Biomass is basically organic material derived from living organisms and therefore, like any living organism on Earth, contains oxygen, carbon, hydrogen, nitrogen and many others elements in smaller quantities. Content of individual components depends on the origin of biomass and determines the possibility of direct use for energy purposes or need for further processing. However, fossil fuels also consist of organic matter plus their energy density is higher and processing them into liquid or gaseous fuels is easier than processing biomass.

Why use biomass?

Advantages of using biomass for energy purposes in comparison with fossil fuels can be divided into three main groups:

Environmental benefits

Using fossil fuels results in reintroducing carbon and other contaminants sequestered millions of years ago back into the atmosphere. Carbon introduced into the atmosphere from the combustion of biomass or biomass converted to fuels was recently taken up by the growing plants. So by using biomass we can achieve “CO2 neutral combustion”, but only when we grow as much biomass as we use for energy purposes. (Harvesting forests does not apply). It is also possible to reduce the amount of organic wastes by transforming them into usable fuel.

Energy independence

Resources of fossil fuels (especially oil and natural gas) are unequally distributed across the globe, and their reserves are depleting. The ever-increasing demand for energy increases fuel prices and may lead to lack of fuel supply for individual countries. Production and use of biomass is possible almost anywhere in the world, therefore its use can contribute to greater energy independence of individual countries.

Economical benefits

In regions or countries using own produced biomass instead of imported fossil fuels money stays within their own budgets. Using biomass as an energy source can be a great benefit for farmers and agricultural industry.

Biomass for energy purposes

Depending on the region a large number of different biomass sources and a wide variety of ways to convert it to energy is possible. Sources of biomass can be divided into three main groups:

  • Energy crops
    • Short rotation coppice (i.e. willows, poplar)
    • Grasses and non-wood energy crops (i.e. Miscanthus)
    • Non food agricultural crops for biofuel production (i.e. rapeseed, corn)
  • Waste and residue
    • Dry agricultural residues (straw, corn stover, poultry litter)
    • Wet agricultural residues (animal slurry and farmyard manure, grass silage)
    • Wastes (industrial waste, sludge, food waste, waste vegetable oil)
  • Forest products
    • Wood obtained from forestry work or forestry clearance

Methods of biomass conversion into energy or fuels

Thermal conversion

By thermochemical conversion of biomass we can produce heat, electric power or synthetic fuels. Commonly used technologies for these purposes are direct combustion, thermal gasification and pyrolysis.

Direct combustion of biomass is the oldest known method to produce heat. Mankind has come a long way from the discovery of fire to present day and possesses the technology that allows for much more comfortable and efficient methods of heat extraction than a simple campfire. For heating households, public buildings or districts with biomass the best way is to use stoves or boilers with restricted air flow which enable effective and convenient heat production. For electrical power production the best way is to combine it with heat production (Cogeneration, CHP) in a power station. Small scale power generation is generally inefficient, producing a considerable amount of collateral heat.

Thermal gasification is basically partial combustion where thermal decomposition of biomass takes place in the presence of small amount of oxygen or air. The main goal of this process is to produce gas containing significant amounts of combustible gases (methane, hydrocarbons, hydrogen). Generated gas can be either burned in boilers to produce heat or cleaned and used in combustion turbines for electricity production or less frequently for transport fuel production. For particular biomass types it could be the most economical and efficient method of extracting energy.

Pyrolysis is a process of thermal decomposition of biomass in absence of oxygen, it is endothermic chemical process and needs external heat source. Depending on conditions the main product can be fuel gas or liquid fuel (fast pyrolysis). Final applications are similar to those of gasification.

Biogas production

Biogas is produced by anaerobic digestion in a biogas plant (anaerobic digester). During this process bacteria break down organic material in the absence of air, producing a biogas containing methane. Depending on conditions (types of used bacteria, anaerobic digester and biomass) produced biogas can contain up to 75% of methane. In addition, a solid residue similar to compost and liquid liquor, which can be used as a fertilizer, is produced as a byproduct.

Biofuels production

Biodiesel is produced through transesterification reaction of fatty acids (from vegetable oils or animal fat) with alcohol in the presence of catalyst. After reaction and separation from glycerol (byproduct) it can be put directly to a car tank or mixed with regular diesel. Chemically biodiesel is a mixture of mono-alkyl esters of long chain fatty acids. The most common production method is a batch process operating in low temperature and atmospheric pressure with use of cheapest alcohol-methanol (in some cases ethanol and higher alcohols are used) and base catalyst. Supercritical process is an alternative catalyst-free method which needs high temperature and pressure but production costs are similar or lower then for catalytic batch process. There are other uncommon methods of biodiesel production by using ultrasounds, microwave or enzymes.

Bioethanol is produced by alcoholic fermentation, in the same process that is used in brewing, wine making or spirits production. In conventional fermentation yeasts are converting starch and sugar rich biomass directly to ethanol. It is possible to make use of a much larger proportion of available biomass by using acids or enzymatic hydrolysis of cellulose and hemicellulose in order to convert it to fermentable sugars. After distillation bioethanol can be added directly to conventional petroleum in concentration up to 10%. The concentration increase can be achieved by modification of vehicles or transformation of bioethanol (i.e. to biobutanol).

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Potential obstacles

Main disadvantage of thermally converted biomass is its low energy density. In addition, many types of biomass have high moisture content and are hydrophilic. Therefore, special furnace design, storage space or frequent biomass deliveries are needed.

Source of biomass has to be located in many cases near the utilization facility (e.g. power plant) to make transport costs economically justified.

Agricultural biomass utilization involves risks such as influence of weather conditions on product amount, quality and price similar as for food crops production.

Main risk involving biomass delivered from energy crops and forestry is soil exhaustion and deforestation.

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