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Why deal with the use of non-timber solid biomass as a fuel?
For an increasing number of people around the world, access to firewood or charcoal is becoming more expensive, unreliable, and difficult. They are thus often forced to look to alternatives sources of fuel. Non-timber solid biomass is one such alternative source.
What are conditions for the use of non-timber solid biomass as a fuel?
In general, any material that a plant has generated consists of hydrocarbons and can be used as a fuel. But not all of this biomass comes in the form of ‘firewood,’ which we picture as any wood in the form of logs or sticks that is collected or cut from trees or shrubs.
Some plant-materials have a similar composition than wood, but are small in size and not stick-shaped. For instance, forest residues, wood-chips, saw-dust, nutshells, pods, are all such plant materials.
On the other hand, there are plant materials which have a different composition than wood. They have less lignin, a higher content of silica, and often other components like starch, fats, or proteins. Examples for these materials are bark, softer leaves, straw, grass, agricultural residues (stalks, husks) or plant products such as grains, seeds, nuts etc.
For the use of any solid biomass as a fuel for cooking in a normal combustion process, they should adhere to the following characteristics:
- The biomass should be “dry”, which means moisture content preferably below 20%. Any water in the fuel evaporates at the expense of heat availability during actual cooking. High moisture content also makes the stove operation less stable.
- The biomass should be “energy-dense”. If the fuel has low energy density, the same cooking tasks requires the burning of much higher volumes as compared to firewood. This may result in inconveniences for the user. He/she either has to accept a very big stove (batch feed) or a very cumbersome cooking process (frequent refuelling continuous feed stove).
There are also some non-technical factors to be considered in the choices of biomass fuels:
- The (production of the) fuel should not compete with resources necessary for food production (like land, water, labour, fertiliser etc.) or a higher value use, such as a building material.
- Fast growing fuels should not negatively impact the biodiversity of the locality.
- Any fuel must be economically viable in the long-run.
- The supply of any biomass should be sustainably managed, so that it can be a truly renewable energy source
Sources of Non-timber Solid Biomass
The list of usable feedstock is nearly endless and depends on what is readily available in a certain location. Agricultural residues are generated in large volumes season by season and often discarded as waste. Crop residues are the largest source of non-timber biomass fuel. They include straw, stem, stalk, leaves, husk, shell, peel, lint, stones, pulp, stubble, etc, which come from cereals cotton, groundnut, jute, legumes, coffee, cacao, olive, tea, fruits, and palm oil.
The following table from FAO gives some ideas of where to look for appropriate feedstock. Slaughterhouse and municipal by-products are not recommended for cooking, due to high variability and the presence of potentially toxic ingredients.
FAO: Classification Biofuel sources
Table 2: Classification of Biofuel sources by different characteristics
Cooking with Unprocessed Non-timber Solid Biomass
In the developing world, most agricultural residues that are burnt as fuel are used in their natural state with some pre-treatment like drying, and cutting. Compared to wood-fuels, crop residues typically have a high content of volatile matter and ash, lower density and lower energy values. Conventional ‘stoves’ are mostly designed to burn firewood or charcoal. The direct use of unprocessed solid biomass waste for cooking in wood-fuel stoves has some advantages and disadvantages.
- Agricultural residues are available for free or at low cost to poor rural families.
- Use as fuel is a good way of waste management, instead of consuming precious land-fill space, creating huge rotting piles or burning them in situ thus destroying the soil organisms on the fields
- Residues are often available close to the household, thus with low impact on women’s time for harvesting and transport
- Agricultural wastes are often easier to light than wood and charcoal
- Using agricultural residues to substitute or complement firewood puts less stress on timber-resources
- Agricultural residues often require appropriate stoves to burn well, e.g. gasifier stoves.
- Residues are often very bulky and storage requires more space inside a house or shelter
- The seasonal availability of crop residues can be limit for the use.
- The burning time per volume/weight of fuel is often shorter. For the same cooking task, more fuel is required as compared to wood.
- If they contain larger proportions of oils or proteins, the burning properties change (e.g. smoke) and need to be addressed with an appropriate stove technology
Processing Solid Biomass for Fuel
Many non-timber solid biomass materials are of low energy-density. Their direct usage is inconvenient as compared to stickwood. One option to improve their properties for cooking is densification.
Compacted and densified fuel, also known as processing, has several advantages:
- It has a higher heating value per volume (more carbon per volume)
- It reduces transport costs (more fuel, less air to be transported)
- It has more predictable performance in a stove due to more uniform size, shape, density etc.
- It is often easier and cleaner to handle (less dust, easier packing etc.)
- It is more convenient as it comes in the right size ready-to-use (no chopping required)
- It has better storability (less moisture absorption, less mould, less spontaneous fires through self-ignition, less insect-infestation than natural fuel)
- It can be a solution to waste management problems
- It adds value to low-value residues, often creating employment in the process
However, densified biomass is not the magic bullet! To produce this fuel, special equipment and labor are required, which need to be paid for. This increases the cost of the fuel.
Hence densified biomass is likely to be viable only where…
- fuel is already a commodity (like in many urban areas)
- households have purchasing power
- there is a large source of un-used ‘wasted’ residual biomass (which does not compete with the use as manure or compost)
- there is a feasible link between the source of biomass and the market of the densified fuel (relation of distance, transport costs and the value of the fuel)
- fuel densification can be run as an income generating business
- there is electricity so that larger scale operations can be done. Without electricity, only manual production at a small scale is feasible
Various binding and compaction methods are used to ‘glue’ the loose biomass material together to form a compact dense shape, which does not immediately fall apart during drying, handling and use as fuel. The intended use of the product and the envisaged scale of operation determine size, shape, and the needed degree of compaction of the product.
Many factors influence the feasibility of biomass densification in a given scenario. The following table gives guidance for the choice of densification options according to the desired pressure and intended throughput per hour. It reflects methods of feedstock preparation and compaction, binders, etc. The availability of required inputs like water, electricity, capital, labour, space etc. is critical to the success of any densification project. These can potentially be limiting factors for the feasibility of a densification option and should be used initially as part of the ‘make-or-break’ arguments. Please note that the factors described are in a continuum and have no clearly defined concrete values that would determine a clear-cut boundary to the next category.
Guiding tool to identify appropriate biomass densification options:
Guiding tool to identify appropriate biomass densification options (Developed by Christa Roth)
Low-pressure moulding by hand or low-cost light levers require a wet preparation of the feedstock and drying space after production. It might yield enough output for single household consumption or a family-business. Economies-of-scale with outputs of densified product above 1,000 kg/h require capital-intensive machinery and tri-phase electricity supply above 20 KvA, which might be a limiting factors in certain locations. Detailed examples for these options with photos, sketches, plans and links to manufacturers of machinery are presented in the Module 3 of the Manual Cooking with gas from soild biomass.
The processes of biomass densification can be clustered in three main groups:
- The wet, ambient temperature, low pressure (10-15 bar) process: an added binder is optional, as binding is effected through random rearrangement of softened and detached natural fibres in a wide variety of agricultural residues and in other waste feedstock. The process accepts sawdust, rice husks, bagasse, coffee/ peanut shells, and other granular feedstock as well as charcoal dust and crumbs -or purposefully charred agricultural residues- as part of the matrix, as long as the fibres can encapsulate them into a tight non-elastic mass when compressed. Emphasis is on careful blending and pre-preparation of feedstock for pliability, combustibility and other behaviours. Once the principles are mastered, a far wider variety of ingredients are possible as compared to other processes. Densification and shaping can be done using a hand to squeeze the material into shape, or human force to press the material in a mould. Over 25 designs of hand operated or mechanized versions of presses are in use, based on various ways to create pressure: levers, hydraulic jacks, screw platens, treadle/peddles etc. Costs range from $50 to $750. The density of product is commonly 0.3 to 0.5 gm/cc.
- The moist-dry ambient temperature, low-medium pressure (10-50 bar) process: The next level would usually start at a similar pressure as the previous process, but the pressure would go far beyond, depending on the type of machinery. The process uses some form of binder (clay, starch, banana peel paste, waxes, glues, molasses, etc.). Temperatures are near room-temperature but the water is minimal or absent. The relatively dry feedstock mix allows the use of loose augurs (‘screws’) and rams or pillow compression cylinders, as well as the above "wet process" presses. Over 10 designs of hand driven or mechanized presses using various augurs and rams are in use. Costs can start at 50 USD for hand-driven devices. Fuel density ranges from .3 to .7 gm/cc. The product range includes waxed logs and products from char dust products, finding increased acceptance e.g. in Asia.
- Dry high-pressure process: The next kind of densification involves a great jump in pressure (400 to 600 bars), and requires drying equipment to ensure a moisture content below 20%. Compression by ram or augur often requires added heating jackets which raise the barrel/cylinder/die temperatures up to around 200° Celsius. This combination of pressure and temperature effectively scorches the exterior wall of the resulting log, and tends to melt the lignin of the biomass to accomplish binding. The process requires an assured supply of feedstock of a known type, grade and moisture content. These are more industrialized machines costing between 3,000 and 30,000 USD.
The term ‘briquette’ is used for a sizeable ‘chunk’ of densified product of any shape and compaction level where the smallest side-length is above 2 cm size. If the final product of a high-pressure compaction is a short roundish stick of 6-12 mm diameter, the term ‘pellet’ is used. Pellets are shaped by pressing dry biomass at very high pressure through a die with many holes (like an oversize spaghetti-maker).
Various briquette- and pellet- presses are available mostly for the industrialized world. Fuel densities can even go beyond 1.0 gm/cc, as some highly densified briquettes and pellets are heavier than water and don’t float (an easy test to determine fuel density). There is a risk that dense but super dry pellets and briquettes tend to crumble apart in more humid conditions, as they regain moisture.
In general the product quality increases with rising compaction pressure, which entails:
- Higher temperatures: causing the lignin contained in the feedstock itself to ‘melt’, so it can act like wax as the sole binder. Added binders become unnecessary.
- Less water needed for the feedstock preparation: thus less drying time and space needed afterwards; lower moisture content of product, thus higher heating values
- Rising electricity requirements and higher costs for investment and operation
- Decreasing labour intensity which reduces job creation in the production phase, with the potential of more local job creation in the fuel distribution chain
Carbonised biomass briquettes
Another processing method is the carbonisation of raw biomass and the shaping into briquettes similar to conventional charcoal lumps. There are two paths that lead there:
- Carbonising the biomass, milling up the char, adding a binder and shaping the briquette. This is also a good way to recycle the fine residues that are left when handling conventional charcoal char or the char created in pyrolytic TLUD gasifier-cookstoves as a byproduct of cooking (link to the gasifier manual). Binders can be any porridge from a starchy substance (flour, banana peels, etc.) or clay.
- Making a briquette first (see previous chapter) and then carbonise the entire briquette in a kiln.
Biomass Briquettes Production and Marketing
The advantages of biomass briquetting are by no means limited to its use in modern industrial plants or solid fuel boilers. Indeed, in developing countries a far bigger percentage of the population cover their energy needs with biomass alone, where their primary need is for heat energy for cooking. International development cooperation has accordingly long been focussed on improving the basic energy supply in many countries around the world. It is notable that biomass briquettes have played a bigger part in many projects over the recent years, such as those for distributing better stove technologies, for example. Next to adapted cooking behaviours and improved cooking appliances, the fuel can play one important role in improving the overall situation of households. Biomass briquettes can be produced out of many field or process residues and burning them in cooking appliances instead of traditional fuels as logged and collected wood or charcoal can be an interesting alternative for business makers but also for fuel clients. The article "biomass briquettes - production and marketing" provides an overview of the main findings about biomass briquettes as fuel, following the value chain from the raw material to the final market. It focuses on economical questions that have to be considered when starting a business or support business development with biomass briquettes.
This article was originally published by GIZ HERA. It is basically based on experiences, lessons learned and information gathered by GIZ cook stove projects. You can find more information about the authors and experts of the original “Cooking Energy Compendium” in the Imprint.
- ↑ Unified Bioenergy Terminology, ftp://ftp.fao.org/docrep/fao/007/j4504e/j4504e00.pdf, page 9
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