Aug-2022

Biofeed co-processing for fuel production

Comprehensive review of corrosion operating concerns when co-processing renewables and bio-oils through existing hydrotreaters, FCC units, amine units, and other assets.

Scott Sayles, Matthew Caserta, Stephen DeLude and Al Keller
Becht

Article Summary

Renewable feedstocks: processing steps
The processing of renewable feedstocks in a refinery requires the following stages/steps:
- Feed selection
- Pretreatment
- Storage
- Processing
- Treating
- Blending.

Each step has aspects unique to the feed being considered. An overview of production requirements indicates that biofuel production must consider the balance between food source utilisation and managing climate change. Feedstock selection is also a policy decision between private and public sectors. The use of non-food sources is the preferred path towards renewable fuels. Renewable feed supply and availability are equally challenging.

Renewable feed requires oxygen removal and olefins saturation to produce fully fungible fuel components. As a result, hydrogen and hydroprocessing capacity must increase to accommodate additional demand. The economic incentive to produce renewable feeds depends on uncertain feed and product markets. Biofeed processing and co-processing may provide significant sustainability and economic benefits for fuels producers but may not be competitive without subsidies.

Biofeeds are not all the same. Specifications and quality control need to be established consistent with the chosen processing scheme. For example, if not stored properly, biofeeds can degrade quickly (bio-action). Biofeeds are successfully co-processed in refinery units but only up to unique constraints for each specific unit.

As biofeed concentration increases, increasing hydrogen demand, corrosion issues, and water/amine systems should be assessed based on selected processing options. These changes are predictable and can be effectively implemented with a proper engineering overview. The final renewable diesel or jet require blend recipes and/or additive requirements that will change (quality). The final blends must meet all product quality requirements.

Renewable fuels challenges
Challenges face renewables fuels processing due to unique feed and product quality. Non-edible feeds are preferred but have additional processing challenges. In general, non-edible feeds have lower CI scores, which are better for CO2 reduction.

Blends containing oleochemicals (FAME) produced from edible feeds are less desirable as blending components. The future use of FAME is questionable with the drive to create a fungible fuel market using hydrotreated renewable products.

Non-edible feeds may require thermochemical upgrading due to the low (C+H)/O ratio. The ratio defines the efficiency of the molecule to release heat and not form water1. Feeds such as wood or forestry waste have low (C+H)/O ratios and require gasification, pyrolysis, and/or Fisher Tropsch synthesis. The fast/catalytic pyrolysis, and hydrothermal liquefaction are all options that may require post-conversion and treating in traditional refining processes. Conceptually, the processing options are a function of the (C+H)/O ratio and the existing process units.

Hydrogen removal processes such as pyrolysis or thermal cracking have the lowest liquid yields. Catalytic or FCC processes are a hydrogen removal reaction, allowing hydrogen transfer with improved liquid selectivity. Finally, hydrogen addition processes have the highest liquid yield and can achieve greater than 100 vol%. This bespoke carbon efficiency is shown graphically in Figure 1.

Feedstock supply
Feedstock quality must match the process line-up being considered. The types of feeds define the renewable feed processing limitations for each unit. The issues encountered are like the current fossil fuels crude purchasing optimisation systems. The renewable challenge is to get feedstock to the processing facilities on a scalable basis, along with associated costs in a sustainable matter.

A reasonable scale biofeed facility would be less than 15 MBPD, with the range being 5-60 MBPD. The best possible economic outcome is to leverage existing fossil fuel refineries and the associated supply chain. The use of existing storage and transportation to market is desirable. The feedstocks are different enough in composition that the feedstock storage considerations are modified compared to fossil fuels.

The feeds with longer chain fatty acids favour diesel, whereas shorter chain feedstocks favour jet (see Figure 2). The range of saturate (high (C+H)/O), for example, is tallow oil, while low (C+H)/O is represented by tall oil, which is also more difficult to process.

Often, raw feeds cost more than resultant products and may not be competitive without price incentives.

Hydrogen dilemma
To meet future biofuels and renewable demands, US hydrotreating capacity will likely have to double, and hydrogen generation will need to double or triple. There is an anticipated increase in hydrogen generation from the current capacity of 3 billion scf/day to 11 billion scf/day (see Figure 3). There is a desire to shift from grey to blue to green hydrogen. Oxygen content creates H2O, CO2, and CO byproducts that must be managed in hydrotreaters. This results in catalyst activity, corrosion, and removal concerns.

Product qualities
The two main products from renewable feed processing are diesel and/or jet. The units also produce naphtha, LPG, and light ends. The green naphtha market is in the beginning stages. Propane LPG is a fungible product with fossil fuels. However, currently the market is still developing, but the focus in this article is on renewable diesel and jet.

Renewable diesel quality
Renewable diesel production is fully fungible with fossil diesel. The energy density requirements are similar, with the target being a C/H or H/C target (~2 H/C mol/mol or 6.5 C/H wt/wt). The linear alkanes produced from the treat reactor are converted to isomers, which have a high cetane blending value.

Renewable jet quality
Renewable jet has low aromatics and requires blending with fossil fuels to meet the minimum aromatics content requirement. ASTM D7566 provides renewable jet specifications. Blending renewable and fossil jet meets ASTM D1655, producing sustainable aviation fuel, or SAF.