Cooking with Agricultural Residues

Introduction

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.

What are Agricultural Residues?

Agricultural residues are generated in large volumes after each harvest season and are often discarded as waste. Solid crop residues are the largest source of non-timber biomass fuel: straw, stems, stalks, leaves, husks, shells, peels, lint, stones, pulp, stubble, and the like from all sorts of crops. The largest amounts are derived from annual plants such as cereals (e.g. rice, wheat, maize/corn, sorghum, barley, millet), cotton and legumes (e.g. pigeon peas, bean, soy, groundnut) complemented by woody pruning from perennial plantations like coffee, tea, cacao, olive, fruits (e.g. banana, mango, coco, cashew). In the developing countries most agricultural residues and residues from processing of agricultural products are used as fuels in their natural state with limited pre-treatment such as drying and cutting; compacting is rather rare. Compared to wood-fuels, crop residues typically have a high ash content, lower energy density and shorter burning time.

The list of usable feedstock is nearly endless and depends on what is readily available in a certain location. The following table originally from FAO gives some ideas of where to look for appropriate feedstock.

Cooking with Agricultural Residues

Compared to woodfuels, 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. Micro-gasifiers are a good option to use agricultural residues, mostly in a densified way, because they are able to burn small energy carriers in a very efficient way.

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 refueling 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, labor, fertilizer etc.) and food itself or a higher value use as fuel, such as a building material.
  
• Fast growing fuels should not negatively impact the biodiversity of the locality.
• The supply of any biomass should be sustainably managed, so that it can be a truly renewable energy source.
• Any fuel must be economically viable in the long-run.
  

The direct use of unprocessed solid biomass waste for cooking in woodfuel stoves has some advantages and disadvantages:

Advantages

• Agricultural residues are often available free of charge to poor rural families.
• Controlled burning in a stove is cleaner and more environmentally friendly than uncontrolled burning.
• The ash can be taken back to the field and used as plant nutrient.
• Lower impact on women’s time for harvesting.
• Agricultural wastes are often easier to light than wood and charcoal.

Disadvantages:

• When burnt in open fires or traditional improved stoves, residues can cause extreme air pollution. But they do burn well in gasifier stoves.
• They are often bulky and have to be carried to the homes. Storage requires more space inside a house or shelter.
• The seasonal availability of crop residues can limit their continuous use.
• The burning time is often shorter.

Densification Options for Agricultural Residues

Many non-timber solid biomass materials are of low energy-density. Their direct usage is inconvenient as compared to stick-wood. One option to improve their properties for cooking is densification.

The term ‘briquette’ is commonly used for a sizeable ‘chunk’ of densified product of any shape and compaction level where the smallest side-length is above 25 mm in size. Briquettes are normally pressed into a mold that grants the briquette its final shape. For more information on biomass briquettes, their production and marketing please see the article “Biomass Briquettes – Production and Marketing” on energypedia.

The term ‘pellet’ is used for short, roundish sticks of 6-25 mm in diameter as a product of a high-pressure agglomeration process. Pellets are shaped by pressing dry biomass at very high pressure through a die with many round holes (like an oversized spaghetti maker).