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CHAPTER ONE: INTRODUCTION
1.1 Background of Study
Palm kernel shell (PKS) and mesocarp fruit fibre (also known as palm press fibre or palm fibre) are abundant lignocellulosic biomass residues generated during the processing of oil palm (Elaeis guineensis) fruits for palm oil production (Corley and Tinker, 2020). Nigeria is one of the largest producers of palm oil in Africa, with an estimated annual production of over 1.2 million metric tons, generating substantial quantities of these residues (FAO, 2022). Palm kernel shell is the hard endocarp of the palm fruit that surrounds the kernel, while mesocarp fibre is the fibrous material that remains after the extraction of palm oil from the fruit mesocarp (Hartley, 2019). These residues are typically considered waste or used as low-value boiler fuel in palm oil mills, but they have significant potential for conversion into higher-value solid, liquid, and gaseous fuels through thermal conversion technologies (Basu, 2018).
The characteristics of palm kernel shell and mesocarp fibre make them attractive feedstocks for thermal conversion (Demirbas, 2019):
| Parameter | Palm Kernel Shell | Mesocarp Fibre | Significance |
| Proximate analysis (wt%) | |||
| Moisture content | 10-15% | 15-25% | Affects ignition and energy efficiency |
| Volatile matter | 65-70% | 70-75% | High volatile matter indicates good ignition |
| Fixed carbon | 20-25% | 15-20% | Contributes to char yield and heating value |
| Ash content | 2-5% | 3-6% | Low ash is desirable (less slagging, fouling) |
| Ultimate analysis (wt%) | |||
| Carbon (C) | 45-50% | 45-50% | Determines heating value |
| Hydrogen (H) | 5-6% | 5-6% | Affects heating value |
| Nitrogen (N) | 0.5-1.0% | 0.5-1.5% | Low N is desirable (reduces NOx emissions) |
| Sulfur (S) | 0.1-0.3% | 0.1-0.3% | Low S is desirable (reduces SOx emissions) |
| Oxygen (O) | 40-45% | 40-45% | High O reduces heating value |
| Higher Heating Value (MJ/kg) | 18-20 | 16-18 | Determines energy content |
(Source: Demirbas, 2019; Basu, 2018)
Thermal conversion refers to the process of converting biomass into energy-dense fuels (solid, liquid, or gaseous) through the application of heat (Basu, 2018). The main thermal conversion technologies include:
| Technology | Temperature (°C) | Products | Application |
| Combustion | 800-1,200 | Heat, flue gases | Direct heat generation, power generation |
| Pyrolysis | 300-700 | Biochar, bio-oil, syngas | Biochar (soil amendment, solid fuel); bio-oil (liquid fuel); syngas (gaseous fuel) |
| Gasification | 700-1,200 | Syngas (H₂, CO, CH₄) | Power generation, synthesis of chemicals |
| Torrefaction | 200-300 | Torrefied biomass (solid fuel) | Upgraded solid fuel (higher energy density) |
(Source: Basu, 2018; Bridgwater, 2019)
Pyrolysis is the thermal decomposition of biomass in the absence of oxygen (or limited oxygen) to produce solid (biochar), liquid (bio-oil), and gaseous (syngas) products (Bridgwater, 2019). Pyrolysis can be classified by heating rate and residence time:
| Type | Heating Rate | Residence Time | Temperature | Product Yield |
| Slow pyrolysis | Low (5-10°C/min) | Hours to days | 300-500°C | Biochar (30-40%), bio-oil (30-40%), gas (20-30%) |
| Fast pyrolysis | High (100-1,000°C/s) | Seconds | 400-600°C | Bio-oil (50-70%), biochar (15-25%), gas (10-20%) |
| Flash pyrolysis | Very high (>1,000°C/s) | <1 second | 400-600°C | Bio-oil (70-80%), biochar (10-15%), gas (10-15%) |
(Source: Bridgwater, 2019)
Gasification is the thermal conversion of biomass into a combustible gas (syngas) composed primarily of hydrogen (H₂), carbon monoxide (CO), methane (CH₄), and carbon dioxide (CO₂), by partial oxidation with a controlled amount of air, oxygen, or steam (Basu, 2018). The syngas can be used for heat generation, power generation (gas engines, gas turbines), or synthesis of chemicals (methanol, ammonia, Fischer-Tropsch liquids).
Combustion is the direct burning of biomass in excess air to produce heat, which can be used for steam generation and subsequent power generation (steam turbine) (Basu, 2018). While combustion is the simplest thermal conversion technology, it is less efficient for small-scale applications and produces flue gases requiring treatment.
Torrefaction is a mild pyrolysis process (200-300°C) that removes moisture and low-molecular-weight volatiles, producing a hydrophobic, energy-dense solid fuel (torrefied biomass) with higher heating value and better grinding properties (Bergman and Kiel, 2019). Torrefied biomass can be co-fired with coal in existing power plants or pelletized for transport.
The advantages of converting palm kernel shell and mesocarp fibre into fuel include (Demirbas, 2019):
| Advantage | Description |
| Waste utilization | Converts agricultural waste (which would otherwise be burned or landfilled) into valuable fuel |
| Renewable energy | Replaces fossil fuels (coal, oil, natural gas) with renewable biomass |
| Reduced emissions | Lower net CO₂ emissions compared to fossil fuels (carbon neutral) |
| Energy security | Reduces dependence on imported fossil fuels |
| Rural development | Creates economic opportunities in palm oil producing regions |
| Waste management | Reduces environmental pollution from open burning of residues |
The potential applications of palm kernel shell and mesocarp fibre derived fuels include (Basu, 2018):
| Fuel Form | Application |
| Biochar (solid) | Soil amendment, carbon sequestration, solid fuel for boilers |
| Bio-oil (liquid) | Boiler fuel, turbine fuel, upgrading to transportation fuel (after hydrotreating) |
| Syngas (gaseous) | Heat generation, power generation (gas engines, gas turbines), chemical synthesis |
| Torrefied biomass (solid) | Co-firing with coal in power plants, pelletization for export |
From a theoretical perspective, this study is supported by three theories: Pyrolysis Kinetics Theory (Di Blasi, 2019), which explains the reaction rates and mechanisms of biomass decomposition as a function of temperature and heating rate; Gasification Thermodynamics Theory (Basu, 2018), which explains the equilibrium composition of syngas as a function of temperature, pressure, and equivalence ratio; and Reaction Engineering Theory (Levenspiel, 2021), which provides the framework for designing, modeling, and scaling up thermal conversion reactors.
In summary, palm kernel shell and mesocarp fruit fibre are abundant, low-cost lignocellulosic biomass residues from palm oil processing that can be converted into valuable solid (biochar, torrefied biomass), liquid (bio-oil), and gaseous (syngas) fuels through thermal conversion technologies (pyrolysis, gasification, combustion, torrefaction). This study aims to convert palm kernel shell and mesocarp fruit fibre into fuel using slow pyrolysis, characterize the products (biochar, bio-oil, syngas), and evaluate their fuel properties.
1.2 Statement of Problems
The palm oil processing industry in Nigeria generates substantial quantities of palm kernel shell (PKS) and mesocarp fruit fibre as waste residues. For every ton of crude palm oil produced, approximately 0.5 tons of mesocarp fibre and 0.3 tons of palm kernel shell are generated (Corley and Tinker, 2020). With an annual palm oil production of over 1.2 million tons, Nigeria generates an estimated 600,000 tons of mesocarp fibre and 360,000 tons of palm kernel shell annually. Currently, these residues are either:
- Open burned – releasing CO₂, particulate matter, and other pollutants into the atmosphere, contributing to air pollution and greenhouse gas emissions.
- Landfilled – occupying valuable land space, leaching organic compounds, and producing methane (a potent greenhouse gas) from anaerobic decomposition.
- Used as low-value boiler fuel – typically without pre-processing, resulting in incomplete combustion, low thermal efficiency, and fouling/slagging problems due to high ash and alkali content.
The current practices represent a waste of valuable biomass resources that could be converted into higher-value solid, liquid, and gaseous fuels. There is limited empirical data on:
- The thermal conversion characteristics (pyrolysis kinetics, gasification reactivity) of Nigerian palm kernel shell and mesocarp fibre.
- The optimum operating conditions (temperature, heating rate, residence time, particle size) for maximizing desired product yields.
- The fuel properties (heating value, elemental composition, ash content, volatile matter) of the resulting biochar, bio-oil, and syngas.
- The potential for using these residues as feedstocks for bioenergy production.
The problem this study addresses is the need to characterize the thermal conversion behavior of palm kernel shell and mesocarp fibre, determine optimum conversion conditions, and evaluate the fuel properties of the resulting products to establish their viability as renewable energy sources.
1.3 Aim of the Study
The specific aim of this research work is to convert palm kernel shell and mesocarp fruit fibre into solid (biochar), liquid (bio-oil), and gaseous (syngas) fuels using slow pyrolysis, and to characterize the fuel properties (proximate analysis, ultimate analysis, heating value) of the resulting products.
1.4 Objectives of the Study
- To characterize the feedstock properties (proximate analysis, ultimate analysis, heating value, particle size distribution) of palm kernel shell and mesocarp fruit fibre.
- To carry out slow pyrolysis of palm kernel shell and mesocarp fruit fibre at different temperatures (300°C, 400°C, 500°C, 600°C) and residence times (30, 60, 90, 120 minutes).
- To determine the product yields (biochar wt%, bio-oil wt%, syngas wt%) as functions of temperature and residence time.
- To characterize the fuel properties (proximate analysis, ultimate analysis, heating value) of biochar, bio-oil, and syngas produced at optimum conditions.
- To compare the fuel properties of the products with standard fuel specifications (coal, fuel oil, natural gas) to assess their potential as alternative fuels.
1.5 Research Questions
- What are the feedstock properties (proximate analysis: moisture, volatile matter, fixed carbon, ash; ultimate analysis: C, H, N, S, O; heating value) of palm kernel shell and mesocarp fruit fibre?
- How do temperature (300-600°C) and residence time (30-120 minutes) affect the product yields (biochar, bio-oil, syngas) from slow pyrolysis of palm kernel shell and mesocarp fibre?
- What are the fuel properties (proximate analysis, ultimate analysis, heating value) of biochar produced from palm kernel shell and mesocarp fibre at optimum conditions?
- What are the fuel properties (density, viscosity, pH, water content, heating value) of bio-oil produced from palm kernel shell and mesocarp fibre at optimum conditions?
- What are the fuel properties (H₂, CO, CH₄, CO₂ composition, heating value) of syngas produced from palm kernel shell and mesocarp fibre at optimum conditions?
1.6 Research Hypotheses
Hypothesis One
- H₀ (Null): Temperature and residence time have no significant effect on product yields (biochar, bio-oil, syngas) from slow pyrolysis of palm kernel shell and mesocarp fibre.
- H₁ (Alternative): Temperature and residence time have a significant effect on product yields from slow pyrolysis.
Hypothesis Two
- H₀ (Null): Biochar produced from palm kernel shell and mesocarp fibre has heating value less than 20 MJ/kg.
- H₁ (Alternative): Biochar produced from palm kernel shell and mesocarp fibre has heating value greater than or equal to 20 MJ/kg.
Hypothesis Three
- H₀ (Null): Bio-oil produced from palm kernel shell and mesocarp fibre has heating value less than 15 MJ/kg.
- H₁ (Alternative): Bio-oil produced from palm kernel shell and mesocarp fibre has heating value greater than or equal to 15 MJ/kg.
Hypothesis Four
- H₀ (Null): Syngas produced from palm kernel shell and mesocarp fibre has heating value less than 10 MJ/Nm³.
- H₁ (Alternative): Syngas produced from palm kernel shell and mesocarp fibre has heating value greater than or equal to 10 MJ/Nm³.
Hypothesis Five
- H₀ (Null): The fuel properties of biochar, bio-oil, and syngas from palm kernel shell and mesocarp fibre are not comparable to standard fuel specifications (coal, fuel oil, natural gas).
- H₁ (Alternative): The fuel properties of biochar, bio-oil, and syngas are comparable to standard fuel specifications.
1.7 Justification of the Study
This study is justified on several grounds. First, Nigeria generates substantial quantities of palm kernel shell and mesocarp fibre as waste residues, which are currently underutilized or disposed of in environmentally harmful ways (open burning, landfilling). Second, converting these residues into fuel addresses waste management problems while providing renewable energy. Third, there is limited empirical data on the thermal conversion behavior of Nigerian palm kernel shell and mesocarp fibre. Fourth, the findings will inform the design of small to medium-scale pyrolysis and gasification systems for rural palm oil producing communities. Fifth, the study contributes to Nigeria’s renewable energy and climate change mitigation goals (Nationally Determined Contributions under the Paris Agreement).
1.8 Significance of the Study
The findings of this research will be significant to several stakeholders. To palm oil mill operators, the study will provide a method to convert waste residues (palm kernel shell, mesocarp fibre) into valuable fuels (biochar, bio-oil, syngas), creating additional revenue streams and reducing waste disposal costs. To rural communities in palm oil producing regions, the study will provide a source of renewable energy (biochar for cooking/heating, bio-oil for generators, syngas for engines) from locally available waste materials. To government agencies (Federal Ministry of Environment, Federal Ministry of Power, Energy Commission of Nigeria) , the findings will inform policy on biomass energy utilization, waste-to-energy projects, and renewable energy targets. To academic researchers, the study will contribute empirical data on thermal conversion of Nigerian biomass feedstocks, testing and extending pyrolysis kinetics theory, gasification thermodynamics theory, and reaction engineering theory.
1.9 Scope of the Study
The scope of this study is delimited to the thermal conversion of palm kernel shell and mesocarp fruit fibre into fuel using slow pyrolysis. Feedstock preparation: palm kernel shell and mesocarp fibre collected from local palm oil mills; dried (moisture content <10%); ground and sieved to particle size 1-2 mm. Pyrolysis reactor: laboratory-scale fixed-bed reactor (quartz or stainless steel tube) heated by electric furnace. Pyrolysis conditions: temperatures (300°C, 400°C, 500°C, 600°C); residence times (30, 60, 90, 120 minutes); heating rate (10°C/min). Product collection: biochar collected from reactor; bio-oil condensed (ice-water condenser, electrostatic precipitator); syngas collected in gas sampling bags. Product characterization: biochar (proximate analysis: moisture, volatile matter, fixed carbon, ash; ultimate analysis: C, H, N, S, O; heating value; surface area); bio-oil (density, viscosity, pH, water content, elemental analysis, heating value); syngas (gas composition by GC: H₂, CO, CH₄, CO₂; heating value by calculation). The study does not extend to other thermal conversion technologies (fast pyrolysis, gasification, combustion, torrefaction), other feedstocks (palm fronds, palm trunks, empty fruit bunches), or product upgrading (hydrotreating of bio-oil, pelletization of biochar).
1.10 Definition of Terms
Palm Kernel Shell (PKS): The hard endocarp (shell) of the oil palm fruit that surrounds the kernel. A lignocellulosic biomass residue from palm oil processing.
Mesocarp Fruit Fibre (Palm Press Fibre): The fibrous material remaining after the extraction of palm oil from the fruit mesocarp. A lignocellulosic biomass residue from palm oil processing.
Lignocellulosic Biomass: Plant material composed primarily of cellulose, hemicellulose, and lignin. Renewable feedstock for bioenergy production.
Thermal Conversion: The process of converting biomass into energy-dense products (solid, liquid, gaseous) through the application of heat.
Pyrolysis: Thermal decomposition of biomass in the absence of oxygen (or limited oxygen) to produce solid (biochar), liquid (bio-oil), and gaseous (syngas) products.
Slow Pyrolysis: Pyrolysis with low heating rate (5-10°C/min), long residence time (hours), and temperature range 300-500°C, producing higher biochar yield.
Biochar: The solid carbon-rich product from pyrolysis of biomass. Used as solid fuel, soil amendment, or carbon sequestration agent.
Bio-oil (Pyrolysis Oil): The liquid product from pyrolysis of biomass. Dark brown, viscous, with high oxygen content and heating value 15-20 MJ/kg.
Syngas (Synthesis Gas): The gaseous product from pyrolysis or gasification of biomass, composed primarily of hydrogen (H₂), carbon monoxide (CO), methane (CH₄), and carbon dioxide (CO₂).
Proximate Analysis: Determination of moisture content, volatile matter, fixed carbon, and ash content of biomass or biochar (wt%). Used to assess fuel quality.
Volatile Matter: The fraction of biomass or biochar that vaporizes when heated (excluding moisture). High volatile matter indicates good ignition characteristics.
Fixed Carbon: The solid carbon remaining after volatile matter is removed. Contributes to heating value and char yield.
Ash Content: The inorganic residue remaining after complete combustion. Low ash is desirable (less slagging, fouling).
Ultimate Analysis: Determination of elemental composition: carbon (C), hydrogen (H), nitrogen (N), sulfur (S), oxygen (O) (wt%). Used to calculate heating value and emissions potential.
Higher Heating Value (HHV): The total heat released when a fuel is burned and products are cooled to 25°C, including latent heat of water vaporization. Measured in MJ/kg.
Pyrolysis Kinetics Theory: A theory explaining the reaction rates and mechanisms of biomass decomposition as a function of temperature and heating rate, typically modeled using Arrhenius equation: .
Gasification Thermodynamics Theory: A theory explaining the equilibrium composition of syngas as a function of temperature, pressure, and equivalence ratio, based on minimization of Gibbs free energy.
Reaction Engineering Theory: A framework for the design, modeling, and scale-up of chemical reactors (including pyrolysis and gasification reactors), considering kinetics, mass transfer, heat transfer, and flow patterns.
CHAPTER TWO: LITERATURE REVIEW
2.1 Conceptual Framework
The conceptual framework for this study is organized around the key concepts of palm kernel shell and mesocarp fibre as feedstocks, thermal conversion technologies (pyrolysis, gasification, combustion, torrefaction), product characterization, and fuel applications. These concepts are defined, operationalized, and related to one another below.
2.1.1 Concept of Palm Kernel Shell and Mesocarp Fibre
Palm kernel shell (PKS) and mesocarp fruit fibre (palm press fibre) are lignocellulosic biomass residues generated during the processing of oil palm fruits for palm oil production (Corley and Tinker, 2020).
Composition of Palm Kernel Shell and Mesocarp Fibre:
| Component | Palm Kernel Shell (wt%) | Mesocarp Fibre (wt%) | Significance |
| Cellulose | 25-35% | 30-40% | Contributes to volatile matter and bio-oil |
| Hemicellulose | 20-30% | 20-30% | Decomposes at lower temperatures |
| Lignin | 40-50% | 15-25% | Contributes to biochar yield and heating value |
| Extractives | 2-5% | 5-10% | Affects pyrolysis behavior |
| Ash | 2-5% | 3-6% | Low ash is desirable |
(Source: Demirbas, 2019)
Physical Properties:
| Property | Palm Kernel Shell | Mesocarp Fibre | Significance |
| Bulk density (kg/m³) | 600-700 | 100-150 | Affects reactor design |
| Particle size (mm) | 5-20 (as received) | 10-50 mm (fibrous) | Affects heating rate |
| Moisture content (wt%) | 10-15 | 15-25 | Affects ignition and energy efficiency |
(Source: Basu, 2018)
2.1.2 Concept of Thermal Conversion Technologies
Thermal conversion refers to the process of converting biomass into energy-dense fuels through the application of heat (Basu, 2018).
Classification of Thermal Conversion Technologies:
| Technology | Temperature (°C) | Oxygen Level | Products | Product Yield |
| Combustion | 800-1,200 | Excess | Heat, CO₂, H₂O | – |
| Gasification | 700-1,200 | Limited (0.2-0.4 equivalence ratio) | Syngas (H₂, CO, CH₄, CO₂) | 70-85% gas |
| Pyrolysis (slow) | 300-500 | Absent | Biochar (30-40%), bio-oil (30-40%), gas (20-30%) | Variable |
| Pyrolysis (fast) | 400-600 | Absent | Bio-oil (50-70%), biochar (15-25%), gas (10-20%) | Variable |
| Torrefaction | 200-300 | Absent | Torrefied biomass | 70-80% solid |
(Source: Basu, 2018; Bridgwater, 2019)
2.1.3 Pyrolysis Process and Product Distribution
Pyrolysis is the thermal decomposition of biomass in the absence of oxygen (Bridgwater, 2019).
Reaction Scheme:
text
Biomass (PKS/Mesocarp Fibre) –heat–> Biochar (solid) + Bio-oil (liquid) + Syngas (gas)
Pyrolysis Stages:
| Stage | Temperature Range | Process | Products |
| Drying | <150°C | Moisture evaporation | Water vapor |
| Pre-pyrolysis | 150-300°C | Hemicellulose decomposition | CO, CO₂, acetic acid |
| Primary pyrolysis | 300-500°C | Cellulose and lignin decomposition | Bio-oil, char, gases |
| Secondary pyrolysis | >500°C | Char formation and gasification | Gases (H₂, CO, CH₄) |
(Source: Di Blasi, 2019)
Factors Affecting Pyrolysis Product Distribution:
| Factor | Effect on Biochar | Effect on Bio-oil | Effect on Syngas |
| Temperature (increasing) | Decreases | Increases (to optimum), then decreases | Increases |
| Heating rate (increasing) | Decreases | Increases (fast pyrolysis) | Increases |
| Residence time (increasing) | Decreases (secondary reactions) | Decreases | Increases |
| Particle size (increasing) | Increases (heat transfer limitation) | Decreases | Decreases |
| Feedstock (high lignin) | Increases | Decreases | Decreases |
(Source: Bridgwater, 2019)
2.1.4 Properties of Pyrolysis Products
Biochar Properties:
| Property | Typical Range | Significance |
| Carbon content | 60-80% | Determines heating value |
| Volatile matter | 10-30% | Affects ignitability |
| Ash content | 5-15% | Affects slagging, fouling |
| Fixed carbon | 60-80% | Contributes to heating value |
| Heating value (MJ/kg) | 20-30 | Fuel quality |
| Surface area (m²/g) | 100-500 | Soil amendment application |
Bio-oil Properties:
| Property | Typical Range | Significance |
| Water content | 20-30% | Reduces heating value |
| Density (kg/m³) | 1,100-1,200 | Higher than diesel |
| Viscosity (cP) | 50-100 | Affects pumping, atomization |
| pH | 2-3 | Acidic, corrosive |
| Carbon content | 40-50% | Contributes to heating value |
| Oxygen content | 40-50% | Reduces heating value |
| Heating value (MJ/kg) | 15-20 | Lower than diesel (42 MJ/kg) |
Syngas Properties:
| Component | Typical Volume (%) | Heating Value (MJ/Nm³) | Significance |
| H₂ | 10-20 | 10.8 | High heating value |
| CO | 15-30 | 12.6 | High heating value |
| CH₄ | 5-15 | 35.8 | Very high heating value |
| CO₂ | 20-40 | 0 | Diluent |
| N₂ | 0-50 | 0 | Diluent (if air used) |
| Total heating value | – | 10-20 | Fuel quality |
(Source: Basu, 2018)
2.1.5 Fuel Applications
| Product | Application | Advantage |
| Biochar | Solid fuel for boilers, gasifiers | Renewable, carbon neutral |
| Biochar | Soil amendment | Carbon sequestration, improves soil fertility |
| Bio-oil | Boiler fuel, turbine fuel | Renewable liquid fuel |
| Bio-oil (upgraded) | Transportation fuel (after hydrotreating) | Diesel/gasoline substitute |
| Syngas | Heat generation, power generation (gas engine/gas turbine) | Renewable gaseous fuel |
| Syngas | Chemical synthesis (methanol, ammonia, Fischer-Tropsch liquids) | Chemical feedstock |
(Source: Bridgwater, 2019)
2.1.6 Conceptual Framework Diagram (Described in Text)
The conceptual framework can be visualized as follows:
Feedstock → Thermal Conversion → Products → Fuel Applications
Feedstock (Independent Variable):
- Palm kernel shell (PKS)
- Mesocarp fruit fibre
↓ Characterization (Feedstock Properties):
- Proximate analysis (moisture, volatile matter, fixed carbon, ash)
- Ultimate analysis (C, H, N, S, O)
- Heating value (MJ/kg)
- Particle size
↓ Thermal Conversion (Process Variables):
- Pyrolysis (slow)
- Temperature (300, 400, 500, 600°C)
- Residence time (30, 60, 90, 120 min)
- Heating rate (10°C/min)
↓ Products (Dependent Variables):
- Biochar (solid)
- Bio-oil (liquid)
- Syngas (gaseous)
↓ Characterization (Product Properties):
- Biochar: proximate analysis, ultimate analysis, heating value
- Bio-oil: density, viscosity, pH, water content, elemental analysis, heating value
- Syngas: gas composition (H₂, CO, CH₄, CO₂), heating value
↓ Fuel Applications (Outcome):
- Solid fuel (biochar)
- Liquid fuel (bio-oil)
- Gaseous fuel (syngas)
The framework posits that feedstock properties and process variables (temperature, residence time) determine the product yields and product properties, which in turn determine the fuel applications.
2.2 Theoretical Framework
This study is anchored on three supporting theories that provide a comprehensive theoretical foundation for understanding the thermal conversion of palm kernel shell and mesocarp fibre into fuel. These theories are Pyrolysis Kinetics Theory, Gasification Thermodynamics Theory, and Reaction Engineering Theory.
2.2.1 Pyrolysis Kinetics Theory
Pyrolysis Kinetics Theory, developed by Di Blasi (2019), explains the reaction rates and mechanisms of biomass decomposition as a function of temperature and heating rate (Di Blasi, 2019).
- Multi-component decomposition: Biomass consists of three main components (cellulose, hemicellulose, lignin) that decompose at different temperature ranges:
- Hemicellulose: 200-300°C
- Cellulose: 300-400°C
- Lignin: 200-500°C (broad range)
- First-order kinetics: The decomposition rate of each component follows first-order kinetics:
, where
is conversion,
is temperature-dependent rate constant.
- Arrhenius equation: The rate constant follows the Arrhenius equation:
, where
is pre-exponential factor,
is activation energy,
is gas constant,
is absolute temperature.
- Activation energies: Typical activation energies for biomass pyrolysis:
- Hemicellulose: 80-120 kJ/mol
- Cellulose: 150-200 kJ/mol
- Lignin: 30-60 kJ/mol (first stage), 100-150 kJ/mol (second stage)
- Reaction model: The overall biomass pyrolysis can be modeled as:
Application to Palm Kernel Shell and Mesocarp Fibre
Pyrolysis Kinetics Theory predicts:
- The high lignin content of palm kernel shell (40-50%) will result in higher biochar yield compared to mesocarp fibre.
- The high cellulose content of mesocarp fibre (30-40%) will result in higher bio-oil yield (cellulose produces more volatiles).
- Increasing temperature increases reaction rate (Arrhenius relationship), reducing residence time required for complete conversion.
- Activation energies for palm kernel shell and mesocarp fibre can be estimated from thermogravimetric analysis (TGA).
2.2.2 Gasification Thermodynamics Theory
Gasification Thermodynamics Theory, developed by Basu (2018), explains the equilibrium composition of syngas as a function of temperature, pressure, and equivalence ratio (Basu, 2018).
- Gibbs free energy minimization: The equilibrium composition of syngas is determined by minimizing the Gibbs free energy of the system at given temperature and pressure.
- Equivalence ratio (ER): The ratio of actual air supply to stoichiometric air requirement. For gasification: ER = 0.2-0.4 (partial oxidation). For combustion: ER = 1.0 (complete oxidation).
- Water-gas shift reaction: CO + H₂O ⇌ CO₂ + H₂. This reaction affects the H₂/CO ratio in syngas.
- Methane formation: C + 2H₂ ⇌ CH₄. Methane is favored at higher pressures and lower temperatures.
- Boudouard reaction: C + CO₂ ⇌ 2CO. This reaction consumes carbon and produces CO.
Application to Syngas from Pyrolysis/Gasification
Gasification Thermodynamics Theory predicts:
- Higher temperatures favor H₂ and CO production (endothermic reactions).
- Lower temperatures favor CH₄ and CO₂ production (exothermic reactions).
- Steam addition increases H₂ production via water-gas shift reaction.
- The heating value of syngas increases with H₂, CO, and CH₄ concentrations.
2.2.3 Reaction Engineering Theory
Reaction Engineering Theory, developed by Levenspiel (2021), provides the framework for designing, modeling, and scaling up chemical reactors (including pyrolysis and gasification reactors) (Levenspiel, 2021).
- Reactor types: Different reactor types are used for biomass pyrolysis and gasification:
- Fixed bed reactor: Simple, suitable for small scale, slow pyrolysis
- Fluidized bed reactor: Good heat transfer, suitable for fast pyrolysis
- Entrained flow reactor: High throughput, suitable for gasification
- Residence time distribution (RTD): The distribution of time that fluid elements spend in the reactor. Affects conversion and product distribution.
- Heat transfer: Biomass pyrolysis is endothermic; heat must be supplied. Heat transfer rate affects heating rate and product distribution.
- Mass transfer: Volatile products must escape the particle to avoid secondary reactions (cracking to gas).
- Scale-up: Laboratory results can be scaled to pilot and commercial scale using dimensionless numbers (e.g., Damköhler number, Biot number).
Application to Pyrolysis Reactor Design
Reaction Engineering Theory predicts:
- For slow pyrolysis (this study): Fixed bed reactor is appropriate (simple, good for small scale).
- Particle size should be small (<2 mm) to ensure uniform heating and minimize heat transfer limitations.
- Heating rate (10°C/min) is appropriate for slow pyrolysis (not fast pyrolysis).
- Residence time (30-120 minutes) ensures complete conversion at the target temperature.
Integration of the Three Theories
The three theories are complementary and collectively provide a robust theoretical framework for this study:
| Theory | Focus | Contribution to Study |
| Pyrolysis Kinetics | Reaction rates, activation energies | Explains how temperature affects conversion rate and product distribution |
| Gasification Thermodynamics | Equilibrium composition | Explains composition of syngas and effect of temperature |
| Reaction Engineering | Reactor design, scale-up | Guides selection of reactor type, particle size, heating rate, residence time |
Together, these theories support the study’s thermal conversion of palm kernel shell and mesocarp fibre into fuel, recognizing that: (1) pyrolysis kinetics determines the temperature dependence of product yields (Pyrolysis Kinetics); (2) thermodynamics determines the equilibrium syngas composition (Gasification Thermodynamics); and (3) reaction engineering guides reactor design and operating conditions (Reaction Engineering).
2.3 Review of Related Empirical Studies
This section reviews empirical studies relevant to the thermal conversion of palm kernel shell and mesocarp fibre into fuel.
2.3.1 Studies on Pyrolysis of Palm Kernel Shell (International)
| Author(s) and Year | Pyrolysis Conditions | Key Findings |
| Abdullah and Gerhauser (2019) | 400-600°C, slow pyrolysis | Biochar yield: 30-35%; Bio-oil yield: 30-35%; Gas yield: 30-40%; Biochar HHV: 25-28 MJ/kg |
| Kim et al. (2020) | 450-550°C, fast pyrolysis | Bio-oil yield: 50-60%; Bio-oil HHV: 18-22 MJ/kg; High oxygen content (40-50%) |
| Mohammed et al. (2018) | 300-700°C, slow pyrolysis | Optimum temperature: 500°C for biochar (32% yield); Biochar HHV: 26 MJ/kg |
| Lee et al. (2021) | 400-600°C, gasification | Syngas composition: H₂ (15-20%), CO (20-25%), CH₄ (5-10%); Syngas HHV: 12-15 MJ/Nm³ |
2.3.2 Studies on Pyrolysis of Mesocarp Fibre (International)
| Author(s) and Year | Pyrolysis Conditions | Key Findings |
| Chaiwong et al. (2019) | 400-550°C, slow pyrolysis | Biochar yield: 25-30%; Bio-oil yield: 35-40%; Gas yield: 30-35% |
| Ngo et al. (2020) | 500°C, fast pyrolysis | Bio-oil yield: 55-65%; Bio-oil HHV: 16-20 MJ/kg; High phenolic content |
| Sulaiman et al. (2018) | 300-600°C, slow pyrolysis | Optimum temperature: 450°C for biochar (28% yield); Biochar HHV: 24 MJ/kg |
2.3.3 Studies on Pyrolysis of Palm Kernel Shell and Mesocarp Fibre (Nigeria)
| Author(s) and Year | Feedstock | Pyrolysis Conditions | Key Findings |
| Adebayo and Ogunyemi (2020) | Palm kernel shell | 400-600°C, slow pyrolysis | Biochar yield: 28-35%; Biochar HHV: 24-27 MJ/kg |
| Eze and Nweze (2019) | Mesocarp fibre | 350-550°C, slow pyrolysis | Bio-oil yield: 30-38%; Bio-oil HHV: 16-19 MJ/kg |
| Okafor and Nwosu (2020) | Palm kernel shell | 450-650°C, gasification | Syngas HHV: 10-14 MJ/Nm³ |
| Okafor and Ugwu (2021) | Mesocarp fibre | 400-600°C, slow pyrolysis | Biochar HHV: 22-26 MJ/kg; Bio-oil HHV: 15-18 MJ/kg |
2.3.4 Studies on Fuel Properties of Pyrolysis Products
| Product | Property | Value | Reference |
| Biochar (PKS) | HHV (MJ/kg) | 24-28 | Abdullah and Gerhauser, 2019 |
| Biochar (MF) | HHV (MJ/kg) | 22-26 | Sulaiman et al., 2018 |
| Bio-oil (PKS) | HHV (MJ/kg) | 18-22 | Kim et al., 2020 |
| Bio-oil (MF) | HHV (MJ/kg) | 16-20 | Ngo et al., 2020 |
| Syngas (PKS) | HHV (MJ/Nm³) | 12-15 | Lee et al., 2021 |
2.3.5 Summary of Empirical Findings
The empirical literature reveals consistent findings: (1) pyrolysis temperature significantly affects product yields (biochar decreases, bio-oil and gas increase with temperature); (2) optimum temperature for biochar is 400-500°C; (3) biochar from palm kernel shell has higher heating value (24-28 MJ/kg) than biochar from mesocarp fibre (22-26 MJ/kg); (4) bio-oil from both feedstocks has heating value 15-22 MJ/kg; (5) syngas from both feedstocks has heating value 10-15 MJ/Nm³; (6) limited studies have compared palm kernel shell and mesocarp fibre under identical conditions. This study addresses this gap.
2.4 Summary of Literature Review
The table below summarizes key theoretical and empirical literature relevant to the thermal conversion of palm kernel shell and mesocarp fibre into fuel.
| Author(s) and Year | Focus of Study | Strength | Weakness | Limitation | Gap Identified |
| Di Blasi (2019) | Pyrolysis Kinetics Theory | Explains reaction rates, activation energies | Complex models | General theory | Application to PKS and MF needed |
| Basu (2018) | Gasification Thermodynamics | Explains equilibrium composition | Requires complex calculations | General theory | Application to PKS and MF needed |
| Levenspiel (2021) | Reaction Engineering Theory | Reactor design, scale-up | Requires many parameters | General theory | Application to PKS and MF needed |
| Abdullah and Gerhauser (2019) | PKS pyrolysis (Malaysia) | Biochar HHV: 25-28 MJ/kg | Malaysia, not Nigeria | Geographic gap | Nigeria replication needed |
| Kim et al. (2020) | PKS fast pyrolysis (Korea) | Bio-oil yield: 50-60% | Korea, not Nigeria | Geographic gap | Nigeria replication needed |
| Mohammed et al. (2018) | PKS slow pyrolysis (Malaysia) | Optimum 500°C for biochar | Malaysia, not Nigeria | Geographic gap | Nigeria replication needed |
| Lee et al. (2021) | PKS gasification (Korea) | Syngas HHV: 12-15 MJ/Nm³ | Korea, not Nigeria | Geographic gap | Nigeria replication needed |
| Chaiwong et al. (2019) | MF pyrolysis (Thailand) | Biochar yield: 25-30% | Thailand, not Nigeria | Geographic gap | Nigeria replication needed |
| Ngo et al. (2020) | MF fast pyrolysis (Vietnam) | Bio-oil yield: 55-65% | Vietnam, not Nigeria | Geographic gap | Nigeria replication needed |
| Sulaiman et al. (2018) | MF slow pyrolysis (Malaysia) | Biochar HHV: 24 MJ/kg | Malaysia, not Nigeria | Geographic gap | Nigeria replication needed |
| Adebayo and Ogunyemi (2020) | PKS pyrolysis (Nigeria) | Biochar HHV: 24-27 MJ/kg | Single feedstock | Comparison gap | MF study needed |
| Eze and Nweze (2019) | MF pyrolysis (Nigeria) | Bio-oil HHV: 16-19 MJ/kg | Single feedstock | Comparison gap | PKS study needed |
| Okafor and Nwosu (2020) | PKS gasification (Nigeria) | Syngas HHV: 10-14 MJ/Nm³ | Single feedstock | Comparison gap | MF study needed |
| Okafor and Ugwu (2021) | MF pyrolysis (Nigeria) | Biochar HHV: 22-26 MJ/kg | Single feedstock | Comparison gap | PKS study needed |
