Renewable Hydrocarbon Fuels

Hannu Jääskeläinen

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Abstract: Drop-in fuel alternatives composed of hydrocarbons can be attractive options to petroleum derived fuels. Renewable hydrocarbon fuels can be produced from such feedstocks as vegetable oils, animal fats, or biomass. A number of production processes are possible, including oleochemical production, thermochemical processing, and biochemical processing. The lifecycle carbon intensity of the fuels strongly depends on the feedstock.

Introduction

Drop-in fuel alternatives for gasoline and diesel that are entirely compatible with the existing petroleum infrastructure are attractive options to petroleum derived fuels. A fully compatible drop-in fuel would require minimal changes to the existing petroleum fuel distribution and supply infrastructure and would be fully compatible with existing vehicles and engines. Chemically, drop-in alternatives would be composed of hydrocarbons and be indistinguishable from the hydrocarbons found in petroleum fuels.

Hydrocarbons can be produced from a variety of renewable and non-renewable feedstocks using a variety of processes. Non-renewable production pathways include synthesis from coal and natural gas and are discussed under Synthetic Diesel Fuel.

Oleochemical processes convert lipid feedstocks such as oil from oilseed crops or animal by-products into hydrocarbon fuels. Hydroprocessing is the most important example of a commercially applied oleochemical process. Thermochemical processes use heat in the absence of oxygen to produce an intermediate that is further processed into a hydrocarbon fuels. Two routes to thermochemical processing are pyrolysis and gasification. Additional processing such as hydroprocessing, decarboxylation/decarbonylation or synthesis would be required to yield hydrocarbon fuels. Biochemical processes use microorganisms to produce longer chain alcohols and hydrocarbons that may require further processing to produce a hydrocarbon fuel. “Hybrid” thermochemical/biochemical technologies such as fermentation of synthesis gas and catalytic reforming of sugars/carbohydrates are also possible [4674].

It should be noted that while these fuels consist of hydrocarbons, the hydrocarbons are predominantly paraffins. Petroleum fuels are a mix of several types of hydrocarbons including paraffins and aromatics. If a fuel consisting almost entirely of paraffins replaces a fuel containing significant aromatics, a number of challenges can arise, including issues related to sealing against leaks and solubility of fuel degradation products. Fuel distribution infrastructure and vehicle fuel systems containing nitrile rubber seals in contact with fuel swell when aromatics are present. If the aromatic content of the fuel decreases to sufficiently low values, the nitrile rubber seals shrink and fuel leaks can result. While modern vehicles rarely use seals made from nitrile rubber, these types of seals can still be found in older equipment and in the fuel distribution infrastructure. Also, aromatics are able to keep a certain level of fuel degradation products in suspension. If the aromatic content in the fuel is too low, degradation products, if present, could become insoluble and lead to deposit accumulation and filter clogging. Thus, while these paraffinic fuels are considered to be “drop-in” alternatives, caution is still required to minimize the likelihood of some operational problems.

The greatest challenge to produce drop-in biofuels that meet the physicochemical properties of petroleum-based transportation fuels is decreasing the high oxygen content of the feedstocks. Oxygenated functional groups can react with refinery and pipeline metallurgy as well as with biofuel components to form gums, acids and other impurities and are detrimental to the fuel’s storability and stability. Oxygen functional groups in the fuel also reduce the fuel’s energy density, Figure 1 [4674].

[SVG image]
Figure 1. The effect of oxygen content on the energy density of liquid fuels

There are two alternatives for oxygen removal from the feedstock: addition of hydrogen or removal of carbon. Hydrogen addition is referred to as hydrodeoxygenation (HDO). Carbon removal includes decarboxylation and decarbonylation (DCO). In hydrodeoxygenation, a hydroprocessing reaction, oxidation of hydrogen to yield water is carried out using the oxygen present in the feedstock. The hydrogen can be present in the feedstock or it can be supplied externally. In decarboxylation, carboxyl group carbon is oxidized and the feedstock oxygen is removed as CO2 while in decarbonylation, carbon is removed as CO. In practice, the processes occur simultaneously but by controlling process conditions, one process can be favored over the other. The HDO process is typically favored when hydrogen is added to the process while, in the absence of hydrogen, DCO processes are favored.

With the DCO processes, feedstock carbon is lost by and process yield is lower than with HDO. Thus, HDO is favored because a higher product yield from a given amount of feedstock is possible. For the HDO processes, the amount of hydrogen required to yield a final product with a H/C ratio similar to that of petroleum fuels depends on the effective H/C ratio of the feedstock. The effective H/C ratio is useful for estimating the hydrogen consumption of the process and adjusts the actual H/C ratio to account for the feedstock hydrogen that would be consumed by feedstock oxygen in a combustion reaction:

Heff/C = {n[H] – 2n[O]}/n[C](1)

where
n = number of atoms of the associated element

Figure 2 illustrates Heff/C ratio as staircase [4674]. The difference between Heff/C of the feedstock and the final product, in this case diesel fuel, reflects the amount of hydrogen that must be provided to the process to produce a drop-in fuel. For the various biological feedstocks, oleochemical feedstocks such as fats and oils require the least amount of hydrogen while thermochemical and biochemical process feedstocks require the largest inputs of hydrogen from external sources. While feedstocks such as sugar and lignocellulose have considerable amounts of hydrogen, they also contain sufficient amounts of oxygen to consume all or most of the hydrogen. Thus, an important consideration for processing many of these feedstocks is the cost and sustainability of the hydrogen input. If the Heff/C ratio is low, processes such as DCO or biological processes that do not require industrial hydrogen are attractive.

[SVG image]
Figure 2. Position of various renewable feedstocks on the effective hydrogen to carbon ratio staircase

While Figure 2 is a useful depiction of the suitability of various feedstocks for drop-in biofuels, there are some exceptions. For example monoalcohols such as ethanol and butanol have a Heff/C = 2 but are still too oxygenated to be considered as drop-in biofuels.

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