Biomass chemicals encompass a class of chemicals derived from renewable and organic sources. The majority of biomass materials comprise plant-based products or byproducts, although animal-derived substances like fats or manures are also included.
While biomass chemicals garnered renewed attention during the oil crises of the 1970s, they are not recent innovations. In fact, the chemical industry once heavily relied on wood, sugars, starches, and animal fats for producing chemical products.
Certain biomass chemicals remain vital components of the chemical industry. Cellulose fibers, for instance, continue to dominate significant markets despite facing competition from petroleum-based materials over the years.
Many other biomass chemical processes were operational until the 1960s and still persist in developing nations or countries lacking convenient access to petroleum.
Notable examples of important biomass chemicals include ethanol, acetone/butanol, itaconic acid, lactic acid, xanthan gum, sorbitol, industrial starches, fatty acids and alcohols sourced from vegetables and animals, glycerol, soap, furfural and its derivatives, rayon, cellophane, carboxymethylcellulose, lignosulfonates, tall oil, vanillin, and methanol.
During the oil shocks of the 1970s, several chemical companies revisited the realm of biomass chemicals. In the United States, ethanol derived from grain fermentation received significant attention, as it was actively supported by the federal government as a gasoline extender and enhancer of octane levels.
Other biomass-based commodity chemicals that received focus during that period included methanol, acetic acid, butanol/acetone, and ethanol derivatives.
With petroleum prices stabilizing, most chemical producers in the United States have abandoned their interests in biomass commodity chemicals. However, a few manufacturers are currently exploring new avenues in biomass specialty chemicals, such as innovative starch polymers, novel lignin chemicals, and polysaccharide gums.
The fluctuating interest in biomass chemicals mirrors, to some extent, the fluctuation of petroleum prices. Nonetheless, other characteristics inherent to biomass chemicals have contributed to a cautious approach toward their redevelopment.
Table of Contents
1. Biomass Raw Materials
The category of biomass feedstock is extensive and includes various sources such as grains, sugar crops, oilseed crops, agricultural wastes, liquid and solid byproducts of food processing, wood, wood chips, bark, mill residues, forest residues, pulping liquors, animal fats, manures, algae, and even exotic crops like guayule, jojoba, and euphorbia.
All biomass materials consist of carbon, hydrogen, and oxygen, yet the chemical composition of each material type is distinct. Due to the significant oxygen content (up to 40%) found in most biomass materials, they are generally regarded as valuable sources of oxygenated chemicals.
The diversity of biomass materials necessitates the design of specific biomass chemical processes for each material type. In contrast to petroleum, which is a simpler and more uniform substance, biomass derived from different sources cannot be interchanged.
The production and collection of biomass feedstocks also present challenges. When a well-established biomass feedstock like starch is used, production and collection are straightforward because there already exists a starch-processing industry.
However, utilizing starch for the production of new chemicals or for increasing the production of conventional starch-based chemicals may lead to competition with existing starch markets. The same reasoning applies to the use of timber or pulp-grade wood as a feedstock for chemical production.
Choosing a nonconventional biomass feedstock, such as agricultural residues, poses logistical problems for collecting dispersed materials. Generally, biomass wastes are bulky, scattered, and economically challenging to collect and transport over long distances.
The currently operational biomass chemical processes primarily utilize conventional biomass feedstocks like sugar, starch, and wood, as well as biomass wastes such as forest residues, pulping liquors, and agricultural residues.
Research is currently underway to explore the potential of new crops like guayule, jojoba, euphorbia, or crambe that can be cultivated on currently uncultivated land exclusively for chemical production.
Some of these projects, including the establishment of short-rotation forestry initiatives, still require further development of production and collection infrastructures, processing industries, and markets.
2. Technology for producing chemicals from biomass
Because of the complex composition of biomass materials, the conversion of these substances into chemicals often lacks efficiency. Plant materials, for instance, consist of three main components: cellulose, hemicellulose, and lignin.
When a chemical process utilizes only one component, while the others become waste requiring disposal, the overall process becomes economically unfavorable. In traditional wood pulping, for example, cellulose is separated from lignin and hemicellulose fractions, which are typically burned for energy or occasionally transformed into specialty chemicals.
To ensure the success of biomass chemical plans in the future, new wood chemical processes would likely need to find higher-value applications for lignin and hemicellulose. Some experts in the field of biomass have proposed the concept of a “biomass refinery” as a necessary component for such plans.
Separating the components of lignocellulosic materials without extensive use of chemicals and energy, as done in traditional wood pulping, presents a challenging task. The development of new process technologies for converting the six-carbon sugars of wood cellulose into ethanol has been significantly hindered by this separation issue, despite intensive research efforts.
Biomass chemicals derived from purified components of lignocellulose are often produced through fermentation. Sugar or starch is typically used as feedstock for these chemicals, including ethanol, butanol/acetone, citric acid, and xanthan gum.
Although fermentation is a well-established process, its broader application to chemical production encounters some challenges. A typical fermentation process generates a dilute product stream containing only 10% or less of the desired product, which necessitates subsequent concentration.
Ethanol, for example, is commonly separated from the fermentation mixture through energy-intensive distillation. Although progress has been made in ethanol separation research and development, the problem has not been completely resolved.
Biomass chemicals can also be produced through chemical conversion routes. Industrial starches modified through chemical processes, as well as starch-based polymers, are manufactured using this approach. Additionally, vegetable-oil-based fatty acids and alcohols are produced via chemical process routes.
Due to the difficulty and cost associated with separating biomass components, some biomass chemicals are created through thermochemical routes that utilize the entire material. The oldest and most well-known example of this type of process is the destructive distillation of wood to produce methanol. In newer processes, biomass is gasified into synthesis gas and then reformed into methanol.
Biomass can also be gasified into methane or pyrolyzed into tars and liquid hydrocarbon products. However, most of the work on gasification and pyrolysis for chemical production is still in the laboratory or early stages of pilot plant development.
Another chemical conversion route involving the entire biomass material is anaerobic digestion of biomass solids or liquids. Originally developed as a method for treating liquid sewage, anaerobic digestion that produces methane was initially conceived as a less energy-intensive means of reducing the chemical oxygen demand of waste. This process was also explored as a way to produce methane of pipeline-quality.
3. Biomass Chemical Products
3.1. Ethanol and Its Derivatives:
Ethanol has been a widely studied biomass chemical. In the early 1980s, the United States supported the production of fermentation ethanol as a fuel extender through a fuel tax exemption. This incentive led to significant growth in the production of fuel and industrial-grade fermentation ethanol.
Industrial-grade fermentation ethanol started to replace synthetic ethanol in the chemicals market, and it was predicted that fermentation ethanol would dominate the industrial ethanol market by 1990.
Fuel ethanol is typically produced through batch fermentation of starches or sugars using the yeast Saccharomyces cerevisiae. Continuous fermentation processes were also being developed for commercialization.
Ethanol can be converted into other large-volume commodity chemicals. One example is the conversion of ethanol to ethylene by passing ethanol vapor over a heated fixed-bed alumina catalyst. The resulting ethylene-water mixture is separated into its components. This method, although expensive, is used in Brazil, where there is a government-supported fermentation ethanol program.
Acetaldehyde, which is currently produced via a petrochemical route, can also be derived from ethanol. Ethanol is vaporized and passed over a heated, fixed-bed chromium-activated copper catalyst. Acetaldehyde can be further converted to n-butanol via acetaldol and crotonaldehyde, although this process is costly.
Butadiene, a chemical previously produced from ethanol in a single-step process, involved passing acetaldehyde and ethanol over a tantalum oxide on silica gel catalyst. However, the energy-intensive distillation required for product purification made this process expensive.
3.2. Other Fermentation Chemicals
Apart from ethanol, there are various other chemical fermentation processes. One example is the butanol/acetone fermentation, which was once of great industrial importance, particularly in South Africa.
This fermentation process utilizes grain or molasses as raw materials and employs the bacterium Clostridium acetobutylicum. The process yields a mixture of butanol/acetone/ethanol, which needs to be separated through distillation.
Submerged aerobic fermentations have been developed for antibiotic production since World War II. These fermentations have enabled the production of chemicals like citric acid, gluconic acid, itaconic acid, xanthan gum, and other polysaccharides with applications as thickeners, emulsifiers, stabilizers, and potentially in oil recovery or clean-up.
3.3. Sugar Chemicals
Dextrose, obtained from starch, is an important raw material for manufacturing sorbitol, a chemical widely used in food, dentifrices, cosmetics, and as a chemical intermediate. Sucrose polyols also play a role in urethane foam manufacture.
3.4. Starch Chemicals
Industrial starch derived from grains and potatoes finds numerous applications. It can be used as is or modified through processes like depolymerization to produce dextrins or through chemical reactions that substitute hydroxyl groups with other substituents.
Modified starches include oxidized starch, acetates, phosphates, amino alkyl ethers, and other ethers. These low-cost materials are used as paper and textile sizes and binders.
Some more extensively modified starch chemicals have been introduced, such as starch-acrylonitrile graft copolymer, which has specialty uses in personal care products and medical applications. Starch can also be converted into α-methyl glucoside, a starch polyol used in the production of rigid urethane foam.
3.5. Chemicals Derived from Oilseeds
Oilseeds such as linseed, soybean, cottonseed, coconut, palm, and other plant oils and animal fats have always been significant raw materials in chemical manufacturing. These oils contain a mixture of triglycerides of fatty acids with carbon chains ranging from 8 to 20.
Despite facing competition from petrochemical counterparts, biomass fatty acids still maintain a considerable market share in surfactants, paint binders, and plasticizers.
The production of natural fatty acids involves hydrolyzing fats or plant oils under high-pressure steam conditions, requiring substantial investments in high-pressure equipment and vacuum distillation capacity for acid separation.
However, these products find substantial markets in surfactants, plasticizers, greases, synthetic lubricants, cosmetics, toiletries, textile chemicals, emulsifiers, secondary petroleum recovery products, and mineral beneficiation.
Triglycerides also yield long-chain fatty alcohols with applications as lubricants and surfactants. Soap, another natural oil product, has lost ground to synthetic detergents, but glycerol, a byproduct of soap or fatty acid production, remains competitive against synthetic glycerol.
Fatty acid derivatives, such as azelaic and pelargonic acids, obtained through the ozonolysis of oleic acid, have found niches in specialty markets. These acids are used in modified polyester fibers, elastomers, adhesives, plasticizers, synthetic lubricants, and surface coatings.
Nylon 11, derived from ricinoleic acid found in castor oil, is marketed as a plastic for molded parts in automobiles, conveying equipment, and other machinery. Other specialized nylons can also be derived from natural fatty acids, such as nylon 9 from soybean oil and nylon 1313 from erucic acid sourced from crambe oil.
Jojoba oil, extracted from the seeds of the jojoba plant, an exotic desert shrub, finds applications in cosmetics. Its chemical structure closely resembles that of sperm oil, which is now restricted in many nations due to conservation efforts for the near-extinct sperm whale.
Jojoba oil producers are exploring opportunities to introduce this ester of a long-chain fatty acid and a long-chain alcohol into the high-pressure lubricant market and potentially the automobile lubricant market.
3.6. Furfural and Its Derivatives
Furfural (2-furaldehyde) and its derivatives, including furfuryl alcohol, furan resins, and tetrahydrofuran, are produced in numerous countries using biomass materials such as corn cobs, wheat and oat hulls, and other sources.
Furfural is derived from pentoses, five-carbon sugars present in hemicellulose, a major component of lignocellulosic materials. The production process involves steam treatment followed by dehydration. Furfural was considered a potential product for developing countries with abundant biomass resources in the early 1980s, and several nations have successfully established production plants.
Approximately one-third of the furfural produced is used as a solvent, while the remaining two-thirds serve as intermediates in the manufacturing of furfural derivatives.
The most important derivative is furfuryl alcohol, which is used in the production of furan resins for foundry sand binders.
Another derivative, tetrahydrofuran, can also be produced through petrochemical methods. Expanding furfural production could lead to the discovery of additional uses, such as the production of adiponitrile.
3.7. Cellulose Polymers
Cellulose polymers, including rayon, cellulose acetate, cellulose ester, cellophane, and modified cellulose, continue to be significant despite facing strong competition from petrochemical alternatives.
Interest in cellulose arises from research on alternative fuels that emerged from the oil crises of the 1970s. Cellulose is considered a potential source of hexose sugars, which can be fermented to produce ethanol.
Research on cellulose-to-ethanol conversion has led to novel lignocellulose fractionation schemes, which may contribute to the development of pulping technologies that preserve the value of hemicellulose and lignin fractions in lignocellulosic materials.
3.8. Lignin Chemicals
Paper pulp mills generate substantial waste streams containing lignin and hemicellulose fractions from wood. Typically, these waste streams, which contain chemicals from the pulping process, are primarily used for their energy value through combustion. However, a small portion can be converted into various chemicals.
Neutral sulfite semichemical pulping produces black liquor, from which acetic acid and formic acid can be extracted. Kraft black pulping liquor may serve as a source of dimethyl sulfoxide, produced by heating the liquor to convert lignin methyl groups into methyl sulfide, followed by oxidation to dimethyl sulfide.
Sulfonated lignins, a major component of Kraft black liquor, can be recovered as lignosulfonates, which are utilized as dispersants, sequestrants, emulsion stabilizers, and humectants. Crude calcium lignosulfonate can be oxidized to vanillin, providing competition for the extract obtained from the vanilla bean.
Tall oil, a byproduct of the Kraft process used for pulping softwoods, is a mixture of rosin and a group of fatty acids.
New lignocellulose separation processes, such as steam explosion, have enabled the production of unmodified, low-molecular-mass lignin. Although this material is still undergoing extensive testing, it shows promise as a partial substitute for phenolic resoles in phenol-formaldehyde adhesives.
Other potential uses for this type of lignin include being a binder for animal feeds, an encapsulating agent, a feed grain coating, chemicals for road dust treatment, reinforcing fillers in rubbers, and a setting agent in Portland cement.
Advocates of “biomass refining” propose hydrocracking and hydroalkylation of lignin to produce phenol and benzene.
3.9. Methanol
Traditional methods of biomass methanol production, known as destructive distillation, involve heating wood in steel ovens, resulting in the production of charcoal, acetic acid, methanol, tar, and oil. However, this process is not economically feasible in industrialized countries, although it may be viable in less developed nations.
New methods of methanol production require biomass gasification, often using oxygen-blown gasifiers, to produce synthesis gas, which is subsequently reformed to produce methanol. Several projects are currently in the pilot plant or demonstration stage.
3.10. Methane
Various methods can be employed to produce methane from biomass. The most widely used method is anaerobic digestion, initially developed for the treatment of low-strength organic liquid wastes.
Anaerobic digestion is a slow fermentation process in which bacteria convert organic matter first into organic acids and then into methane. Extensive experimentation is currently being conducted in this field, with the primary applications found in waste treatment.
While methane production is often secondary in these applications, with the gas being used to fuel the process, the process can also be utilized solely for methane production.
References
- Biomass Chemicals; Ullmann’s Encyclopedia of Industrial Chemistry. – https://onlinelibrary.wiley.com/doi/10.1002/14356007.a04_099
- Chemicals from Biomass. – https://link.springer.com/referenceworkentry/10.1007/978-1-4614-6431-0_28-2
