In general, biomass-to-energy conversion technologies have to deal with a feedstock that can be highly variable in mass and energy density, size, moisture content, and intermittent supply. Therefore, modern industrial technologies are often hybrid fossil-fuel/biomass technologies that use fossil fuel for drying, preheating, and maintaining fuel supply when the biomass supply is interrupted.
Method of Biomass conversion
There are three types of biomass conversion technologies currently available that may result in specific energy and potential renewable products:
- Direct combustion processes
- Thermochemical Processes
- Biochemical processes
1. Direct combustion processes
Direct combustion furnaces can be divided into two broad categories and are used for producing either direct heat or steam. Dutch ovens, spreader-stoker, and fuel cell furnaces employ two stages.
The first stage is for drying and possible partial gasification, and the second for complete combustion. More advanced versions of these systems use rotating or vibrating grates to facilitate ash removal, with some requiring water cooling.
The second group includes suspension and fluidized bed furnaces which are generally used with fine particle biomass feedstocks and liquids. In suspension furnaces, the particles are burnt whilst being kept in suspension by the injection of turbulent preheated air which may already have the biomass particles mixed in it.
In fluidized bed combustors, a boiling bed of pre-heated sand (at temperatures of 500 to 900°C) provides the combustion medium, into which the biomass fuel is either dropped (if it is dense enough to sink into the boiling sand) or injected if particulate or fluid.
1.1 Co-firing
A modern practice that has allowed biomass feedstocks an early and cheap entry point into the energy market is the practice of co-firing a fossil fuel (usually coal) with a biomass feedstock. Co-firing has a number of advantages, especially where electricity production is an output.
Firstly, where the conversion facility is situated near an agro-industrial or forestry product processing plant, large quantities of low-cost biomass residues are available. These residues can represent a low-cost fuel feedstock although there may be other opportunity costs.
Secondly, it is now widely accepted that fossil-fuel power plants are usually highly polluting in terms of Sulphur, CO2, and other GHGs. Using the existing equipment, perhaps with some modifications, and co-firing with biomass may represent a cost-effective means for meeting more stringent emissions targets.
Biomass fuel’s low Sulphur and nitrogen (relative to coal) content and nearly zero net CO2 emission levels allow biomass to offset the higher Sulphur and carbon contents of the fossil fuel.
Thirdly, if an agro-industrial or forestry processing plant wishes to make more efficient use of the residues generated by co-producing electricity, but has a highly seasonal component to its operating schedule, co-firing with a fossil fuel may allow the economic generation of electricity all year round.
Agro-industrial processors such as the sugarcane sugar industry can produce large amounts of electricity during the harvesting and processing season, however, during the off-season, the plant will remain idle.
This has two drawbacks, firstly, it is an inefficient use of equipment that has a limited lifetime, and secondly, electrical distribution utilities will not pay the full premium for electrical supplies which can’t be relied on for year-round production.
In other words, the distribution utility needs to guarantee year-round supply and may, therefore, have to invest in its own production capacity to cover the off-season gap in supply with associated costs in equipment and fuel.
If, however, the agro-processors can guarantee electrical supply year-round through the burning of alternative fuel then it will make efficient use of its equipment and will receive premium payments for its electricity by the distribution facility.
2. Thermochemical Processes
Thermal biomass conversion processes use heat as the dominant mechanism to upgrade biomass into a better and more practical fuel.
- Pyrolysis
- Carbonization
- Gasification
- Catalytic Liquefaction
2.1 Pyrolysis
The biomass feedstock is subjected to high temperatures at low oxygen levels, thus inhibiting complete combustion, and may be carried out under pressure.
Biomass is degraded to single carbon molecules (CH4 and CO) and H2 producing a gaseous mixture called “producer gas.” Carbon dioxide may be produced as well, but under the pyrolytic conditions of the reactor it is reduced back to CO and H2O; this water further aids the reaction.
Liquid phase products result from temperatures that are too low to crack all the long-chain carbon molecules so resulting in the production of tars, oils, methanol, acetone, etc. Once all the volatiles have been driven off, the residual biomass is in the form of char which is virtually pure carbon.
Pyrolysis has received attention recently for the production of liquid fuels from cellulosic feedstocks by “fast” and “flash” pyrolysis in which the biomass has a short residence time in the reactor.
A more detailed understanding of the physical and chemical properties governing the pyrolytic reactions has allowed the optimization of reactor conditions necessary for these types of pyrolysis.
Further work is now concentrating on the use of high-pressure reactor conditions to produce hydrogen and on low-pressure catalytic techniques (requiring zeolites) for alcohol production from pyrolytic oil.
2.2 Carbonization
This is an age-old pyrolytic process optimized for the production of charcoal. Traditional methods of charcoal production have centered on the use of earth mounds or covered pits into which the wood is piled.
Control of the reaction conditions is often crude and relies heavily on experience. The conversion efficiency using these traditional techniques is believed to be very low; on a weight basis, Openshaw estimates that the wood to the charcoal conversion rate for such techniques ranges from 6 to 12 tons of wood per ton of charcoal.
During carbonization most of the volatile components of the wood are eliminated; this process is also called “dry wood distillation.” Carbon accumulates mainly due to a reduction in the levels of hydrogen and oxygen in the wood.
The wood undergoes a number of physio-chemical changes as the temperature rises. Between 100 and 170°C most of the water is evaporated; between 170°C and 270°C gases develop containing condensable vapors, CO and CO2.
These condensable vapors (long-chain carbon molecules) form pyrolysis oil, which can then be used for the production of chemicals or as fuel after cooling and scrubbing. Between 270°C and 280°C, an exothermic reaction develops which can be detected by the spontaneous generation of heat.
The modernization of charcoal production has led to large increases in production efficiencies with large-scale industrial production in Brazil now achieving efficiencies of over 30% (by weight).
There are three basic types of charcoal-making
- Internally heated (by controlled combustion of the raw material),
- Externally heated (using fuelwood or fossil fuels), and
- Hot circulating gas (retort or converter gas, used for the production of chemicals).
Internally heated charcoal kilns are the most common form of the charcoal kiln. It is estimated that 10 to 20% of the wood (by weight) is sacrificed, a further 60% (by weight) is lost through the conversion to, and release of, gases to the atmosphere from these kilns.
Externally heated reactors allow oxygen to be completely excluded and thus provide better quality charcoal on a larger scale. They do, however, require the use of an external fuel source, which may be provided from the “producer gas” once pyrolysis is initiated.
Recirculating heated gas systems offer the potential to generate large quantities of charcoal and associated by-products but are presently limited by high investment costs for large-scale plants.
2.3 Gasification
High temperatures and a controlled environment lead to virtually all the raw material being converted to gas. This takes place in two stages. In the first stage, the biomass is partially combusted to form producer gas and charcoal.
In the second stage, the C02 and H2O produced in the first stage is chemically reduced by the charcoal, forming CO and H2. The composition of the gas is 18 to 20% H2, an equal portion of CO, 2 to 3% CH4, 8 to 10% CO2, and the rest nitrogen. These stages are spatially separated in the gasifier, with gasifier design very much dependent on the feedstock characteristics.
Gasification requires temperatures of about 800°C and is carried out in closed top or open top gasifiers. These gasifiers can be operated at atmospheric pressure or higher. The energy density of the gas is generally less than 5.6 MJ/m3, which is low in comparison to natural gas at 38 MJ/m3, providing only 60% the power rating of diesel when used in a modified diesel engine.
Gasification technology has existed since the turn of the century when coal was extensively gasified in the UK and elsewhere for use in power generation and in houses for cooking and lighting. Gasifiers were used extensively for transport in Europe during World War II due to shortages of oil, with a closed top design predominating.
2.4 Catalytic Liquefaction
This technology has the potential to produce higher quality products of greater energy density. These products should also require less processing to produce marketable products.
Catalytic liquefaction is a low-temperature, high-pressure thermochemical biomass conversion process carried out in the liquid phase. It requires either a catalyst or a high hydrogen partial pressure. Technical problems have so far limited the opportunities of this technology.
3. Biochemical processes
The use of micro-organisms for the production of ethanol is an ancient art. However, in more recent times such organisms have become regarded as biochemical “factories” for the treatment and conversion of most forms of human-generated organic waste.
Microbial engineering has encouraged the use of fermentation technologies (aerobic and anaerobic) for use in the production of energy (biogas) and fertilizer, and for the use in the removal of unwanted products from water and waste streams.
- Anaerobic Fermentation
- Methane Production in Landfills
- Ethanol Fermentation
- Biodiesel
3.1 Anaerobic Fermentation
Anaerobic reactors are generally used for the production of methane-rich biogas from manure (human and animal) and crop residues. They utilize mixed methanogenic bacterial cultures which are characterized by defined optimal temperature ranges for growth. These mixed cultures allow digesters to be operated over a wide temperature range i.e. above 0°C up to 60°C.
When functioning well, the bacteria convert about 90% of the feedstock energy content into biogas (containing about 55% methane), which is a readily useable energy source for cooking and lighting. The sludge produced after the manure has passed through the digester is non-toxic and odorless.
Also, it has lost relatively little of its nitrogen or other nutrients during the digestion process thus, making a good fertilizer. In fact, compared to cattle manure left to dry in the field the digester sludge has a higher nitrogen content; many of the nitrogen compounds in fresh manure become volatized whilst drying in the sun.
On the other hand, in the digested sludge little of the nitrogen is volatized, and some of the nitrogen is converted into urea. Urea is more readily accessible by plants than many of the nitrogen compounds found in dung, and thus the fertilizer value of the sludge may actually be higher than that of fresh dung.
Anaerobic digesters of various types were widely distributed throughout India and China. Extension programmed promote biogas plants as ideal candidates for rural village use due to their energy and fertilizer production potential along with their improved health benefits.
Health benefits primarily arise from the cleaner combustion products of biogas as opposed to other biomass or fossil fuels that may be used in the domestic environment, these two countries now have an estimated 5 to 6 million units in use.
Reliability problems have arisen from a number of problems i.e. construction defects, the mixed nature of the bacterial population, the digesters requirements for water and the maintenance of the optimum nitrogen ratio of the medium. Another problem is the digester’s demand for dung, which may have alternative uses.
Modern designs have answered many of these problems and digesters are again becoming useful, especially with regard to the potential of digesters to remove toxic nutrients such as nitrates from water supplies; levels of which are now much more stringently controlled in many industrialized countries.
The combination of energy production with the ability to enhance crop yields make biogas technology a good candidate for more widespread use now that reliable operation can be demonstrated. Recent Danish commercial experience with large scale digesters provides a useful example.
3.2 Methane Production in Landfills.
Anaerobic digestion in landfills is brought about by the microbial decomposition of the organic matter in refuse. The levels of organic matter produced per capita vary considerably from developed to developing countries e.g. the percentage of Municipal Solid Waste (MSW) which is putrescible in Sierra Leone is about 90%, compared to about 60% for US MSW.
The reduced levels of putrescible in US MSW are a result of the increased proportions of plastics, metals, and glass, mostly from packaging. Landfill-generated gas is on average half methane and half carbon dioxide with an energy content of 18 to 19 MJ/m3. Its production does not occur under pressure, and thus recovery processes must be active.
Commercial production of land gas can also aid with the leaching problems now increasingly associated with landfill sites. Local communities neighboring landfill sites are becoming more aware of the potential for heavy metals and nutrients to leach into aquifers.
Landfill processing reduces the volume of sludge to be disposed of, and the nutrient content, thus facilitating proper disposal.
Methane is a powerful greenhouse gas, with substantial amounts being derived from unutilized methane production from landfill sites. Its recovery, therefore, not only results in the stabilization of the landfill site, allowing faster reuse of the land but also serves to lessen the impact of biospheric methane emissions on global warming.
3.3 Ethanol Fermentation
Ethanol is mainly used as a substitute for imported oil in order to reduce their dependence on imported energy supplies. The substantial gains made in fermentation technologies now make the production of ethanol for use as a petroleum substitute and fuel enhancer, both economically competitive (given certain assumptions) and environmentally beneficial.
For example, subsidies for alcohol production in Brazil are now becoming regarded as detrimental to the stability of the ethanol market, and thus obsolete, In Zimbabwe, foreign exchange savings are seen as a major bonus, which along with the employment and environmental benefits have made the long term future and expansion of the this programmed a priority for the Zimbabwean government.
The most commonly used feedstock in developing countries is sugarcane, due to its high productivity when supplied with sufficient water. Where water availability is limited, sweet sorghum or cassava may become the preferred feedstocks.
Other advantages of sugarcane feedstock include the high residue energy potential and modern management practices which make sustainable and environmentally benign production possible whilst at the same time allowing continued production of sugar. Other feedstocks include saccharide-rich sugar beet, and carbohydrate-rich potatoes, wheat, and maize.
One of the most promising fermentation technologies to be identified recently is the “Bio still” process which uses centrifugal yeast reclamation, and continuous evaporative removal of the ethanol. This allows the fermentation medium to be continuously sterilized and minimizes water use.
The Bio still process markedly lowers the production of stillage, whilst the non-stop nature of the fermentation process allows substrate concentrations to be constantly kept at optimal levels, and therefore fermentation efficiency is maximized. {Hall, 1991} Improved varieties of yeast, produced through clonal selection techniques have also raised the tolerance levels of the yeast to alcohol concentrations, again improving efficiency.
3.4 Biodiesel
The use of vegetable oils for combustion in diesel engines has occurred for over 100 years. In fact, Rudolf Diesel tested his first prototype on vegetable oils, which can be used, “raw”, in an emergency. Whilst it is feasible to run diesel engines on raw vegetable oils, in general, the oils must first be chemically transformed to resemble petroleum-based diesel more closely.
The raw oil can be obtained from a variety of annual and perennial plant species. Perennials include oil palms, coconut palms, physic nut, and Chinese Tallow Tree. Annuals include sunflower, groundnut, soybean, and rapeseed. Many of these plants can produce high yields of oil, with positive energy and carbon balances.
Transformation of the raw oil is necessary to avoid problems associated with variations in the feedstock. The oil can undergo thermal or catalytic cracking, Kolbe electrolysis, or transesterification processes in order to obtain better characteristics.
Untreated oil causes problems through incomplete combustion, resulting in the buildup of sooty residues, waxes, gums etc. Also, incorrect viscosities can result in poor atomization of the oil also resulting in poor combustion. Oil polymerization can lead to deposition on the cylinder walls.
Generally, the chemical processing required to avoid these problems is simple, and in the case of soybean oil may be carried out in existing petroleum refineries. The use of diesel-powered vehicles is widespread throughout agriculture, and biodiesel provides an environmentally friendly, CO2-neutral alternative. It is now being widely promoted in the EC and elsewhere, as its use does not require major modification to existing diesel engines.
