EXAMENSARBETE INOM TEKNIK, GRUNDNIVÅ, 15 HP
STOCKHOLM, SVERIGE 2020
Agricultural waste and wood waste for pyrolysis and
biochar - an assessment for Rwanda
JENNY ELIASSON
VIKTOR CARLSSON
Abstract
A high priority in order to combat climate change is disposal of waste. In low-income countries, a large portion of biomass residues generated in the forestry, agricultural and industrial sectors could be usable, instead of being seen as waste. For instance, it could be converted into biochar, which is proven to have many environmental benefits. In Rwanda, the agricultural sector employs 80% of the population and accounts for 35% of GDP. This sector, together with later refinement of crops and forestry production, cause large amounts of residue that many times is considered as waste. In this report, a literature study was conducted to evaluate possible biochar production from agricultural and wood wastes in Rwanda. Characteristics that determine if a biomass could be suitable for a biochar
production were identified as C, H, O, N, S, hemicellulose, cellulose, lignin, ash and moisture
content, residue-to-product ratio, and low heating value. These characteristics were assessed
for the chosen Rwandan agricultural and wood wastes, by compiling values from published
reports. The result shows that there are large volumes of residues that have potential for
biochar production instead of being seen as waste in Rwanda. Biochar production from these
wastes could enable environmental benefits for Rwanda, although further investigation of
each single biomass could be needed in order to see if it is practically, technically and
financially possible to do in reality.
Sammanfattning
För att bekämpa klimatförändringen är avfallshantering en hög prioritet. I låginkomstländer kan en stor andel av biomassarester som genereras i skogsbruk, jordbruks- och
industrisektorer vara användbara, istället för att ses som avfall. Till exempel skulle det kunna omvandlas till biokol, som har visats sig ha många miljömässiga fördelar. I Rwanda arbetar 80% av befolkningen inom jordbrukssektorn och den står för 35% av BNP. Denna sektor, tillsammans med förädling av grödor och skogsbruksproduktion, orsakar stora mängder rester som många gånger betraktas som avfall. I denna rapport genomfördes en litteraturstudie för att utvärdera möjlig produktion av biokol från jordbruks- och träavfall i Rwanda. Egenskaper som avgör om en biomassa kan vara lämplig för en biokolsproduktion identifierades som C-, H-, O-, N-, S-, hemicellulosa-, cellulosa-, lignin-, ask- och fukthalt, samt andel avfall som uppstår i förhållande till färdig produkt och värmevärde. Dessa egenskaper utvärderades för det valda jordbruks- och träavfallet genom att sammanställa värden från publicerade
rapporter. Resultatet visar att det finns stora volymer rester som har potential för
biokolsproduktion istället för att ses som avfall i Rwanda. En biokolsproduktion från dessa avfall skulle kunna ge miljömässiga fördelar för Rwanda, även om ytterligare undersökning av varje enskild biomassa skulle behövas för att se om det är praktiskt, tekniskt och
ekonomiskt möjligt att genomföra i verkligheten.
Acknowledgements
We would like to especially thank Cecilia Sundberg for being our supervisor during this
project. She has been an inspiring mentor with her great knowledge and experiences, and this
project could never have been completed without her. We would also like to thank the two
members of the academic staff at University of Rwanda for helping us to get in contact with
relevant stakeholder and providing us with valuable information.
Acronyms and Abbreviations
A.D. Air dried
C Carbon
CH
4Methane
CO Carbon monoxide
CO
2Carbon dioxide
FAO Food and Agriculture Organisation of the United Nations
GHG Greenhouse gas
H Hydrogen
HHV High heating value
HTC Hydrothermal carbonization
LHV Low heating value
MC Moisture content
MSW Municipal solid waste
N Nitrogen
NA Not Available
NISR National Institute of Statistics Rwanda
NO
XNitrogen oxides
N
2O Nitrous oxide
O Oxygen
RPR Residue-to-product ratio
S Sulfur
SDG Sustainable development goal
SO
2Sulfur dioxide
t Metric tonne
wt.% Weight percent
Table of Content
1. Introduction 1
2. Aim 2
3. Limitations 3
4. Background 4
4.1 Thermochemical conversion processes 4
4.1.1 Pyrolysis 4
4.1.2 Dry Torrefaction 4
4.1.3 Gasification 5
4.1.4 Hydrothermal carbonization 5
4.2 Pyrolysis of waste materials 5
4.2.1 Food waste and agricultural waste 5
4.2.2 Wood waste 6
4.3 Overview Rwanda 6
4.3.1 Environmental policy 6
4.3.2 Agricultural policy 7
4.3.3 Energy supply 7
4.3.4 Agricultural production 7
4.3.5 Forestry production 8
5. Method 9
6. Results 10
6.1 Feedstock characteristics 10
6.1.1 Heating value 10
6.1.2 Moisture content 10
6.1.3 Ash content 10
6.1.4 Chemical composition 11
6.1.5 Particle size 11
6.2 Feedstock characteristics of agricultural and wood waste in Rwanda 12
7. Discussion 19
7.1 Pre-treatment 19
7.2 Waste sources 19
7.3 Current waste management in agroindustries 19
7.4 Environmental benefits 20
7.5 Sources of error 20
7.6 Further studies 21
8. Conclusion 23
References 24
Appendix 1 27
Appendix 1A – Spreadsheet of agricultural waste 27
Appendix 2 29
Appendix 2A – Reference numbers agricultural and wood waste spreadsheets 29 Appendix 2B – References agricultural and wood waste spreadsheets 31
Appendix 3 36
Appendix 3A – Interview with a member of the academic staff at University of
Rwanda (person 1). 36
Appendix 3B – Interview with a member of the academic staff at University of
Rwanda (person 2). 37
Appendix 3C – Interview with a Rwandan cassava factory 37
List of figures
Figure 1. Production of major crops in Rwanda.. ... 8
Figure 2. LHV of the biomasses. ... 12
Figure 3. HHV of the biomasses. ... 12
Figure 4. The composition of O, H and C in the biomasses. ... 13
Figure 5. The composition of S and N in the biomasses. ... 14
Figure 6. The structural composition of the biomasses. ... 15
Figure 7. The moisture content of the biomasses. ... 16
Figure 8. The ash content of the biomasses. ... 17
Figure 9. The production of major crops and the amount of residue they are estimated to
cause. ... 18
1. Introduction
In order to combat climate change, disposal of waste and excess product is of high
importance. The world’s population is rapidly increasing, while economic development and improvements of living standards are high priorities (Czajczyńska et al., 2017). This results in an increase of consumption per capita, generating larger quantities of waste, and thereby causing challenges to sustain growth within the planet’s limited amount of resources. The production-consumption cycle needs a transition towards a circular economy, where recycling and reuse of waste is included (Otoo and Drechsel, 2018).
Today, Africa is the least urbanized continent in the world, but at the same time has the fastest urbanization rate (Aryampa et al., 2019). When Africa’s population is growing, food and agricultural production will rise and the waste generation will increase (Scholz et al., n.d.). However, the large volumes of biomass residues generated in the forestry, agricultural and industrial sectors in low-income countries do not necessarily have to be considered as waste, which it is today. A large portion of this is in fact usable, instead of unnecessary burning, burying or storage (Mwampamba et al., 2013).
An attractive alternative for waste disposal is pyrolysis, which is the heating of biomass materials in the absence of oxygen (Kambo and Dutta, 2015). The process results in waste reduction and produce valuable energetic and/or chemical products (Conesa et al., 2009). One of the products of pyrolysis is biochar, a carbon-rich solid, which is proven to have many environmental benefits. Applying biochar to the soil can improve the soil structure, as it improves the retention of nutrients and water in the soil and create habitats for symbiotic microorganisms, which could increase the amount of harvest (Czajczyńska et al., 2017). This could be a huge improvement for smallholder farmers who have degraded soils. Furthermore, biochar is very stable, which makes it suitable for carbon sequestration and therefore
reducing greenhouse gas emissions, if it is used in soil and does not get burned (Kung et al., 2015). While CO
2from fresh biomass is re-emitted to the atmosphere in the range of months to years, the CO
2in biochar reemitts in scales of decades to millennia (Scholz et al., n.d.).
Looking at a low-income country perspective, there are therefore various reasons for
introducing biochar systems.
2. Aim
The aim is to identify agricultural residues and wood wastes in Rwanda with such properties that they are considered to add value for further processing into biochar.
Specific sub-goals are the following:
- Identify and quantify available wastes
- Assess their properties in relation to demand for pyrolysis with biochar production
3. Limitations
There are various kinds of waste materials that could be feasible for biochar production.
Wastes that are considered relevant for this study are agricultural waste and wood waste. The reason for this is that the agricultural sector is the biggest sector in Rwanda, causing a large amount of crop residues both on site and in agroindustries. Furthermore, a large portion of Rwanda’s energy mix consists of firewood, but since small-sized branches, bark and leaves generally are not used in in the firewood production, a large amount of wood waste is produced on site. In addition, the municipal solid waste (MSW) composition for Rwanda is estimated to consist of 70 wt.% of organic waste; food and green waste (Kabera et al., 2019), which means it could be a large potential source.
Furthermore, there are more factors than only the biomass characteristics that determines if a
biochar production could be feasible. These factors are not considered in the report, including
for instance accessibility, production cost, technical limitations, where the waste occurs, and
distance required to transport biomass to the pyrolysis site.
4. Background
4.1 Thermochemical conversion processes
There are various of thermochemical conversion processes that can use waste as a feedstock (biomass) to produce biochar. These methods are proven to have many environmental
advantages, including waste minimization, pollution reduction and energy recovery (Guida et al., 2019). The processes considered relevant for this study are described below. As pyrolysis is considered the most relevant for this study since it produces biochar, it is explained in more detail, while the other processes are briefly explained.
4.1.1 Pyrolysis
Pyrolysis is the heating of organic materials in the absence of oxygen (Kambo and Dutta, 2015). It is a thermochemical decomposition process taking place at temperatures between 300-600°C. The process results in three main products: carbon-rich solid product called biochar, volatile liquids and gases. The gases consist of carbon monoxide (CO), carbon dioxide (CO
2), hydrogen (H), methane (CH
4) and higher hydrocarbons called biogas (Kung et al., 2015). Depending on temperature, reaction time and heating rate, pyrolysis can be
subdivided into four categories: slow, fast, flash and intermediate, where slow and fast are the most common types. The yield of solid char depends on these factors.
The difference between fast and slow pyrolysis is the time and temperature. In fast pyrolysis, the temperature and heating rates are high. The process can be done in seconds and the resulting product consists mainly of bio-oils (Morgan et al., 2015). In slow pyrolysis, the process can go on for hours and the heating rate and temperature are lower; a temperature under 450°C is common. The purpose with slow pyrolysis is mainly to produce biochar. It’s estimated that fast pyrolysis produces 60-75% of bio-oil, 15-25% biochar and 10-20% gases (Moneim et al., 2018), while slow pyrolysis produces about 35% biochar, 30% bio-oils and 35% gases (Kung et al., 2015). The products in both fast and slow pyrolysis can be further processed and produce higher-quality fuels (Kung et al., 2015). Further processed bio-oil could for instance be utilized as raw material for petrochemical production, biochar could be made into briquettes which can be used for cooking and the gas produced can be burned as energy or a source for heat supply in the pyrolysis process (Liu et al., 2016).
Pyrolysis has several advantages compared to combustion. Its process temperature is lower, the scale of the pyrolysis plant is more flexible compared to other combustion plants and the composition of the pyrolysis product can be modified by changing the temperature and heating rate. (Czajczyńska et al., 2017). Due to the absence of oxygen and lower process temperature, the air pollutant emissions of pyrolysis could be lower compared to combustion, although emissions of other compounds simultaneously could increase with a lower oxygen ratio (Conesa et al., 2009).
4.1.2 Dry Torrefaction
In dry torrefaction, the operation is made at temperatures of 200-300° C (Chen and Jhou,
2020). The thermo-technical characteristics can be compared to coal and energy content that
is retained in the torrefied biomass range from 70-90%. However, the torrefaction product
cannot be referred as biochar, as it is more like a preprocessing step for making biochar
(Kambo and Dutta, 2015). The properties of the torrefaction result is in-between that of raw
biomass and biochar.
4.1.3 Gasification
In gasification, biomass is heated at a very high temperature range of 600-1200°C in a combustion chamber (Kambo and Dutta, 2015). The primary product in this process, around 85%, is a mixture of gases referred to as syngas (Zabaleta et al., 2018). The biochar produced from gasification could contain high amounts of alkali and alkaline earth metal and
polyaromatic hydrocarbons, which are toxic (Kambo and Dutta, 2015). Therefore, this biochar may be problematic to use in soil that intend to be used in agriculture.
4.1.4 Hydrothermal carbonization
Hydrothermal carbonization (HTC) is a thermochemical process suitable of converting organic feedstock into a high carbon rich solid product (Kambo and Dutta, 2015). The process often use biomass with high moisture content as a feedstock. The HTC is performed at 180-260°C under pressure for 5-240 min. Experimental batches show results of 45-75%
char from the initially present biomass (Lu et al., 2012).
4.2 Pyrolysis of waste materials
There is a large range of waste materials that could be suitable for pyrolysis and biochar production. Various types of organic waste can be used, including waste from wood, agriculture, forestry and pulping industry (Czajczyńska et al., 2017). Feasible materials within the municipal solid waste (MSW) are paper, cloth, plastics, food waste and garden waste. In order to implement pyrolysis in MSW, materials that cannot be used for energy recovery have to be removed (Sipra et al., 2018). Wastes that are considered relevant for this study are agricultural waste and wood waste.
4.2.1 Food waste and agricultural waste
Although pyrolysis of food waste has a good potential, the effectiveness depends greatly of the composition of the waste (Czajczyńska et al., 2017). The components in food waste are lipids, carbohydrates, amino acids, phosphates, vitamins and other substances containing carbon. There are several types of food waste, including agricultural waste, catering waste, animal by-products and mixed domestic food waste, all with different feasibility for
pyrolysis. The use of domestic food waste in pyrolysis is limited due to the high variation of the composition and high chlorine content due to the salt present in the food (Sipra et al., 2018). In addition, food waste usually contains more nitrogen than other feedstocks due to the protein in the food (amino acids), which is not suitable as a feedstock since a large portion of it is lost during the pyrolysis process and could result in toxic emissions. Debono et. al observed that the nitrogen content in char is between 17-26% of the initial nitrogen content in food waste, wood waste and sewage sludge (Debono and Villot, 2015).
However, there have been various studies showing the potential of agricultural waste. Fruits
and vegetable peels and other residues from for example spinach, bananas, peas and tomatoes
have a great potential as feedstock to biochar production (Soltan et al., 2019). Both slow and
fast pyrolysis can be used for the agricultural waste, depending of what the wanted products
are; for biochar production, slow pyrolysis is more suitable. The same crops could however
have different values due to for instance crop type, moisture content of the residues, weather
variation, water availability, soil fertility and farming practices (Ullah et al., 2015).
4.2.2 Wood waste
Wood biomass commonly mainly consists of hemicellulose, cellulose and lignin, together with some extractives (Czajczyńska et al., 2017). It is divided into two main groups; soft and hard wood. Pyrolysis of wood usually needs a temperature of at least 300-375°C. As the biomass is gradually heated, complex transitions occur, which is essential to know in order to properly design and control the pyrolysis process (Guida et al., 2019). The pyrolysis products from wood biomass generally has a high oxygen content as the feedstock consists of a large portion of oxygen (Ware et al., 2017).
Possible sources for wood waste are residues from the timber industry such as logging companies, sawmills, veneer and panel factories. Many tropical countries have a large amount of waste from wood industries, and it is especially an untapped source in Africa (Dam, 2017). In addition, residues are caused from forestry production and garden waste. For instance, in charcoal production, small-sized branches, bark and leaves are generally not used, which causes a large amount of wood waste on sites. This could stand for a large portion of the whole tree; 30-40 wt.% of an eucalyptus tree (Amutio et al., 2015). However, this could also have a potential to be used for charcoal production by collecting, drying and grinding the waste (Dam, 2017).
Furthermore, residues from pruning of trees could have a potential for biochar production. A study of teak tree pruning made in Spain estimated that the average biomass residue from tree pruning is 16.93 kg wood and 1.37 kg of leaves per tree/year (Pérez Arévalo and Velázquez Martí, 2020). This can be compared to a study in Tajikistan, where data from the country’s
“Institute of Horticulture and Vegetable Growing” showed that about 156 apricot trees were grown in one ha, which resulted in approximately 15-20 kg/tree of residues after pruning the trees every year (Akhmedov et al., 2019). Tree pruning residues has shown to be suitable as an energy fuel source. For instance a study in Croatia viewed the opportunity to use pruning residues as energy source for heating (Bilandzija et al., 2012). Analysis was made on the combustible and non-combustible matters of the following fruit trees: apple, pear, apricots, peach and nectarine, sweet cherry, sour cherry, prune, walnut, hazelnut, almond, fig, grapevine, and olive, which were generally shown to have suitable characteristics.
4.3 Overview Rwanda
4.3.1 Environmental policy
With the strategy “Green Growth and Climate Resilience National Strategy for Climate Change and Low Carbon Development” Rwanda’s government is committed to combat the climate changes (King et al., 2011). Nevertheless, Rwanda has one of the lowest emissions per capita in the world; estimated at 0.4 tCO2e/person, compared to the global average of 6.7 tCO2e/person. Rwanda is highly vulnerable to climate change due to its dependence on the agricultural sector, which employs 80% of the population and stands for 35% of the GDP.
With a population over 11 million, Rwanda has the highest populated density in Africa; 525 per km
2, and is expected to grow to 26 million in 2050 (“Rwanda Population (2020) - Worldometer,” n.d.). As the population grows, so does the agricultural sector, which has doubled since 2007 (King et al., 2011). The agricultural sector is in the top three sources for Rwanda’s greenhouse gas (GHG) emissions. A significant reason for this is the large
quantities of chemical fertilisers that are used, releasing N
2O emissions as well as other GHG
emissions during fertiliser manufacturing process and transportation. In fact, the soils in the
agricultural sector stands for 57% of the N
2O emissions. Two of Rwanda’s policy goals
within the biomass sector in order to reduce their emissions are “switching fuel from traditional biomass energy carriers to modern biomass” and “increase access to cleaner cooking fuels. This includes biogas, LPG, charcoal and biomass pyrolysis stove” (Puri and Rincón, n.d.).
4.3.2 Agricultural policy
Rwanda has a “crop intensification programme”, which aims to increase productivity,
irrigation coverage and soil quality in the agricultural sector (Cantore, n.d.). It also focuses on sustainability in the long term since a positive development in the agriculture will benefit the GDP. One of the ambitions is to reduce the high demand of inorganic fertiliser by “applying an integrated approach to soil fertility and nutrient management, which employs
agroecology, resource recovery and reuse, and fertiliser enriched composts” This could reduce GHG emissions, oil dependence and costs for farmers (King et al., 2011). An
intensification of sustainability in the agricultural sector is seen as a key component for a low carbon and resilient agricultural sector for the future. Furthermore, research on biofertilisers technologies such as organic fertiliser is a prioritized goal within Rwanda’s Strategic Plan for Agriculture Transformation 2018-2024 (Ministry of Agriculture and Animal Resources, 2018).
4.3.3 Energy supply
Biomass currently accounts for the majority of Rwanda’s energy mix; 83% of the primary energy use, where the majority consists of firewood (REG, 2020). The majority of the population is dependent on wood fuel for cooking, which is expected to be unchanged until electricity supply is available everywhere in the country and affordable for the whole population (Champion and Grieshop, 2019). However, Rwanda faces a wood fuel deficit.
While the demand of wood (firewood and charcoal) is 1.93 kg/person/day, the production is only 0.46 kg/capita/day (Otoo and Drechsel, 2018). At the same time, the firewood
production result in deforestation. Between 1990 and 2010, Rwanda lost 37% of its forest cover. As the population is increasing, the deforestation is intensified, which will result in more environmental degradation.
4.3.4 Agricultural production
Estimations are made that 59% of the country's total area is agricultural land (NISR, 2019).
Nearly 80% of the rural population is farmers and the average landhold is about 0.59 hectares. Since the intensification programme started in 2007, the agricultural sector has grown noticeable in staple food crops. This is significant since the food availability in Rwanda is improving as the smallholder productivity increases (Food and Agriculture Organization, 2013). According to the National Institute of Statistics of Rwanda, the agricultural year in Rwanda is divided into three seasons:
Season A: Starts in September to February the following year.
Season B: March to June
Season C: Small agricultural season (“mainly for vegetables and sweet potato grown in swamps and Irish potato grown in volcanic agro-ecological zone”) (NISR, 2019).
Figure 1 shows the agricultural production in Rwanda. The major crops are banana
1, sweet potato, cassava, potato, beans and maize (Appendix 1A).
4.3.5 Forestry production
The forest cover of Rwanda is 17%, corresponding to 450 000 ha, where 46% is natural forests and the remaining public and private plantation (Otoo and Drechsel, 2018). Of the plantations, 25% is owned by private citizens and 9% by institutions. The majority, 65%, is owned by state or district, where 30% of the state plantations are left for soil protection and can therefore not be harvested. The plantations are dominated by various species of
Eucalyptus, but other tree species that could be found are for instance Pinus patula and Callitris robusta (Kinyanjui et al., 2018). The wood demand in Rwanda consists of 49%
charcoal, 28% firewood and only 23% for non-energy usage. The current wood fuel and charcoal production in Rwanda comes mainly from woodlots in smallholder plots, where the eucalyptus trees are the most suitable and common specie (Dam, 2017; Nahayo et al., 2013).
It is not produced in natural forest anymore in order to conserve it and prevent deforestation.
It is estimated that 90% of the woodlots consist of eucalyptus trees (Ndayambaje et al., 2014).
Beyond deforestation of natural forests, Rwanda currently faces various other challenges within the forest sector, such as lack of forest management plans and knowledge of national forest stock, wasteful conversion and consumption of timber, dominance and under-
utilizations of Eucalyptus and uneven forest cover distribution (Ajewole et al., 2016).
1 Banana production also consisted of cooking banana, dessert banana and banana for beer. It was not entirely clear if these categories were sub-categories of the total banana production or if they were another category.
They were therefore assumed to be sub-categories.
Figure 1. Production in metric tonne (t) of major crops in Rwanda. Unlabeled fractions in the figure include the crops groundnut, peas, soybeans, coffee and tea.
5. Method
This report is based on a literature study, which was chosen as an appropriate method since values of various crops and wood wastes could be compiled. The primary literature study was conducted by general searches of the food and forest production in Rwanda, to get an
overview of the situation in Rwanda's agricultural and forestry sector. Main crops and wood waste considered relevant for the study were chosen and listed in a spreadsheet. The kind of residue caused by the crops, such as straw and peel, was identified, if possible. Important feedstock characteristics in order to implement pyrolysis were identified, and these characteristics were assessed for the chosen agricultural and wood wastes, by compiling values from previous reports. The characteristics chosen were ultimate analysis (proportion of C, H, O, N and S), structural composition (proportion of hemicellulose, cellulose and lignin), ash content, heating value and moisture content (air dried). When estimating the heating value, the low heating value (LHV) was taken. If only the high heating value (HHV) was present, this was taken instead, which then is mentioned.
The quantity of the agricultural wastes was estimated by the residue-to-product ratio (RPR).
Since the RPR varied significantly in different reports compared to other characteristics, a mean was taken from 1-10 sources, and this value was multiplied with the annual production of the crops in Rwanda. Relevant data from the spreadsheet were then compiled to various figures in order to easier compare the results from different biomasses. From these
characteristics, an assessment of the feasibility for pyrolysis and biochar production of the wastes was made.
The result was supplemented by information from relevant stakeholders in Rwanda in order to get further insights of the potential of implementing biochar production from the identified wastes. The interviews were held through email, were the stakeholders answered a few questions regarding the current waste treatment and the possibility of a biochar production, with possible follow up questions. The stakeholders included a cassava factory and two members of the academic staff at University of Rwanda.
Literature used in the report were found through scientific search engines, mainly Web of
Science, Google Scholar and KTH Primo. A few sources were found through the research
institute EAWAG, where they had relevant publications of projects regarding carbonization
of urban bio-waste through pyrolysis. Statistics of the crop production in Rwanda were taken
from the Food and Agriculture Organisation of the United Nations (FAO) and National
Institute of Statistics Rwanda (NISR), since they provide a large database of agricultural
statistics from Rwanda. The quantity of wood waste produced could not be determined due to
limited data in the forestry production. Main key search terms used were Biochar, Pyrolysis,
Biochar feedstock characteristics, Agricultural waste, Composition crop residues, Wood
waste, Tree pruning, Agricultural production Rwanda, Forestry production Rwanda, RPR,
Energy mix Rwanda. Various searches have been made on specific crops and wood wastes,
such as Cassava waste structural composition, Eucalyptus waste ultimate analysis.
6. Results
This section presents the result, divided into two sections. The first section identifies important feedstock characteristics for pyrolysis and the second identifies characteristics of biomasses in Rwanda.
6.1 Feedstock characteristics
In order to implement pyrolysis, there are various significant characteristics that are required for the feedstock. The characteristics that are considered relevant are explained below. These have been chosen since they have a large impact on the yield of the biochar, and they
determine if the biomass could be used for biochar production or not.
6.1.1 Heating value
The heating value of a biomass can be defined by the higher heating value (HHV) or the lower heating value (LHV) (Saidur et al., 2011). HHV is the energy content of the feedstock on a dry basis. LHV is the energy content on a wet basis; HHV subtracted by the energy required to evaporate the moisture content in the biomass. A feedstock with high heating value is suitable for pyrolysis. The heating value is affected by the moisture content, ash content and chemical composition.
6.1.2 Moisture content
Biomass contains a proportion of water, which can exist as water vapor, chemically bound water and free liquid water (Tripathi et al., 2016). Depending on the moisture content,
biomass is categorized as wet or dry, which will need different pretreatment methods(Kambo and Dutta, 2015). Examples of wet biomass are fruits, vegetables and animal waste, while dry biomass could be agricultural residues and some wood species. When biomass is heated, the water contained in the biomass will evaporate before the thermochemical decomposition can start (Zabaleta et al., 2018). The higher the moisture content, the more heat energy will be required in order to dry the biomass. In other words, LHV is decreasing and lowering the flame temperature with a higher moisture content (“Moisture Content of the Biomass - an overview | ScienceDirect Topics,” n.d.). In addition, a higher moisture content increases the formation of tar, a viscous liquid, and as a result reduces the biochar formation. Therefore, a low moisture content is preferred for biochar production, and biomass containing more than 30% water is not suitable as a feedstock (Tripathi et al., 2016). Wet biomass needs to be dried before the thermochemical conversion process in order to reduce energy loss. Drying the feedstock to a moisture content of 10-15% is often required (Zabaleta et al., 2018). The drying could be done either in a machine or out in the sun, if the climate is hot and arid enough. However, the moisture reduction by air drying and sun drying is usually limited to a certain extent, which means mechanical drying is needed if a further moisture reduction is required (Tripathi et al., 2016). The drying could for example be 24 hours in around 100°C for oak and corn (Vakalis et al., 2019), and 48 hours under the sun (in Egypt) for rice straw (Moneim et al., 2018).
6.1.3 Ash content
Ash is the inorganic component of the biomass; the incombustible mineral material (“Ash
Content - an overview | ScienceDirect Topics,” n.d.). A feedstock with low ash content is
suitable for pyrolysis, as it increases the heating value of the product (Lohri et al., 2015). A
high ash content results in poor combustion, disposal problems, higher processing costs and
more fouling or aggregation on the reactors (Chen et al., 2015). Therefore, a biomass with
high ash content is not considered suitable as a feedstock for thermo-chemical conversion processes. A low ash content together with a high lignin content could higher the mechanical strength of the produced biochar (Downie et al., 2012). Agricultural residues generally contain more minerals compared to wood biomass and therefore often have a higher ash content.
6.1.4 Chemical composition
The composition of a biomass can be determined in various ways, including ultimate analysis and structural composition. Ultimate analysis is the proportion of the elemental components, which in biomass includes mainly carbon (C), oxygen (O), followed by hydrogen (H), sulfur (S), nitrogen (N) and some extractives such as alkali metals, alkaline earth metals and heavy metals. Other elements including magnesium, chlorine and potassium could also be present, depending on the source of biomass (Tripathi et al., 2016). The structural composition in biomass is mainly the polymers lignin, cellulose and hemicellulose (Ware et al., 2017). The proportion is generally measured in weight percent (wt.%) and as the composition of the elements differ, the product’s physical properties and chemical composition will vary significantly.
The amount of C, O and H is significant as those are the main contributors to the energy content and therefore controls the fuel property of the biomass. The proportion of these elements in the biomass is used to estimate its heating value (Tripathi et al., 2016). A high C and O content is preferred, as it generally increases the char formation and the heating value of the product. However, it’s not suitable with a too high O content. If the O content is very high (>70%) and the C content is significantly low (>25%), it can imply that the feedstock has a poor energy density (García et al., 2012). The N and S contents are preferably low. A high content of N and S will release a larger amount of nitrogen oxides (NO
X) and sulfur dioxide (SO
2) emissions, which are toxic (Chen et al., 2015).
Furthermore, a high lignin content is favourable to produce high char yield due to its higher thermal stability compared to cellulose and hemicellulose (Lohri et al., 2015). While
hemicellulose and cellulose break down at 200-250°C and 240-350°C respectively, lignin decomposes gradually at a larger temperature range; between 280-500°C (Czajczyńska et al., 2017). While cellulose and hemicellulose accounts for the volatile products, lignin accounts for the char yield. Although both cellulose and lignin enhance the biochar formation, biomass that consist of a larger portion of lignin relative to cellulose could have a larger biochar production. Therefore, the char production decreased as the cellulose content increased (Tripathi et al., 2016). Furthermore, a high lignin content generally correlate to a high heating value (Saidur et al., 2011).
6.1.5 Particle size
The particle size has a significant impact on the heating rate and yields of the products. A
feedstock with large particle size has a reduced heating rate since the biomass wall works as
an insulator, which means that it takes longer time for the pyrolysis process to occur. If the
batches are too big and dense, their walls will be pyrolysed while the inner core is untouched
(Zabaleta et al., 2018). Therefore, processes where a high heating rate is required, such as fast
pyrolysis, requires smaller particle sizes; fine dust or powder. Slow pyrolysis, however, could
have a feedstock with larger particle sizes, even up to many centimeters in diameter (Downie
The feedstock could need pre-treatment such as grinding in order to homogenize the biomass in size. This is causes the lignocellulosic structure to evenly break down, which increase the pyrolysis efficiency (Zabaleta et al., 2018). Furthermore, compressing of the feedstock could also be needed before the pyrolysis occurs, as it could make the process more energy efficient and could generate a larger amount of char. For example, Zabaleta et al. showed that sawdust briquettes performed better than non-compressed sawdust in pyrolysis process (Zabaleta et al., 2018). A particle size larger than 1 cm is shown to be suitable for the feedstock in slow pyrolysis.
6.2 Feedstock characteristics of agricultural and wood waste in Rwanda
Figure 2. LHV of the biomasses.
Figure 3. HHV of the biomasses.
The heating value is a significant factor that indicate if a biomass is suitable as a feedstock. In figure 2, rice straw stands out with a lower LHV compared to the other biomasses, indicating that it may be less suitable for pyrolysis. Groundnut residues, maize stalks, soybean residues, avocado stone and different types of wood residues such as eucalyptus on the other hand have higher values and could therefore be more suitable for pyrolysis. In figure 3, HHV of biomass residues are shown from crops were LHV could not be found. None of these have a
noticeably low HHV and could therefore in terms of heating value be considered favorable for pyrolysis. However, the HHV of eucalyptus chips, avocado branches, orange tree leaves and branches are still lower than the biomasses with the highest LHV, which means the biomasses with a high LHV could be more suitable than these biomasses.
The C content generally stands for the highest proportion, around 40-50%, followed by a slightly lower O content and a H content around 5-6%. The high content of these components means the biomasses could be suitable for biochar production. Some biomasses have a significantly higher O content, including sorghum stalks, potato residues and banana pseudo- stem. However, the O content is not considered to be too high, and the biomasses could still be suitable for pyrolysis. Sugarcane top and mango seed have a slightly lower content of C and O, which could imply that they have a poorer energy density and therefore are not as suitable as a feedstock. This is also underpinned by the data shown in figure 2, since mango seeds have slightly lower LHV compared to the other biomasses. The LHV or HHV for
Figure 4. The composition of O, H and C in the biomasses.
The content of N and S is overall very low. Especially the S content is low, and the biomasses with the highest proportion still only have about 0.4%. However, the wastes including leaves;
eucalyptus leaves, coffee tree leaves, tea waste, orange tree leaves and pineapple leaves, seem to have a higher N content, as well as cassava peels and banana peels. This means that using these wastes as a feedstock for pyrolysis probably will result in more toxic NO
Xand SO
2emissions, which is not favorable.
Figure 5. The composition of S and N in the biomasses.
The cellulose content is generally the highest, followed by hemicellulose and lignin. Residues that are seen to have a higher lignin content (above 25%) are cassava stalks and rhizome, groundnut husks, avocado seeds, eucalyptus bark and tea waste, which is favorable as it increases the char yield. The peels from fruits and cassava are on the other hand shown to have low lignin content, and fruit peels also have a very low hemicellulose and cellulose content. Some biomasses have a significantly low overall content of hemicellulose, cellulose and lignin, under 50%, including lemon and orange peel, eucalyptus leaves and coffee tree leaves. This shows that these biomasses include more extractives, which could lower the yield of the products.
Figure 6. The structural composition of the biomasses.
The results of moisture content indicate that some biomasses are more suitable than others.
The wood biomasses are shown to generally have a lower moisture content than the crop residues, which is preferred as a higher moisture content reduces the char formation.
Different kinds of stalks and seeds seems to generally have a high moisture content, together with cassava residues. Still, no biomass has over 20% or more, which still could be
considered feasible for biochar production since earlier reports have suggested to not use feedstock containing more than 30 % moisture. However, even though all the biomasses could be feasible for biochar production, the ones with a lower moisture content are preferred. Therefore, those under 10% could probably be considered more favorable. If pyrolysis is done with feedstock of high moisture content it is affecting the process negatively and lowering flame temperature.
Figure 7. The moisture content of the biomasses.
The ash content is overall low for the biomasses, although it’s above 10% for rice husks, wheat straw, lemon tree leaves and orange tree leaves. As expected, the agricultural crops generally have a higher ash content than the wood biomasses, since these generally contain more minerals. A high ash content could cause problems for combustion chambers and therefore biomass with low ash content is preferred. Maize cobs, sorghum stalk, sweet potato residues, groundnut husk, sugar cane bagasse, mango seed and coffee husk all stand out of the agricultural residues with low ash contents and could therefore be suitable in this context.
Figure 8. The ash content of the biomasses.
Figure 9 shows that the largest residue volumes overall are linked to the most produced crops.
It is clear from the graph that a high RPR value creates great amounts of waste. Especially the waste from bananas stands out as it generates a larger amount of waste individually than other crops do combined. Beans, maize and sorghum are also some crops that have a RPR value higher than 1, which makes them large waste generators. Even if cassava and potato have a low RPR value, they still generate great amounts of waste owing to the large production.
Figure 9. The production of major crops and the amount of residue they are estimated to cause.
