Impact of torrefaction on the fuel properties of lignite, coconut shells, cassava peels and their blends

Thesis Overview

<p> INTRODUCTION<br>1.2 Coal<br>One of the most important field of study in the realm of science and technology is that of<br>fuel because the whole of our world’s civilization is based upon an unceasing availability of<br>power. Coal (from the Old English term col, which meant “mineral of fossilized carbon” since the<br>13th century) [1] is a combustible black or brownish-black sedimentary rock usually occurring in<br>rock strata in layers or veins called coal beds or coal seams. It is one of the most important of the<br>primary fossil fuels and is composed primarily of carbon along with variable quantities of other<br>elements, chiefly hydrogen, sulfur, oxygen, and nitrogen [2].<br>Although fossil fuels have their origin in ancient biomass, they are not considered biomass<br>by the generally accepted definition because they contain carbon that has been “out” of the carbon<br>cycle for a very long time. Their combustion therefore disturbs the carbon dioxide content in the<br>atmosphere. Their structure varies based on their age and also the amount of pressure applied over<br>time.<br>Coal is the most abundant fuel in the fossil family [2]. United States has more coal reserves<br>than any other country in the world. In fact, one-fourth of all known coal in the world is in the<br>United States, with large deposits located in 38 states [2]. Like all fossil fuels, coal can be burned to<br>release energy.<br>1.2 Coal Formation<br>14<br>Coal forms from the accumulation of plant debris usually in a swamp environment. When<br>plant dies and falls into the swamp, the standing water of the swamp protects it from decay.<br>Swamp waters are usually deficient in oxygen, which would react with the plant debris and cause it<br>to decay. This lack of oxygen allows the plant debris to persist. In addition, insects and other<br>organisms that might consume the plant debris on land do not survive well under water in an<br>oxygen deficient environment. To form the thick layer of plant debris required to produce a coal<br>seam, the rate of plant debris accumulation must be greater than the rate of decay. Once a thick<br>layer of plant debris is formed, it must be buried by sediments such as mud or sand. The weight of<br>these materials compacts the plant debris and aids in its transformation into coal. About ten feet of<br>plant debris will compact into just one foot of coal. Plant debris accumulates very<br>slowly. Therefore, accumulating ten feet of plant debris will take a long time. The fifty feet of<br>plant debris needed to make a five-foot thick coal seam would require thousands of years to<br>accumulate.<br>Due to the variety of materials buried over time in the creation of fossil fuels and the<br>length of time the coal was forming, several types were created. Depending upon its composition,<br>each type of coal burns differently and releases different types of emissions. The four types (or<br>“ranks”) of coal mined today are lignite, sub-bituminous, bituminous, and anthracite. Coal forms<br>when dead plant matter is converted into peat, which in turn is converted into lignite, then subbituminous<br>coal, bituminous coal and lastly anthracite. This involves biochemical and geological<br>processes (diagenesis and catagenesis respectively). The major methods of mining coals are<br>surface (opencast or open cut) mining, underground (deep) mining and underground gasification.<br>Lignite is a soft brownish-black coal; it forms the lowest rank of the coal family. It has<br>higher moisture and less carbon content than the higher rank coal. Nigeria has the largest deposit<br>15<br>of lignite in Africa e.g. Garinmaiganga and Tai mines in Gombe state [3]. Sub-bituminous is a dull<br>black coal. It gives off a little more energy (heat) than lignite when burned. In Nigeria, subbituminous<br>coal is mined in Onekama and Okpara mine in Enugu state and Okaba mine in Benue<br>state. [4] Bituminous coal has more energy than sub-bituminous but Anthracite is the hardest coal<br>and gives off the greatest amount of heat upon combustion. Unfortunately, in Nigeria, as elsewhere<br>in the world, there is little anthracite coal to be mined.<br>1.3 Coal Combustion<br>Coal combustion is the burning of coal in the presence of oxygen. This aims at heat<br>(energy) generation. When the combustible materials such as carbon, hydrogen or compounds<br>containing these are ignited in the presence of air (oxygen), combustion takes place. The<br>combination of carbon, hydrogen and sulfur with oxygen may be expressed by the following<br>equations.<br>C + O2 CO2 ……………..… (1)<br>2H2 + O2 2H2O ………….….. (2)<br>S + O2 SO2…………….… (3)<br>Sulfur in coal burns off as gaseous sulfur which combines with oxygen to form SO2 and probably<br>SO3 on further oxidation. The combustion of coal releases several environmental pollutant such as<br>SO2, NO2, CO, CO2, and CH4<br>[5].<br>1.4 Environmental Impacts of Coal<br>16<br>The environmental impact of coal includes issues such as land use, waste management,<br>water, and air pollution. Starting from coal mining, blasting, processing, transportation and use of<br>its products, SO2, NO2, CO, CO2, and CH4 are formed [6, 7]. These gases are hazardous to<br>health. They affect the vegetation and aquatics when diffused to streams, rivers and air [8-10]. In<br>addition to atmospheric pollution, coal combustion produces hundreds of millions of tonnes of<br>solid waste products annually, including fly ash, bottom ash, and flue-gas desulfurization<br>sludge [11-13].<br>1.5 Biomass<br>Biomass is a renewable energy source not only because its energy comes from the sun but<br>also because biomass can re-grow over a relatively short period of time. Through the process of<br>photosynthesis, chlorophyll in plants captures the sun’s energy by converting carbon dioxide from<br>the air and water from the ground into carbohydrates—complex compounds composed of carbon,<br>hydrogen, and oxygen [14]. When these carbohydrates are burnt, they turn back into carbon dioxide<br>and water and release the energy they captured from the sun [15-18]. In this way, biomass functions<br>as a sort of natural battery for storing solar energy. Biomass is a biological material derived from<br>living or recently living organisms. It often refers to plants or plant-based materials which are<br>specifically called lignocellulose biomass [19-24]. It refers to organic matter that has stored energy<br>through the process of photosynthesis [25, 26]. It exists in one form as plants and may be transferred<br>through the food chain to animal’s bodies and their wastes. All of which can be converted for<br>everyday human use through processes such as combustion, which releases the carbon dioxide<br>17<br>stored in the plant material [27, 28]. Many of the biomass fuels used today come in the form of wood<br>products, dried vegetation, crop residues and aquatic plants.<br>1.6 Justification of the Study<br>The importance of energy for a nation’s development cannot be overemphasized. This is<br>because energy is the cornerstone of economic and social development. In Nigeria, the energy<br>demand is high and is increasing geometrically while the supply remains inadequate. The energy<br>supply mix must thus be diversified through promoting and developing the abundant energy<br>resources present in the country to enhance the security of supply.<br>Coal which generates 40% of the world’s electricity has however been neglected for a long<br>time in Nigeria because the existed coal power production facilities degraded the environment<br>through pollution. Alternatively, co-firing biomass along with coal offers advantages but mostly<br>boilers are specifically designed for coals of certain ranks such as bituminous and subbituminous<br>coal. This is because bituminous and sub bituminous coal has higher carbon and lower<br>moisture contents compared with lignite. There are less similar ranking for lignite and biomass.<br>And since their physical properties are highly diverse, so are the costs for getting these fuels from<br>the field or into the boiler. Biomass and lignite have a relatively low-energy density and high<br>moisture content and as such tend to rot during storage. Biomass has the tendency to have a fibrous<br>nature that can make it difficult to grind into small particles. In order to successfully co-fire<br>biomass with lignite, both fuels need pretreatment to increase the heating value, hydrophobicity,<br>bulk density, stability during storage and grindability.<br>18<br>Torrefaction has been proposed as a method to improve biomass`s properties for<br>gasification and combustion. The purpose of this research is to evaluate the impact of such a<br>treatment on the fuel properties of lignite, cassava peels and coconut shell.<br>1.7 Objectives of the Study<br>The objectives of this study are as follows:<br>1. To produce suitable torrefied biomass that can improve the fuel properties of lignite.<br>2. To investigate the effect of torrefaction temperature and residence time on the physical and<br>chemical properties of lignite, coconut shells and cassava peels.<br>3. To investigate the effect of torrefaction on the pore size, fiber content, topography and<br>the morphology structure of lignite, coconut shells and cassava peels.<br>4. To determine the influence of blend ratio on the fuel properties of the torrefied lignite, coconut<br>shells and cassava peels.<br>5. To investigate the effect of torrefaction on the possible reduction of pollutant elements from the<br>lignite, coconut shells and cassava peels.<br>19 <br></p>