Most of the industries produce solid combustible wastes which are of flammable or toxic in nature. These wastes which are risky in character can do much inauspicious consequence to the nature and every bit good as to the world. This paper shows the risky waste processing and besides energy recovery from those wastes by utilizing in cement kilns. Because of the high operating temperatures, long abode times and their ability to supply favorable conditions for the chemical combination of inorganic residues into the active compounds, these procedure can supply ideal conditions for environmental sound disposal of combustible risky waste stuffs. The chief advantages of utilizing risky wastes in cement kilns include energy recovery, preservation of fossil fuels, decrease in production costs and besides the usage of already bing installations.
Industrial wastes have been ever a job to the universe due to its nature. Some of the wastes can be disposed decently without any harmful effects. But some of the solid and risky wastes can non be decently managed and cause serious jobs to many states around the universe. These wastes pollute air, H2O and land which will impact the natural balance of nature in a negative manner. Inturn this will do jobs to populating existences including human life. The production of these risky wastes can non be avoided as these may be the byproducts of ineluctable merchandises for human twenty-four hours to twenty-four hours life. Even if a procedure is optimised for minimal waste production, there will be jobs in safe direction of harmful wastes. Recycling is an option for these risky wastes. But due to the proficient and economical restrictions, many of these solid and risky wastes can non be recycled. Landfill is another option for risky waste disposal. This method is merely a impermanent storage of these harmful wastes. But these harmful wastes will stay as it is and may breathe toxic gases and chemicals. Hence this procedure can non be considered as a lasting solution for risky waste direction [ 1 ] .
Incineration is considered as the most appropriate method for solid and risky waste disposal which are environmentally safe. Incineration is the procedure that treats waste through burning of organic stuffs. This is an highly high temperature intervention and is rather frequently called as thermic intervention. The waste merchandises are turned into ash, particulates, flue gases and heat that can besides be used to bring forth electric power. The harmful pollutants left behind from the fluke gases are dispersed into the ambiance. Even though these incinerators are capable of disposing high grade of risky wastes, incineration of some wastes can make new emanation beginning and typically do non retrieve the resource value of waste stuffs, therefore necessitating extra non-renewable fuel to run. Besides the demand of immense capital for the building of a commercial incinerator makes this procedure a backseat [ 5 ] .
Apart from merely incineration the risky wastes, if energy is recovered from waste stuffs, it will be a great accomplishment. The solid and risky wastes can be used as auxiliary fuel or natural stuff permutation in industrial furnaces such as cement kilns. This can be one of the best solutions for safely destructing the risky wastes and at the same time retrieving their resource values.
II. Hazardous Waste
Hazardous waste is hard to specify ; there are different types of definitions to risky waste based on composing, physical signifier, chemical, biological etc.
Harmonizing to UNEP, Hazardous wastes average wastes, by ground of their chemical activity or toxic, explosive, caustic, or other features, cause danger or probably will do danger to wellness or the environment [ 7 ] .
A. Categorization of risky waste
There are different ways of sorting risky waste based on lists, beginning, risky features and chemical, biological and physical belongingss. The usage of lists classification provides more information about the belongingss, jeopardies and possible disposal options and offers less information about their beginnings. The procedure of utilizing origin place the wastes has restrictions but non supply the precise thought about composing. Hazardous features can be used to sort the risky waste based on the chief four groups. They are toxic and eco-toxic, caustic, burnable and reactive. Chemical, biological and physical belongingss are besides utile for categorization of risky waste depends on the inorganic wastes, organic wastes, oily wastes and sludge ‘s.
B. Beginnings of risky waste
Hazardous wastes obtained from big figure of beginnings in the present society from big graduated table to little graduated table and even a normal family. The major risky is from the industries like excavation and metallurgic industries, cars and crude oil, agricultural and piscaries, atomic power, pharmaceutical, plastic, fabricated metal, pigment, chemical and electronic. The minor risky waste nil but non-industrial beginnings i.e. , from infirmaries, nursing places and family. These two are the chief beginnings for the risky waste.
C. Effectss of risky waste
Normally coevals of waste means loss of merchandise. There are different effects of risky waste, which are contaminated land are non utile for future usage, taint of land H2O for longer footings, waste stuffs can migrate to the disposal countries ; toxins are bioaccumulate and besides tend to travel up the nutrient concatenation [ 7 ] . These are the different effects of the risky waste on the present society and it is really expensive to clean up contaminated sites.
D. Environment impacts and wellness hazards
Environmental impacts and wellness hazards originating from risky wastes are similar to all states. Hazardous wastes are potentially environment and wellness threatening constituents. Environment impacts like gaseous emanations or land taint or chemical production cause ecological harm and ecological instability in the environment. There are besides wellness hazards to human existences, when the risky wastes are exposed straight or indirectly on the human existences cause wellness jobs like physical injury, decease and unwellness. The major hazard is to the people working in the fabrication industry.
III. Cement Production Process
The chief chemical constituents required for the cement production are Ca, Si, aluminum and Fe. Calcium is obtained from limestone, Si and aluminum are provided by proper mixture of clay, shale or sand and Fe is provided by Fe ore. These natural stuffs have different chemical composing with different proportions i.e. , 75 % Ca carbonate, 15 % Si dioxide, 3 % aluminum oxide, 2 % ferrous oxide and 5 % other minerals [ 1 ] .
The four major procedures for the cement production are acquisition and transit of natural stuffs, readying of natural stuffs, pyroprocessing of natural stuffs to from cement cinder and crunching cinder into cement. Now the natural stuffs are crushed, land and proportioned so the needed mixture and chemical composing for the pyroprocessing system.
Fig.1: Cement Kiln. Beginning: “ hypertext transfer protocol: //www.sczgcn.com/proshow.asp? id=581 ”
The pyroprocessing takes topographic point in a cylindrical rotary kiln. This kiln is an extended steel cylinder runing in size up to 250m in length and 8m in diameter lined with furnace lining brick and inclined at an angle 3-6 grades shown in fig.1. Rotation of kiln is different for older and newer versions. For older kiln, rotates at about 70 revolutions per hr and for newer kiln, rotates at about 180 revolutions per hr [ 1 ] . The natural stuffs are feed at the elevated terminal of the kiln and moved easy by the rotary motion of kiln down towards the firing terminal where heat is obtained from coal, gas or oil fire.
There are three different thermic zones operated in the kiln and those are drying and preheating zone, calcining zone, firing zone or clinkering zone. The temperature in the drying and preheating zone is C, H2O content get evaporated when the stuff is in this zone. After this zone so followed by calcining zone where C dioxide is driven off from the limestone stuff, therefore organizing Ca oxide and the provender temperatures in this zone range C. Reach calcination in the combustion or clinkering zone and operated temperature at C, chemical reactions lead to the formation of cinder [ 1 ] . Clinker contains different chemical composite compounds and those are tricalcium silicate, dicalcium silicate, tricalcium aluminate, tricalcium alumino ferrite.
At this phase the stuff is in a semi-liquid province, when it is cooled in the cinder ice chest so it is referred to cement cinder. The cinder is so finely grounded with the add-on of 5 % gypsum to give concluding merchandise cement.
During the cement fabrication procedure, it is of import that the temperature of the provender stuff reaches C to obtain the cinder. For accomplishing this clinkering temperature, burning gas temperature must transcend C in the firing zone. The abode clip of the burning gas in the firing zone of the kiln ranges 2-5 seconds depending on the size of the kiln and the abode times of the overall gas during the procedure can make 10 seconds [ 1 ] .
The operation of the cement kiln is in a counter current mode. The flow of burning gas and dust is in counter mode to the stuff, go forthing the elevated terminal of the kiln. Exhaust gases comprises of N, C dioxide, sulfur, H2O and N oxide together with the all right atoms of the provender and cinder stuff. These all right atoms are entrained in the gases fluxing through the kiln and can be removed by utilizing the electrostatic precipitators or baghouse filters fitted in the kiln and return back as provender stuff to the kiln, sold as a cement byproduct to other industries or disposed in a landfill. The alkalic nature of the provender stuff in the cement kiln acts as trap for sulfur dioxide [ 1 ] . Thus, after go throughing through the pollution control devices, C dioxide and H2O vapor which are the combustible gases, get discharged through the stack.
IV. Use of Hazardous Wastes in Cement Kiln
While utilizing the solid and risky wastes in cement kilns, the proficient facets and environmental facets must be taken into history. The proficient facets include the cement kiln features and the benefits while utilizing these wastes in cement kilns.
A. Cement Kiln Characteristics
The major features of a cement kiln which makes it most suited for risky waste direction are
( 1 ) High temperature and long keeping period
( 2 ) Natural alkaline environment
( 3 ) Minimum sum of waste produced
( 4 ) High thermic capacity
The organic risky waste will acquire destroyed at really high temperatures. Hence the cement kiln must be operated at high temperatures to interrupt down those organic wastes. The temperature of stuffs must be at a lower limit of 1500 grades for bring forthing cement cinder and the temperature of burning gas must make 1700 grades. These high burning gas temperatures and long gas residue times combined with intense turbulency inside the kiln guarantee efficient devastation of most stable organic compounds. For most of the organic compounds, the devastation temperature will be 1200 grades for a residue clip of 2 seconds under oxidizing conditions. Hence these conditions of cement kiln are plenty for the solid and risky wastes to acquire interrupt down.
In cement kiln, the substance nature will be alkalic. And some of the risky wastes will be acidic in nature. Hence these acidic gases will acquire neutralized in the kiln with the presence of limestone by the alkaline substances. This makes an added advantage of avoiding any scrubbing devices for neutralizing.
Ash is one of the byproducts produced in cement kiln which need to be disposed. Dust is besides recycled or disposed of. Any noncombustible stuff, such as metals in the waste becomes incorporated in the merchandise.
The big size of cement kiln and the heated stuff inside kiln guarantee that important temperature oscillations in kiln temperature are non possible. Hence if there is an exigency shut down due to operational jobs, the flow of any organic waste can be halted before the temperature autumn below critical values [ 1 ] .
B. Benefits of utilizing cement kiln for waste direction
Some of the benefits of firing wastes in cement kilns are
( 1 ) Energy recovery
( 2 ) Conservation of non-renewable dodo fuel
( 3 ) Decrease in production costs
( 4 ) Use of installations already bing [ 6 ]
In cement kilns, the solid and risky wastes are burned at high temperature and the thermic energy is used to bring forth energy. As energy is recovered from wastes, cement industries are much more interested towards the combustion of risky wastes at that place by cut downing the environmental impacts.
As the risky wastes are used as fuels for combustion, the usage of non renewable fossil fuels can be reduced. If used in adequate sums, the waste can replace fossil fuels such as coal, gas or oil upto 50 % . The fuel cost is the chief portion which determines the entire cost of cinder production. So by utilizing the waste stuffs as fuel, the fabrication costs can be reduced significantly.
Another of import benefit is that the use of bing engineerings. Kiln procedure is already employed in cement fabrication procedure. There is no demand to put in a new works for firing the solid and risky wastes. A new incinerator will be around $ 50 million to build. Hence the cement works is a good option for disposing the risky wastes and there by retrieving energy from that wastes.
V. Environment Aspects
There are some environmental facets to be taken in consideration while utilizing the risky wastes in cement kilns. Some of them are
( A ) Destruction and Removal Efficiency ( DRE ) for Principle Organic Hazardous Constituents ( POHCs ) .
( B ) Destruction and Removal Efficiency ( DRE ) for PCBs.
( C ) Products of Incomplete Combustion ( PICs ) .
( D ) Particulate Emissions.
( Tocopherol ) Acidic Gas Emissions.
( F ) Metal Emissions [ 1 ] .
A. DREs for POHs
Destruction and removal efficiency is defined as the ratio of rule organic risky components come ining the burning device in the waste watercourse to the components go forthing the stack. During trial Burnss, it was found that DRE to be 99.99 % . if there is any fluctuation from this value, so it must be caused by jobs associated with finding of DRE s in cement kilns. For illustration, if the natural stuffs contain compound selected to cipher the DRE, the concentration of this compound will be under estimated. Besides if the compound is present in natural provender come ining cooler terminal, the compound gets volatilized, therefore by doing the DRE degree depression. The choice of POHC which is a byproduct of uncomplete burning produced from conventional fuel leads to low DRE. Even though there are few jobs associated with low DRE, the overall public presentation is rather adequate to run into criterions [ 1 ] .
B. DRE s for PCBs
PCBs are stable compounds and are hard to be destroyed. But cement kilns can destruct PCB which shows its capableness of destructing bulk of organic compounds in risky waste fuel. The trial burn shows the DRE degree for PCBs will be about 99.9999 % [ 1 ] .
C. Products of Incomplete Combustion
These include the dibenzo-p-dioxins ( PCDD ) and dibenzofurans ( PCDF ) . These chemical emanations will be harmful. But trial Burnss did non observe any PCDDs or PCDFs in the stack gas. Combustion temperature is the chief factor which influences the emanation of these gases. As the cement kiln is working at high burning temperature and long gas keeping period, the emanations of PCDDs and PCDFs are non important [ 1 ] .
D. Particulate Emissions
Particulate emanations are caused by firing chlorinated wastes. Hydrochloric acid will be formed due to the combustion of chlorinated wastes and this acerb reacts with alkalic constituents in the kiln to organize base salts. These salts will acquire condensed at cooler terminal. These alkali salts are all right atoms with different electric resistance to normal atoms which will ensue in air pollution. The base salts are carried to aerate pollution control devices by the gases in the cement kiln. The electrostatic precipitator is adjusted to counterbalance the difference in electric resistance. By trial Burnss and by recent trials, when the ESPs are adjusted, the particulate emanation is nil when the chlorinated wastes are burned [ 1 ] .
E. Acidic Gas Emissions
The production of HCL gas by the combustion of chlorinated dissolvers reacts with alkalic environment and is acquiring neutralized. From the trials, it is indicated that upto 95-99 % of Cl come ining the kiln are retained by the procedure solids. The part from constituents such as ammonium, K and Na chlorides are besides makes the degree of chlorine ions to increase.
F. Metal Emissions
As the cement works procedure uses elements from Earth ‘s crust which includes metals, there must be some operational experiences with metals of concern. Metallic elements can be volatile or furnace lining. These stubborn metals have high thaw points and hence is absorbed into the minerals. The consequence of furnace lining metals is relatively low due to the procedure of big sums of natural stuffs. The volatile metals get vaporized and are condensed into a smoke. By the usage of efficient dust aggregation systems, the all right particulates such as limestone and clay atoms are trapped. Lead is a volatile metal caused by firing chlorinated dissolvers. By trial Burnss it is observed that cement kilns can retain 99 % of the lead in the procedure solids. By chemical and physical immobilisation, cement kilns manage the metals in a good method [ 1 ] .
VI. TYPES Of Waste Suitable For Cement Kiln
As mentioned earlier, revolving cement kiln is one of the biggest traveling industrial equipment on Earth with its inside temperature lifting up to 1700oC. For successful processing of cement cinders, it is of import to keep the temperature of natural stuff Federal to cement kiln at 1500oC which can be easy achieved by utilizing conventional beginnings of fuel like coal, oil etc. However, while utilizing risky waste stuff as fuel in a cement kiln, it is of import that the stuff can accomplish the needed temperature degree on burning.
The usage of Hazardous waste as a fuel for cement kiln has been practiced by industries for past three decennaries nevertheless there are still cipher is wholly certain about the effects of incinerating risky waste in cement kiln on environment and human wellness. Harmonizing to a publication [ 8 ] , cement kilns produce grate sum of risky air-borne atoms every bit good as toxic gases like C mono-oxide, when utilizing conventional fossil fuels for the procedure of burning. If Hazardous Waste Materials ( HWM ) are used in topographic point, they will merely intensify the degree of emanation of toxic gases like sulfur dioxide, N oxides and airborne atoms.
Incinerating Hazardous waste in cement kiln is likely the best solution available so far, to dispose HWM, nevertheless in order to avoid any calamity while practising this process, it is of import to comprehend the result of every measure involved in this procedure. First, it is of import to recognize the technological spread between developed and developing states. The cement kilns used in some developed states are technologically beforehand and hence integrate more waste filtering mechanisms which makes them suited to utilize more toxic type of HWM as fuel, whereas cement kilns used in developing states may non hold such engineering to treat toxic waste from incineration of HWM. Hence it is of import to categorize all these treating workss by a individual planetary monitoring organic structure which will tag their suitableness with specific type of HWM.
Harmonizing to Basel ‘s convention, there are assorted classs under which HWM can be defined. Based on their chemical composing, toxicity and physical belongingss, they are placed under Annex I to Annex IV. However, by and large talking, risky waste can be categorised based on their province i.e.
Gas HWM [ 2 ] .
A. Solid Hazardous Waste Material.
For the past three decennaries, cement kilns have been utilizing assorted solid HWM generated from family every bit good as industrial waste. Municipal solid waste, depending on their Cl contain, can cover up to 30 % of entire fuel ingestion. [ 9 ] Similarly, discarded gum elastic Surs have proved to be a better replacement of coal. About 88 % of a Sur is composed of C and O because of which it contains high heating value [ 10 ] . They contain 34900 kJ/Kg of energy, which is 1.25 % more than 27900 kJ/kg of coal [ 11 ] which makes them a better replacement to be used as fuel in cement kilns. Other so these, sewer sludge and exhausted potlinings from aluminum industries are besides being used as fuel for cement kilns.
B.Liquid Hazardous Waste Materials
Assorted industries across the Earth, like paper industry, oil processing workss, cement industries, pesticide fabrication industries etc. produces 1000s of liters of liquid waste as byproduct which is extremely flammable and can used to fix good quality fuel for cement kilns [ 12 ] . Liquid waste like metal cleansing fluid, machining lubricator, waste oil from cars and similar petrochemical waste can supply high burning energy for cement kilns [ 2 ] .
C. Gas Hazardous Waste Materials
Gaseous HWM can besides be used as fuel in assorted cement kilns. Topographic points like landfills and cloacas produce high sum of waste gases like methane which are extremely flammable and can be used as fuel for cement kilns. However, transit of gaseous HWM is a complex procedure and hence requires proper planning, so as to be executed safely [ 2 ] .
Due to their economic benefits, usage of jeopardy waste as fuel for cement kilns is common these yearss. Even though, these installations for disposing HWM are many times bigger and efficient than any standard jeopardy waste incinerator, there are many high toxic HWMs which should merely be disposed utilizing specifically designed incinerators.
VII. Performance Of Cement Kiln
A revolving cement kiln runs 24 hours a twenty-four hours and is merely stopped one time or twice a twelvemonth for indispensable care. This is because chilling and reheating the kiln is both clip and energy consuming procedure. Therefore, any unexpected dislocation in system during production can do a major economic reverse for the production company. A dislocation in fuel flow of the system causes an “ disturbance ” which defines the efficiency of any given HWM. The risky waste used as fuel for these kilns must supply a uninterrupted flow of heat in order to keep the internal temperature of the system i.e. it should bring on minimal figure of disturbances in the cement kiln during a procedure rhythm.
VIII. Recent Developments and Current Status in Hazardous Waste Management
Griffith University has undertaken a research on waste direction in Australia and clearly stated the usage of cement kilns for pull offing risky wastes. There are about 20 cement workss throughout Australia, where most of the wastes are stored. The ground given by the environmental regulators for non developing this engineering in Australia is that the cement industry would non be acute to collaborate. If the authorities make sufficient attack cement industry which did non endanger their concern involvements, they would volitionally accept the petition. The benefits which can be made from waste direction must be given more importance [ 1 ] .
This is the status in many states. Industries are concerned that it will be a load if they start making this waste direction and they have to put more. In Belgium, a cement works owned by Ciments d’Obourg produces about 1360000 metric tons of cinder per twelvemonth. The sum of waste will be about 140000 metric tons of liquid and solid replacements. At nowadays more than half of the energy required for works is generated by the waste direction and energy recovered from that. In Switzerland, Swiss cement comprises of 11 cement cinder bring forthing workss. In 1991, about 15 % of the fuels are replaced by the waste stuffs. Now it ‘s covering more than 25 % of entire fuel ingestion. In US, approximately 25 % of cement workss are firing risky wastes. In 1991, 1.3 million metric tons of risky wastes were burned replacement over 1 million metric tons of fossil fuel. In France, it is calculated that the usage of waste derived fuel in a cement kiln saves 300000 metric tons o fuel.
Since the Kilnburn international conference in 1992, there are several cement industries confident in implementing this engineering in Australia. Some of the cement workss already started utilizing risky wastes as a fuel, which will inturn cut down the fuel cost and pull offing the wastes.
The European Directive on Hazardous Waste Incineration, which was issued in December 1994, recognises that co-incineration in cement kilns is a feasible solution for waste intervention disposal and energy recovery. In France, this Directive has been translated into a new jurisprudence ( December 1996 ) which imposes rigorous environmental control on both the waste classes accepted into cement kilns and the stack emanations allowed. In the USA, the Environmental Protection Agency ( EPA ) has proposed new emanation criterions for cement kilns that procedure risky waste-derived fuel. These emanation criterions will be even more rigorous than those applied to risky waste incinerators.
These ordinances seek to guarantee environmental quality, and command mechanisms that allow cement kiln processing to be a feasible, to the full qualified method for the intervention of, and energy recovery from risky waste, as demonstrated by in-depth surveies and legion pilot tests [ 3 ] .
In this paper, the assorted types of risky wastes, its impact on environment, cement works ; advantages of utilizing risky wastes in cement kiln are depicted clearly.
The chief features of a cement kiln for utilizing risky wastes as fuel are high temperature and long keeping period, natural alkaline environment, minimal waste production and high thermic capacity. The benefits of utilizing risky wastes are energy recovery, preservation of fossil fuels, decrease in production costs and use of already bing installations.
The chief advantages in utilizing cement kiln to incinerate risky wastes are
( 1 ) The neutralisation of acidic gases, sulfur oxides and H chloride by active calcium hydroxide in the kiln burden.
( 2 ) The arrested development of hints of heavy metals in the cinder construction.
( 3 ) By-products such as clinker ashes or liquid residues from gas cleansing are non produced.
( 4 ) Fossil fuel usage is diminishes.
( 5 ) The calorific value from waste is extracted.
( 6 ) There is a high fire temperature of 2000 grades.
( 7 ) Excess O is produced during and after burning.
( 8 ) High turbulency and strong thermic currents are generated [ 1 ] .
Trial consequences shows that over 99 % of lead in waste is retained in the procedure solids. 95-99 % of Cl come ining the kiln is retained by the procedure solids cut downing hazard of HCL emanation to the ambiance.
Initiated during the 1970s as an reply to the crude oil crisis, waste incineration utilizing cement kilns has become a sophisticated engineering conveying together the professional cognize how from both cement and waste profession. The proper direction of wastes in cement kilns would be a benefit cement industry, human life and to the nature. The unwanted and risky wastes are non merely destroyed, but it is efficaciously managed and energy is recovered doing the byproducts risky free. Surely the usage of cement kiln will be a great accomplishment for the interest of worlds and their environment.