For many millennia, the main source of energy was biomass, complex hydrocarbons produced by living organisms. Heat came from wood, charcoal, peat, bamboo, animal dung and bones, burnt in open fires and primitive kilns and furnaces. Natural oils, waxes and tallows gave light. Oils were pressed from seeds or nuts or rendered from whale or seal blubber, waxes were plundered from beehives or extracted from berries; tallow came from the carcases of cattle. The Romans had a very effective form of 'central heating' fuelled by biomass. Even today - several centuries after fossil fuels began to be widely exploited -much of the third world still relies on firewood for heating and cooking. This dependence has had adverse environmental effects, particularly in Africa, but as the search for alternatives to fossil fuels intensifies, the potential of some types of biomass has been recognised.
In practice, biomass comes in two forms. Waste materials such as worn timber pallets, paper, forestry and sawmill residues can be processed into chipped, shredded or granulated form for convenience of handling. (In the US, forestry residues are often known as hog fuel or hog mass . ) Food processing by-products such as nutshells, cereal straw and crushed sugarcane (bagasse) have high calorific values. The production of beer, wine, spirits, cheese and other dairy products generates large volumes of energy-rich organic wastes. Other biodegradable wastes with a high organic content - pelleted sewage sludge, agricultural and slaughterhouse waste and the like - yield useful amounts of energy when burnt, although there might be concerns about odours and emissions. Or crops can be grown purely as fuel.
Ideally, these will be plants that flourish on marginal land otherwise unsuitable for modern agriculture, and require the minimal amounts of fertiliser and pesticides. Coppiced willow and poplar, perennial grasses such as miscanthus hybrids and switchgrass, hemp and sugarcane have all been tried, and several are now readily available. Hemp and miscanthus seem to be particularly promising. Hemp also yields valuable fibre and seeds; the residue left after the plants are Netted. to release the fibres can easily be pelletised. Miscanthus has the advantage that it needs little or no drying after spring harvesting before processing into pellets or bales. It also has a low mineral content (see below).
Biomass crops are sometimes marketed as being 'carbon-neutral', i.e. the carbon released when they are burned is the same carbon that was taken up during their growth, so the net effect on the atmosphere is neutral. This is not exactly true: their cultivation requires significant amounts of fossil fuels, as does the manufacture of any artificial fertiliser or pesticide used, and the final processing and transportation to the point of use. However, their overall environmental impact is usually much better than the fossil fuels they replace - especially if they are used locally. And burning material that might otherwise be allowed to rot down in landfill and produce methane, an even more potent greenhouse gas than carbon dioxide, is obviously a good idea.
'Localism' is the essence of biomass energy. In reality there is little point in switching to biomass generation if the fossil fuel consumed during the production, processing and transportation of the biomass is greater than the fossil fuel that the biomass replaces. There can be a case for subsidising the transportation of biomass over long distances in the short-term to build up a customer base that would attract potential local biomass producers to enter the market, but unless local supplies eventually become available in the medium- to long-term the choice of biomass generation would be short-sighted.
Biomass fuels are usually processed only to improve their ease of handling, unlike biofuels, which are biomass crops processed to convert them into more energy dense solid, liquid or gaseous hydrocarbons such as charcoal, producer gas, biodiesel and bioethanol -although the term biofuel is now being used mainly for liquid fuels derived from plant matter and intended as a replacement for liquid fossil fuels (see Chapter 11).
Ease of handling is one of the attractions of using cereals like maize and oat as fuels. Oat in particular is attracting a lot of interest, as at the time of writing (2007), there is a marked surplus of oats in Europe. Its main attraction for farmers is that it can be grown further north than most cereals and requires less herbicide. But, because it lacks gluten and hence is unusable for breadmaking on its own, oat is less in demand than wheat, maize, rice or even barley. There is an obvious ethical issue in burning produce that could feed the starving, but there are cultural and environmental issues to be considered as well. Cereal production is usually heavily subsidised, associated fossil fuel consumption is high, and the vagaries of world cereal markets and government subsidies can make cereal supplies uncertain.
Oats are normally available with a consistent moisture content, usually around 15%. Moisture content is a critical factor in all biomass fuels. There are two ways of calculating this: moisture content wet basis (MCWB) and moisture content dry basis (MCDB). It is very important to know which figure is being quoted. One tonne of biomass with a 60% mcwb will have little more than half the energy content of one tonne at a 60% mcdb, although it is the mcwb figure that is usually quoted.
High moisture content biomass poses a number of problems. For the same calorific value transport costs will be higher. In store, various biochemical processes can take place, health-threatening mould spores can be released, and spontaneous combustion is always a risk (see below). During combustion there can be excessive emissions of carbon monoxide and unburnt hydrocarbons. Very low moisture content is not entirely desirable, however. Dust will always be a problem, and can represent both a fire and explosion hazard.
Moisture content also directly affects the calorific value or heating value of the biomass; it is the gross calorific value or heating value that is normally quoted. Unless the biomass is burnt in a condensing boiler (which recovers much of the heat in the water vapour emitted), the relevant figure is the net calorific value (which is the gross calorific value minus the heat of vaporisation of the water content).
Typical ranges for gross calorific value of biomass are from 15-20 GJ/tonne (gigajoules per tonne). This compares to 15-30 GJ/tonne for coal and 45 GJ/tonne for diesel fuel. Ash and sulphur content is usually much lower than with fossil fuels, and the ash can be re-used as a soil conditioner in many cases. However, bulk density - and hence energy density - is much lower than fossil fuels. Various densification processes such as chipping, shredding and pelletising have been used to improve this figure, but storage volumes will normally be significantly larger than with fossil fuels.
Alkali levels in the biomass must also be considered. High alkali levels can lead to serious problems with slag deposits in the boiler tubes. High mineral levels can also be undesirable, as the resulting ash may pose a disposal problem. Another key decision is on the pre-drying of the biomass before it enters the boiler. Some biomass will be available pre-dryed, either passively, by long-term exposure to the air, or by the application of heat - active drying. Or it can be dried actively or passively after delivery. There are practical limits to what can be achieved by passive drying. Green wood, even when chipped, can rarely be air dried below 30% or so, higher than most boiler installations prefer. Dry chipped or shredded waste kiln-dried or recycled timber is sometimes blended with green wood chips to reduce the overall moisture content. Active drying can reduce moisture contents to the optimum - at a price. However, reducing moisture content before the biomass is burnt increases flame temperature and improves the efficiency of the combustion process. Overall efficiency can be significantly greater, smoke is reduced and there is a lower risk of flue corrosion problems.
Some biomass combustion systems are specifically designed to handle high moisture content biomass. Typically these utilise some of the heat of combustion to dry the fuel as it approaches the combustion zone. Some small biomass boilers come with built-in storage, in which some pre-drying occurs. Otherwise the options are purchasing biomass with a suitable guaranteed maximum moisture content - usually around 15% mcwb - installing an active drying system on site, or providing suitable storage facilities in which air drying can take place.
In an industrial situation, where waste process heat or steam is available, active drying could be a very cost-effective option. The choice is between conventional rotary driers using hot air or flue gases, flash dryers that use high velocity hot air, and systems using superheated steam. Flash dryers and superheated steam dryers (SSD) only work with small particles; single pass rotary dryers can handle larger or more variable material. There is a lower risk of fire or explosion with SSDs.
An increasingly popular treatment for biomass is a combination of SSDs and low temperature pyrolisation or torrefaction (see Chapter 11), sometimes known as airless drying. This produces a denser, more consistent solid material with a low moisture content and high resistance to dusting.
Passive drying is not without its risks. Large volumes of wet biomass will tend to compost, and high temperatures will develop at the centre, high enough in some cases for fire to be initiated. The usual advice is that woodchips should not be heaped up more than 10 m high for this reason. The biomass has to be exposed to the air as much as possible while being protected from the elements. Some materials have poor permeability and will need to be turned and mixed regularly. Apart from fire, the main danger is the growth of moulds within the store.
With woody biomass, an option to chipping and drying is ' ultrasonic wave reduction technology' as typified by the American KDS Micronex system. This uses high-powered ultrasonics to simultaneously disintegrate the wood and vaporise the moisture content. The end product is a fine wood dust dry enough to go straight into the combustion process.
Biomass is available for delivery in a number of forms by a variety of transport systems. Bagged pellets come in sizes from 15-25 kg, or in 1 m3 bags. Tipper trailers or trucks are the most common form of bulk delivery. Inland waterway delivery offers a low carbon alternative to road transport. However, consideration must be given to how bulk biomass is to be offloaded from barges, as this is far from straightforward. It is also important to minimise handling, especially of wood pellets. Excessive handling can lead to the formation of wood dust in unacceptable quantities.
Dry storage is one of the key factors in successful biomass generation. Some boiler manufacturers offer package deals with prefabricated storage and handling facilities. Existing silo capacity has been successfully utilised, and shipping containers have been adapted for storage purposes. On the larger scale, a purpose-built facility is usually the best answer, although a lot depends on the biomass chosen. Good ventilation and drainage are critical. This is particularly the case when a below ground storage hopper is preferred as the most space efficient solution where the biomass arrives in bulk in tipper trucks and trailers. Large stocks will need to be turned over and mixed regularly, to aerate the material and minimise variations in moisture content, but not too often (see above).
Storing and handling bulk woodchips is relatively straightforward, provided sensible precautions are taken (Reproduced with permission from Biomass Energy Centre)
Care must also be taken in selecting the most appropriate method for transferring the biomass from storage into its final destination. Screw auger feeds are popular for granular
Large volumes of damp woodchip can pose the risk of spontaneous combustion (Reproduced with permission from Biomass Energy Centre)
materials such as chips and pellets; front loaders or bucket grabs may be the best solution for shredded waste timber or coppice wood. Gravity feed is less energy intensive and is often preferred for smaller installations. Pelleted materials in particular must be handled with care. There is always the risk of disintegration through friction and impact, especially if the moisture content has changed significantly while in store. Too much dust can jam the feed system, and lead to combustion problems.
Biomass can be burnt to produce either hot air or hot water. It can be combined with other technologies to form a cogeneration or trigeneration facility (see Chapter 14). Traditionally most of the heat from the biomass was used to generate steam, which drove turbines that drove electricity generators. Surplus heat was used for space heating/cooling, often on a district basis. This was only effective on the larger scale. A recent development replaces the turbines with a Stirling engine, best described as an external combustion piston engine (see Chapter 14). Such installations can be independent of national energy grids.
A wide range of boilers and combustion units are available, in sizes from a few kilowatts upwards. Most require electricity to function. All biomass combustion installations will be significantly larger than the equivalent fossil fuel installation, due to the lower energy density of biomass.
Boiler/combustion unit choice should be largely influenced by the type of biomass available. For example, some units will struggle to handle grain, which has a tendency to smother the flames as it enters the burner. Burners that can cope with grain often need to be started with pellets. Biomass installations generally need more supervision and expert intervention than fossil fuel installations, mainly because the fuel is more variable. On the other hand, most biomass boilers/combustion units can handle a wide variety of biomass fuels, allowing flexibility in operation and substitution of biomass sources as availability and prices fluctuate.
One recent development, which falls somewhere between straightforward combustion and gasification (see Chapter 11), is the UK Bioflame system. Gasification and combustion take place in close sequence at atmospheric pressure, and the hot exhaust gases then go to a steam generator that normally drives a steam turbine, although other options are available. A wide range of different biomass sources can be exploited. Current Bioflame installations are relatively large, producing 2-3.5 MW of electricity, and are usually part of a cogeneration or district heating scheme.
The disposal of ash from biomass combustion has to be considered from the outset. Luckily, both fly ash (ash precipitated from the flue gases) and bottom ash usually contain significant levels of calcium, potassium and phosphorus, and hence can be recycled back to the soil to improve fertility and soil texture. Some ashes may have high levels of heavy metals, usually cadmium, and these will need special disposal. Willow is a known cadmium concentrator so willow ash must be monitored carefully. Municipal waste ash is also usually unsuitable as a soil improver. Some fly ashes contain significant carbon, and can be recycled back into the burners. There have been a number of projects to develop economic end uses for ashes unsuitable for soil improvement, such as replacing natural aggregates in low grade concrete products and the like.
Growing interest in the production of biofuels could lead to competition for biomass resources. This in turn could force up prices, eroding what should be biomass's inherent advantage over most fossil fuels. Against that, the increasing demand for biomass as a whole will persuade many industries and local authorities to look closely at their waste streams and assess their potential for energy production. More biomass options are the likely result, more localisation the logical outcome.
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