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Your Precious Yard Trimmings: Biomass and Chemical Applications
Posted on December 3rd, 2015 by Dr. Sina Ebnesajjad in Chemical R&D
It is autumn and people are hauling the tree limbs they have pruned and other yard waste to municipal piles. That or placing them at the curb for trash pick-up disposes the limbs. What a shame, you think, to throw away all that carbon and hydrogen that the trimmings contain. Plant life takes carbon dioxide and water out of the air and uses them to grow plant life. What if we could do something useful with all the tree limbs and plant materials, known as biomass, thrown away every day?
The Opportunity in Biomass
The idea of using biomass as a raw material for fuel and chemicals has been around for nearly a century but the incentives to make it work commercially have not. A small portion of US electricity is produced from burning biomass. Paper mills, for example, consume scrap pulp and wood to generate energy. Corn is also grown to produce ethanol for blending with gasoline although the overall benefits of ethanol blending have been questioned. The issue of Global Warming has had a positive impact on increasing the number of research projects on biomass conversion in recent years.
World production of biomass is estimated at 146 billion metric tons a year, mostly from the growth of wild vegetation. Annual estimates for the US indicate roughly 14 million dry tons (DT) of wood in municipal solid waste and construction debris. According to a study by Oregon State University wood milling residues amounts to 87 million DT per year. US forest harvest residues weigh 64 million DT. The biomass scale is incredibly large.
It seems sensible to take advantage of the carbon dioxide removal machines called trees and plants. Fossil fuels store carbon in their chemical structures locked underground. Burning of oil products and coal burnt adds new carbon to the existing load in the earth atmosphere. The basic photosynthesis reaction actually removes atmospheric carbon dioxide and generates sugars and oxygen using solar energy:
The produced sugars are stored in three types of polymers: starches, cellulose and hemicellulose (lower molecular weight than cellulose). Biomass is composed of 75-90 wt% sugar polymers and the other 10-25 wt% being was mainly lignin.
Sugar polymers such as cellulose and starches can be readily broken down to their constituent monomers by hydrolysis for conversion to ethanol or other chemicals. Lignin is a complex structure containing aromatic groups and is less readily degraded. Lignocellulose is the most abundant biomass on the earth, e.g. found in switch grass. The sugars in lignocellulose are tightly bound to lignin, which must be broken down before its sugars are available for hydrolysis. Pretreatments to remove lignin and hemicellulose can significantly enhance the hydrolysis rate of the cellulose component. Lignin could become an important source for some polyaromatic chemicals.
Approaches to Biomass Conversion
The Royal Belgian Academy Council of Applied Science describes three main approaches for conversion of biomass: thermochemical, chemical and biochemical.
- In the thermochemical route, biomass can be converted to feed material thermally in the presence (gasification) or absence of oxygen (pyrolysis). Gasification exposes the biomass to very high temperatures to produce gases converted into carbon monoxide and hydrogen mixture (Syngas) for conversion by Fischer Tropsch reaction into chemicals and polymers. Pyrolysis is operated at lower temperatures, than gasification, yielding mainly liquid products.
- Chemical conversion of biomass using a catalyst has been the route to produce industrially important chemicals. Inorganic catalysts allow such as acid hydrolysis, trans-esterification reactions to take place efficiently. A new approach involves combined hydrolysis and catalytic hydrogenation of biomass to produce sugars such as sorbitols. These sugars can be converted to industrially useful intermediates and eventually polymers.
- Biochemical conversion involves breaking down the biomass by using enzymatic and/or microbial action, to make the polymeric carbohydrates available as (fermentable) sugars, which can then be converted into biofuels and products using microorganisms (bacteria, yeasts, fungi etc) and enzymes. Cost and difficulty of biochemical conversion are two impediments to this route.
Conversion of mixed biomass has proven complex because of the hundreds of chemicals produced and their impact on the conversion reactions. Consequently, researchers have focused on specific biomass source materials such as softwoods or single species of plants and trees. Also the development of transgenic plants (e.g. poplar, willow, maize,…) with lowered lignin content has been a major step forward.
The variety of chemicals that can be produced from biomass is also beneficial. Conversion of biomass into energy carriers and a wide range of useful chemicals and materials can be carried out in the so-called multi-product biorefineries. A crucial step in developing this industry is to establish integrated biorefineries capable of efficiently converting a broad range of industrial biomass feedstocks simultaneously into affordable biofuels, biopower, and a wide range of biochemicals and biomaterials.
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Dr. Sina Ebnesajjad
President at FluoroConsultants Group, LLC
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