Enclosed Combustor Destruction Efficiency – how does it work?
Enclosed combustors are effective devices to reduce methane (CH4) and VOC emissions from natural gas that might otherwise be vented to the atmosphere by storage tanks, heater treater flash gas, pneumatic devices, glycol dehydration units and facility depressuring. The efficiency of an enclosed combustor is often referred to as the destruction and removal efficiency (DRE).
For open-tipped flares burning flare gas, U.S. environmental regulatory agencies accept 98% as the DRE. No regulatory required performance (stack) testing is required for open-tipped flares operating in the U.S.
The U.S. EPA (Quad O) regulations require enclosed combustors meet a minimum 95% DRE. For NSPS Quad O manufacturer performance tested enclosed combustors, U.S. state environmental regulatory permitting agencies may accept DRE of 99% or greater provided the performance testing yielded such results and other enhance monitoring is conducted.
A Texas Commission on Environmental Quality guidance document (Control Device Requirements Charts for O&G) requests additional testing, monitoring and limits for enclosed combustors to ensure they meet DREs greater than 99%.
EPA Enclosed Combustor Performance Testing
Performance stack testing of enclosed combustors used in oil and gas operations is required by USEPA Quad O regulations. This includes manufacturer performance testing of a specific model or site-specific field performance testing by an oil and gas operator. Most U.S. based oil and gas operators choose to use manufacturer tested enclosed combustors to avoid the regulations’ requirement for initial and routine site-specific performance field testing every 5 years.
The performance tests are used to demonstrate that the enclosed combustor has a minimum 95% DRE. Manufacturer performance testing is used to demonstrate the DRE for a specific model enclosed combustor. Site-specific field testing is conducted at an individual specific application.
Some of the EPA Quad O manufacturer performance testing requirements include:
- Propene (propylene) fuel gas (approx. 2300 BTU/scf; 86 MJ/m3).
- Varying firing rates as a percentage of maximum design.
- Outlet sampling location must be a minimum of four stack diameters downstream from the highest peak flame or other flow disturbance, and a minimum of one stack diameter upstream of the exit or other flow disturbance. This requires some combustors to install an extension on the combustion tube during testing. Actual operating units for these devices will have a shorter tube length resulting in lower combustion efficiency under higher wind speeds over the enclosed combustor outlet and air intake.
- Must have total hydrocarbons emissions less than 10 ppmvw as propane corrected to 3.0 percent CO2.
- Must have CO emissions equal to or less than 10 ppmvd, corrected to 3.0 percent CO2. [This CO limit ensures that the testing will result in DRE greater than 99%.]
Manufacturer testing of a specific enclosed combustor make/model is a controlled test using propene fuel that may not reflect actual field conditions. Quad O tested enclosed combustors may not achieve 99.9%+ DRE when operating in an oil and gas production field facility, even though they may achieve these DRE rates during the EPA performance test in a controlled environment. Factors that can lower DRE in the field operated enclosed combustor include:
- Different field gas and storage tank vapors chemical composition (i.e., water vapor, N2, CO2, methane, ethane, propane, butane+) than propene.
- Different field gas heat value (BTU/scf; MJ/m3) than propene.
- Variability of inlet gas flow rates
- Ambient wind interacting with the air inlets and outlet can lower combustion temperature. This is especially true for combustors that were fitted with an extension during performance testing.
USEPA Control Technology Guidance
The October 2016 USEPA Control Technology Guidelines for Oil and Natural Gas Industry discusses manufacturer tested enclosed combustors’ DRE. Footnote 27 for Section 126.96.36.199. Routing Emissions to a Combustion Device states, “All reported control efficiencies were above 99.9 percent at tested conditions. The EPA notes that the control efficiency achieved in the field is likely to be lower than the control efficiency achieved at a bench test site under controlled conditions, but we believe that these units should have no problem meeting 95 percent control continuously and 98 percent control on average when designed and properly operated to meet 98 percent control.”
Carbon Offsets (Credits) and Enclosed Combustor DRE
As the demand and market for GHG reductions and carbon (GHG) offsets increases, operators are seeking conventional technology to reduce methane (CH4) emissions and obtain carbon offsets. The DRE of an enclosed combustor is especially important when used to generate carbon offsets from CH4 reductions.
To obtain carbon offsets, the emission reduction must meet the additionality test. This means the reduction is voluntary (i.e., not required by regulation or permit) and would not have happened in the absence of a market for offset credits. Also, carbon offsets should be accurate and consistently reliable. Carbon offsets based on an enclosed combustor DRE that is greater than actually achievable for the particular field operations, could make the operator liable for the paying back for the GHG emissions that exceeded the actual DRE.
One way to generate carbon offsets it to operate an enclosed combustor to combust CH4 in the gas stream. Typically, an enclosed combustor will have a DRE greater than conventional open-tipped flares. An open-tipped flare is typically accepted by regulatory air permitting agencies to have a 98% DRE for natural gas (including CH4). Using a device with a DRE greater than 98% may qualify a project for carbon offsets for the extra CH4 combusted. This could yield an extra 1% to 1.99% reduction in CH4 for DREs of 99 to 99.99%.
Use of an enclosed combustor (instead of an open-tipped flare) should meet the additionality test since the use of an enclosed combustor’s would be voluntary (not required by regulation) and because carbon offsets obtained from the application may justify the added cost for an enclosed combustor.
Carbon offset projects claiming DREs greater than 99% that later demonstrated to inaccurate could make the offset user liable for the excess CO2 emissions from the enclosed combustor.
Cimarron’s NSPS Quad O performance tested enclosed combustors have achieved a 99.9% DRE burning propylene, but we do not claim 99.9% DRE for a simple, natural draft enclosed combustor.
It may not be accurate to claim DREs greater than 99% for the typical enclosed combustor used in oil and gas operations.
Enclosed combustors operating at a typical oil and gas production location may not achieve a DRE greater than 99% due to the following:
- Field gas composition variation
- Flow rate variation
- Wind speed interacting with the air inlets
- Ambient temperature variation
- Ambient pressure variation due to site elevation
Note: Cimarron has conducted in 2020 a site-specific DRE testing of an enclosed combustor burning field natural gas at an oil and gas facility located in Colorado. The results yielded a DRE of 99.5% or above.
To ensure combustion DRE greater than 99%, we suggest the following monitoring parameters:
- Inlet pressure
- Flue gas temperature
- Flue gas temperature-controlled louvers to adjust excess air
Cimarron solutions to assist with compliance with the new flaring and venting rules include:
- Measurement of vent gas from storage tanks using our IQR Teams.
- Fugitive leak detection and repair monitoring
- Installation of vent and flare gas meters
- Vapor recovery units (VRU)
- Vapor recovery towers (VRT)
- Flares and enclosed combustors
- Retrofit flares and enclosed combustors with automatic igniters, pilot systems and alarm systems for flare pilot/ignitor malfunction.
- Real-time performance monitoring system technology (SyteLink360TM)
Cimarron – Who We Are
Cimarron’s vision is to work with our clients to create a cleaner environment.
The company engineers and manufactures environmental, production and process equipment for the upstream, midstream and downstream energy industries, as well as environmental control solutions for biogas at wastewater facilities, digester tanks and landfills.
Cimarron offers our customers the know-how and environmental expertise to meet the environmental standards of today and tomorrow. Cimarron is committed to bring value to the Energy industry and their shareholders based on our financial strength, experienced personnel, and engineering capabilities.
As a company, we thrive every day to make a difference through innovation (e.g. ESG), customer focus, and operational efficiency. In addition to being present in all major regions in the US, Cimarron serves more than 45 countries around the world, ranging from offshore to desert. From key operational centers in the United States, Italy and the United Arab Emirates, Cimarron offers ongoing service and support through its own field service personnel and strategic third-party partners, creating a cleaner environment for our customers and their shareholders.
Since its founding in the mid-1970’s in Oklahoma, the company’s product offering has expanded from production equipment to include the largest line of environmental solutions that capture or incinerate fugitive vapors. With the acquisitions of HY-BON/EDI in 2019 and AEREON (including Jordan Technologies) in 2020, Cimarron has added strong brands, products, and services to its portfolio.