TORREFACTION AND CHARACTERIZATION OF MAHOGANY SAWDUST FOR SOLID FUEL PRODUTION

ABSTRACT

As interest in biomass utilization into combustion fuels grows, the torrefaction, becomes ever more important. Torrefaction is aimed to maximize energy by removing undesirable components (water, hemicellulose and volatile matter) which cause poor ignition characteristic, excessive smoking and low combustion efficiency when used for energy generation in boilers and  blast  furnaces.  In  this  research,  the  effect  of torrefaction  on  the  physiochemical  and combustion characteristic of tropical biomass (Mahogany Sawdust) were investigated. The proximate and ultimate analyses of the tropical biomass were determined before torrefaction and at different torrefaction temperature (200, 250 and 300 oC). The Sawdust was subjected to thermal degradation test via Thermogravimetric Analysis (TGA). Fourier Transform Infrared Spectroscopy (FT-IR) analyses  were also  carried  out  to  determine the  presence of  active functional group in the raw biomass and the resulting torrefied biomass.  Electron Microscopy (SEM) analysis was finally carried out to determine the morphologies of the raw and torrefied biomass. The result of biomass weight loss as function of temperature variation revealed that at relatively  low  torrefaction  temperature  of  200    and  250  oC  the  weight  loss  were  very pronounced,  whereas  at  torrefaction  temperature  of  300oC,  the  weight  loss  of  the  tested biomass becomes relatively negligible. In the fixed bed furnace, the change in sawdust mass yield between 10 min was (21.37%) and 30 min (32.14%) at 200oC was about 10.77% and it increased between 200oC and 300oC. The oxygen-carbon ratio deduced from the ultimate analysis of the torrefied biomass displayed a low and concentrated distribution in the Van Krevelin plot than those of raw biomasses. The thermal stability of the biomass feedstock from TGA were found to be in the decreasing order of 300>250>200OC. TGA curves for raw sawdust and torrefied sawdust showed three main decomposition with the curves of torrefied sawdust shifting to higher temperatures. The energy yield of torrefied sawdust is higher than that of untorrefied sawdust. The optimal heating value of the torrefied SD was found to be 28.2 MJkg-1 against 19.5MJkg-1 of the raw SD depicting 44.62% increase of the heating value. The FTIR showed that torrefied sawdust at 300oC has more C=C than sawdust torrefied at 200oC and 250oC. The plot of conversion against temperature and ln g (α) against 1/T for SD using first  order  kinetic  shows  first  order  kinetic  shows  a  linear  relationship  with  regression coefficient    (R2)    of    0.7442.The    torrefied    briquettes    strength    was    in    order    of: 300OC>250OC>200OC. The finding from this study revealed that the Sawdust from mahogany can  be  used  as  a  solid  renewable  and  environmentally  friendly  fuel  as  an  alternative  or potential replacement for coal.

CHAPTER ONE

1.0 INTRODUCTION

1.1 Background to the study.

Energy has always played a significant role in life, survival and development of mankind and one of the major challenges of the 21st century is bridging the gap between energy supply and demand with pure dependable and cheap energy.

There are several sources of energy ranging from fossil to renewable source (Garba et al., 2013). Most of this world energy needs are met through fossil fuel such as coal, oil and natural gas. Fossil fuels are non-renewable and demand for this energy is projected to increase which creates concern for sustainability. To find a balance between growing economy, environmental protection and improved quality of life in future, there is need for renewable energy source. Biomass demand as a better fuel is high but raw biomass has a relatively low density, energy and  contains  too  much  moisture  which  can  rot  during  storage,  and  also  is  difficult  to comminute into small particles. If agricultural wastes are properly recycled, it would rid the environment of solid waste and enhance the aesthetics.

Energy and fuel are the important links in civilization and human development. For effective economic growth and development of a country, the energy has to be adequate and affordable. Also,  the  economic  development  of  a  country can be  examined  through  the  method  of consumption of the energy and richness of energy availability (Mahmoud et al., 2012).  The extensive use of fossil fuels for energy has become a major cause of global warming due to increasing emission of carbon dioxide into the atmosphere.

In Nigeria, the major source of energy in the rural community is fuel wood. According to (Davies, 2013) the demand and need for fuel wood was projected to rise to 213.4×  103 metric tonnes, while the supply will decrease to about 28.4×  103 metric tonnes by the year 2030. The uncontrolled level of cutting of wood for fire wood and charcoal for combustion and for other domestic  and  industrial  uses  is  now  a  serious  problem  in  Nigeria.  This  has  led  to environmental degradation, deforestation, and misuse of soil forests and water resources. Bio- energy is one among the highly promising energy resources that is renewable which hold not just the key to the present fuel problems but also a future solution (Omemu et al., 2008).

Biomass is organic matter derived from living, or recently living organisms. As an energy source, biomass can either be used directly through combustion to produce heat, or indirectly after converting it to various forms of bio fuel. Burning biomass appears not to be the only way in  which  its  energy  can  be  released;  it  can  be  transformed  to  other  means  of  energy. Conversion of biomass to bio fuel for instance, can be achieved by different methods which include thermal, chemical and biochemical methods. In a statement made by Pondel (2015), biomass is generally characterized by its high moisture content and volatility and also by its low Higher Heating Value (HHV) and energy density levels compared to fossil fuels. From history, humans have used biomass-derived energy since the time when people began burning wood to make fire. Even today, biomass is the only source of fuel for domestic use in many developing countries. The potential for the use of biomass as energy source in Nigeria is very high because about 80% of Nigerians are rural and semi-urban dwellers and they solely depend on biomass for their energy needs (Onuegbu et al., 2011).

Also, a high quantity of agricultural and forestry residues produced annually have not been properly utilized or have been vastly under-utilized. The practice that is common is to set ablaze these residues or abandon them to decompose. Coconut shell for instance, is an agricultural waste which has not been fully utilized. Previous studies by (Jekayinfa and Omisakin 2015) found that coconut shell has a calorific value between 18.1 and 20.8 MJ/Kg which  is  relatively  high  and  it  is  coupled  with  relative  low  ash  content  of  3.5-6%.  To efficiently use this biomass material, different conversion methods have been employed to enhance its fuel characteristics. Also, it can be combined with other biomass materials.(Its benefits are that it is cheap, can be found in areas where wood is scarce, it is renewable and contain a reasonable amount of energy. Cow dung, for example, which when converted into biogas contains around 50% methane and 30% carbon dioxide by mass, which means fewer energy extractions when burnt directly. Cow dung is estimated to have an approximated energy density of 12 MJ/Kg if it is burned with an efficiency of 100 % Demirbas (2015).

A technology that meets all these challenges mentioned above and also improves the quality of biomass as fuel is torrefaction. Torrefaction and briquetting of biomass process improves the handling characteristics of the biomass, enhances its volumetric calorific value and reduces transportation cost, (Sandip, 2014). This technology among so many advantages is eco-friendly and  mahogany  is  the  most  used  wood  in  Nigeria  making  the  sawdust  from  it  generally available in a large quantity.

Torrefaction is a pre-treatment technology aimed at processing biomass fuels to facilitate or enable their use in thermochemical processes. It is a mild form of pyrolysis, where biomass is subjected to temperatures around 230-320  ℃ in a non-oxidizing atmosphere (Muhammad et al., 2014).  As  studied by Sandip  (2014),  the  main  components  of biomass  are cellulose, hemicellulose and lignin which are collectively called lignocellulose. These lignocellulose bio- materials can be briquetted without binder. The torrefaction process enhances the effectiveness of  biomass  as  solid  fuel  which  leads  to  improvement  of  combustion  properties  such  as increased calorific value, adiabatic flame temperatures and reduced volatility (Muhammad et al., 2014). Torrefaction also leads to increased hydrophobicity, better grinding and decrease in biological degradation activity. The degree to which the biomass properties are altered depends on the supposed ―severity‖ of torrefaction, which relates to the residence time and temperature used in the process (Yash et al., 2014).

The chemical structure of the torrefied material is altered, which produces water, carbon monoxide, carbon dioxide, methanol, and acetic acid. Investigations cited in the open literature are often related to farming application or conversion to value added-product. For instance, Omemu et al. (2008) studied the effect of different pretreatment of maize cob on growth of the six different fungi. Ashish et al. (2005) evaluated the biochemical parameters in reduced time and temperature in order to optimize cellulose production by Aspergillus terreus on ground nut shell. Mahmoud et al. (2012) and Oyetola et al. (2006) reported the use of ground nut shell and rice husk in sand-crete block production.

The fundamental mechanism in all these processes is devolatilization.   Devolatilization of biomass materials is the decomposition of the biomass into permanent gases, condensable vapours and solid residue. Biomass Devolatilization starts at a temperature of about 227oC, and fast rates are attained at about 300oC and the process is terminated at 300-320 oC (Shafizadeh, 2009).  Once the devolatilization species enter the gas phase, the volatile species may interact depending on the biomass property and it composition, heating condition, surrounding gas atmosphere and residence time distribution. Renewable denotes energy from sources that are naturally replenishing but flow limited. They are virtually in exhausted in duration but limited in amount of energy that is available per time. Majorly this form of energy source includes solar, wind, geo thermal, hydropower and biomass.

Pyrolysis represents the chemical decomposition of organic matters by heating in the absence of oxygen. The biomass decomposes into vapor, aerosol and char; the proportion of these three states depends on temperature and duration of pyrolysis. This process can be fast, slow or mild depending  on  the  time,  temperature  and  the  properties  of  product  desired.  Thermal  fast pyrolysis gives the highest yield of liquid and could be demonstrated as gasification and liquidfication. To obtain some desired improved fuel quality of biomass, there is need for mild pyrolysis to be applied in conversion and the major thermo chemical conversion process for it is known as Torrefaction.

1.2 Statement of the Research Problem

Energy is an essential tool for economic growth and development. However, the energy should be available and affordable. The present energy sources are nonrenewable, expensive, finite and not environmentally friendly. Therefore the need for a cheaper, renewable and environmentally friendly fuel source.

Also, Sawdust has been used over the years as a fuel especially in rural part of Nigeria where wood is scarce. Unfortunately, burning sawdust efficiently is more difficult than burning wood efficiently. It produces some pollutants and is a major health hazard in countries where it is burnt indoors with limited ventilation. There is therefore the need to improve the fuel characteristics of this sawdust for complete utilization of the energy contained in it. This research  presents  a  technology that  has  the  potential  to  ameliorate  many  or  all  of  these deficiencies through a form of thermal processing known as ―Torrefaction‖.

1.3 Aim and Objectives

The aim of the study was to carry out torrefaction and characterization of mahogany saw dust for solid fuel production.

The objectives of this study were to:

i.         collect mahogany saw dust, removal of impurities and drying.

ii.        carry out the ultimate analysis and proximate analysis of the raw biomass.

iii.        carry out fix bed pyrolysis experiment on Saw dust to enhance the fuel properties of the biomass.

iv.        carry out the ultimate analysis  and   proximate analysis of the raw and   torrefied biomass

v.         determine of the mass yield and energy yield and of the torrefied biomass.

vi.       characterize of the biomass and torrefied biomass using FTIR, TGA and SEM.

vii.      determine the kinetic parameter of the samples

1.4 Justification for the Study.

The importance of energy in economic growth and development cannot be over emphasized, however, for this energy to be effective, it has to affordable and available. This research used biomass material (Mahogany wood saw dust) which are so abundant and also cheap. The biomass material being cheap and found locally in almost all saw mills within the country without any need for importation will help in enhancing the gross domestic product of the nation and also help in converting waste to wealth thereby combating the waste management problem which has been a major setback for the country. Also, the fuel produced which is environmentally friendly will serve as an alternative to the fossil fuel thereby solving the problems associated with fossil fuels.

1.5 Scope of the Study

The scope of this work is limited to collection of raw mahogany Saw Dust (SD), removal of impurities, carrying out proximate and ultimate analysis, torrefaction at different temperatures and resident time and characterisation of the solid fuel using TGA, FTIR, and SEM .

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