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Product Code: 00006764

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This study presents the numerical simulation of biomass pyrolysis under isothermal condition following a two-step parallel kinetic reaction scheme in a single reactor. Biochar, one of the end products of this process is considered an efficient vector for soil conditioning and sequestering carbon to offset atmospheric carbon dioxide. The properties of mild steel and poultry litter were used to develop a numerical model using finite element method (FEM). This model considered heat transfer in a novel batch reactor and mass transfer during pyrolysis of biomass. The numerical solutions to the system of pyrolysis kinetic model were used to investigate the mass loss of pyrolysis products at different residence time and temperatures. The simulation study found that the reactor should be heated on all sides (furnace heating) for maximum heat transfer in order to achieve a two-step reaction in which biochar can be produced at two stages. Also, significant mass loss occurred at 573.15K between 7200s and 9000s (0.0847kg and 0.051kg). Biochar yield declined at higher temperatures as 33.72%, 27.84%, and 22.28% at 573.15K, 673.15K and 773.15K during primary reaction. At secondary reaction char yield varies between 9.18%, 11.56 and 12.08%. The model was validated by comparing the predicted values with experimental result from literature. Additionally, good agreements were established between the present result and that of past works. It is therefore expected that this study will enhance the understanding of the pyrolysis process involving primary and secondary reactions in a single reactor by giving physical insights into the various factors and the parameters affecting the process including the heat transfer rate within the reactor body.

Title  page                                                                                            i
Declaration ii
Certification iii
Dedication iv
Acknowledgement v
Table of contents                                                               vi
List of tables                                                                                        viii
List of figures                                                                                ix
Abstract    x

1.1. Background to the Study                                                         1
1.2. Statement of Problem                                                                  3
1.3. Aim and Objectives of the Study                                4
1.4. Justification                                                             4
1.5. Scope of Study                                                 5

2.1. Biochar Production in Nigeria 6
2.2. Overview of Pyrolysis                                                                  10
2.3. Pyrolysis Kinetics                                                          14
2.3.1. One-component pyrolysis models                                   15
2.3.2. Multi-component pyrolysis model                                    16
2.4. Heat and Mass Transfer Modeling                                    19
2.5. Numerical Modeling and Simulation of Pyrolysis Process          23
2.6. Pyrolysis Reactors                                                                25
2.7. Feedstock    27

3.1. Research Design 28
3.2. Model Development 28
3.2.1.   Geometric design of reactor                              29
3.2.2.   Pyrolysis kinetics                                                 32
3.2.3.   Theoretical background 33
3.2.4.   Error analysis 36
3.2.5.   Numerical modeling 36
3.2.6.   Building the model in Comsol-Multi-Physics       37
3.2.7.   Numerical model description                                         38
3.2.8.   Parameters selected for numerical model simulation 40

4.1. Numerical Solution to Heat and Mass Transfer Model      43
4.1.1 Heat transfer in the batch reactor 43
4.1.2 Heat transfer analysis in biomass particle 46
4.1.3 mass transfer analysis                                                48 mass loss rate at various temperatures 51
4.2 Validation of Numerical Model 60
4.2.1 error analysis 61

5.1. Conclusions 64
5.2. Recommendations 65
5.3. Contribution to Knowledge 66
References 67
Appendices 76


2.1: Biochar production in Nigeria 9
3.1: Geometric characteristics of the reactor 31
3.2: Thermo-physical properties of mild steel used for the heat transfer analysis 40
3.3: Thermo-physical properties of poultry litter 40
3.4: Thermo-physical properties of pyrolysis products 41
3.5: Residence times and temperature used for simulation 41
3.6: Constant parameters used in simulation 42
4.1: Mass of char produced from primary and secondary reactions 51
4.2: Pyrolysis process yield 52
4.3: Comparison of char yield from primary reaction 60
4.4: Summary of error analysis for biochar yield 61


2.1: (a) Biochar from orange peels (b) and albedo 7

2.2: Pyrolysis process 10

2.3: Products obtained from a pyrolysis process 11

2.4: One-component wood pyrolysis reaction model 15
2.5: Chan’s kinetic model 16

2.6: The multi-component pyrolysis mechanism 17

2.7: Multi-component mechanism and kinetic constants for wood pyrolysis based on the contribution of the three main components 17

2.8: A multi-step reaction mechanism 17

2.9: Miller and Bellan’s model 18

2.10: Secondary reaction model 18

2.11: Steady, one dimensional heat conduction in a cylindrical layer 22

2.12: (a) Reactor (top) and sieve tray filled with corncob biomass (bottom), (b) furnace 26

3.1: Schematic diagram of research design 29

3.2: Geometry of pyrolysis reactor (2D view)                          31

3.3: Geometry of pyrolysis reactor (3D view) 32

3.4: Modified two-stage parallel reaction model of biomass pyrolysis 33

4.1: (a) The temperature profile at 5400s (b) 7200s (c) 9000s (d) the mesh obtained from finite element analysis obtained when reactor is heated from the bottom only 43

4.2: (a) The heat transfer rate at 573.15K in K/s (b) heat transfer rate (K/s) at 673.15K (c) heat transfer rate at 773.15K obtained from one-dimensional heat source (point source: heating from the bottom only) 44

4.3: The heat transfer rate in K/s at 9000s (a) 573.15K (b) 673.15K (c) 773.15K (d) the temperature edges obtained when reactor is heated from the all sides except the top at 673.15K. 45
4.4: The Temperature profile of hidden entities at 673.15K, 5400s (a) reactor heated from all sides (b) reactor heated from the bottom only. 46

4.5: The heat transfer rate in K/s at (a) 573.15K, 7200s (b) 573.15K, 9000s (c) 673.15K, 5400s (d) 673.15K, 9000s (e) 773.15K. 7200s (f) 773.15K, 9000s obtained when the biomass particle is heated (g) the mesh and element size of the poultry litter particle (h) particle geometry 49

4.6: Mass loss variation of pyrolysis products from numerical simulation (a)  (b)  (c)   at 7200s, 5400 and 7200s (d)  (e)  (f)   at 9000s respectively. 58

4.7: The residual concentration of pyrolysis products and rate of reaction constant from COMSOL multi-physics: temperatures (a) and (b) 573.15 K at 7200s (c) and (d) 573.15 K at 9000s (e) and (f) 673.15 K at 5400S (g) and (h) 673.15 K at 9000s (i) and (j) 773.15K at 7200s (k) and (l) 773.15K at 9000s. 58

4.9 The comparison between predicted biochar yield and experimental result 61



Crop growth and yield have declined significantly due to soil fertility problems, there is evidence of inefficacious use of fertilizer most especially nitrogen loss from leaching. The continuous decline in soil fertility and the costs of fertilizer are major impediments to crop production in smallholder farms. Lack of ample amounts of nitrogen in most soils puts a setback on the farmers' goals of increasing crop yield per unit area. Naturally, biochar deposits this nitrogen to the soil. Biochar is a pyrogenic black carbon that has aroused increased interest in the academic arena (Verheijen et al., 2009; Yao et al., 2013). 

The International Biochar Initiative (IBI) (; Mohan et al., 2014), states that, biochar is a black solid substance obtained from the combustion of biomass. Various heating rates produce different quality of biochar for use as fuel and adsorbents (Masto et al., 2013). Several studies have suggested that when biochar is applied to soils, it enhances soil fertility and crop productivity. It also, increases soil nutrients and water holding capacity, and reduced emissions of other greenhouse gases from soils (Cohen-Ofri et al., 2006; Verheijen et al., 2009; Yao et al., 2011), it also mitigates global warming. Biochar may be added to soil as a conditioner or carbon sink to reduce greenhouse CO2 emissions from decaying biomass (Mulabagal et al., 2020). However, its application to agricultural soil is considered a promising strategy to sustain fertility, while concurrently sequestering atmospheric CO2 (Woolf et al., 2010; Biederman and Harpole, 2013). It increases nutrient availability, microbial activity, soil organic matter, water retention, and crop yields, while mitigating greenhouse emissions, minimizing nutrient leaching, and soil erosion (Yeboah et al., 2009; Woolf et al., 2010; Mohan et al., 2006). All soils contain some amount of biochar deposited through natural occurrence, such as grassland and forest fires (Skjemstad et al., 2002). In fact, areas high in naturally occurring biochar, such as the west coast of the Mississippi river and east of the rocky-mountains in North American Prairie are some of the most fertile soils discovered in the world (O'Neill et al., 2009). In 1870, James Orton, an American geologist, observed that alongside the typically grey, acidic soils of the basin there are large fragments of 'black and very fertile' soil. A study confirmed that the half-life of biochar found in coastal temperate forest in Western Vancouver has been calculated to be 6623 years (Lehmann and Joseph, 2009) while biochar stocks found after Savannah in Zimbabwe had an average residence time of only a few decades. However, the global reason for biochar decline is due to climate change and urbanization which puts pressure on agricultural land resources. 
Although, alternative methods are adopted to produce biochar. Biochar can be produced as a high carbon content by-product, when biomass, such as plant wastes or animal waste undergoes combustion in a closed container with little or no air (Lehmann and Joseph, 2009). Traditionally biochar is known as charcoal when produced for fuel. However, the name biochar is used when the material is produced specifically for application to soil as a part of an agronomic or environmental management program (Brown, 2009; Deal et al., 2012). In more technical terms, biochar is produced by thermal decomposition of organic material under reduced amount of oxygen, and at relatively low temperatures (Lehmann and Joseph, 2009). This process is known as pyrolysis. Pyrolysis of biomass is carried out in reactors of different designs leading to the production of not only biochar but also bio-oil (tar) and syn-gas (gas) which is useful in electricity generation and other industrial applications. Although, significant investigations have been carried out on this area with various reactor configurations. The configuration of the reactors in most cases is used to determine the product yield and the number of by-product separated from the parent material. Additionally, different kind of biomass have been pyrolyzed most especially wood (detailed research has been carried out on wood), others include plastic, corn cob and stover, while few researchers have studied the pyrolysis of poultry litter etc. However, literature search has shown that most reactors are designed to produce only three products (Allyson, 2011). Again, the interest in most design is the production of mostly bio-oil and syn-gas. However, the technology for converting bio-oil and syn-gas to a useable form is not yet very much available in the country. In Nigeria, traditional methods of pyrolysis are inefficient in addressing the issues of environmental pollution, ease of production and exposure of workers to related risks (Hammed et al., 2014). The only product that can directly be utilized in its present form from this process for agricultural production is the biochar. Therefore, the emphasis in this research work is to initiate a novel biochar production process through a two-step parallel kinetics. The implication therefore, is to design a reactor configuration different from other existing ones to be able to synchronize this two-step process to produce more quantity of biochar.

Pyrolysis plants are very expensive to build as well as the experimental process. In fact, studies have shown that experimental investigation of pyrolysis is generally expensive due to costs for design, construction and operation and also very time-consuming (Roegiers, 2016). However, significant improvement of computational power allows numerical modeling of biomass pyrolysis as a cost and time saving alternative. Moreover, computational simulations provide a more detailed insight in the various aspects of pyrolysis processes under different conditions (Roegiers, 2016). Computational Fluid Dynamics (CFD) modeling can be used to simulate the mass flow in a pyrolysis reactor, coupled with a kinetic model to represent the reaction kinetics. However, modeling of pyrolysis requires a detailed knowledge of the complex mass flow of multiphase hydrodynamics and insight in the reaction kinetics of pyrolysis. Most studies have focused on the pyrolysis of wood, corn cobs, corn stover etc. only few researchers have studied the pyrolysis of poultry litter. However, most works aimed at single-step pyrolysis mechanism involving the primary products (char, tar and gas) only.

Since, the technology of converting gas and tar to useable form is not available in the country presently, only biochar can be used directly for agricultural purposes. It is therefore imperative to design a reactor for biochar production following a multi-step pyrolysis kinetics which makes this study novel. So that biochar can be produced at two stages in a single reactor. This can be achieved by designing a batch reactor and performing a coupled heat and mass transfer analysis using numerical method since pyrolysis plants and experiment are very expensive as stated earlier in this section. 

The aim of this study is to enhance the understanding of the pyrolysis process involving a two-step parallel kinetic reaction scheme by giving physical insights into the pertinent variables affecting the process with an improvement on reactor geometrical characteristics. To achieve the aim of this research, five tasks which form the objectives of the study will be undertaken:
i.) design of a novel lab-scale batch reactor 

ii.) analysis of the biomass pyrolysis kinetics

iii.) prediction of the mass loss rate in the biomass 

iv.) numerical modeling and simulation of heat and mass transfer 

v.) validation of the reactor geometry and numerical model


Despite the advantages of biochar for increasing soil fertility, there are food safety and environmental concerns about its application in agricultural sites in its unmodified form (Wilkinson et al., 2003; Chan et al., 2008). The misuse of biochar may result in environmental problems (Gay et al., 2003). However, biochar characteristics and properties are greatly affected by pyrolysis process and its parameters (mainly process temperature and residence time). These factors are particularly important in determining the nature of the final product and, consequently, its potential value in terms of carbon sequestration, agronomic performance, and/or environmental remediation. The use of biochar has other benefits as well, such as increased water holding capacity and higher availability of nutrients, resulting in healthier vegetation (Beck et al., 2011). Healthier vegetation will also make the urban space more livable and enjoyable for humans and wildlife (Bolund and Hunhammar, 1999). 
Most pyrolysis plant design configuration emphasizes on the production of bio-oil and gas as a result this poses a great degree of mass loss which would have resulted in biochar production. In this study, the main focus is more on biochar production as an eco-friendly nitrogenous fertilizer for agricultural production (soil amendment or conditioning). Therefore, there is need to re-conFig. the reactors to produce more biochar than bio-oil and gas.

This research work is focused on numerical modeling and simulation of coupled heat and mass transfer of a novel batch pyrolysis reactor under isothermal heating conditions via slow heating rates. Poultry litter has been selected as the parent material (biomass) for the prediction of the pyrolysis products of a two-step kinetic reaction (primary and secondary reactions) to determine the mass loss, concentration variation and heat transfer rates at different temperatures and residence time. Poultry litter was selected as the feedstock due its high nitrogen content. Also, mild steel was considered as the material for the reactor simulation of the reactor body due its availability in the country.

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