For thousands of years, the thermal process of fire has been a source of warmth and cooking fuel, while the biochemical fermentation of fruits, grains, and vegetables has provided food and beverage preservation. These biomass conversion pathways are ingrained in the human experience, and they continue to evolve with technology to meet our demands for energy.
As the flow chart depicts, biomass can be converted into fuels, chemicals, or electric power through both the thermochemical and biochemical processes. Thermochemical processes utilize heat and catalysts while biochemical processes utilize enzymes and microorganisms.
Despite their long histories, neither process has been able to transform lignocellulose biomass (without combustion) at the massive industrial scale like the petroleum and petrochemical industries have in processing hydrocarbons. In fact, the success of the petroleum industries may be the reason that certain biomass conversion processes have been underdeveloped to date. But the rise of petroleum fuels is also one of the main drivers of the current focus on biomass transformation due to the former’s role in pollution and climate change.
Biofuels, which have been produced through biochemical processing of starch and sugar crops, have had the largest impact globally, especially as a fuel additive in Brazil (cane) and the United States (corn). However, thermochemical processing has several advantages over biochemical processes, including dramatically shorter reaction times, lower catalyst costs, a wider array of fuel input options, and a much lower risk of process contamination.
Biochemical conversion utilizes bacteria and enzymes to break down biomass molecules. The process is slower by an order of magnitude than gasification (hours or days versus minutes or seconds) but it does not require a significant amount of external energy. The three main routes for biochemical conversion are:
1) Anaerobic/Aerobic Digestion
In anaerobic digestion, the bacteria access oxygen from the biomass itself, not from the air, to produce methane and carbon dioxide. Aerobic digestion, or composting, breaks down biomass in the presence of oxygen while using microorganisms that take the oxygen from the air, producing carbon dioxide, heat, and a solid digestate.
2) Fermentation
Fermentation converts the biomass partially into sugars using acids and/or enzymes. The sugars can be converted into ethanol or other chemicals with the addition of yeast. Whereas anaerobic digestion produces gases, the principal products of fermentation are liquids. The fermentation of starch and sugar-based feedstocks has been fully commercialized via corn and cane-based ethanol production in the US, Brazil, and elsewhere. Ethanol is used as a blendstock for gasoline-powered vehicles, making up nearly 10% of the gasoline supply in most markets in the US.
3) Enzymatic/Acid Hydrolysis
Cellulosic ethanol, which would rely upon the conversion of the non-starch/sugar portions of biomass (lignocellulose), has not been as successful as the pretreatment of the feedstocks through hydrolysis (acid, enzymatic, and hydrothermal). It has proven to be less efficient and economical in breaking down the inputs into simple sugars that could be accessible by the yeast and bacteria in the process.
Thermochemical conversion processes include combustion, gasification, pyrolysis, and solvent liquefaction. Each process functions at a distinct degree of oxidation, temperature, heating rate and reaction time to produce electric power, heat, fuels, and/or chemicals.
1) Combustion
Combustion was one of the first advanced uses of biomass conversion. Combustion is an exothermic (heat-producing) reaction between oxygen and the hydrocarbon in biomass. The biomass is converted into heat, water, and carbon dioxide. Biomass combustion remains a major source of energy production throughout the world and has replaced coal as a renewable source of energy in many power plants.
The advantages of combustion include the extreme simplicity of process operation: burning. Since biomass combustion is discouraged or banned in certain regions due to the release of polluting contaminations, gasification and other processes may be favored due to lower concentrations of CO2, SO2, NOx and solid waste in the end products, in addition to ease of fuel transport and flexibility in applications (gas, liquid, chemical production).
2) Gasification
Gasification is defined as a high-temperature conversion of carbonaceous materials into a combustible gas mixture under reducing conditions. Through gasification, a heterogeneous solid material can be converted into gaseous fuels intermediate (producer gas and syngas) that can be used for heating, industrial processes, electricity generation, and liquid fuel production.
Gasification of biomass has four key steps:
The catalyst required for gasification typically consists of air, oxygen, steam, or a mixture of those three. The key benefits of using biomass as an energy source include the fact that the components, when released, do not constitute a net contribution back into the atmosphere as well as the reduction on the dependence of non-renewable or imported fuel sources.
3) Pyrolysis
Pyrolysis involves the conversion of biomass into hydrocarbon liquids, gases, or solids in the total absence of oxygen at temperatures ranging from (400 - 900 oC). Pyrolysis can be segmented into three process types; torrefaction, slow pyrolysis, and fast pyrolysis each with different temperatures, pressures, and reaction times.
Slow pyrolysis will produce gases and solid biochars while fast pyrolysis will produce liquids. The product of fast pyrolysis, called bio-oil, is an energy-rich liquid recovered from condensable vapors and aerosols. Bio-oil consists of a mixture of oxygenated organic compounds including carbolic acids, alcohols, aldehydes, esters, saccharides, and other compounds.
Pyrolysis can be thought of as a standalone process or a precursor process to gasification or other technologies where the gas or liquid product of pyrolysis is used as an intermediate feedstock in the production of more complex products downstream.
4) Hydrothermal Liquefaction
Hydrothermal liquefaction is a relatively low-temperature (300–400°C), a high-pressure process that produces bio-oil from relatively wet biomass in the presence of a catalyst and hydrogen. Biomass with high water content may be directly utilized without energy-intensive pretreatment and converted into a bio-oil and platform chemicals. The bio-oil has certain similarities to petroleum crude and can be upgraded to the whole distillate range of petroleum-derived fuel products. Hydrothermal liquefaction (also known as direct liquefaction) is essentially pyrolysis in hot liquid water. Liquefaction’s main use case involves the conversion of bio-organic waste with high water content, including wet primary and secondary sludges.
Advantages of hydrothermal liquefaction include:
1) the ability to convert wet biomass without expensive pretreatment or drying.
2) substantially reduced greenhouse gas emissions vs other technologies.
3) production of a crude oil type product that can be distilled into a range of petroleum fuels and chemicals.
4) recognition of the technology by the US Department of Energy and other agencies.
5) thermal conversion efficiency of over 80%.
6) a direct path to convert bio-organic waste to heavy transportation fuels.
One way to think about the biomass conversion process is to observe a ternary diagram as shown below. Each corner of the triangle represents a 100% concentration of a given element, in this case carbon, hydrogen, or oxygen. Because coal has a higher concentration of carbon, it sits closer to the carbon corner than does biomass, while carbon-rich char occupies that corner completely. Biomass contains more hydrogen and oxygen than does coal, situating it nearly in the center of the diagram. The thermodynamic processes will break down the components of the biomass and begin to move them into different areas of this component mixture map, depending upon the makeup of the feedstock and process being used.
For example, slow pyrolysis (carbonization) will move the product toward the carbon corner through the formation of char. Fast pyrolysis moves it toward hydrogen, but away from oxygen, which results in a higher liquid product. Oxygen gasification moves the product toward the oxygen corner while steam gasification moves the product away from the carbon corner. Hydrogenation adds hydrogen, moving the product toward the hydrogen corner.
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