L21. Pathways to Products: Thermochemical
In our last lesson we talked about fermentation as one pathway to getting products out of biomass. We learned that sugar and starch crops are the feedstocks of fermentation 99% of the time because getting sugar from lignocellulose is tricky. Here, we will discuss other pathways lignocellulosic biomass can take to become bioproducts and biofuels. We will also learn about how oleaginous (fancy word for oily) biomass is processed.
What is Thermochemical Conversion?
If you remember from the last lesson, before we could ferment our biomass we first had to go through several steps to break it down into usable sugars. This was especially true for lignocellulosic biomass, which is far more challenging to deconstruct than starch. Thermochemical conversion is a different way to break down lignocellulose from larger molecules into smaller molecules, this time using heat and/or catalysts. Note: catalysts Links to an external site. are just substances that speed up a chemical reaction.
There are many different types of products we can make from thermochemical conversion products including solids, liquids, and gases. When we talk about thermochemical conversion, it’s important to think about heat and oxygen. This is because heat and oxygen almost completely control what kind of conversion will occur. The lowest level of conversion is pyrolysis - it requires no oxygen and just enough heat to start getting molecules to break. Gasification is the middle level - it requires more heat and the addition of a small amount of oxygen. The next level is combustion. When you give the system a lot of heat and all the oxygen it can handle, it combusts (burns). Think about a fire: when the fire in your fireplace or bonfire pit is dwindling, you fan it or blow on it to increase the oxygen flow for combustion.
Figure 1. The thermal process changes as you add heat and oxygen.
We choose our process based on what end product we want to make. If we want to make liquids and solids, we go with pyrolysis. If we need gas, we use gasification, and if we want heat, we use combustion. Note that since these processes occur on a spectrum, you can’t make them happen in isolation. In a flame, for example, some level of pyrolysis, gasification, and combustion is all going on. That means we can optimize our process to get as much of our desired primary product as possible, but there will always be some secondary products being made. In this way, thermochemical conversion is kind of messy. We’ll work our way through these three conversions starting with combustion, since it’s the most familiar to us.
Combustion
Combustion is the process of burning something. This is what is happening in the fireplace or on the gas stove. If you see a flame, you are seeing combustion. Chemically, that means we convert carbon (in this case, our biomass), oxygen, and energy into carbon dioxide, water, and heat. Combustion is simple, and it is the most widespread energy conversion on Earth. In 2012, 60% of the wood produced globally was not used for wood or paper or some other bioproduct, but rather used as fuel for heating, primarily through combustion. Other examples of burning biomass for fuel from past lessons include sawdust and wood scraps, lignin, and bagasse.
Gasification
Gasification is the conversion of carbon-rich materials (e.g. biomass) under high heat and very low oxygen into a flammable gas that’s rich in carbon monoxide (CO) and hydrogen (H). This gas is called synthetic gas or syngas, and is mainly used as an intermediate to produce different chemicals, such as liquid fuels. Historically, syngas (synthesis gas) was used for home heating and cooking before natural gas. So this is not a new technology, but rather, we are rediscovering it. The following short video from the US Department of Energy explains gasification, and how syngas can also be converted into a liquid.
You saw in the video that the gas must go through several cleaning and conditioning steps before it can be used to synthesize chemicals. Further processing is typically required to make sure the gas has the proper composition; specifically, hydrogen gas and carbon monoxide must be present in a 2:1 ratio. Chemicals that can be made from syngas include ammonia, methane, dimethyl ether, methanol, and other alcohols like ethanol. Through several additional processing steps, syngas can actually be turned into gasoline and diesel, but it is far more expensive to produce gasoline or diesel this way than to refine it from petroleum
Fun fact: Germany used this technology during World War II when it did not have access to petroleum-rich areas of the world, and South Africa used it when faced with a world oil embargo during apartheid. Photo is a car running on wood using a gassifier. Not pretty or convenient but it works! The gas produced is used as fuel for the car engine.
Unfortunately, creating ethanol fuel via gasification is expensive. Part of the reason for the high cost is that the process is not very efficient. Not only do we have to plant, harvest, and pretreat the biomass, there are the steps of gasification, contaminant removal, product separation, and liquid condensation. If we want ethanol, the cheapest option is still sugar fermentation.
Pyrolysis
Pyrolysis is the rapid breakdown of organic compounds (like those in biomass) in the absence of oxygen to produce liquid, gas, and char. The video below provides an excellent summary of the different types of pyrolysis, and the resulting products. He talks very slowly, so we would recommend watching at 1.25 or 1.5 speed. We have the video automatically set to start at 1:34, and you can stop watching at 12:35 when he begins the section called "Next Lecture - Biomass to Parts".
As with gasification, the product generated from pyrolysis (bio-oil) must undergo further processing before it is ready to be used as a fuel. We call this process chemical upgrading. The chemical upgrading of bio-oil is still being developed, but there are some general strategies. The bio-oil undergoes two stages of hydrotreating to remove oxygen and separate the oil into light and heavy fractions. The heavy oil is then hydrocracked to break down large polymers into fuel-range molecules.
Summary: Thermochemical conversion of lignocellulose
We’ve learned that thermochemical processing doesn’t produce things, it breaks them down. A different way to think about it is that the product these processes are creating is “deconstructed” lignocellulosic biomass. If we want to create chemicals that don’t look like little pieces of cellulose or lignin, it's up to us to take all of the deconstructed pieces, the C’s and O’s and H’s, and put them back together into our desired products. Through further processing, we are able to create commodity chemicals or chemically upgrade the deconstructed biomass to transportation fuels. The main issue facing all thermochemical products is that petroleum is cheap, so it is much less expensive to create fuels by refining petroleum than through any of the processes described above.
How Are Plant Oils Converted?
Oleaginous biomass includes traditional oil seed crops like canola, rapeseed, soybeans, and sunflower, in addition to less traditional sources like microalgae, animal manure, and sewage. What we are interested in getting from these sources are lipids - a broad category of organic 
compounds that include fats, fatty acids, cholesterol, and waxes.
Extracting Oil from Seeds
Extracting lipids from oil seeds is pretty straightforward. If the seeds have an oil content of 20% or higher, they are mechanically pressed to release the oil from the seeds. The leftover parts of the seeds are used in animal feed. At lower oil concentrations, a solvent Links to an external site. is used to extract the lipids. Algae, something we will talk about in Lesson 24, also contains oils. To extract oil from algae it must be dried first. After drying, the same oil extraction processes - mechanical pressing or solvent extraction - can be used.
Transesterification
Triglycerides are another category of lipids that can be used directly as fuels in diesel engines. This means it is possible to run a diesel engine off of the vegetable oil you cook with. (Mythbusters actually tried this Links to an external site..) However, it’s better to chemically modify the triglycerides to improve their fuel properties. The most common modification to get from triglycerides to biodiesel is called transesterification. This may sound complicated, but it's actually a very simple process that can be done by anyone in a lab. This next quick video shows how easy it is to make biodiesel from oil (transesterfication). Note: these students should be wearing eye protection. The process creates some fumes and can explode easily. Don't try it at home :)
In transesterification, triglycerides are reacted with methanol and a catalyst to produce methyl esters (biodiesel) and a coproduct called glycerol (a common ingredient in soaps and cosmetics). Once the glycerol is removed, the methyl esters have similar combustion properties to fuels designed for diesel engines, hence the name biodiesel. But keep in mind that biodiesel is chemically different from petroleum-derived diesel and therefore is not a perfect substitute.
Upgrading to diesel
Alternatively, triglycerides and other lipids can be hydrotreated (like we talked about under pyrolysis) to produce synthetic diesel. Hydrotreating lipids produces long, oxygen free, organic molecules made exclusively of hydrogen and carbon (which we call hydrocarbons). We can describe these hydrocarbons as diesel fuel (sometimes referred to as renewable diesel) now because they have the same number of carbon atoms as the hydrocarbons in petroleum-derived diesel. Hydrotreating vegetable oils and animal fat to produce renewable diesel has already been demonstrated at a large scale, but it will not be widely deployed until we find a low-cost lipid feedstock.
Other options
Although oleaginous biomass is rich in lipids, it also contains large amounts of carbohydrates and protein. For example, algae is 40% lipids, 30% protein, and 20% carbohydrates. To extract only the lipids and send the rest of the biomass to the landfill would be a very wasteful use of a biorenewable resource. An option would be to process the remnants leftover after lipid extraction, or the whole algal biomass, into a liquid bio-oil via pyrolysis. You should know by now that there is not much waste in any of these processes!
The Biorefinery Concept
Biorefining is a term used to describe the sustainable processing of biomass into a spectrum of marketable products and energy. This encompasses a wide range of technologies used to break down biomass resources (wood, grasses, corn, etc.) into their building block components (carbohydrates, lignin, lipids, etc.) which can then be converted to value-added products (fuels, chemicals, etc.) The following table summarizes what we've covered in this course so far within the framework of these biorefining steps.
A biorefinery is a facility (or network of facilities) that integrates biomass conversion processes and equipment to produce transportation fuels, power, and chemicals from biomass using the two pathways of fermentation or thermochemical conversion. This concept is analogous to today’s petroleum refinery, which produces multiple fuels and products from petroleum.
It's important to note that some chemicals produce through biorefining are identical to those produced from petroleum. There are also chemicals produced from biomass that are completely different from petroleum products, but can serve the same function. Think about it like a plastic bag and a paper bag - they both carry groceries (same function), but they're made up of completely different materials.
The chemicals ethylene, acetic acid, and methane gas are identical whether they are produced from biomass or petroleum. On the other hand, ethanol cannot be produced from petroleum but it serves the same function as gasoline (they both can be used to power our vehicles). Same function, but chemically they are very, very different.
Here's a final schematic to illustrate how a biorefinery might work in practice. Wood chip biomass comes in and is pretreated. Some of the pretreated biomass undergoes enzymatic hydrolysis to produce 6-carbon sugars which can then be turned into PLA (a type of bioplastic we will discuss in the next lesson). The 5-carbon sugars produced during pretreatment go straight to fermentation where they are converted to ethanol. The leftover lignin and residues from ethanol fermentation can be combusted to produce steam that will power the energy-intensive pretreatment stage. As you can see, one source of biomass comes in, and through several different pathways, a variety of energy types and products can be produced.
Conclusions
Over the past two lectures we've seen that there are many pathways to convert biomass into biofuels and bioproducts. The main issue with the commercialization of these processes is economics. Petroleum is cheap as are any petroleum-based products. To compete requires cheap feedstocks and very efficient processes.
We learned that fermentation of corn grain to ethanol and other products is economically viable. We also learned that there are cheap lignocellulosic feedstocks from forestry and agriculture that could be used for bioproducts. Unfortunately, the cost of converting these feedstocks to products is still not competitive with petroleum.
Research continues to make these processes more efficient and products more cost effective. In the next unit we will discuss some some environmental policies that are in place to help expand the use of biofuels. It also important to note that consumers are also pushing for bioproducts. This is probably the main driver behind the push to develop and use bioplastics as discussed in our next lesson!