liquid fuels part 2: it all starts with carbon
Where are the sustainable liquid fuels? Are there problems with the feedstock, the process or both?
Liquid fuels are based on either a hydrocarbon or an alcohol, and both of these are based on carbon and hydrogen. Of these two constituents carbon is of interest here. If you want a sustainable liquid fuel you need a sustainable feedstock i.e. source of carbon. That is the starting point.
The end fuel produced from a sustainable feedstock should produce no, or near no, net carbon when it is used. Recently, grown crops fit this requirement. Those with a high sugar content have been used in the production of ethanal. However, the use of such crops has been shunned as they could otherwise be used for food. Next up; non-crop based biomass. Work continues today on these, but the lower sugar content makes fuel production more challenging.
There are other high carbon content feedstocks that should be considered. I am thinking about tires, plastic, wood waste, household waste, or just about anything with a decent fraction carbon. These are not sustainable by the classic definition, but there are some benefits to their use. First, it would be a second use, reducing the consumption of virgin fossil fuels for energy. Yes, you still have to deal with the carbon dioxide, but it has now seen two cycles. Second, reuse or recycling of these feedstocks is difficult. Third, they may prove hazardous to keep around, becoming either landfill or, worse, pollution, think plastic ocean waste. Thus, while such feedstocks do not fit the classic definition of sustainable they are plentiful and I would argue considering them is advantageous.
Are the processes tor producing liquid fuels proving to be difficult? The basic premise, or goal, of any process is to break chemical bonds to “liberate” the carbon, without reacting with oxygen i.e. combustion, or worse partial combustion. This is no small order. Let’s use tires as an example. The carbon chains in man-made rubber are linked together, or cross-linked, by sulphur. These cross-linking bonds are difficult to break. X-ray-based systems, pyrolysis and super-critical water, to name a few, have all been researched. We are though not there yet. Another difficulty is non-homogeneity in the feedstock. Plasma-gasification worked in early trials with wood chips, but it appears to have difficulty with household waste.
In the end, the quest for sustainable liquid fuels has not been easy. By the very nature of the feedstock, whether they are low-sugar biomass or hydrocarbon based products, the road to a fuel is difficult. To this end the technologies are generally new, novel and require a long development time. There are though new entrants joining the quest and learning from those that have gone before.