Effect of Fuel Combustion in Libyan Power Stations and Cement Industries on Environment

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    Vol. 3 (4) Oct – Dec 2012 1442 International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701  _____________________________________________ Research  Article   Effect of Fuel Combustion in Libyan Power Stations and Cement Industries on Environment Iftikhar Ahmad and Mohamed Nuri Rahuma Department of Chemistry, Faculty of Science, Benghazi University, Benghazi (Libiya). ABSTRACT The modern oil-fired heavy industries such as power stations and cement industries, which use heavy fuel oil as a fuel, are highly concentrated sources of combustion gases. The flue gases comprise a combination of gases such as carbon dioxide (CO 2 ), sulphur dioxide (SO 2 ), nitrogen oxides (NO x ), carbon monoxide (CO), particulate matters and others. Most of the power stations and cement factories in Libya use heavy fuel oil as energy source while some of them are using natural gas as fuel. This paper describes the composition of heavy fuel oil produced in different Libyan refineries and also describes the combustion chemistry of these heavy fuel oils. The effects of flue gases generated during combustion of heavy fuel oil on environment and human health have been discussed in this paper. The methods and techniques to minimise the impact of emissions have been presented. The alternative fuels and their availability in Libya has been discussed. 1. INTRODUCTION The modern oil-fired power stations and cement factories are source of combustion gases and special measures have to be taken to protect the environment from pollution by them. These comprise a combination of control of the unwanted emission at source, for example by dispersal of the  bulk of the flue gases from very tall stack, and by removal of dusts. The effectiveness of tall stacks has been discussed in detail by other workers 1 . Carbon dioxide is the main discharged gas from the stack. Carbon dioxide is the least regarded as a  pollutant, although since the early seventies the  possible long-term climatic effect, on a global scale, of adding to the long-established atmospheric concentration of carbon dioxide has become a subject of speculation and inquiry 2 . The principal reason is that the measured atmospheric concentration of carbon dioxide at different latitudes has increased and about half of the carbon dioxide emitted appears to have remained in the atmosphere during the available  period of observation, the rest being absorbed by vegetation, soil and oceans. A primary effect of increasing the carbon dioxide concentration is to increase the absorption of infrared radiation from the earth’s surface and so lead to warming of the atmosphere (the so-called green house effect). Such a temperature rise would certainly cause important climatic changes with massive consequences such as flooding of coastal areas and major cities. Along with the carbon dioxide as it emerges from the stack there will be water vapour, other gases such as sulphur dioxide, nitrogen oxides and very fine dust carried along with the gases which will emerge from the stack top at velocity as high as 30 m/s. The amount of pollutants from stack gases depends upon the composition of the fuel. Most of the Libyan power stations and cement factories use heavy fuel oil as fuel. Table-1 shows the quality control tests of heavy fuel oil produced in Libyan refineries. Table-2 gives the impurities in a typical heavy residual fuel oil 3 .  Table 1: Quality control properties of Libyan heavy fuel oils Test Method Test Description Result Az-Zawiya Refinery Tobruk Refinery ASTM D4052 Specific gravity at 15.6/15.6 o C API gravity Density at 15 o C, kg/l 0.9231 21.8 0.9225 0.9126 23.6 0.9121 IP 170 Flash point, o C 162 162 ASTM D4007 Water and sediment content, vol.% <0.05 <0.05 Calculation Calorific value (gross), kcal/kg 19598 10691 ASTM D189 Conradson carbon residue, wt.% 5.12 9.34 ASTM D482 Ash content, wt.% <0.001 <0.001` ASTM D97 Pour point, o C + 36 +39 IP 377 Mod Vanadium. ppm 6 1    Vol. 3 (4) Oct – Dec 2012 1443 International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701 Sulphur is an important impurity in heavy fuel oil. It burns to sulphur dioxide and up to one percent of this may oxidise in the hot gases to sulphur trioxide, which eventually unites with the water to form sulphuric acid. Nitrogen is another important impurity in heavy fuel oil. Much of it is oxidised to nitric oxide, NO. At the very hottest parts of a flame nitrogen from the air reacts with oxygen to form the equilibrium amount of nitric oxide appropriate to that temperature. A small but highly variable fraction of the NO is oxidised to the nitrogen dioxide, NO 2 /N 2 O 4  that is the  physiologically active form, NO being itself inert. This oxidation is thermodynamically favoured when the flue gases mix with air through it occurs extremely slowly. It has become customary to refer to nitrogen oxides as NO x  since NO is capable of slow oxidation and can be regarded as a potential source of the more active oxide. In this paper references has been made very briefly to a number of features of oil-fired factories emissions which have clear chemical interest. A selection of these will now be discussed in greater detail. Table 2: Impurities in typical fuel oil Impurity Amount (in ppm) Impurity Amount (in ppm) Sulphur  Nitrogen Silicon Vanadium Iron  Nickel Calcium Potassium Aluminium Sodium Chlorine Magnesium 2700 1500 300 150 100 50 100 50 75 50 25 12 Zin Phosphorous Chromium Cobalt Manganese Copper Lead Selenium Cadmium Antimony Arsenic Mercury 4 4 3 3 2.5 2.5 2 1 0.2 0.02 0.01 0.01 2.   Gas Reactions in the Furnace The gaseous species whose concentrations in emissions are most affected by flame reactions are sulphur trioxide (eventually sulphuric acid), nitrogen oxides, carbon mono oxide and minor amounts of hydrocarbon gases. The flame  parameters, which affect the concentration of these species, are the temperature to which gas mixtures are exposed and the duration of such exposure, the gas mixing, i.e., the excess oxygen concentration during the combustion process and finally the rate of cooling. In a flame of particulate fuel and air the combustion parameters will vary across the width of the reaction zone. Some regions will be rich, heated less by reaction than by radiation and convention, and pyrolysis of the fuel may occur more than in other zones of the flame where oxygen will be in excess. Such different zones will be mixed by violent short-range turbulence. The task of accurately modeling such a complex process in which chemical reactions and mixing are closely involved is a daunting one but considerable success has been achieved with simplified models, which assume that the higher temperatures in a flame most of the chemical species in dynamic equilibrium with each other and that it is valid to apply known data for the kinetic parameters of the reaction of interest. This approach has made it possible to deduce the effect of cooling and mixing processes on the final gas composition. Thus, Billingsley et. al 4  took account of the interaction of forty six gas phase reactions with a bearing on the formation of NO, SO 3  and CO in a combustor operating under a variety of conditions of pressure, fuel/air ratio, fuel composition and temperature history. The calculations show that the way to limit the formation of NO is so to arrange the mixing of the gases that some part-cooled combustion gas is recycled to dilute the flame gases and diminish the top temperature achieved. This technique has been applied successfully to boiler plant in many countries. It is also clear from the calculations that SO 3  can be minimised by limiting the concentration of residual oxygen present after the flame and during the cooling of the flue gases from say, 1650 K to 1000 K. 3.   Smokes and Tarry Organic Materials By regulation the smoke permitted from a power station stack is severely limited. The total permitted solids can only be achieved if plant conditions –  burner efficiency, excess oxygen, gas mixing in the furnace – are such as to consume the fuel particles and avoid the formation of soot and tars almost entirely. The ‘unburnt carbon’ particles are usually greater than 10  m in diameter and do not emerge from the chimney. Smaller particles, which may be associated with the fine dust do not usually exceed 20 mg/m 3  when the plant is on steady load. At    Vol. 3 (4) Oct – Dec 2012 1444 International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-3701 ground level this is a negligible contribution to  background smoke and soot. Attention has been focussed upon it, however, because of the well-known presence in such soot particles of the carcinogenic polycyclic aromatic hydrocarbons of which the most dangerous by far is benzo-a-pyrene. Power stations and cement factories are not, of course, the only sources of this type of compound. Whenever combustion of fuel oil and fossil fuels takes place formation and releases of these compounds will occur, to a degree which reflects the efficiency of the combustion process. Libya has no data of how much benzo-a-pyrene goes to atmosphere from power stations, cement factories and other sources. Allen 5  concluded that in USA  power stations expelled a total of 0.8 tonnes/year whereas residential heating provided 390 tonnes and garden refuse burning 75 tonnes. It is clear that good flame mixing and high intensity combustion in a power station furnace are effective in destroying complex aromatic structures and minimizing benzo-a-pyrenes. 4.   Long Range Transport of Industries Flue Gases During the last decade the possibility that industrial emissions could be transported over long distance has become the focus of great interest both in Europe and North America. A central feature of the claims that have been made is that precipitation (rain and snow) is acidic, that there is a trend of increasing acidity and an expanding area of acidity on a regional scale which matches the increased emissions of sulphur dioxide and nitrogen oxides from industry. Such claims are of major concerns to electrical power utilities which, through combustion of fuels, are major emitters of both SO 2  and NO x . Sophisticated methods have been successfully developed to protect the near-field environment from these pollutants by ensuring that the flue gases are sufficiently diluted by the air and it has been argued that such measures are likely to  be equally effective in removing risk of pollution from the far field. Within the mixing layer primary pollutants can be removed by direct contact with land or sea surfaces, which is called dry deposition, or by  being washed out by rain or snow, i.e., by  precipitation scavenging. They can also undergo chemical transformation into secondary products which can then be removed by the same processes. In the case of SO 2  the dominant removal process is directed dry deposition of the gas by oxidation to sulphuric acid and subsequent deposition as  particles of the acid also contributes. Oxidation involves a complex sequence of gas phase reactions in which free radicals, generated photochemically  by sunlight, play an important role. Many of the  primary reactions occurring have been characterized in detail in laboratory kinetic studies so that it is now possible to model in some detail the chemical evaluation of a plume, under conditions where photochemical reactions  predominate. The principal chemical step is the reaction of SO 2  with hydroxyl radicals: OH + SO 2  HSO 3  Leading ultimately to sulphate aerosol. The major reactions generating hydroxyl are O 3  O (D) + O 2  O (D) + H 2 O OH + OH HNO 2 OH + NO O + RH OH + R From which it is evident that the fate of SO 2  is connected with other minor atmospheric constituents such as ozone, hydrocarbons and oxides of nitrogen and it is inappropriate to attempt to describe the fate of a single pollutant in isolation. 5.   Chemical Measures to Minimize the Impact of Emissions (a)   Combustion with additives to retain sulphur By adding lime or magnesium hydroxide to the combustion gases it is possible to retain a high  proportion of sulphur – as calcium or magnesium sulphate in the solid ash. Unfortunately the short residence time available in the boiler means that the added lime must be in the form of very fine (e.g., < 10  m) particles to ensure that it has time to react to a reasonable extent. An alternative  procedure is so burn the fuel in a fluidized bed to which the coarsely crushed lime is added. Experience has shown that the lime stone or dolomite can be used since there is time to decompose the carbonate as well as for the  particles of additive to react in depth with the sulphur oxides. A ratio of Ca/S of 2:1 is said to be sufficient to retain nearly 90% of sulphur in the fuel as calcium sulphate with the ash. (b)   Flue gas desulphurization Flue gas desulphurization plants are being installed in existing as well as new power stations  particularly in Japan, USA and Europe. The technique most often used is to wash the flue gases with the lime slurry. The resultant calcium sulphite or sulphate if air is blown through it, must then be removed. A regenerable absorbent process based on the Na 2 SO 3 /NaHSO 3  conversion is also  possible, it can be operated to produce either sulphur or sulphuric acid as the end product.    Vol. 3 (4) Oct – Dec 2012 1445 International Journal of Research in Pharmaceutical and Biomedical Sciences ISSN: 2229-37016. Alternative Fuels (a) Natural Gas  Natural gas is the least polluting fossil fuel per unit of heat produced, with respect to both greenhouse gas emissions and other combustion products. Any market share won by natural gas from other fossil fuels will therefore promote the cause of environmental protection. Resources of Natural Gas Up-to-date proved reserves of natural gas of the whole world comprise 15 trillion m 3  and considering current and expected levels of  production this fact provides better conditions for reserves of natural gas compared with oil. International Gas Union (IGU) estimates the resources of natural gas approximately up to 400 trillion m3 though it is clear that their transfer into the category of proved ones will require a great scope of exploration work. Moreover, it is important to note that international classification of reserves takes into consideration economic efficiency of their being recovered under conditions of current price level. The proven gas reserves of Libya comprise 46.4 trillion cubic feet. Total proved gas reserve estimates are the sum of all non associated gas, associated and dissolved gas, plus all of the recoverable gas volumes available in the under ground storage reservoirs.  Natural gas reserves are classified in two categories  based on the reservoir occurrence: (a) Non associated gas – it is defined as free natural gas not in contact with crude oil in reservoirs associated, (b) Associated gas - it occurs in crude oil reservoirs either as free gas or in solution with crude oil. Table 3 shows 6  world gas proven reserves, consumption and production. Besides conventional gas resources the proved reserves of which are sufficient for 60 year and, taking into account new discoveries, for 90 – 100 year period of time there are so-called non-conventional natural gas reserves the completion of which will secure an assured development of gas industry beyond the limits of 21 st  century. They are:    tight gas;    coalbed methane;    gas dissolved in formation waters;    gas hydrate;    deep gas. Unconventional gas reserves are some tens and may be even hundred thousand trillion m 3 . Table 3: World gas proven reserves, production and consumption (1998) Region Reserves (Trillion ft 3 ) Production (Billion m 3 ) Consumption (Billion m 3 ) P/C R/P Africa 361.1 101.2 48.7 2.08 >100 Asia/Pacific 359.5 245.8 259.0 0.95 41.4 Federation of Soviet Union (FSU) 2002.6 643.9 529.0 1.22 83.4 Latin America 2190.0 86.7 86.0 1.01 71.5 Middle East 1749.6 181.0 171.8 1.05 >100  North America 294.6 739.0 718.9 1.03 11.4 Western Europe 183.9 274.3 927.1 0.30 18.4 Total 5170.3 2271.8 2240.5 1.01 63.0 (b) Dimethyl ether (DME) DME is another type of clean fuel which can be  produced from natural gas and can be used as a substitute for power plant fuel. The Tropsae  process is the process for direct synthesis of DME from natural gas. It is a plant of production of methanol and subsequent conversion into DME in one integrated synthesis section. This process layout eliminates the need to isolate and purify methanol as an intermediate before further  processing into DME. The Tropsae DME process is based on well proven technology and process scheme, similar to the one used for producing methanol. The plant comprises three process sections, viz. 1.   Synthesis gas preparation by Auto Thermal Reforming (ATR) 2.   Oxygenate synthesis (combined synthesis of methanol and DME) 3.   Product separation and purification. Production of liquid fuels from natural gas requires very large capacities in order to benefit from economy of scale to the maximum degree. The most suitable reforming technology 7  for this  purpose is the Topsoe ATR technology, which  permits single line production units exceeding 7,500 MTPD DME. The Topsoe ATR technology has been developed tremendously over the last decade, and recently it has been demonstrated industrially at a steam/carbon ratio of only 0.6.
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