, and furnaces. The low-to-medium temperature range has used the Organic Rankin Cycle (ORC) and the Kalina Cycle for electricity generation. Sources of waste heat have included gas turbine exhaust, boiler exhaust, cement kilns, and hot water. The ORC system has been available for 50 years and uses an environmentally friendly air conditioning fluid or other organic fluid as the working fluid to turn a generator. The Kalina Cycle uses a combination of water and ammonia as the working fluid. Both the ORC and Kalina approach have the intrinsic advantage that the working fluid is vaporized at lower temperatures than required in the standard steam cycle.
In an ORC cycle a heat stream (i.e. hot water) enters a heat exchanger (radiator) where the heat conducts from the primary heat stream to a secondary working fluid. The secondary fluid, vaporized and at high temperature and pressure, is directed into a turbine, twin screw expander (as with the GM), or other power block connected to a generator. Rotation of the turbine or expander turns the generator and creates electricity. The secondary fluid is then condensed to a liquid via an air or water cooling system. This liquid is then pumped to a higher pressure and returned to the heat exchanger to repeat the cycle. A preheater may sometimes be used to extract some of the heat remaining in the secondary stream prior to it entering the condenser in order to preheat the fluid between the pump and the exchanger.
End Use for Recovered Heat
Electricity production is the focus for waste heat recovery in upstream and midstream oil and gas operations. When the conversation turns to electricity, production terms such as kilowatts (kW) and megawatts (MW) are used to represent the power (energy/time) output of a generator. By contrast 1 bbl of oil or 1 mcf of gas has energy equivalents of 1,700 kWh and 303 kWh respectively. If you do the math one can show that a 10 MW power plant running over a 24-hour period produces 240 Mwh, which is equivalent to 141 bbl oil or 792 mcf of gas.
However, waste heat capture can be used in other processes, such as combustion air preheating, boiler feedwater preheating, load preheating, steam generation, space heating, water heating, and transfer to liquid or gaseous process streams.
Numerous advantages exist to waste heat capture. In the case of preheating combustion air or feedwater, the amount of energy required to heat the water to its final temperature is reduced. Heat captured for a drying oven can replace the burning of fossil energy that would otherwise have been used in the oven. Furnace efficiency can be improved by as much as 50 percent in combustion air preheating. Capacity requirements for a facility’s thermal conversion devices can be reduced, leading to reductions in capital costs. Waste heat energy recovery is also considered to be a greenhouse-gas-free source of energy, which can be a favorable impact for a company that is required to reduce these emissions.
“Green Energy” and Economics
Given the high incidence of wasted heat energy, Gulf Coast Green Energy (GCGE) dedicated itself to the procurement of equipment that would allow a company producing waste heat to capture that heat for electricity generation and use on site. GCGE works with manufacturers, such as ElectraTherm and its “Green Machine,” to produce electricity using a small skid-mounted unit that can be easily transported to the location where waste heat is available. The GM can produce up to 65 kW of power from temperatures ranging from 190 to 240oF, assuming the cooling fluid (air or water) temperature is sufficiently low. This ORC system uses R245fa as its working fluid and a robust twin screw expander as its power block, a technology well understood and used in industry since the late 19th century. For heat extracted from hot water, a flow of 60 to 200 gpm (2,100 to 6,900 bbl/d) is required, depending on the temperature of the hot water. Equivalent energy input ranges from 2,200,000 to 2,940,000 Btu/hr. The GM has been used to produce electricity from an oil well in Mississippi (Denbury Resources), at a compression station in South Texas (Valerus Compresson), and from applications at other companies, including ConocoPhillips in Canada.
The Energy Efficiency and Renewable Energy (EERC) component of the DOE has reported that economic viability of a heat recovery system can be explored at a basic level by calculating the associated simple payback period:
Simple payback periods of less than one year to five years are often realized. However the economics can vary from site to site depending on the source and amount of waste heat available.
For this reason GCGE developed a cost analysis procedure so that the project could be evaluated for determining if capturing the waste heat would show economic benefit for the client. Information regarding project description and reasons for project investment, project site conditions, heat source type, and cooling source type become important to economically evaluate a site.
For example the oil well project in Mississippi with Denbury Resources, Inc., was conducted as demonstration with funding provided by the Research Partnership to Secure Energy for America (RPSEA). Hot water (204oF) coproduced with the oil was passed though the GM ORC system to produce a gross output of 19 to 22 kWe from a brine flow rate of 120 gpm. The project cost was $230,000, which included additional R&D expenses, and had an IRR of 12%, resulting in a net revenue potential of $450,000 over the life of the equipment. This power output was able to offset about 20 percent of the power needs to run a down-hole pump. A larger and better sized air-cooled condenser for the high ambient temperature would have increased output by as much as 40 percent. From the ongoing experience of these units, GCGE has observed that an attractive payback can be had at oil and gas sites where cost of power is over $.07/kWh.
Thus the oil and gas industry has the potential to profit in numerous ways by using existing technology to capture waste heat energy and generate electrical power. This can offset their direct energy costs by reducing fuel consumption in compressors or prime movers or in the cost of purchased electrical power. Waste heat energy capture can assist the company regarding environmental restrictions that may exist as waste heat is considered to be a greenhouse-gas-free energy source.