The Right Time for Rich Burn
Natural gas production in the U.S. averaged more than 100 billion cubic feet per day (bcf/d) in October and November as 2022 is likely to go down in history with a record average of 98.0 bcf/d for the year. The Energy Information Administration (EIA) forecasts production to grow at approximately two percent in 2023, averaging between 100 bcf/d and 101 bcf/d.
Record Flows of LNG
A commitment by the U.S. to supply an additional 530 bcf of liquified natural gas (LNG) to European countries to support European efforts to expand their gas supply sources had U.S. LNG flowing to Europe at record levels throughout 2022. A study released by the American Petroleum Institute (API) and the International Association of Oil and Gas Producers (IOGP) found that Europe’s demand for LNG is projected to increase 150 percent from 2021 – 2040.
In parallel, there is ever-increasing recognition that the world needs to limit the impact of climate change and as the second most abundant greenhouse gas (GHG) after carbon dioxide, methane is a big target. Investors, shareholders, governments, and communities alike are scrutinizing the carbon intensity associated with the production of fossil fuels and this is driving energy producers to seek new and innovative ways to reduce their methane emissions intensity.
Natural gas-fired engines, also known as prime movers, providing mechanical power for the compressors located along the three million miles of pipelines zigzagging across the Lower 48 are typically separated into three distinct classes: 2-cycle (stroke) lean burn, 4-stroke lean burn and 4-stroke rich burn and have a lifespan of 50-60 years. Why do the three distinct classes matter?
Greenhouse Gas Reporting Program (GHGRP)
They matter because the U.S. Environmental Protection Agency (EPA) is proposing amendments to specific provisions of the Greenhouse Gas Reporting Program (GHGRP) to improve the quality of data collected under the program. Since 2011, the EPA’s GHGRP has collected annual emissions data from nearly 8,000 large industrial facilities and other sources in the U.S. that emit 25,000 metric tons of CO2 equivalent (mtCO2e) or more per year. According to the EPA, the proposed revisions would further enhance the quality of the data from the Petroleum and Natural Gas Systems source category (Subpart W) so that the GHGRP continues to serve as a tool for both EPA and the public to understand emissions from this sector.
Studies cited in the proposed GHGRP rule indicate that a significant portion of emissions can result from unburned methane entrained in the exhaust of natural gas compressor engines—also referred to as “combustion slip” or “methane slip”. The studies further assert that emissions from natural gas compressor engines included in the GHGRP are significantly underestimated because they do not account for combustion slip.
Revised Methodologies
Therefore, the EPA is proposing to revise methodologies for determining combustion emissions from compressor engines to account for combustion slip. Reporters would use subpart-W specific emission factors by engine design class (e.g., 2-stroke lean-burn, 4-stroke lean-burn, 4-stroke rich-burn, or other) in the proposed new Table W-9. In the original proposal, the EPA proposed that the revisions would become effective on January 1, 2023, and that reporters would implement the changes beginning with reports prepared for the 2023 reporting year and submitted April 1, 2024. As of this publication, it is unclear when the new emission factors will go into effect. See Figure 1 for comparisons of current versus proposed emission factors.
As is made apparent by the proposed emission factors, four-stroke, rich-burn engines simply and inherently emit less methane. Rich-burn engines operate near the stoichiometric air-to-fuel ratio (16:1) where theoretically 100% of the fuel and oxygen is consumed during combustion. In reality, this is about a 99.7% combustion efficiency and results in a more than 90% methane emission reduction. With the appropriate exhaust after-treatment systems, the emission values level out, and almost the same conditions can be achieved for lean- and rich-burn engines.
Inflation Reduction Act (IRA)
In yet another parallel, in August 2022, President Biden signed the Inflation Reduction Act (IRA) which includes a charge on methane emissions from selected entities in the oil and gas industry. The methane emissions charge applies to facilities that are required to report their GHG into the GHGRP. Beginning January 1, 2024, the charge starts at $900 per metric ton of methane and increases to $1500 after two years which equates to $36 and $60 per metric ton of carbon dioxide equivalent, respectively.
With the proposed changes to the GHGRP and the IRA’s methane charge, now is the “right time for rich-burn.”
This article was authored by INNIO's Waukesha Engine and was originally published on COMPRESSORtech2 on January 26, 2023.