Using ethane on site can avoid poor transport options
Edward Woods, Consultant
Photo courtesy of Enerplus Resources.
Many operators of gas processing facilities that strip natural gas liquids (NGLs) from wet gas are finding themselves unsure of what to do with stranded ethane as a result of an increasing glut, minimal growth in market demand and no cost-effective transport options. Gas processing operators are looking at the possibility of holding stranded ethane, limiting their ability to process gas. By extension, their customers face an unattractive prospect of shutting in production and/or curtailing drilling programs. Combustion turbines, located onsite, can consume ethane, power gas processing facilities, provide heat to gas fracturing towers and allow continued gas production.
Situation
Growth in shale gas fields and unconventional plays has significantly contributed to our nation's natural gas reserves. The Marcellus Shale is one example of this booming market.
In the western Marcellus, gas coming out of the ground is "wet" - having a mix of natural gas and NGLs. Constituents within NGLs include ethane, propane, butane, and pentanes. To meet market requirements, the NGLs need to be separated, or stripped, from the methane to ensure the natural gas sent to consumers has a consistent BTU content. This "stripping" process requires heat to crack the gas and electricity to drive circulation pumps and compressors.
The majority component of the NGLs is ethane, which is used in the manufacture of plastics, anti-freeze, and detergents, to name a few. Development in the Marcellus and other fields has led to an oversupply of ethane in the market, depressing prices to as low as $0.23/gallon ($3.45/MMBTU).
Transport of ethane is costly and can exceed the value of the product if it needs to be moved more than a couple hundred miles to market. As a result, operators of stripping plants are "rejecting" as much ethane as possible back into the pipeline as one alternative to manage the oversupply. Limits on BTU content of pipeline gas restrict the amount of rejected ethane.
Complication
Expanded drilling programs and no new demand for ethane are on the horizon. It is predicted that an additional 200,000 bbls of ethane per day will be entering the market next year, putting additional downward price pressure on the commodity for the foreseeable future. If the ethane distribution system is maxed out and there is no home for sub-spec gas or excess ethane, operators will not be able to dispose of the commodity and be forced to limit gas production.
Stripper plants located any distance from a market have to deal with stranded ethane. These facilities, due to their location, have high electricity costs and frequent power outages due to storms and high demand on the grid.
Faced with not being able to transport ethane into the market has the knock-on effect of the stripping facilities not being able to process gas, leading to shut-in wells and reduced drilling programs. As a result, owners of stripping plants are looking at new ways to utilize ethane and in the process, optimize their operations.
Options and considerations
With a market price of ethane at $0.23/gallon ($3.45/MMBTU), it is on par with that of natural gas, making it attractive as a fuel. Use it as a fuel at the point of stripping from the gas stream for onsite power generation and the benefits multiply. The ability to operate without being impacted by restrictions in the ethane disposal market, allows customers to continue with gas field development activities and it becomes a very attractive proposition.
Ethane is a "hot" gas, with an energy content of 1,783 BTU/scf. It has a fast flame front and produces high exhaust temperatures. Reviewing prime movers, and their ability to burn ethane to produce power and heat finds few available options:
Reciprocating engines - Gaseous-fueled reciprocating engines are built in a wide range of power outputs, making them easy to size for the power demand. High compression ratios of reciprocating engines and internal components such as pistons, valves and heads would rapidly wear due to high combustion and exhaust gas temperatures associated with using ethane as a fuel. One project to fuel engines at a gas processing plant with up to 88% ethane was initiated in 2011, but there is no other mention of the project that would indicate success.
Combustion turbines - Combustion turbines are ideally suited for powering gas processing plants. Both are designed for continuous operation with minimal intervention and low maintenance. The majority of combustion turbines configured for oil and gas applications are simple-cycle machines with high pressure ratios between incoming air and the combustion stage. The high pressure ratio of approximately 14:1 found in most simple-cycle turbines is to reduce emissions and increase thermal efficiency. This pressure ratio limits the BTU range of fuels used in the turbine to pipeline gas and wellhead gas (900-1400 BTU/scf). Use of high-energy fuels, such as ethane, can rapidly damage combustion components within the turbine.
Developments in combustion turbine technology - Low pressure ratio turbines are able to operate on a wide range of fuels up to 1,900 BTU/scf, but have not been widely accepted due to their poor efficiency. Recent developments with low pressure ratio turbines include the use of a recuperator to improve thermal efficiency. A recuperator is a heat exchanger mounted in the exhaust stream that transfers heat energy to combustion intake air. Efficiencies of low pressure ratio turbines with recuperators rival that of high pressure ratio turbines and are considered a proven technology. These technological developments result in a low pressure ratio turbine with a recuperator can operate on ethane or natural gas, whichever is more economical with the efficiency of their high pressure ratio counterparts. These turbines can be fueled with ethane, allowing them to be a power source at gas processing facilities. Two turbine platforms that utilize these technologies are the FlexEnergy MT and Dresser-Rand KG2.
Solution
The glut of ethane for the foreseeable future will place continued price pressure on the product. Transportation costs of moving the product to market, if there is one, will rapidly negate any expectation of revenue when it is sold. Using ethane as a fuel in combustion turbines at the point of extraction from the gas stream is a viable solution and supports upstream development activities. Combustion turbines with an integrated heat recovery system using onsite fuel can reduce operational costs and has the potential to reduce emissions when replacing older on-site boilers. With that being said, one now would need to determine how to maximize this opportunity.
To take advantage of the onsite fuel and reap the economic benefits, the combustion turbine package(s) will need to be sized to power requirements of the facility, have acceptable installation costs, connect to the fuel supply, and interface with the grid. Overall, these items may seem complicated, but when looking at the details, they are not.
Sizing for power requirements - To improve reliability and be able to rotate turbines during annual maintenance, it is recommended that at least two turbines be used to power a gas processing facility.
Turbine installation - Installation costs can vary from site to site. For this reason, it is recommended that the turbine package be designed for outdoor operation and skid-mounted. A skid-mounted turbine package will allow for placement on a level gravel pad. Should operational requirements change, the skidded turbine can be hoisted onto a trailer and transported elsewhere.
Fuel supply connections - Temperature and pressure of ethane can vary, depending on the fractionation process and where in the system it is tapped to fuel the turbine. One needs to identify these parameters when selecting a fuel gas pressure regulator for the turbine. In extreme instances, a fuel heater may be required to bring the fuel temperature above minimum turbine requirements.
Hot fluid circuit connections - Hot fluid systems used at gas fractionating facilities can benefit from utilizing exhaust heat from the turbine. On a turbine with an integrate hot water system, connections to the existing facility hot water loop are completed by incorporating the turbine hot water with the facility hot water loop. In some instances, a heat exchanger may need to be used to isolate systems.
Electric system connections - Most utilities, and the state public utilities commission they operate under have "net metering" rules. These are regulations and guidance for interfacing with the grid. Should a stripping plant operator choose to export excess power to the grid, the rules and regulations are designed to prevent damage and risk to health of maintenance personnel working on power lines. To properly interface with the grid, a utility paralleling switchgear will be required. A switchgear in this configuration will allow the turbines to power the facility and if needed, draw any additional from the grid. In the event of excess power and a resell agreement is in place, it can be configured to allow export to the grid.
Renewable energy - Some states offer incentives for distributed power generation and CHP. Depending on the state, the turbine and its configuration, the CHP system may qualify for tax credits, renewable energy credits or both.
What would an ideal system look like? To maximize ethane consumption, combined output power of the turbines would match the maximum electrical load of the facility. Heat recovered from the turbine exhaust will generally equal or exceed heat requirements for the gas fractionation process.
Placement of the skid-mounted turbine packages will be in close proximity to the electrical distribution system and facility boilers. A packed, level gravel pad is sufficient for turbine placement. A concrete foundation will suffice as well.
Upon connecting fuel lines, electrical cables and communications, the turbines undergo commissioning to confirm performance. After start-up, the system can be monitored for service needs and operational abnormalities. Factory Certified Technicians will be readily dispatched should the need arise. Normal turbine maintenance is 1x/year to change air filters and gearbox oil.
Summary
Stranded ethane can be used in select turbines as a fuel to provide power and heat to stripper plant operations. When properly configured, combustion turbines are cost-effective, add value and provide multiple benefits.
About the author
Edward Woods has over 20 years' experience in business development, marketing, product development, product management, and value creation in power generation and emissions technologies in the oil and gas and power generation industries. He earned a bachelor's degree in mechanical engineering technology and a master of science in management from Purdue University and an MBA from Tilburg University in Holland. Woods is a member of the Purdue University College of Technology Industrial Advisory Committee and has been awarded numerous patents for power generation and emissions reduction technologies. This paper was written under contract to Keystone Drill Services, Somerset, PA.
