What is biomass energy? What are biofuels?

Biomass energy production, at its core, is the use of: photosynthesis to capture solar energy, organic carbon compounds to store the energy, chemical and/or biological processes to extract and convert the stored biomass energy into fossil equivalent fuels called biofuels, and finally, combustion of the biofuels. Biofuels are fuels that are derived from the biological material of living or recently living organisms called 'biomass'. In the modern era most biofuels are derived directly from the biomass of photosynthetic organisms, such as ethanol from the stalks of the sugarcane plant, whereas in centuries past biofuels were sometimes derived from photosynthesis indirectly, such as from animal biomass, e.g. oils from the blubber of whales. Some biomass can be used directly as biofuel without further processing, such as the dry cellulose from trees used in wood stoves. However, most biofuels with higher energy densities such as ethanol, require processing of the raw biomass into biofuel. In cases where the biomass has to be processed, it is called 'feedstock'. For ethanol, common feedstocks are sugarcane, corn, and sugar beet.For diesels, common feedstocks are rapeseed (aka canola), soybeans, and palm nuts. For bio-oil the feedstock can be a wide array of hydrocarbon containing materials.

Photosynthesis

            Biomass energy production begins with the use of photosynthesis to capture solar energy. Nature evolved a system to harness the energy in solar photons hundreds of millions of years ago, which we call photosynthesis. Photons strike antenna chlorophyll molecules in plants, which excites electrons in the photo-reaction centers attached to the antennas to higher energy levels. The electrons are then separated by the chlorophyll photo-reaction center from the electron holes they leave behind and the two are able to react separately with different molecules on opposite sides of the photo-reaction center, with the electron reducing CO₂ and protons (H⁺) to carbohydrates (CH₂O) and with the electron hole oxidizing H₂O to oxygen (O₂), protons (H⁺), and electrons (e^-)[1].

Chlorophyll's structure is such that it absorbs light primarily in the 400-700 nm region of the radiation spectrum, called Photosynthetically Active Radiation, which contains 50% of the energy in solar photons. The plant itself only physically absorbs 80% of incident photons in the 400-700 nm range, with the rest lost to reflection, transmission, and absorption by other molecules. Of this 40% of the total incident solar energy which makes it to the chlorophyll antenna system, only 28 percent is converted to carbohydrates, the rest are lost in electron transfer and chemical steps. Thus, the maximum possible conversion efficiency from sunlight to carbohydrates is 50% * 80% * 28% = 11%. Land-based plants uses 40% of the energy for its own metabolic needs, leaving 11% * 60% = 6.7% as the portion of incoming sunlight which is stored as photosynthetic energy. This maximum possible conversion efficiency is for C₄ plants (named such because their first photosynthetic product is a 4-carbon sugar), which include corn, sugar cane, switchgrass, and sorghum; C₃ plants, which account for 95% of the global plant biomass (named such because their first photosynthetic product is a 3-carbon sugar), have a maximum possible photosynthetic efficiency approximately half as efficient (~3.4% maximum) because of differences in their photosynthetic systems[2].

      Note however, the 6.7% and 3.4% figures are maximum possible photosynthetic efficiencies based on the biophysics of land-based plants; inefficiencies must be added for insufficient or excess insolation, insufficient nutrients, insufficient water, nighttime photorespiration, and disease. In practice maximum sustained photosynthetic efficiencies under natural light and atmospheric conditions were measured at 3.5% for C₄ crops and 2% for C₃ crops[3]. A simple exercise shows us that, in practice, average photosynthetic efficiencies of crops are far lower than even those values: 2% average efficiency for potatoes (a C3 crop), receiving an average of 100 W/m² (total yearly solar energy averaged over entire year for the United Kingdom, a highest yield potato producer), would result in 153 tonnes/ha fresh weight of potatoes per hectare; in actuality a yield of 45 tonnes/ha fresh weight of potatoes is observed, translating to 0.6% photosynthetic efficiency or approximately 1/3 of the maximum value[4]. Corn (a C4 crop) has approximately a 1% average photosynthetic efficiency[5]. For the purposes of this analysis we will use a value of 1% for average photosynthetic efficiency of land-based plants. Marine-based photosynthetic organisms (technically not plants), such as algae and cyanobacteria have higher average photosynthetic efficiencies of 2-3%[6][7] because their aqueous surroundings provide easier access to nutrients and water, meaning fewer structures for waste transport, nutrient transport, water retention, and rigidity are necessary, which means less photosynthetic carbohydrate product has to be diverted to the maintenance and repair of these structures.

Storage of energy in organic carbon compounds

            The captured energy is initially stored as carbohydrates through photosynthesis. Carbohydrates encompass a wide range of molecules, ranging from simple sugars such as glucose, to complex sugars such as amylose (a starch), to complex carbohydrates such as cellulose (indigestible by non-ruminants such as man but digestible by ruminants such as cows, as well as certain bacteria), and lignin (indigestible by all organisms with the exception of certain fungi), and are used for two primary purposes, energy storage for the plant (generally in the simple sugar or starch form), and structural integrity (plant walls made of cellulose and lignin).

Simple carbohydrates can be converted into triglycerides in the plants through a biosynthetic process called lipogenesis. Lipogenesis requires ATP, a biological energy transfer agent; this lowers the conversion efficiency of incident sunlight to stored energy even further when the final biological storage medium is composed of triglycerides. Triglycerides are commonly known as fats and oils.

Extraction and conversion of biomass energy into fossil equivalent fuels

            While carbohydrates are useful biological energy storage mediums, they usually have to be extracted from the plant biomass. This is because most plant biomass has large quantities of water, and the carbohydrates are either stored in aqueous internal solutions in the plant (as is the case with glucose and starches), or they are moist. For example, a very simple form of carbohydrate biomass energy is wood, which has to be dried after cutting down a living tree.

After physical extraction the carbohydrates are usually in a form not useful for combustion, they contain significant water content, and so they are dried. For wood this means wood drying, for liquid fuels this means converting the carbohydrates to a form that allows removal of the water, usually an aqueous alcohol, which is then distilled to 95.6% alcohol (by volume), and then purified to 99.5 anhydrous alcohol using a molecular sieve. Alternatively, the use of plant oil for the biofuel allows the avoidance of distillation, because oil and water naturally are immiscible. The oil is extracted from the crushed plant through the use of a solvent in which the oil is soluble.

Carbohydrates in their raw form cannot be directly combusted in internal combustion engines, either because they are dissolved in the plants aqueous internal fluids (water suppresses combustion) or are solid if separated from the internal fluids (solids are not useful for internal combustion engines where the combustion gases also perform the movement of engine parts because they clogs the engine). A form which is liquid at room temperature and which contains no water is necessary for useful internal engine combustion. Note that solid carbohydrates can be used in external combustion engines (i.e. power plants), because the working gases which move the turbine or piston are separate from the combustion gases. This is done in Brazil, where the leftover cellulose from sugar cane harvest, called bagasse, is burned in the sugar and ethanol refineries to generate electricity which is then sold to the power grid (and to provide heat for the distillation process).

            Liquid triglycerides, such as oils from plants, can be directly combusted in internal and external combustion engines, but require engine modification because the triglycerides have a significantly different molecular structure than petroleum-based liquid fuels, which results in high viscosities which are incompatible with traditional diesel engines and provide extremely poor cold weather performance. Typically the highest value for any biofuel is as a direct petroleum-based liquid fuel replacement, namely because direct replacement implies little engine modification, and because liquid fuels carry a premium because of their portability (and hence ease of use in mobile internal combustion engines such as car engines). While carbohydrates are typically converted to an alcohol (usually ethanol), triglycerides are typically converted to long chain esters, which functions similarly to the long chain petroleum fuel we call diesel.

Combustion of the biofuels

            The energy stored in the biofuels must be extracted somehow. Extraction of energy from the biofuels typically is done via combustion. Combustion is the reaction of a fuel with oxygen which results in the release of energy. The bonds in O₂ and the bonds in the fuel are broken to form the component atoms in a transition state, this requires energy input because free atoms are less stable than bonded atoms (which can be initially provided by a spark and later provided by the second stage of the reaction). In the transition state the atoms recombine to form the product molecules, this releases energy because bonded atoms are more stable than free atoms. As long as the formation of the product releases more energy than the input energy to break the reactant, the reaction will propagate and there will be energy release in the form of heat.

Internal combustion engines utilize the combustion gases directly, and the energy release in the form of heat increases the pressure inside the reaction chamber until the pressure reaches a point where it can move the piston. External combustion engines have the combustion occur external to the turbine or piston chamber, and transfer heat to a working fluid (usually liquid water which forms steam, but can also be air or other fluids). The fluid increases in volume as heated and thus the pressure inside the turbine chamber increases. The vapor expands and drives the turbine forward, losing some of its energy. It is then cooled, which condenses the fluid which is recycled into the system.