IMPACT OF TORREFACTION ON THE FUEL PROPERTIES OF LIGNITE COCONUT SHELLS CASSAVA PEELS AND THEIR BLENDS

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AB STRACT

The effect  of torrefaction temperature  and residence  time  on the fuel properties  of lignite,  biomass (coconut  shells and cassava peels) and their blends was investigated.  The samples were subjected  to three  torrecfaction  temperatures   (200,  260  and  300C)  and  at  two  residence  times  (10  and  20 minutes) using programmable  muffle furnace.  Blends of torrefied  lignite and biomass were prepared in two different ratios (80:20  and 70:30).  The energy content, proximate  and ultimate  analyses of the samples  were  determined  using ASTM  methods.  Scanning  electron  microscope  (SEM) was used to evaluate  the pore size,  fiber content,  topography  and the morphology  of the samples.  The potential

emissions of SO CO and NO, from the torrefied  samples were evaluated using emission estimation

model for fossil fuel electric power  generation.  The proximate  analysis  showed that the ash (8.0 %) and moisture  (30.0 % ) contents  of lignite were  higher than that of the biomass.  The coconut  shells and  cassava  peels  had  higher  volatile  matter  of 72.9 % and  68.1  % respectively  and  much  lower fixed  carbon.   The  data  showed  that  release  of volatile  matter  decreased   at  severe  torrefaction condition.  The  content  of fixed  carbon  and  energy  increased  with  the  severity  of the  torrefaction condition  except  for cassava peels which decreased  at 300 C.  Oneway  analysis  of variance  on the results  of the proximate  analysis  showed  that there was significant  difference  (P<0.05)  between  the volatile matter,  fixed carbon,  energy and ash content of lignite,  coconut  shells and cassava peels,  but no  significant  difference  between  the moisture  and solid yield.  For the blends,  volatile  matter  was found to be higher than that  of lignite  alone.  Increase  of biomass  ratio  in the blends  decreased  the carbon,              nitrogen,              oxygen              and              sulfur              content              of             the samples.  Lignite/coconut shells (70:30)  had better fuel properties  compared  to (80:20)   and lignite/ca ssava peels (at both ratios).   Results of the ultimate analysis showed that after torrefaction there were large  reduction  in   oxygen   and   hydrogen   content.   However,  15 %  carbon,  26 %  nitrogen   and

72 %   sulfur was  reduced from cassava peels  while  lignite  recorded  an increase of 40 % carbon,5

6 % nitrogen and 48 % sulfur after torrefaction.  The SEM image showed that torrefied  lignite had a uniform   and  denser  structure  compared to the raw.   The  torrefied coconut   shells  showed a destroyed and less fibrous structure than the raw while the torrefied cassava  peels showed a smootsu rface.  The fiber length of   lignite,  coconut  shells     and     cassava peels decreased after  torrefaction. Results of the  emission   potential   showed  that  emissions of SO.CO  and NO  from   lignite   and coconut shells  increased  after  torrefaction,  while  cassava peels decreased.   It was also found that b lending biomass and lignite reduced emissions  of SO,  CO  and NO,      from      lignite.  Torrefaction improved the fuel properties  of lignite and biomass  such as heating value grindability, hydrophobicit y,  and uniformity.  Blending the two fuels   (lignite/biomass)    provided   a     way   to compensate   the negative  effects   of  each  other.  Therefore,  producers   of  power  and  heat  should  explore  the  use of torrefied lignite,  coconut shells,  cassava peels and their blends as suitable fuels.

CHAPTER  ONE

INTRODUCTION

1.2   Coal

One ofthe most important  field of study in the realm of science and technology is that of fuel because  the whole  of our world’s  civilization  is  based  upon  an unceasing  availability  of power. Coal (from the Old English term col, which meant “mineral of fossilized carbon”  since the

13th century)”   is a combustible black or brownish-black  sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams.  It is one of the most important of the primary fossil fuels and is composed primarily of carbon along with variable quantities of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen ‘.

Although fossil fuels have their origin in ancient biomass, they are not considered biomass by the generally accepted definition because they contain carbon that has been “out” of the carbon cycle for a very long time. Their combustion therefore disturbs the carbon dioxide content in the atmosphere.  Their structure varies based on their age and also the amount of pressure applied over time.

Coal is the most abundant fuel in the fossil family ‘. United States has more coal reserves than any other country in the world.  In fact,  one-fourth of all known coal in the world is in the United States, with large deposits located in 38 states “. Like all fossil fuels, coal can be burned to release energy.

1.2 Coal Formation

Coal forms from the accumulation  of plant debris usually  in a swamp environment.  When plant  dies  and  falls  into  the  swamp,  the  standing  water  of the  swamp  protects  it  from decay. Swamp waters are usually deficient in oxygen,  which would react with the plant debris and cause it to  decay.  This  lack  of oxygen  allows  the plant  debris  to  persist.  In  addition,  insects  and  other organisms  that  might  consume  the plant  debris  on  land  do not  survive  well  under  water  in  an oxygen deficient  environment.  To form the thick layer of plant  debris required  to produce  a coal seam,  the rate  of plant  debris accumulation  must be greater  than the rate  of decay.  Once a thick layer of plant debris is formed,  it must be buried by sediments such as mud or sand.  The weight of these materials compacts the plant debris and aids in its transformation  into coal.  About ten feet of plant   debris   will   compact    into   just    one   foot   of   coal.    Plant   debris   accumulates    very slowly.  Therefore,  accumulating  ten  feet of plant  debris will  take  a long  time.  The  fifty feet of plant  debris  needed  to  make  a  five-foot  thick  coal  seam  would  require  thousands  of years  to accumulate.

Due  to  the  variety  of materials  buried  over  time  in  the  creation  of fossil  fuels  and  the length of time the coal was forming,  several types were created.  Depending upon its composition, each type of coal bums  differently  and releases  different  types of emissions.  The  four types  (or “ranks”)  of coal mined today are lignite,  sub-bituminous,  bituminous,  and anthracite.  Coal forms when dead plant  matter  is  converted  into peat, which  in tum  is converted  into  lignite, then  sub• bituminous  coal, bituminous  coal and lastly anthracite. This involves biochemical  and geological processes  (diagenesis  and  catagenesis  respectively).    The  major  methods  of mining  coals  are surface (opencast or open cut) mining, underground  (deep) mining and underground  gasification.

Lignite  is a soft brownish-black  coal;  it  forms  the  lowest rank  of the coal  family.  It  has higher moisture  and less carbon content than the higher rank  coal. Nigeria  has the largest deposit

of lignite in Africa e.g.  Garinmaiganga  and Tai mines in Gombe state ‘.  Sub-bituminous  is a dull black  coal.  It  gives  off  a  little  more  energy  (heat)  than  lignite  when  burned.  In Nigeria,  sub-

bituminous coal is mined in Onekama and Okpara  mine in Enugu  state and Okaba mine in Benue

state.  ”  Bituminous  coal has more energy than sub-bituminous  but Anthracite  is the hardest coal and gives off the greatest amount of heat upon combustion.  Unfortunately,  in Nigeria,  as elsewhere

in the world, there is little anthracite coal to be mined.

1.3 Coal Combustion

Coal combustion  is the  burning  of coal  in  the  presence  of oxygen.  This  aims  at  heat (energy) generation.  When the combustible materials  such as carbon,  hydrogen  or compounds containing  these  are  ignited  in  the  presence  of air  (oxygen),  combustion  takes  place.  The combination  of carbon,  hydrogen  and sulfur  with  oxygen  may be expressed  by the  following equations.

CO2                            (1)

2H,0……………… (2)

Sulfur in coal burns off as gaseous sulfur which combines with oxygen to form SO and probably SO, on further oxidation.  The combustion of coal releases several environmental pollutant such as SO5, NO,, CO, CO, and CH, [5] .

1.4 Environmental Impacts of Coal

The  environmental   impact  of coal  includes  issues  such  as  land use,  waste  management, water,  and air pollution.  Starting  from coal mining,  blasting, processing,  transportation  and use of its  products,   SO,   NO,   CO,  CO»,  and  CH,   are  formed  ”  “l,  These  gases  are  hazardous   to

health. They  affect the vegetation  and  aquatics  when  diffused to streams, rivers  and  aie lllp addition  to  atmospheric  pollution,  coal  combustion produces  hundreds  of millions  of tonnes  of

solid  waste   products   annually,   including   fly  ash,   bottom   ash,   and   flue-gas   desulfurization

s ludge [11-13] .

1.5 Biomass

Biomass  is a renewable  energy source not only because  its energy comes  from the sun but also because  biomass  can re-grow  over a relatively  short period  of time.  Through  the process  of photosynthesis,  chlorophyll  in plants  captures  the sun’s energy by converting  carbon dioxide  from the air and water  from the ground  into carbohydrates-complex compounds  composed  of carbon, hydrogen,  and oxygen!’l,  When these carbohydrates  are burnt, they turn back into carbon dioxide

and water and release the energy they captured  from the sun lhll,  jn this way,  biomass  functions as a sort of natural  battery  for storing solar energy.  Biomass  is a biological  material  derived  from

living  or recently  living  organisms.  It often  refers  to  plants  or plant-based  materials  which  are

specifically  called  lignocellulose  biomass  l!2+l_ pt refers to organic matter that has stored energy through the process of photosynthesise  I.2l_  ft exists in one form as plants and may be transferred through  the  food  chain to  animal’s bodies  and  their  wastes.  All of which  can  be  converted  for everyday  human  use  through  processes  such  as  combustion,  which  releases  the  carbon  dioxide stored in the plant materiaj  ·?]   Many of the biomass fuels used today come in the form of wood products, dried vegetation, crop residues and aquatic plants.

1.6 Justification of the Study

The importance  of energy for a nation’s  development  cannot be overemphasized.  This is because  energy  is the cornerstone  of economic  and social development.  In Nigeria, the energy demand is high and is increasing geometrically while the supply remains inadequate. The energy supply  mix  must  thus  be  diversified  through  promoting  and  developing  the  abundant  energy resources present in the country to enhance the security of supply.

Coal which generates 40% of the world’s electricity has however been neglected for a long time in Nigeria  because  the existed  coal power  production  facilities degraded  the environment through pollution. Alternatively,  co-firing biomass along with coal offers  advantages but mostly boilers  are  specifically   designed   for  coals  of certain  ranks  such  as  bituminous   and  sub• bituminous coal. This is because bituminous and sub bituminous coal has higher carbon and lower moisture contents compared with lignite. There  are  less similar ranking  for lignite and biomass. And since their physical properties are highly diverse,  so are the costs for getting these fuels from the field or into the boiler.   Biomass  and lignite have a relatively  low-energy  density and high moisture content and as such tend to rot during storage. Biomass has the tendency to have a fibrous nature  that  can make  it  difficult  to  grind  into  small particles. In order  to  successfully  co- fire biomass with lignite,  both fuels need pretreatment  to increase  the heating value, hydrophobicity, bulk density, stability during storage and grindability.

Torrefaction   has   been   proposed   as   a   method   to   improve   biomass’s   properties   for gasification  and  combustion. The  purpose  of this  research  is  to  evaluate  the  impact  of such  a treatment on the fuel properties of lignite,  cassava peels and coconut shell.

1.7 Objectives of the Study

The objectives of this study are as follows:

1.    To produce suitable torrefied biomass that can improve the fuel properties oflignite.

2.   To  investigate  the  effect  of torrefaction temperature   and residence  time  on the  physical  and chemical properties of lignite, coconut shells and cassava peels.

3.   To  investigate   the  effect   of  torrefaction  on  the  pore   size,   fiber  content,   topography   and the morphology structure oflignite, coconut shells and cassava peels.

4.   To determine the influence  of blend ratio on the fuel properties  of the torrefied  lignite,  coconut shells and cassava peels.

5.   To investigate the effect  of torrefaction on the possible reduction  of pollutant  elements from the lignite, coconut shells and cassava peels.



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