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What happens when we burn biomass?

Burning biomass is essentially a reversal of photosynthesis. Let's take a look at the biomass burning process and its benefits.
JON LOVETT: Simple, traditional cooking fires use three stones. The fire is lit between the stones, the cooking pot is balanced on top of the stones, and the wood is fed in-between the gaps in the stones. This type of fire is effective but inefficient. Much of the heat is lost, and it produces a lot of smoke that contributes to indoor air pollution if the fire is inside a house. Even outside a house, the fire is very smoky. Fuel-efficient cookstoves contain the fire in a stove and increase the amount of heat going directly to the cooking pot, thereby increasing the efficiency and reducing the amount of wood needed. The wood also burns at a higher temperature, reducing the amount of smoke.
This saves on the time needed for wood collection, decreases deforestation from firewood collection, and also reduces indoor air pollution. Fuel-efficient cookstoves can also be designed for charcoal. As charcoal is expensive, this reduces the costs. In the following video, Agnes Naluwagga, coordinator with Uganda’s Regional Testing and Knowledge Center, describes some of the common types of cookstove.
AGNES NALUWAGGA: So here, we have an example of a traditional stove that’s commonly used in Uganda, a traditional stove that uses charcoal. You can see that the combustion chamber is quite big. It doesn’t even have any ceramic or clay liners, so there’s a lot of heat loss. So you find these [INAUDIBLE] these are stoves that you’ll find used. But now, we’ve seen a transition. There are some improvements. We can see in the local market, you’ll find this improvement where now people– this is, I would say this is semi-improved. It’s a clay-lined stove, smaller combustion chamber, using charcoal, commonly.
And then you have a lot of local manufacture within the region, within Uganda, where you have– this is an improved, what we are calling a locally-manufactured improved [? local ?] stove. So it’s using charcoal. You can see it’s lined, so it has an element of heat-saving insulation. And then the combustion chamber is also quite small. And for this stove, you can also use what we call carbonates briquettes. So this is an example. Carbonates briquettes, that is not charcoal, it just uses charcoal dust. It can use agro-waste. So this fuel is also used for this type of stove, or you can even use it on this stove. So there are many of these that are locally made.
An example, you can see there are so many over there. This is an improved wood stove. Now you can see this stuff, you can see the combustion chamber where your fuel enters is quite small. You can even use three or four pieces of wood to cook, and your food will cook really well. It’s well-designed symmetrically. And an advantage about the stove is it even has a thermal electric generator behind here.
You find that as it cooks, it also generates electricity. And that electricity can be used to charge a phone and a light bulb.
JON LOVETT: The process of making charcoal is called pyrolysis. Traditionally, charcoal is created by piling branches, covering them with earth to make a mound, and then setting fire to the branches inside the mound. After a while, the mound is taken down and the charcoal taken out. Pyrolysis is also used in gasification, during which biomass is heated without oxygen to around 700 degrees centigrade. And at this temperature, it breaks down and releases carbon monoxide, hydrogen, and carbon dioxide. This gas is called a syngas, and it can be used directly to power engines, or it can be converted into synthetic fuel for later use. Anaerobic digestion is another way of burning biomass.
The process rots down the biomass in a digester to produce methane. This methane can then be burnt as a cooking fuel or used to power internal combustion engines to produce power for generating electricity or other uses. Anaerobic digesters work best if some or all of the biomass is pretreated by feeding it to livestock, such as cows or pigs. The slurry from the livestock is then fed into the digester. And of course, the livestock also converts energy from the biomass into useful products of meat and milk.

Burning biomass is essentially a reversal of photosynthesis.

Photosynthesis is the process by which plants can reverse the disorder of entropy by using energy from the sun to combine water and carbon dioxide to produce sugar and oxygen. The plant then creates complex carbohydrates, such as starch and cellulose, from the sugars and combines them with other elements derived from nutrients to make compounds such as lignin, which is an important component of wood.

The biomass burning process

When biomass is burnt, oxygen combines with the carbon stored by the plant to release energy in the form of heat and light with the production of carbon dioxide and water, thereby increasing entropy again.

The burning process also leaves behind ash, char, and other residues such as tar. Wood ash is mostly calcium carbonate (CaCO3) if the fire is below 750 degrees Celsius, or calcium oxide (CaO) if the temperature is hotter than 750 C. Other components of wood ash include potash, which is various salts of potassium, and trace elements of iron, manganese, zinc, copper and some heavy metals.

Char is the carbon that remains from burning as black lumps. The amount of char is increased when the wood is burnt without oxygen in pyrolysis and is called ‘biochar’.

Benefits of burning biomass

The alkali nature of the wood ash from calcium carbonate, and the important plant nutrients in the potash, together with the char, make wood ash a good fertilizer for agriculture or market gardening. It can also be used to make other products such as ‘lye’ (potassium hydroxide) for soap manufacture, or in ceramic glazes during the firing of pottery.

Biochar has been shown to significantly increase the fertility of soils and also acts as a long-term store of carbon when dug into the soil.

Tar is created when the wood is burnt at lower temperatures, usually when the wood is a bit wet. It is made up of phenols and related compounds. Tars can be used to create creosote for preventing rotting of wood and tarry smoke is used as a way of preserving meat and fish in ‘smokers’.

The residues left behind from burning depends on the type of biomass (the feedstock) burnt, the temperature at which it is burnt and the amount of oxygen available during combustion.

Taking your learning further

If you are interested in learning more about the burning of biomass, sign up for the full course – Renewable Energy: Achieving Sustainability through Bioenergy – today.

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Renewable Energy: Achieving Sustainability through Bioenergy

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