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What Does Carbon Negative Mean Pdf


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A pdf of a Powerpoint presentation on the meaning of "carbon negative".

A pdf of a Powerpoint presentation on the meaning of "carbon negative".

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  • 1. 6/8/09. Page 1 What Does Carbon Negative Mean? Richard S. Stein University of Massachusetts, Amherst There appears to be some misunderstanding of the meaning of “carbon negative”. Most consider it to refer to a process which reduces the CO2 concentration in the atmosphere. This is in contrast to what happens when fossil fuels like coal and oil are burned which produce CO2 by their reaction with the oxygen in the air, resulting in an increase in its atmospheric concentration. This is “carbon positive”. Processes resulting in no change in atmospheric CO2 are properly called “carbon neutral”. This would be true for photovoltaic, wind, and geothermal processes. There is some controversy whether biofuels are carbon neutral. Carbon dioxide is consumed in the growth of biomaterials, where, through the catalytic reactions occurring through the absorption of light, it combines with water to produce energy containing organic molecules such as sugars, starch, and cellulose. When these are burned, they react with oxygen to produce CO2 and liberate energy. Ideally, the energy evolved is exactly equal to that absorbed in the photosynthesis, and the amount of CO2 evolved equals that which had been absorbed, In this sense, the change is “carbon neutral”. However, one must consider the entire process. Energy is consumed in the cultivation, harvesting, and collecting of the biomass, so if this is considered, less energy is obtained on burning than the total amount of energy used to grow and deliver the biomass. Coal and oil are formed from biomass through geological processes involving the influence of heat and pressure occurring over many thousands of years, whereas the burning of the derived fossil fuel usually occurs in short time periods. In this case, the CO2 which is liberated during this short time of burning is that which had been absorbed over many, many years. The process would only be
  • 2. 6/8/09. Page 2 carbon neutral if the material burned was that which was derived from biomass which grew during the time period of burning. If one obtains power from burning biomass directly, it is essential, in order to be carbon neutral, that this restriction be observed. The amount of biomass being burned should be limited to that which grew during the period of burning. Harvesting of biomass following this restriction is referred to as “sustainable harvesting”. Its collection must be made from a larger area of growth to meet this requirement. It is more expensive to do this, so doing so under profit motivation may not suffice. Regulations are needed to assure that it be done. As indicated, burning under conditions where more CO2 is produced is “carbon positive”. Itʼs environmental impact can only be reduced if this CO2 is prevented from entering the atmosphere. Ways to do this have involved: 1. Pumping it into the ocean. 2. Pumping it into underground cavities. 3. Adsorption. 4. Sequestering through chemical reactions. CO2 has limited solubility in water with which it reacts to form carbonic acid. This increases the acidity of the water which can be detrimental to marine life and cause bleaching and dissolving of coral and and crustacean shells. It is thought that oceans are approaching their capacity for capturing CO2 in this way. An alternative is to liquefy the CO2 and pump it to deep regions of the ocean where the pressure is sufficiently great for it to be stable Pumping to underground cavities is being employed, but numbers of suitable locations are limited and it is costly to deliver it to remote ones. A possibility is to use it to replace depleted underground oil deposits and even to use it to force oil to the surface. There is concern about stability, since geological phenomena are not well enough understood to be confident that it will not leak back to the atmosphere or be catastrophically released as a result of earthquakes, etc.
  • 3. 6/8/09. Page 3 It has been suggested that it could be adsorbed in materials like carbon nanotubes, but at present, the cost of doing so would be prohibitive. There are chemicals that will react with CO2 to form stable solids. For example, lime, CaO, will do so to form calcium carbonate, CaCO3. However, most lime is made from limestone in a process which releases CO2. Cement making processes have been proposed which trap the CO2 in the cement. However, economics is again a problem, and one has yet to find a cost effective procedure. A practice which is encouraged is to leave agricultural waste on the field where it serves to enhance the organic content of the soil, improving its agricultural value. However, these organic components have been shown to decompose in a few years, releasing their CO2. The CO2 capture has a short term benefit and could be regarded as “carbon negative” during this time period, but in the long run, it is not. The use of biochar has been proposed as a means of achieving carbon negativity. The biomass, grown with the absorption of CO2, is pyrolyzed (heated in the absence of or with limited air), Combustable vapors are evolved which serve to continue the heating. The biomass gets converted to biochar, a form of charcoal having useful value. It serves as an agricultural additive, and when mixed with fertilizer and mulch, promotes agricultural growth. It is believed to adsorb the nutrients on its surface and porous interior and serve as a matrix for the growth of fungi and bacteria, enhancing the growth of the biomass. The biochar has been shown to remain in the soil for very long periods of time, sone being found there derived from this practice employed by natives in the Amazon thousands of years ago. Hence, it serves top capture the carbon from the CO2 in a stable form, thus
  • 4. 6/8/09. Page 4 serving to reduce the CO2 concentration in the atmosphere. This is really “carbon negative”. In addition to this benefit, it has been shown that less fertilizer is required when combined with biochar in that the fertilizer is concentrated in the biochar matrix where it is more effective. Its being being bound to the biochar prevents the loss of excess to streams and rivers where it may pollute. To achieve carbon negativity, one should combine biochar use with sustainable harvesting so there is a net removal of CO2 from the atmosphere with long term sequestering in the soil. While less heat is produced during biochar production than would be of the biomass were just burned, its agricultural and soil enhancement benefits more than compensate. Its use can be cost effective for the farmer and of value to society.