Refinery greenhouse gas reduction pathways

While refining certainly does emit greenhouse gases, which can be reduced, the reality is that most emissions do not come from oil refining, but from the combustion of those fuels in power generation and transportation. This article discusses ways to immediately reduce emissions in refineries, and it suggests ways to strategically use refineries to drive significantly larger emission reductions across multiple sectors.

The refining process

Before discussing GHG reduction strategies, it is important to understand how a refinery operates. Refiners face the rather difficult challenge of transforming crude oil into specific petrochemicals and fuels. Crude oil contains a broad range of hydrocarbons, ranging from gaseous methane (CH₄) to very complex paraffins (C₄₀H₈₂ and higher). Separation of products is generally accomplished using energy intensive distillation.

When the resulting products do not match market demand, the larger molecular fractions are chemically split (cracked) to make smaller hydrocarbons, which are more easily transformed to the desired fuel mix (Figure 1). There are many cracking operations in a typical refinery, but a large one is the fluid catalytic cracker (FCC), which takes the heavier, carbon-heavy residuals and breaks them into shorter chain hydrocarbons that can be converted to gasoline and/or fuel oil.

Figure 1: A refinery starts with crude oil (left) and separates it into a number of fractions. Hydrogen and energy are then applied to ultimately reformulate those components into various marketable fuels, oils, and petrochemicals.

All these processing steps require a great deal of energy and hydrogen. Hydrogen is necessary in many of the steps to drive the chemical reactions which reformulate the chemical fractions. Since free hydrogen is rarely present in crude oil, it must be created by reforming CH₄ and steam (H₂O) into hydrogen (H₂) and carbon dioxide (CO₂). Energy needs are chiefly met by burning natural gas and waste refinery fuel gas, which is generated by many of the processing operations and largely consists of methane and various impurities.

GHG emissions in a typical refinery come from a variety of sources, but the bulk of the gases come from combustion (which furnishes the refinery’s energy needs), along with CO₂ emissions from the methane reformer and fluid catalytic cracker (Figure 2).

Figure 2: Greenhouse gas emissions from a refinery mostly result from energy-producing fuel gas combustion. The hydrogen-producing methane reformer and fluid catalytic cracker also generate significant carbon dioxide.

The FCC and methane reformer are large point sources of CO₂, and combustion operations occur throughout the refinery in a variety of heaters, boilers, and other gas-fired equipment.

Refinery GHG reduction strategies

There are short-term, long-term, and very long-term pathways for GHG reductions within refineries. There are also broader strategic initiatives external to refineries that can eliminate significantly more GHG emissions.

Short-term strategies

Some sources of refinery GHG reductions can be reduced immediately and often pay for themselves. Fugitive emissions from control valves, rotating equipment, and piping pollute the environment, and cost the refinery money in lost product and energy. Improved valve and pump seals can immediately pay for themselves in reduced product losses, while often reducing the need for environmentally-mandated inspections. Unnecessary flaring and losses in relief devices can also be addressed with better device monitoring and 'smarter' operations through advanced control strategies. Of course, any energy conservation project will usually pay immediate financial dividends and reduce GHG emissions as well.

Long-term strategies

CO₂ emissions from the FCC and methane reformer are large, reasonably pure, concentrated point sources, and are thus good candidates for CO₂ capture. Mature technologies like amine recovery can be utilised to strip flue gas CO₂, which can then be pressurised, transported, and injected into suitable underground formations for long-term storage, often in conjunction with enhanced oil recovery.

The challenges here are financial and infrastructure related. The capital and operating cost of carbon capture are significant, and such a project is not economically viable without some outside influence, as discussed below. The other problem is a lack of CO₂ transport and storage infrastructure.

Tackling emissions from fuel combustion is more problematic since the burners are scattered across the complex. Electrification of smaller, lower temperature heaters is one option. Diluting or even replacing natural gas with hydrogen for high-temperature processes is another. The first option requires sources of clean electricity, currently in short supply, and the second option requires more hydrogen, which could be furnished by expanded methane reformer production, provided the resulting CO₂ is captured so the resulting total GHG emissions are reduced.

Another viable avenue for GHG reduction includes conversion of smaller refineries to utilise bio feedstocks—such as lipids, animal fats, and agricultural waste streams—to create biofuels. This pathway requires the appropriate feedstocks to be available, as well as specific refinery equipment that is located near those feedstocks. This strategy is being implemented today in very specific instances where the right combination of feeds and equipment is available. As bio-related feedstocks are identified and increased, this strategy will gain traction.

Very long-term strategies

There are emerging technologies that utilise hydrogen and CO₂ as feedstocks to create synthetic fuels. Because the fuel is built from CO₂ that is already present, the net CO₂ emission from combustion is substantially reduced. Unfortunately, these processes are only just being investigated and are not ready for large-scale production.

 

One other very long-term strategy will begin to occur as the electrical conversion of vehicles progresses and fuel demand falls. When that occurs, a refinery can be more selective of the crudes they process and the products they make. For example, sweet crude requires far less energy and emissions to process versus sour crudes. Some feedstocks, like oil sands, are particularly energy and emission intensive, and their use can be cut.

Big picture GHG strategies

Refineries certainly emit GHG gases as they process crude oil, but the bulk of GHG emissions come from burning those fuels in the transportation and power generation sectors (Figure 3). Therefore, significant GHG reductions can only occur if the energy source for those sectors is less carbon intensive.

Figure 3: In the US, the bulk of GHG emission result from the combustion of fuel in the transportation and power sectors. Industry emits about 24% of the total, and refineries are a relatively small part of that sector.

Solar and wind-powered energy sources are excellent candidates for clean energy, but both these sources only produce intermittent power, and both require very large tracts of land. Hydropower is another source, but the environmental impact of dams has its own issues, and it seems unlikely that more large dams will be built.

Hydrogen is an obvious solution. It burns cleanly (creating only pure water emissions), can be converted into electricity via fuel cells, has a very high energy value, and can be stored and transported easily. Transition to hydrogen or a hydrogen/methane mix can drive significant GHG reductions in the transportation and power generation industries.

Hydrogen can be made at scale using electrolysis, which uses electricity to split water into hydrogen and oxygen, or through methane reforming discussed previously. Hydrogen created from zero-carbon, electricity-powered hydrolysis is called green hydrogen. Grey hydrogen is created through methane reforming, with CO₂ emitted to the environment.

A major challenge with hydrogen is creating enough of it in an environmentally sound manner to make a difference in net GHG emissions. That is where refineries can play a major role. If carbon capture and storage can be employed with methane reformer, this creates blue hydrogen, which is far less environmentally damaging. Refineries are already well versed in creating and working with hydrogen, so if the CO₂ emissions can be eliminated, refineries can supply clean hydrogen and bridge the gap, while clean electrical sources and better hydrogen production processes are developed.

Who pays?

That is the trillion-dollar question because most GHG emission reduction methods are quite costly. It is unrealistic to think that companies will volunteer to start paying mountains of money to fund CO₂ transport infrastructure, undertake significant equipment overhauls, and start using much more expensive and problematic feedstocks. A successful transition will therefore ultimately require government action.

Certain efforts — including biofuel production, CO₂ infrastructure, and GHG reduction projects — will likely be encouraged through subsidies and tax credits. Likewise, most GHG emissions will ultimately be taxed, putting companies on an even playing field, and providing stronger incentives and justification to reduce their emissions.

In the end, everyone will likely pay more to help fund this energy transition. Eventually technology will improve, and efficiencies of scale will come into play, but in the short term, energy prices will rise. However, those cost increases will be minor in comparison to the potential costs associated with unchecked global warming.

Written by Diana de la Cruz and Bruce Ofori, Emerson.

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Read the article online at: https://www.hydrocarbonengineering.com/special-reports/21022024/refinery-greenhouse-gas-reduction-pathways/