Biomass can be defined as material of biological origin excluding material embedded in geologic formation and/or fossilized. This definition refers to the short carbon cycle, the life cycle of biological materials (e.g. plants, algae, marine organisms, forestry, micro-organisms, animals, and biological waste from households, agriculture, animals, and food/feed production).
The Earth’s biomass represents an enormous store of energy. It has been estimated that about one eighth of the total biomass produced annually would provide all of humanity’s current demand for energy. Furthermore, since biomass can be regrown, it is a potentially renewable resource.
One of the most attractive aspects of biomass energy is that it doesn’t contribute to the enhanced greenhouse effect, provided that the biomass is harvested sustainably. Biobased products are produced in a biorefinery that integrates biomass
conversion processes. The concept of a biorefinery is analogous to a petroleum refinery where a feedstock, crude oil, is converted into fuels and a wide range of valuable chemicals and materials. In the case of a biorefinery, biomass is used as a feedstock to produce biofuels and a wide range of valuable chemicals and materials.
Biobased products can be manufactured from various biomass feedstocks. Two categories of feedstock and products dominate: first and second generation. First-generation products are manufactured from edible biomass such as starch-rich or oily plants. Second-generation products utilize biomass consisting of the residual non-food parts of current crops or other non-food sources, such as perennial grasses or algae (products from algae are sometimes also referred to as third-generation products). These are generally considered as having a significantly higher potential to replace fossil-based products.
Sugar / starch crops
The most common type of biorefinery today uses sugar- or starch-rich crops. Sugar crops such as sugar cane, sugar beet, and sweet sorghum store the chemical energy as simple sugars (mono- and disaccharides), which can be easily extracted from the plant material for subsequent fermentation to bioethanol or biobased chemicals. Sugar cane is restricted to certain locations like Brazil due to weather and soil requirements.
Starch-rich crops, such as corn, wheat and cassava (manioc), store the energy as starch, a polysaccharide. Starch can be hydrolyzed enzymatically to deliver a sugar solution, which can subsequently be fermented and processed into biofuels and biobased chemicals. The processing of many starch-rich crops also delivers, as a side product, valuable animal feed that is rich in pro- tein and energy.
The use of first-generation sugar/starch crops for production of biobased products (primarily bioethanol) has generated an important issue regarding land use change. According to the World Economic Forum, in 2010 there were about 400 operational first-generation biorefineries around the world. Yet, despite the initial success of this biofuel feedstock, its future is debated because GHG emissions from land-use change are likely to exceed, or at least offset, much of the GHG savings obtained by using biofuels for transport.
Vegetable oil/animal fat
The most common raw materials for biodiesel (fatty acid alkyl esters) are vegetable oils and animal fats (triglycerides). The process used is transesterification with an alcohol. Two categories of oil exist: pure plant oil (PPO) and waste vegetable oil (WVO). Pure plant oil stems from dedicated crops such as rapeseed, soybean, palm and sunflower seeds. Animal fats are attractive feedstocks for biodiesel because their cost is lower than that of vegetal oil. Use of waste vegetable oil or animal fats is an effective method of recycling daily wastes.
Sustainable and economic production of biodiesel from palm oil has proven to be a challenge. This is due to the significant land-use change and sustainability issues linked to pure plant oil production, and the high costs associated with the refinement of waste oil.
Lignocellulosic biomass (or simply biomass) refers to inedible plant materials made up primarily of cellulose, hemicelluloses, and lignin. It represents the vast bulk of plant material. It includes:
- agricultural waste, such as straw, corn stover (leaves and stalks after harvest), corn cob (the hard cylindrical core that bears the kernels of an ear of corn), bagasse (dry dusty pulp that remains after juice is extracted from sugar cane), molasses (thick, dark syrup from the processing of sugar cane or sugar beet);
- forestry wastes, such as wood chips;
- fraction of municipal and industrial (paper) wastes;
- fast-growing energy crops, such as miscanthus, switchgrass, short-rotation poplar or willow coppice.
These feedstocks exclude direct land-use change. Such biomass resources typically contain on dry weight basis 40–60% cellulose, 20–40% hemicelluloses, and 10–25% lignin.
Second-generation feedstock is very likely to be used industrially in the future for the production of fuels and other biobased products. However, it is more difficult than expected to convert lignocellulosic biomass into useful products.
The Jatropha curcas tree produces seeds containing 27–40% inedible oil, which can be converted to biodiesel. An assessment of Jatropha curcas sustainability reveals a positive effect on the environment and GHG emissions, provided cultivation occurs on wasteland or degraded ground. Due to its labor-intensive production chain, it is thought that Jatropha is one of the drivers for rural development.
Microalgae are single-cell photosynthetic organisms known for their rapid growth and high-energy content. Depending on the species, their sizes can range from a few μm to a few hundreds of μm.
Some algal strains are capable of doubling their mass several times per day. In some cases, more than half of that mass consists of lipids or triacylglycerides, the same material as in vegetable oils.
Microalgae have attracted considerable attention in recent years due to their potential value as renewable energy resources. Focus has been on storage triglycerides, which can be used to synthesize biodiesel via transesterification. The remaining carbohydrate content can also be converted to bioethanol via fermentation.
The advantages of using algae-derived fuels are numerous. First, they can provide between 10 and 100 times more oil per acre than other second-generation biofuel feedstock, and the resulting oil content of some micro-algae exceeds 80% of the dry weight of algae biomass, almost 20 times that of tradi- tional feedstock. They need not compete with arable land. Furthermore, they are highly productive, quick to cultivate, and require only CO2, sunlight, and water to grow.
Like plants, algae use the sunlight for the process of photosynthesis. They can be produced in two ways: ponds and photobioreactors (PBR). Open ponds are the simplest of algae growing systems. An alternative to open ponds are close ponds where the control over the environment is much better than that for the open ponds. Close ponds cost more than the open ponds and considerably less than photobioreactors for similar areas of operation. A photobioreactor is a closed equipment which provides a controlled environment and enables high productivity of algae.
However, several barriers remain to be overcome before the large-scale production of microalgae-derived biofuels can become a reality. Producing low-cost microalgal biodiesel primarily requires improvements to algal biology through genetic and metabolic engineering, to achieve optimum char- acteristics, such as high growth rate, high lipid content, and ease of extraction. Advances in photobioreactor engineering are expected to lower the cost of production.
Depending on the nature of the feedstock and the desired output, biorefineries employ a variety of conversion technologies.
First-generation liquid biofuels include bioethanol from sugar/starch and biodiesel from oil/fats. They are currently made mostly using conventional technology. They have generated the “food versus fuel” issue that has claimed that the production of biofuel has diverted crops away from food to fuel.
First-generation chemicals and materials from sugar/starch include polymers, such as polylactic acid (PLA), and chemical building blocks, such as succinic acid and 1, 3 propanediol. Chemicals from vegetable oils include fatty acids and esters. Bioethanol itself is a building block for the production of polymers, such as polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP) or polyethylene terephthalate (PET).
Transesterification and other conversion of triglycerides
Transesterification of plant oil or animal fats is a process by which triglycerides are reacted generally with methanol in the presence of a strong base catalyst to produce fatty acid methyl esters (FAME) as biodiesel and glycerol.
Besides the transesterification process, the Neste process entails direct hydrogenation of plant oil or animal fat triglycerides into the corresponding alkanes, thus removing oxygen from the oil.
Sugar in sugar crops can be directly fermented to produce ethanol. The fermentation of sugar solutions from starch crops usually requires an enzymatic hydrolysis of starch with use of amylases to liberate the sugars from the plant material. The hydrolysis is followed by the microbial fermentation to produce ethanol.
Lignocellulosic biomass is not produced for food, and hence makes the “food versus fuel” debate much less of an issue.
Second-generation biofuels include cellulosic ethanol, BtL (biomass to liquid) diesel, and other BtL fuels. These biofuels are still generally in the demonstration plant stage, even for the most advanced.
Internationally, there is increasing interest in the use of cellulosic biomass for the production of second-generation chemicals and materials. Currently such chemicals and materials are generally in the research and development or demonstration stage.
There are two primary pathways for producing biobased products from lignocellulosic biomass: biochemical and thermochemical. Biochemical conversion basically involves hydrolysis of the polysaccharides in the biomass, and fermentation of the resulting sugars into ethanol. Thermochemical conversion typically involves gasification or other thermal treatment of the biomass followed by catalytic synthesis or fermentation of the resulting gas or liquid into ethanol, diesel, and other fluids.
Before fermentation, lignocellulosic feedstock processing needs to separate the cellulose and hemicelluloses from the non-fermentable lignin. This is usually performed mechanically, followed by acid, alkali, and/or steam treatment. While the lignin is currently mostly burned to generate energy, cellulose and hemicelluloses are hydrolyzed enzymatically with use of cellulases to deliver sugar solutions for subsequent fermentation.
Gasification and Fischer-Tropsch process
Gasification of biomass converts carbonaceous materials into synthesis gas, namely H2 and CO, known as syngas. Gasification is achieved at high temperatures in the presence of a limited amount of oxygen. The resulting syngas can then be used in the Fischer-Tropsch process to produce hydrocarbons.
The conversion of syngas via the Fischer-Tropsch process involves the catalytic conversion of syngas into liquid hydrocarbons ranging from methane to wax. A selective distribution of products is achievable via control of temperature, pressure, and the type of catalyst. When applied to biomass and biofuel production, the process is commonly referred to BtL.
Pyrolysis is the thermal decomposition of the biomass in the absence of oxygen to produce char, gas, and a liquid product (bio-oil) rich in oxygenated hydrocarbons. It can be upgraded to lower the oxygen content and transported using the same infrastructure used by the oil industry. Bio-oil and upgraded oil can be used in applications ranging from value-added chemicals to transportation fuels.
Biodegradable waste or energy crops can be converted into a gaseous fuel called biogas, comprised primarily of methane and CO2. Commercial processes typically run via anaerobic digestion or fermentation by anaerobic organisms. This process is used as a renewable substitute for commercial natural gas.
Extract of the book: Lignocellulosic Biorefineries By Jean-Luc Wertz and Olivier Bédué Published by the Presses Polytechniques et universitaires romandes