May-2022

Fuel gas hydrocarbon recovery as carbon abatement strategy

Fuel or off-gas conditioning represents a winning combination of carbon abatement and improved financial returns

Justin Stark
Chevron Corporation

Article Summary

MACC framework and process improvement. According to the International Energy Agency (IEA), direct industrial process emissions comprised 8.7 Gt CO2 in 2020, and this is expected to increase due to a recovering global economy (IEA, 2021). However, these emissions must fall by roughly 1.2% annually to align with the IEA’s Sustainable Development Scenario (IEA, 2021). Emissions from energy-intensive industries are generally considered some of the most arduous to decarbonise due to their need for high-grade process heat, long project development cycles, and the lack of viable commercial-scale lower-carbon replacement process technologies.

That said, there are opportunities for operational and process improvement in industrial process facilities that are profitable endeavours while cutting significant carbon emissions. The Marginal Abatement Cost Curve (MACC) framework is often employed to track and prioritise such projects, as shown in Figure 1 in a study by McKinsey (McKinsey & Co, 2007). 

Initiatives are tracked by cost of abatement, with the cheapest and highest priority projects on the left-most part of the graph and higher abatement cost initiatives to the right. As shown, industrial process improvements mostly have ‘negative’ abatement costs associated with them, meaning they provide positive earnings or cash for the responsible enterprise. The use of critical benchmarking, through a firm such as Solomon Associates, can provide insights that facilitate project prioritisation and further process intensification studies (Solomon Associates, 2022).

Gas recovery and process heat management
Although many processes fit in the ‘industrial process improvement’ bucket in Figure 1, gas recovery is one such process employed in gas plants, large refineries, and chemical plants. Through Solomon Associates benchmarking on gas processing facilities, insights can be gained on heavy molecule recovery plant efficiency, including energy and non-energy efficiencies. 

Three key components that can cut scope 1 emissions significantly are the degree of heavy molecule recovery (i.e. removing propane and butane out of a heavy natural gas stream), the extent of heat integration, and the energy efficiency within a unit. Typically, these gas recovery units are designed to economically recover some amount of propane or butane from the natural gas stream before that gas is then burned as fuel (or sold to be burned as fuel by some other entity). It is common for the overall recovery of propane to be less than 50%.In contrast, the recovery of ethane was often ignored in older facilities due to the high cost of recovery and lower margin of direct ethane sales when the facility was built.

Unfortunately, the heavier the overall fuel or natural gas stream is, generally the higher its carbon intensity as a fuel. Data from the U.S. Environmental Protection Agency (EPA) report the emissions intensity in kg/MMBTU of CO2 by hydrocarbon molecule (U.S. EPA, 2014). Natural gas, primarily methane with some residual ethane, has an emissions intensity of 53.06 kg CO2/MMBTU, while the emissions intensity of heavier molecules is higher; for example, butane is 64.77 kg CO2/MMBTU (JISEA, 2016). For fired heaters that demand millions of BTU per hour, such a range of intensities can lead to differences in stack emissions on the order of several thousand metric tonnes of CO2 per year. The impact can be staggering, with overall gas and other fuel-fired process heat and steam generation representing the lion’s share of direct industrial carbon emissions at 52%, according to the EPA. 

With new incentives for carbon emissions reduction, along with changing and tightening markets for many commodities compared to the prevailing market conditions when many facilities were built, recovering these heavy molecules from the natural gas stream leads to profitable reductions in scope 1 carbon emissions. As one demonstration of this, two natural gas streams were modelled in Aspen HYSYS as fuel for a conventional furnace, with a process line-up as shown in Figure 2. The furnace in this model represents the fired heater at the front of a crude distillation unit processing 4.8 MTPA, heating this crude from 230°C to 340°C. The stream compositions and the resulting emissions for the same overall energy consumption are shown in Figure 2. The detailed stream compositions are shown in Table 1. 
The much leaner fuel gas stream is representative of that found from conditioned process gas in an oil refinery, with membrane or cryogenic processing pretreatment to remove almost all the C3+ and much of the ethane/ethylene content. The resulting reduction in furnace CO2 emissions is on the order of ~6% or near 6,700 tonnes per year of CO2. The impact can be significant depending on the particulars of the plant configuration. The emissions reduction above is compounded across many process heaters in the facility since they typically draw from the same fuel gas source or header. For demonstration purposes, two similarly sized crude unit feed furnaces and a corresponding delayed coker could see an emissions reduction of roughly 45,000 tonnes per year of CO2, with the same change in fuel composition as above. This does not include any other support fired heaters or gas-fired steam generators required to operate these units.

Example process schema and available technologies 
Many available technologies and processes can be used to recover these heavier molecules from natural gas streams, with their typical recoveries presented in Table 2.

In most cases, cryogenic separation plants are already designed for near-maximum recovery of ethane, while improved recovery is possible through operational changes or modest upgrades. Cryogenic separation uses a combination of refrigeration (often with propane) and a large pressure drop followed by separation in a demethaniser column. Pressure reduction occurs with a machine called a turboexpander to cool the gas to -85°C or lower, removing much of the ethane. Conversely, refrigeration with propane for dewpoint control cools a gas stream to near -35°C to remove almost all C3 and heavier molecules. Although it is difficult to improve this process further, the following are some ideas for better C2+ recovery or energy efficiency overall:
-    Lower demethaniser pressure — the higher the pressure drop across the expander, the lower the demethaniser operating temperature will be, resulting in improved recovery
-    ‘Re-wheeling’ the turboexpander — in many cases, the expander is not operating in its most efficient range or has high recycle flow to prevent surge. Discussing rotor replacement or other modifications with the manufacturer can improve lead to augmented efficiency that improves recovery (by reducing outlet temperature) while reducing compressor horsepower simultaneously
-    Improving heat integration and recovery — using process heat for column reboiler duty or introducing an economiser in the refrigeration loop can, again, improve energy efficiency and recovery. Improving refrigeration compressor operation by converting to an electric machine or changing compressor parameters (for example, installing automatic unloaders on a reciprocating machine or increasing capacity to reduce chiller pressure).