ABSTRACT

The urgent need to protect our forest, to mitigate health hazards faced by the people from the use of firewood for cooking and to find an effective means of managing agro wastes has prompted a research on improving the properties of coal briquette using spear grass (Imperata cylindrica) and elephant grass (Pennisetum purpureum). In the research, proximate analysis and the elemental composition of the plant materials were carried out alongside with a coal sample. Briquettes of different composition were produced by blending the plant materials with the coal at various concentrations: 0%, 10%, 20%, 30%, 40%, 50% and 100%. The physical, mechanical and combustion properties of the briquettes were compared. It was found that the ignition, burning rate and reduction in smoke emission showed improvement with increase in biomass concentration. Compressive strength and cooking efficiency – water boiling time and specific fuel consumption showed initial improvement and rendered to break with briquette containing biomass concentration of 50% for elephant grass briquette. For spear grass, the compressive grass was at maximum at biomass concentration of 30%.     

CHAPTER ONE

 INTRODUCTION

1.1 Background of the Study

Nigeria, like other sub-Saharan countries, has faced forest degradation problems due to combination of factors. Some of the factors are clearing of land for agricultural and industrialization purposes, over grazing, bush fires, drought, over exploitation, ever- increasing deforestation along with the increased in the consumption of fuel wood etc.

About 80% of Nigerians live in the rural or semi-urban areas and they depend solely on fuel wood for their energy needs. Fuel wood accounts for about 37% of the total energy demand of the country. Investigations showed that out of the total wood demand from the forest, 90% goes to fuel wood. Presently, Nigeria reportedly consumes about 43 x 10⁹ kg of fuel wood annually [1] and it will be far more than this by the end of 2010 if the present trend continues [2]. However, it is very obvious that reduction in the use of fuel wood will drastically reduce the pressure mounted on the forest in search of wood. 

Meanwhile, it was reported that the total forest cover of Nigeria is still less than 10% of the land area, which is far below the 25% recommended by the United Nation Development Programme (UNDP) [2]. Therefore, it is imperative that concerted efforts are needed to address this situation.

Furthermore, in the recent years, global warming has become an international concern. Global warming is caused by green house gasses which carbon dioxide is among the major contributors. It was shown that increased emission of CO₂ in the atmosphere in the recent time has exacerbated the global warming [3]. Part of the reasons for this can be explained from the fact that the forest resources which act as major absorbers of CO₂ have been drastically reduced owing to the fact that the rate of deforestation is higher than the afforestation effort in the country.

Apart from environmental effects, the use of fuel wood for cooking has health implications especially on women and children who are disproportionately exposed to the smoke. Women in rural areas frequently with young children carried on their backs or staying around them, spend one to six hours each day cooking with fuel wood. In some areas, the exposure is even higher especially when the cooking is done in an unventilated place or where fuel wood is used for heating of rooms. Generally, biomass smoke contains a large number of pollutants which at varying concentrations pose substantial risk to human health. Among hundreds of the pollutants and irritants are particulate matters, carbon monoxide, formaldehyde and carcinogens such as benzo[α]pyrene, 1,2–butadiene and benzene [4].Studies showed that indoor air pollution levels from combustion of bio fuels in Africa are extremely high, and it is often many times above the standard set by US Environmental Protection Agency (US-EPA) for ambient level of these pollutants [5].

Also, consistent evidence revealed that exposure to biomass smoke increases the risk of a range of common diseases both in children and in adults. The smoke causes acute lower respiratory infection (ALRI) particularly pneumonia in children [6, 7]. Among the women, it causes chronic bronchitis and chronic obstructive pulmonary diseases (COPD) (Progressive and incompletely reversible air ways obstruction) [8,9]. Eyes irritation (sore, red eyes, tears) from the smoke is also a common experience in the use of fuel wood. A hospital based case – control studies proved that a person exposed to smoke of biomass has high risk of cataracts disease [10]. This evidence was further substantiated by an experiment carried out on animals which showed that biomass smoke is capable of damaging eye lens [11]. 

In the whole, it was summed up that the total deaths attributed to the use of fuel wood in Nigeria are about 79,000. Also nearly 45% of the national burden diseases are related to solid fuel use, according to a WHO Survey [2]^. Again, combustion of raw coal has equally been reported to have detrimental effects on both environments and the health of the people. Among other effects, inhalation of coal smoke increases the risk of lung cancer [12].

Frankly speaking, transition to electricity or gas would have been the healthiest solution to these problems but the likelihood of a complete transition in the poorer urban and rural communities in the near future is minimal. Therefore it is pertinent that other intervention measures especially ones recommended by WHO [4] should be adopted to mitigate these health risks to the lowest possible level and equally to relieve the forest resources from pressure mounted on it.    

Fortunately, researches have shown that a cleaner, affordable fuel source which is a substitute to fuel wood can be produced by blending biomass (agricultural residues and wastes) with coal. Nigeria has large coal deposit which has remained untapped since 1950’s, following the discovery of petroleum in the country. Also, millions of tonnes of agricultural wastes are generated in Nigeria annually. But it is unfortunate that farmers still practise “slash–and-burn” agriculture. These agricultural wastes they encounter during clearing of land for farming or during processing of agricultural produce are usually burnt off. By this practice, not only that the useful raw materials are wasted, it further pollutes the environment and reduces soil fertility. 

Fire affects soil below ground biodiversity, geomorphic process, and volatilizes large amount of nutrients and carbon accumulated in the soil organic matter [13]. Furthermore, during process of burning of agricultural wastes on the field, if it is not properly controlled, it can inadvertently lead to bush fire, destroying further the forest which has suffered much from the hand of wood seekers.

Forest fire is one of the severe environmental problems in Nigeria and every year various forest types are burnt as a result of fire set up deliberately or inadvertently through careless or uncared acts. Forest fire destroys the fresh saplings, seedlings and arrest regeneration of native species [13].

However, these health hazard faced by people from the use of fuel wood, along with the agricultural wastes management and reduction of pressure mounted on the forest can be mitigated if Nigeria will switch over to production and utilization of bio-coal briquette; a cleaner and environmental friendly fuel wood substitute made from agricultural wastes and coal. Moreover, this will offer a good potential for utilization of a large coal reserve in Nigeria for economic diversification and employment generation through bio-coal briquette related SMEs.

1.2    Literature Review

1.2.1 Briquetting Process

Briquetting is a mechanical compaction process for increasing the density of bulky materials. This process is used for forming fine particles into a designed shape. It can be regarded as a waste control measure in the case of production of briquettes from agricultural wastes. However, depending on the material of interest, briquetting can be used to provide fuel source as a preventive measure to many ecological problems. Briquetting is a high- pressure process which can be done at elevated temperature [14] or at ambient temperature [15, 16] depending on the technology one wants to employ. 

During this process, fine material is compacted into regular shape and size which does not separate during transportation, storage or combustion. In some briquetting techniques, the materials are simply compressed without addition of adhesive (binderless briquettes) [17, 18] while in some, adhesive material is added to assist in holding the particles of the material together [15, 16].

Generally, briquetting process has focused more on the production of smokeless solid fuels from coal and agricultural wastes. There are various techniques which have been used to produce smokeless solid fuel from coal fine. The most common technique is the use of roller press using only moderate pressure and binder. Note that the machines employed for this process are also used to make other kind of non-fuel briquettes from inorganic materials such as metal ores. However, briquetting of organic materials (agricultural wastes) requires significantly higher pressure as additional force is needed to overcome the natural springness of these materials. Essentially, this involves the destruction of the cell walls through some combination of pressure and heat. High pressure involved in this process suggests that organic briquetting is costlier than coal briquettes.

Various briquetting machines have been designed, ranging from very simple types which are manually operated to more complex ones mechanically or electrically powered. Generally, briquetting operations have developed in two directions, mechanically compression (hydraulic or pistons) and worm screw pressing types.

1.2.2 Historical Background of Briquetting Process [18- 21] Although, compaction of loose combustible materials for fuel making purposes is a technique which has been in existence thousands of years ago but industrial method of briquetting seems to be dated back to eighteenth century. In 1865, report was made on machines used for making fuel briquettes from peats and are recognized as the predecessors of the present briquetting machines. Since then, there has been a wide spread use of briquettes made from brown coal and peat etc. 

The use of organic briquettes (biomass briquettes) started more recently compared to coal briquette. It seams to have been common during World War and during the 1930s depression. The modern mechanical piston briquetting machine was developed in Switzerland based upon German development in the 1930s. Briquetting of saw dust are widespread in many countries in Europe and America during World War II because of fuel shortages. However, after the World War, briquettes were gradually phased out of the market because of availability and cheapness of hydrocarbon fuels. 

As time went on, the use of organic briquette was revitalized due to high energy prices in the 1970s and early 1980s mainly for industrial heating in USA, Canada and Scandinavia, etc. In Japan, the use of briquette seems to be very common especially the use of “Ogalite” fuel briquettes made from saw dust and rice husk. The Japan technology has spread to Taiwan, and from there to other countries such as Thailand, Asia, USA and some other European countries. This type of briquette has been in use in Japan since 1950s as a substitute for charcoal which was then a widespread fuel source.

Furthermore, in Great Britain, the first fuel briquette was manufactured by the Powell Dul Fryn Company in 1938 by heating anthracite chips bound with pitch to a temperature of around 750^oC to produce a briquette known as phurnacite and this production was taken over by the National Coal Board in 1942. They were able to produce half a million tonne of coal annually. It was also reported that the same technique was tried on production of smokeless coal briquette from low-rank coal containing as much as 30-40% volatile materials. But the problem was how to reduce the volatile component of the coal to prevent smoke formation and at the same time, retaining sufficient active constituents to give an easily lighted bright fire with a high radiation. It was found that moderate heating (carbonization) of the low rank coal not only drives off a portion of the volatile matter but appears to change the remaining volatile portion in such a way that it does not smoke even with a volatile content as high as 23%. Phurnacite was produced from low-rank coal by heating the coal bound with pitch to 200^oC. Since then, other technologies for production of smokeless briquettes were developed in Britain. These include the multi heat briquette marketed by the National Coal Board. This brand of smokeless briquette was made by curing pitch bound briquette in a bed of sand which is fluidized and kept at a temperature of about 380^oC. Other types of briquettes developed were home fire and room heat. 

Today, many other developing countries have adopted and developed briquetting technology, owing to high cost and scarcity of fuel. Common types of briquettes so far in use are coal briquettes, peat briquettes, charcoal briquettes and biomass briquettes, etc. Most recently, researches showed that blending of coal and biomass will give rise to a briquette with better combustion properties and pollutants emission reduction. This type of briquette is known as bio-coal briquette. Some authors simply called it biobriquette [16]. 

1.3  Bio-Coal Briquettes

Bio-coal briquette is a type of solid fuel prepared by blending coal, biomass, binder and sulphur fixation agent [23,24]. Other additives may also be added. A research showed that   bio-coal briquettes may be prepared by blending the following [15]:

•          Biomass (25% to 50%)

•          Coal (75% to 50%)

•          Sulphur fixation agent (up to 5%)

•          Binder (up to 5%)

Also, according to clean coal technologies Japan, bio-coal briquettes are prepared by blending: 

•          Biomass (10% to 25%)

•          Coal (75% to 90%)

•          Sulphur fixation agent (depending on the sulphur content of the coal).

In this process, Ca(OH)₂ acts as both sulphur fixation agent and the binder [16]. Also, activators such as iron oxide, potassium manganate and sodium chloride have been reported to have the ability of improving the thermal efficiency of the briquette [26].  

The high pressure involved in the process ensures that the coal particles and the fibrous biomass material interlace and adhere to each other as a result, do not separate from each other during combustion, transportation and storage. During combustion, the low ignition temperature of the biomass simultaneously combusts with the coal. The combined combustion of both gives a favorable ignition and fire properties; emits little dust and soot, generates sandy combustion ash, leaving no clinkers [16, 24]. Also the desulfurizing agent such as Ca(OH)₂ in the briquette effectively reacts with the sulphur content of the coal to fix about 60-80% of it into the ash [16]. It was showed that lime (CaO) as a desulfurizing agent was able to capture up to 90-95% of the total sulphur in the coal, leaving only 5-10% emitted as sulphur oxides [25]. The equation of the reaction is as follows: 

Evidence also revealed that lime when used as desulfurizer also acts as a binder. Also clay has been reported to be a good desulfurizing agent. Clay contains CaO and MgO which acts as desulfurizing agents. Also it contains Fe₂O₃ which has been shown to have a catalytic effect on the sulfation reaction [25].  

There are various biomass resources available for production of biocoal briquettes. Some of them are straw, sugar bagasse (fibrous residue of processed sugar cane), corn stalk, groundnut – shell, wheat straw, palm husk, rice husks, corn cob, forest wastes, and other agricultural wastes. Several researches on bio–coal briquette have been carried out using some of these biomass resources. There are records of researches carried out on production of bio-coal briquettes using sawdust [27], rice straw [28], olive stone [29, 30] and maize cob [24], etc. 

Furthermore, it has been shown that any grades of coal can be used for bio-coal production, even low grade coal containing high sulphur contents [24, 26]. This implies that, by this technology, extra cost of carbonizing low grade coal before briquetting is saved.  

Binder is an adhesive material which helps to hold the particles of the material together in the briquette. Apart from its function to hold the particle from separation, it also protects the briquette against moisture in case of long storage [13]. There are several binders that can be used. Some of them are starch (from various starchy root such as cassava, and cereals), molasses, clay and even tree gum, etc. Some chemical substances have also been used as binding agent for production of briquettes. Some of them are asphalt [31], potassium [32], magnesia [33], ammonium nitrohumate [34] and commercial pitch [35]. Though the use of starch as binder is satisfactory in every respect, it disintegrates under moist or tropical condition. However, the use of small additional hydrocarbon binder such as pitch or bitumen has been reported to improve the water resisting property [36]. Moreover, the nature of the binder has influence in the combustibility of the briquette produced. For instance, briquette produced using clay takes longer time to ignite than the one produced using starch [13]. The reason for this is because of non-combustibility of clay compared to starch.

1.3.1 Characteristics of Bio-Coal Briquettes

(1)    Bio-coal briquette decreases the generation of dust and soot up to one-tenth that of direct combustion of coal [16]. Combustion of coal generates dust and soot because, during the combustion, the volatile components of the coal are released at low temperature (200-400⁰C) as incomplete combusted volatile matter. For bio-coal briquette, since the biomass component of the briquette ignites at low temperature compare to the coal, this ensures that the volatile matter in the coal which would have otherwise been liberated as smoke at low combustion temperature combusts completely. By so doing, there is a significant reduction in the amount of dust and soot generated. Note that smoke is a complex mixture of air-borne solid and liquid particulates as well as gases evolved on pyrolysis or combustion of material [45].

(2)    Bio-coal briquette has a significant shorter ignition time when compared with coal or conventional coal briquette [15]. This is because of the biomass component of the briquette. Biomass has low ignition time.

(3)    Bio-coal briquette has superior combustion-sustaining properties. Because of low expansibility and caking properties of bio-coal briquette, sufficient air flow is maintained between the briquettes during combustion in a fire-place. Hence it has very good combustion-sustaining properties and does not die out in a fireplace or other heater even when the air supply is decreased [16]. This property offers the opportunity of adjusting the combustion rate of the bio-coal briquette easily.

(4)    Bio-coal briquette emits less SO₂. It contains desulfurizing agent and the high pressure involved in the process enables the coal particles to adhere strongly to the desulfurizing agent. During combustion, the desulfurizing agent effectively reacts with the sulphur content of the coal to form a solid compound instead of being released as oxides of sulphur to the atmosphere. However, it is widely accepted that bio-coal briquette technology is one of the most promising technologies for the reduction of SO₂ emission associated with burning of coal [24,37]. 

(5)    Bio-coal briquette has high breaking strength for easy transportation. The high pressure involved in the process coupled with the binder, compressed the raw materials into a rigid mass which does not break easily, hence can be stored and transported safely [16].

(6)    Bio-coal briquette generates sandy ash which can be utilized in agriculture for soil improvement [38]. In the briquette, since the fibrous biomass intertwined with the coal particles, the resulted ash after combustion does not adhere or form clinch-lump, therefore, the ash is always sandy.

(7)    Bio-coal briquette burns nearly perfect; therefore the flame has significant higher temperature than simple biomass burning [39] or coal [26].

1.3.2 Table 1: Comparative Tests of Bio-coal Briquettes [40]