Study of ecological charcoal production from agricultural waste

Wood fuel is undoubtedly the main source of energy for cooking in sub-Saharan Africa as it represents more than three quarter of household energy consumption. The exploitation of wood for fuel purposes contributes to forest degradation. It is becoming urgent to diversify domestic energy sources. Substituting other forms of energy with traditional ones is extremely difficult due to low income of the population and culinary habits. In this context, ecological charcoal seems to be an attractive alternative to wood energy. Agricultural waste was collected, dried to a moisture content of less than 10%. Waste was pyrolysed and the resulting carbonaceous material mixed with a binder and extruded to form briquettes. Three binders were tested: starch, clay and arabic gum. The pyrolysis of biomass generated three by-products: a solid (biochar), liquid and gaseous product. This process took 3 hours 45 minutes to convert 1tonne of waste into 390 kg of biochar and 133 liters of pyrolyser oil. After testing, biochar/binder ratios of 27/1.1, 27/2.7 and 27/2.1 for starch, gum arabic and clay, respectively, at a compaction pressure of 7.8 bar were validated.


Introduction
Energy is the key to development in Africa.It is on energy that the continent will build its industrialisation [1].This is also mentioned in the Sustainable Development Goals, in particular SDG 7, which aims to: "Ensure access to reliable, sustainable and modern energy services for all at an affordable cost" [2].But unfortunately, two billion people around the world still rely on wood for their domestic energy needs.The use of traditional biomass predominates in Africa, where it is burned directly, either for cooking and lighting in the residential sector or to a lesser extent in the industrial sector [3].Over the decades, the heavy reliance on traditional wood and charcoal has led to a massive depletion of forest resources, exacerbated by a growing population, with disastrous economic, social and health consequences for rural and poor populations [4].Moreover, this practice contributes to the proliferation of clandestine wood cutting activities, making charcoal one of the major causes of deforestation.Wood is becoming increasingly difficult to find and alternative energy sources are quite expensive.For example, in Yaounde, the capital of Cameroon, wood is supplied from forests that are now more than 600 kilometres away, whereas 30 years ago it was just 40 kilometres away.The forest is retreating as the trees are cut down.
The concerned people cannot afford modern energy sources, which means that they will soon no longer be able to cook their food.Deforestation accentuates the advance of the desert and climate change.Every year, more than 3.4 million hectares of forest are destroyed in Africa [4].The exclusive use of firewood as domestic fuel has many consequences.Hence the search for a substitute for wood energy is an extreme priority.A possibility is the recovery of waste.Indeed, the residues from agriculture can be transformed and used for energy purposes [5].This solution consists of recovering the organic fraction of agricultural residues, household waste or any other renewable biomass and transforming it into green charcoal briquettes.Results from previous work have shown that pyrolysis would be an effective technique to transform this type of biomass into potential energy sources [6].According to the dictionary of Environment and Sustainable Development "Green coal is a coal produced from biodegradable carbon-rich residues, mainly from agricultural, forestry or household waste.It comes in the form of briquettes or balls of the size of traditional charcoal pieces and could be used in most of the stoves used in the South" [2].Green charcoal can be used as a substitute for charcoal, firewood for domestic cooking or even heat production in industries [7].Carbonisation is the thermochemical decomposition of a material at high temperature and in the absence or lack of oxygen [8].Three products are obtained, and the solid fraction is the one used in this study [9].Unlike complete combustion, carbonisation requires lower processing temperatures and induces a lower level of atmospheric pollutants [10].It aims to produce coal, i.e. a carbon-rich product.Generally, the heat for the reaction is provided by the carbonised material itself.In the field of charcoal production from household waste, several carbonisation systems are used: traditional carbonisation, improved carbonisation, retort carbonisation and hydrothermal carbonisation.There are a number of carbonisation techniques.The most important are: partial combustion carbonisation, injection of hot gases into the load and closed or retort carbonisation.In partial combustion furnaces, the energy for carbonisation is provided by burning part of the charge inside the reactor, which reduces charcoal yields.Not all condensable products and gases are usually recovered.The conversion of biomass to charcoal in this way is poor, as more than 50% of the initial energy is lost.In addition, the disadvantage of this method is that it does not allow the carbonisation of straw, reeds, cotton stalks as well as biomass of small granulometry such as rice husk, coffee parchment and sawdust because of the poor heat transfer in the load to be carbonised and the difficulties in controlling air inlets which can lead to the total ignition of the load.
In hot gas contact carbonisation, the energy for carbonisation is provided by hot gases from an external furnace in direct contact with the feedstock.This method has a good production efficiency of around 30-35% but the cost of the installation is still quite high.It should also be noted that the operation of these reactors is difficult to control without suitable instrumentation.
In retort carbonisers, the charge is placed in an enclosed chamber, and the energy for carbonisation from an external furnace is transmitted through the walls of the chamber.The furnace can be fuelled by any fuel.Once the carbonisation process has started, pyrolysis gases can be injected into the furnace to sustain the pyrolysis.The advantage of these reactors is that they can carbonise plant material of small particle size and have a production efficiency of around 35%.This project is part of a research programme whose medium-term objective is to make available to agricultural producers a process for the energy recovery of agricultural waste.The main objective is to set up a protocol for transforming agricultural residues into ecological charcoal that can be used in farm heating systems and in cooking.First, the agricultural residues with the highest potential to be transformed into charcoal were identified.Then, preliminary tests were carried out to identify the optimal conditions for pyrolysis.Combustion tests were carried out to evaluate different mixtures of powdered coal and binder.Finally, techno-economical analysis was carried out based on the results obtained.

Materials and methods
This study was conducted at the Agricultural Engineering Research Unit of the Faculty of Agronomy and Agricultural Sciences, University of Dschang (West Cameroon).The agricultural waste used in this work was collected from farms around the Bamboutos Mountains.

Raw materials
The raw materials used in this study were agricultural waste (grasses and agricultural residues).The latter are sorted and dried in the sun.High moisture content makes the carbonisation yield low. Large raw materials have to be shredded into small pieces to facilitate carbonisation.

Fig. 1: Biomass used.
This waste is then fed into the pyrolyser.Heating is carried out with the help of biomass.A slow pyrolysis in the absence of oxygen is carried out.

Clean coal production
The green coal was produced by carbonisation in a batch type carboniser equipped with a gas recirculator.It operates in the temperature range of 400°C to 600°C and in an oxygen-free or oxygen-poor environment.In this study a batch pyrolyser was used.These are simple furnaces built from 60 litre metal drums.It is placed in a square tank with a double wall in which a continuous layer of 5 cm of clay is placed.Various adaptations have been made to allow its malleability and efficiency.These include the sealing, which is ensured by a gasket and a clamp, a hinged lid and chimney, welded handles to make it easier to move the kiln, and feet at the base of the kiln to ensure its stability and to regulate the air flow in the combustion chamber.The heat for the pyrolysis was generated by the biomass.The pyrolyser was equipped with a gas circulator, which led the pyrolysis gases to the combustion chamber, thus maintaining the reaction.This pyrolyser was developed as described in reference [11].The process of producing green charcoal involved 4 main steps: waste collection, pyrolysis, briquetting and drying

Addition of binder
To ensure the adhesion between the biochar particles and the strength of the briquettes a binder was added.For this study, three types of binder were tested.These were cassava starch (a), arabic gum (b) and clay (c).

Compacting and drying
After the addition of the various binders, the bio-fillerbinder mixture was fed into an extruder.For all mixtures formed, the compaction pressure of 7.8 bar was maintained.A human powered extruder was used (Figure 4).Once in briquette form, the coal was sun-dried to a moisture content of 8-10 %.

Combustion and controlled cooking tests
The combustion tests were carried out on all types of coal produced, i.e. a total of 12 samples.After these tests, the sample with the best combustion was taken from the coals of the three types of binders and used for the controlled cooking tests.For these tests an improved stove, a single pot, the same dishes and quantity of food were considered.The control fuel was charcoal purchased on the local market.A meal was prepared with each of the three fuels.The chosen dish was rice with tomato sauce.The meals were always prepared with the same amount of food.i.e. 0.5 litre of oil, 1.5 kg of fish, 1.2 kg of vegetables, 1.5 kg of rice, and 3.5 litres of water.For all measurements, we used a precision balance with an error of ±0.01g.The cooking was done outdoors in the shade with an oven and an aluminium pot.The starting time for cooking was always the same (11:30 am).
During the experiment, measurements of fuel, feed and cooking time were taken.Based on the loss in weight between the starting and ending quantities, the amount of water evaporated during cooking was calculated.
At the end of the experiment, the calorific value of the best burning green charcoal and the comparable wood charcoal was determined using a calorimeter bomb.This is measured in order to get an idea of the energy content of fuels formed from agricultural waste.This will allow recommendations to be made on the sales price in comparison with wood charcoal.

Results and discussion
Thermochemical treatment for the production of green charcoal, known as carbonisation, is a technique that can be implemented by industry at different scales.It has been shown to be a versatile treatment for the transformation of different types of agricultural materials (woody residues, grassy biomass).

Pyrolysis balance
The overall pyrolysis balance was evaluated (Fig. 5).In general, the yields of pyrolysis products are consistent with the stated results [12].It has been found that the carbonisation of agricultural waste generates three types of products.These are powdered charcoal (biochar), a condensable fraction (pyrolysis oil and water) and a non-condensable fraction (gas).This result corroborates with results obtained from previous works ( [7], [13], [14]).This study showed that the pyrolysis time influences the quantities of pyrolysis products.Indeed, when pyrolysis lasts 198 minutes (pyrolysis 1), an increase in pyrolysis oil yields of 7% is observed.This increase may be due to the absence of secondary degradation reactions that transform the condensable fraction into a non-condensable fraction.According to reference [15], during pyrolysis, the final equilibrium is strongly dependent on the temperature and heating rate.
On the other hand, when the reaction lasts 225 min (pyrolysis 2), an increase of 13% of the solid fraction (char) and a decrease of 9% of the condensable fraction are observed.This results in 390 kg of char, 134 kg of pyrolysis oil, gas and water per tonne of waste to be pyrolysed.
The powdered charcoal was collected and mixed cold with either cassava starch, arabic gum or clay, and in each of the following ratios: 27/1.1 (~4%); 27/2.1 (~7%) and 27/2.7 (~9%).For each of the mixtures, 5.2 litres of water was used.9 charcoal samples were produced.They were then placed in the sun for two days with an average of 6 hours of sunlight per day.After drying, it was found that charcoal with 4% and 7% arabic gum had a crumbly appearance.At 4% it crumbled completely when trying to lift.The same was observed for the 4% clay coal.

Combustion test
Combustion tests were carried out with each of the 6 coal samples, the coal with starch as a binder showed a longer ignition time and more smoke as the percentage of starch increased.For the coal with 9% clay as binder, great ignition difficulties were observed, followed by interrupted combustion.Therefore, the clay charcoal dosages of 27/2.1 (a), starch charcoal 27/1.1 (b), and arabic gum charcoal 27/2.7 (c) were selected as they showed the best combustion (short ignition time, high burn time, less smoke).

Controlled cooking tests
The three types of eco-friendly charcoal were subjected to a controlled kitchen test.The results were analysed on different aspects.These were the amount of fuel, the cooking time and the amount of fuel used to heat the same amount of food.It has been found that the combustion of green coal takes place in three distinct phases: The first phase is the cold start.It is loaded with 1.2 kg of charcoal and 1.4 kg of eco-coal.The pot is placed on the tripod, which allows the mass of food to be acquired independently of the mass of the pot.A total of 8.2 kg of food (2 rounds of cooking) is poured into the pot.This phase lasted about 5 minutes followed by the hot start phase which takes place between the fifth and sixteenth minute.During the simmering phase, the food was now boiling and the aim was to keep it boiling until a mass of 5.21 kg was reached.

Amount of fuel
The amount of fuel at the beginning of the test was not the same for each type of coal.This initial inequality was due to the weight of the different coals and the capacity of the firebox.Let (C) be the charcoal, (A), (G) and (Ar), ecological charcoal with starch, arabic gum and clay, respectively.It was found that about 16% more biochar is needed than charcoal to cook the same meal.This result differs from the work of [16] when they showed that about 21% more biochar is needed to cook the same amount of food.Other authors such as [17] claim that around 30% more biochar is needed compared to charcoal.
The quality of the binder and the differences in composition have an influence.That is, the amount of fuel is reduced with the type of binder.Of the biochar tested, the one with starch is the closest to charcoal.The total firing time is longer (9 -23%).with biochar than with charcoal.The ignition time is longer (15 -23%).with biochar than with charcoal.This time is higher with charcoal from clay binder.This indicates a particular ignition stress of the briquettes.The simmering time is also higher than with charcoal.This result is corroborated by the work of [16].Indeed, he states that one of the difficulties in using this coal is the difficulty of ignition.He then points out that this can be improved.

Amount of biochar used to heat the same amount of food
The amount of food at the beginning of the experiment is the same for all tests.Although the cooking time and the amount of fuel used differed from test to test, we made sure that the weight of food at the end of the cooking process was the same.These results contradict the statement in reference [16] that there is no difference between starch and clay flour briquettes, so the binders used have no influence on the combustible qualities of the biochar.

Techno-economics
Calculation of the Lower Heating Value (LHV) of the charcoal (LHVcb) and the ecological starch charcoal (LHVce ) used shows that LHVcb is 28,200 kJ/kg compared to 27,100 kJ/kg for LHVce .In other words, 96 % of the PCI of charcoal.The selling price of ecochar will, therefore, have to take into account its LHV and the surplus consumption (16%) compared to wood.Assuming that a kg of wood charcoal costs about 300 CFA francs, 1 kg of ecological charcoal should cost 252 CFA francs to be equivalent to wood charcoal.Particular emphasis could be placed on other pyrolysis products.Indeed, the potential of pyrolysis coproducts, such as pyrolysis oils and non-condensed gases, is enormous.According to reference [6], pyrolysis oils can be used as biocides because during pyrolysis, the molecules and biocidal substances active in certain biomasses are transferred to the oil during pyrolysis.The use of the oil is, therefore, promising, especially in organic farming.

Conclusion
The experiments carried out on the manufacture of charcoal briquettes from agricultural residues aimed to identify the influence of the type of binder and the quantities of binder required for the production of improved quality ecological charcoal.The experiments show a 20% higher efficiency of charcoal compared to biochar.This difference in consumption is partly due to the lower LHV (by 4%) of ecochar compared to charcoal.Between the different types of formulated biochar, a difference was observed.The cassava starch-based biochar has the best combustion and the highest LHV among the manufactured biochar, and is, therefore, an excellent alternative to the use of firewood.The pricing of briquettes on the market will have to take into account these LHV data.

Table 1 .
Amount of fuel tested.

Table 3 .
Amount of food used.