Many industrial processes produce heat as a byproduct that is wasted due to the inability or lack of equipment to capture and use this heat in a productive manner. The oil and gas industry generates such heat at a number of upstream and midstream locations, and that heat can be captured and converted efficiently with portable units.

Geothermal heat from brine

The production of oil and natural gas is a complex process. More than 1.7 million oil and gas wells have been drilled in the US, and more than 350,000 of these wells are at or deeper than 2,745 m (9,005 ft), according to the Energy Information Administration (EIA). These wells represent a vast resource for tapping hot brine still trapped within sedimentary basins. This resource is only beginning to be recognized as a new source for expanding geothermal energy production.

While a new well may produce oil or gas on its own in its youth, eventually pressure drops within the reservoir, resulting in the necessity to pump the resource to the surface. When this occurs, large amounts of brine also can be produced. If the water is sufficiently hot and enough water is produced (5,000 b/d or more), it becomes possible to capture this heat through a binary Organic Rankin Cycle (ORC) system and generate electrical power. Such a commercial unit has been active at a natural gas well in Louisiana where water is being produced to extract the accompanying gas.

In an ORC system, hot water passes through an intake pipe into a heat exchanger. Heat transfers across the heat exchanger into a working fluid whose temperature is raised prior to entering a turbine-generator combination to produce electricity. The exiting (working) fluid, commonly used refrigeration fluid that has dropped in temperature and pressure, is further cooled by air, liquid, or a hybrid system prior to passing through a variable speed pump to reenter the heat exchange system. The hot brine exits the heat exchanger and is injected back into naturally occurring subsurface brine aquifers.

An ORC system captures the heat from hot brine, which is then reinjected into the reservoir. (Images courtesy of Access Energy)

Gas compression

The EIA lists more than 480,000 km (292,320 miles) of pipeline throughout the Lower 48 states of the US. This natural gas pipeline network relies on more than 1,200 natural gas compressor stations to maintain the continu- ous flow of gas between the supply area and consumers. These compressor stations generally run 24/7/365. Not only do they help clean the gas from any solids and unwanted liquids, but they also increase the pressure of the gas, allowing it to flow through the pipeline. Increasing the gas pressure within the line results in the expulsion of heat, which can be captured with an ORC system to generate electricity on site.

Amine gas treatment

Amine gas treatment, also called gas sweetening and acid gas removal, is a process that uses aqueous solutions of various alkylamines (or amines) to remove hydrogen sulfide and carbon dioxide from gases. This process includes an absorber unit and a regenerator unit along with accessory equipment. In the absorber, the amine solution absorbs the HS and COfrom the sour gas to produce a sweetened gas stream. The "enriched" amine is then sent into a regenerator that strips out these materials so that the amine can be reused. The stripped gas from the regenerator is concentrated HS and COthat can be disposed.

Typical operating ranges for the absorber are 35°C to 50°C (95°F to 122°F) and 73 psi to more than 3,000 psi, while the regenerator is in the range of 115°C to 126°C (240°F to 260°F) and 20 psi to 25 psi at the bottom of the regenerator tower. Heat given off during this process can be captured and converted into usable electrical power.

Gas incineration

Incineration is a process involving the combustion of various materials that require destruction at high temperature, such as organic waste from hospitals. In the oil and gas industry, a gas flare is often used at an oil or gas production site to eliminate waste gas that is otherwise not feasible to use or transport. Rather than the conventional flare system, the waste gas can be combined with oxygen to generate heat, CO, and water as final byproducts. Some companies that already incinerate this waste gas suggest a reduction in COemissions of 7.6 tons/day by venting to 1.0 tons/day for 99.99% combustion efficiency. Even at 80% efficiency the reduction is 2.3 tons/day of CO. The heat can be captured through an ORC system to generate usable electricity at the site of incineration.

The Procore Module is a skid-mounted unit sold and serviced by Access Energy, a subsidiary of Calnetix Technologies LLC. It is easily shipped to any location for heat-to-electric generation.

Portable ORC system

With multiple applications available within the oil and gas industry for heat-to-electric production, the development of portable ORC systems provides an easy method for transport and insertion within existing infrastructure. Technologies that include a high-speed permanent magnet generator, magnetic bearings, and power electronics are improvements over past ORC systems. The magnetic generator and bearing components effectively eliminate the need for lubrication and vastly decrease the wear on parts, decreasing maintenance costs. The power electronics not only allow for offsite monitoring of the system but work to maintain frequency and voltage match between produced electricity and grid power. These improvements boost heat-to-power efficiency to 12%. A quick return on investment and a 20-year life for the equipment help provide a 15% to 24% rate of return on equipment investment. With availability of such equipment, the industry can realize significant savings on electricity costs.

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