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Increasing energy requirements is leading to higher fossil fuel consumption and high greenhouse gas emission, which are suspected to cause climate change. CO2 is a primary greenhouse gas and is expected to increase still more steeply over the next few years. The task is enormous: to stabilize and then reduce the level of CO2 emissions by recycling this gas whenever and wherever feasible. Just as former wastes are now seen as valuable recyclable materials, CO2 can prove to be an extremely useful source of biofuel and other goods and services.
On the other hand, large volumes of biomass are going unused, about 200 million tons per year in the US alone. While solar and wind power are being explored, biomass remains an excellent way to meet electricity and liquid fuel needs while recycling CO2. It is renewable, which only adds to the attraction. Biomass can also be used at a sustainable pace, because it contains already stored energy, is stable in price and cheaper than fossil fuel, and can be used whenever needed. Large-scale biomass can also be used to generate power over 20 MW, but the excess heat is cooled off, for a net combustion efficiency of 30%. Various technologies are used, such as bubbling fluidized bed combustion (BFBC) and circulating fluidized bed combustion (CFBC), pulverized fuel combustion, and steam turbines.
Biomass and CO2 Recycling
Some ways of recycling CO2 with biomass include:
Algal Recycling of CO2
The use of CO2 from industrial plants to feed algae, phytoplankton or bacteria enables the production of lipids which can be converted into biofuel, food, nutritional supplements or feed for livestock. This CO2 is captured and recycled so that the same cycle supplies CO2 for multiple products. Diesel fuel can be produced in this manner. Even more, the CO2 which is being purchased for the use of the microbes can be channeled from an industrial process which produces it as an unwanted byproduct. This not only makes commercial sense but also prevents environmental pollution and the need to remove CO2 from the atmosphere, saving tax money for other more profitable uses. This is the tried and tested recycling philosophy at work, and is called Carbon Capture and Utilization (CCU). Algal biofuels result in a CO2 emission decrease of 50-80% through several mechanisms.
Replacement of Fossil Fuel-Dependent Materials with Biomass
Another method is the use of wood in construction rather than steel or concrete. This allows the CO2 already captured by the tree in its lifetime to be locked away as long as the wood lasts, which is often many decades. In addition, by not using the other materials which rely heavily on fossil fuels for their manufacture, spewing millions of tons of CO2 into the air, even more CO2 emission is prevented. Finally, the wood remaining from construction is fed into a biofuel process which allows still more reduction in CO2 emission from the use of fossil fuels, as well as promoting an alternative source of energy which does not involve CO2 emission, and increasing energy security. Forests meant for this purpose must be carefully designated and other forests preserved for their ecological value. Even though older trees do not absorb as much CO2 as actively growing ones do, they are important for their environment and should not be wantonly felled to keep CO2 out of the atmosphere. Biomass waste can be burnt and the residual ash spread over the forest floor for the growth of new trees, by uptake of these minerals in the wood.
This may be the most ecologically sound of these approaches. It consists of conserving natural vegetation which recycles and sequesters CO2 at an impressive rate. This may be in the form of wetlands, grasslands, or forest land. Studies have always shown that green regions absorb far more CO2 than they produce.
Injecting CO2 emissions from industrial plants deep underground into rock formations for long-term or permanent storage. This geologic sequestration is now being carried out but is immensely expensive besides being in a very preliminary stage of development.
Biomass, Biofuels and CO2 Recycling
Biofuel production with biomass has numerous advantages. Biomass is used to generate energy, either directly as fuel in biomass combustion plants, or as the basis for making biofuel. These processes may use woody biomass in the form of bark, industrial or forest wood chips, sawdust, or wood grown in short-rotation woods especially for use as fuel, waste wood, pellets or briquettes. Herbaceous biomass can also be used to generate energy, as straw, cereals, grasses, kernels, grain husks and so on.
Methanol and Dimethyl Ether
One promising pathway is to react CO2 with another greenhouse gas, methane, or just using biogas, to produce biofuel. Both biogas and methane can be produced by fermenting biomass. This results in the formation of a stable and efficient biofuel, namely, methanol or wood alcohol. Apart from its importance in energy production, methanol is also involved in the production of plastics, and thus provides a pathway for permanent CO2 sequestration. Methanol is among the cleanest of biofuels, as is DME, Dimethyl Ether, a simple derivative of methanol, with no soot production on combustion, and suitable as a replacement for heavy-duty diesel and propane.
Advantages of Biofuels in CO2 Recycling
Biomass can be used to manufacture biofuels in a number of pathways, which are either biochemical, depending upon enzymatic action, or thermochemical which depends upon gasification, combustion and pyrolysis. Biomass-based biofuel production helps to reduce CO2 emissions in many ways:
- It prevents gasoline-associated CO2 emissions by replacing it with biofuel which burns more cleanly
- It keeps the CO2 inside the fossil fuels sequestered
- It increases atmospheric CO2 absorption because more wood and other biomass is grown to feed the process
- The process itself uses CO2 at several points to streamline the workflow and save on pretreatment of the slurry, which brings down the production costs to a major extent, but without requiring any major modifications of the equipment or increasing the toxicity of the industrial cycle
- Another biofuel is producer gas which can be simply produced from commonly used combustion systems such as engine exhausts, just by ensuring water condensation and adding appropriate amounts of oxygen. Producing syngas from biomass is called biomass to liquid fuel (BTL) and results in liquid fuels suitable for transportation, which burn very cleanly, contain no sulfur and no nitrogenous compounds. The biofuel production and combustion pathway is incorporated naturally into the earth’s carbon cycle which means an overall lowering of CO2 emission.
Biomass gasification uses recycled CO2 to produce syngas, a mixture of CO2 and sulfur gas. The slow partial burning of biomass yields syngas which is a fuel with low calorific value, containing CO2, H2, CH4 and CO with other tars. Though not a very good fuel in itself, syngas can be fermented or Fischer-Tropsch synthesis to yield liquid fuel.
Bioethanol is a biofuel of primary importance which emits hardly any CO2 on combustion, can be blended with gasoline in 10%, 20% and 22% blends (called E10, E20 and E22 respectively). It can also be converted to ethyl tertiary butyl ether (ETBE), which pushes up the octane number of the fuel to limit the noxious gas emissions from combustion of engine fuel, without having to modify the engine itself.
Research is also going on to use CO2 as a resource to create CO, a very reactive source material for biofuel production, using tungsten diselenide nanocatalysts.
Plastics, Biomass and CO2
Algal plastic made from CO2 is also a reality in the research world, and can replace the toxic processing of petroleum to make plastic products. Polyethylene terephthalate (PET) is one of the most widely used plastic polymers today, and four tons of CO2 are produced during the manufacture of each ton of PET. Hence a new plastic called polyethylene furandicarboxylate (PEF) has been developed which is made from biomass rather than from petroleum, and keeps out oxygen better. These products can also be incinerated to produce CO2 which can be used by plants of all sorts, which then go back into the process to recycle the plastic completely. The commercial viability of this process is being ironed out. Polyhydroxyalkanoates (PHAs) are another group of biodegradable plastics derived from bacteria (Ralstonia eutropha) by carbon fixation which can replace plastics from petroleum.
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