Degradation problem, in the waste to energy plants, based on various types of waste as fuels is very serious and still require technological solution in order to improve their efficiency. This study found that in the aggressive environment of waste to energy plants, different coating compositions with the different techniques tested, still did not give the desired results at the elevated temperature and still these plants, are running at very less efficiency as compared to fossil fuel based power plants. It is found from the previous work that the spray coating like APS, HVOF, HVSFS, Cold Spray coatings reduce the surface degradation in the diesel engines, gas turbines, coal gasification plants and chemical plants. Now a days nano-coating give good results to increase the life and performance of surfaces. Nano scale materials have achieved much attention in recent years due to their outstanding properties compared to those of micron-size counterparts. It is found that particle size strongly influences the particle thermal history as small particles rapidly heat up and also rapidly cool down. It also affects the inter lamellar adhesion of the splats and hence influence the mechanical properties of the coating. But the main hurdle in the nano powder coating is that nano-powders are expensive and not available in the market easily and in bulk. So a suitable method has to be found out to manufacture nano-powders in bulk and less expensive, so that they can be used as commercially viable coatings on the surfaces. This paper reviews the previous research work to understand the reason of less efficiency of the WTE plant and different preventive measures used to increase the efficiency of these plants.
The high economic growth rate of the country on a sustainable basis depends upon augmenting of power generation. There is need for stepping up of power generation, to bridge the gap between demand and supply. Since the first and second energy crisis took place in 1973 and 1979 respectively, energy development and management of energy resources is a serious issue [1]. Today, the developing countries like India are passing through an acute shortage of power (electricity). The crisis is becoming more and more serious with every passing year.
The availability of power is an important part of infrastructure which creates external economies for industrial and business units and overcome indivisibilities for future expansions. Thus adequate availability of power is a factor which attracts new investment to a region from regions and areas chronically suffering from power shortage, to double the multiplier effect in income and job generation[2].
The waste to energy plants (WTE) are power generation plants, where the waste burned under controlled conditions and the heat energy from the flue gases generated, is recovered for the generation of electricity [3-6]. Another advantage of these plants is that the volume of original waste reduces by 95-96% which is otherwise dumped in the land or in the rivers especially in developing countries like India [7 8]. In countries like Japan where land is a scarce resource, 80% of general and industrial waste is treated by this technique and it is also widely used in small and densely populated island Taiwan, which have 22 incinerators combusting large amount of municipal solid waste- around 23,250 tons per day [3, 8, 9].
An average of every 100,000 tones capacity of WTE plant, could produce 8 MW of electricity to the grid, which is enough to meet the needs of about 12000 homes. There are 88 waste-to-energy plants in the U.S. and over 600 worldwide. Worldwide the WTE plants combust close to 143 million metric tons or 10% of the global municipal solid wastes (MSW) and generate about 45 billion kWh of electricity an equal amount of thermal energy for district heating and industrial use [5, 10]. One of the advantages of these plants is that the transmission losses would also be minimized. First WTE plant began to emerge in the mid 1960's [11] and in USA. the importance of these plants were recognized in 1991 and in 1993 there were 125 WTE plants with an estimated generating capacity of 2400MW [12].
So there is a need to seriously explore the ways of increasing the power generation capacity by promoting WTE plants. The developing countries like India emphasized on the need to fully explore the potential of generation of energy by (WTE) plants by the year 2020 [NRSE Policy-2006]. A (WTE), 6-megawatt, thermal power plant being run on agriculture waste in Muktsar District of southwest Punjab state of developing country India, generates 160,000 units of electricity everyday. The major fuel used in this plant, are cotton sticks. This WTE plant is not only helping in handling the power crisis in the state but also generating revenue of three dollars per quintal for the by-product, for the farmers. This use of agriculture waste as 'fuel' in power generation also helps in keeping the environment clean. At Kotbhai village in Gidderbaha assembly segment in Punjab state of India, a 300 kW power plant, based on US technology, will be established. After the take off of this project several villages will be grouped into clusters, so as to form viable waste-toenergy units elsewhere in Punjab. Such initiatives will not only help Punjab, but the entire nation to overcome the power crisis, which has been identified as a key infrastructure bottleneck [source: PSEB Patiala].
To fully utilize the potential of energy generation of WTE plants, the efforts are being made to maximize the energy output from the plants [6]. Since the emergence of first WTE plant in the mid 1960's these plants are operating at very less efficiency as compared to the fossil fuel based plants [1, 11, 13, 14]. Worldwide these plants are operating at low efficiency of around 25 % (steam temperature usually kept below 420oC) as compared to the coal-fired power plants with efficiency of around 47% (steam temperature of 580oC) [13] and to achieve the electric recovery efficiency above 31 %, the steam temperature upto 520 oC has to be achieved [14]. In Europe, temperature of steam in WTE plants, increases from 350 oC to 420-440 oC. In Denmark, there are 32 WTE plants working in the steam temperature range of 380-450oC and steam pressure range of 35-75 bar [11]. The main hurdle in increasing the operating temperature and hence increasing the efficiency of WTE plants is very rapid surface degradation due to the fire side corrosion of the boiler tubes at high temperature and this corrosion problem multiplies as the steam temperature is increased [15]. The corrosion problems in WTE industry are usually rather severe [16], due to the heterogeneous nature of its fuel containing alkali metals such as sodium (Na) and potassium (K), heavy metals such as lead (Pb), tin (Sn) and zinc (Zn) and various chlorine-containing compounds, all of which can form potential corrosive agents [4,5]. Also the waste which is used as a fuel can not be selected for composition and consistency.
Several authors [2, 7, 10, 13, 16-22] suggested that the corrosion in WTE plants, is chlorine based called active oxidation and the deposition of chloride salt mixture accelerates the corrosion at low temperature than the sulfate salt mixture, found in fossil fuel based plants and the degradation problem due to this active oxidation in these plants, require technological solutions. Hence, necessity to preserve the good mechanical properties of alloys used in these plants at elevated temperature under the highly oxidizing and corrosive conditions led to the development of coating materials. Also due to continuously rising cost of the base materials as well as increased material requirements, the coating techniques have been given more importance in recent times. It has been learnt from the published literature that the coatings are designed to serve as a reservoir for the elements forming or contributing to form the surface oxides[23-25]. But due to very aggressive environment of WTE plants different coating compositions with different techniques tested, still did not give the desired results at elevated temperature and also no significant work is done in the medical waste based WTE plant. So there is need to seriously explore the nano-structured coatings in various WTE plant environment to further improve the protection against this aggressive environment.
Now a days nano-coating give good results to increase the life and performance of surfaces. Nano scale materials have achieved much attention in recent years due to their outstanding properties compared to those of micron-size counterparts. It is found that particle size strongly influences the particle thermal history, as small particles rapidly heat up and also rapidly cool down. It also affects the interlamellar adhesion of the splats and hence influencing mechanical properties of the coatings [26-27]. But the first step for synthesis of nano-structured coatings is the preparation of nano-structured feedstock powders. There are number of techniques for producing nano-structured materials which can be further used for nano-coatings on the boiler tubes. Some of the techniques are 1) Mechanical alloying/milling, 2) Sol–gel processing techniques, 3) Sputtering, 4) Vapor deposition, 5) Gas condensation, 6) Spray conversion processing, 7) Crystallization of amorphous alloys, 8) Thermochemical method, 9) Electro-deposition [28].
But the main hurdle in the nano powder coating is that nano-powders are expensive and not available in the market easily and in bulk. of most of these techniques, we obtain nano powders in very less amount, as a large quantity of around 500g Powders is needed for a coating. So a suitable method has to be found out to manufacture nano-powders in bulk and less expensive, so that they can be used as commercially viable coatings on the surfaces. The techniques which can be used commercially for the production of large quantities of nano-structured powders and of var ying compositions, are mechanical alloying/milling and thermochemical techniques, The thermochemical technique is generally used for the synthesis of tungsten carbide (WC)–cobalt (Co) nanostructured powders [28].
The mechanical alloying/milling technique was originally developed by the International Nickel Company in 1966 for the production of oxide dispersion strengthened super alloys. A single milling run can produce nano-structured powder in large quantities of around 20 kg, which is enough for the coating on tubes. The mills used for this purpose are vibratory mill, planetary mill, uniball mill and attritor mill. Out of these mills, only the attritor mill has the highest capacity of powder charge, so this mill is usually used for the production of large quantity of powder, in a specific controlled environment. But not much work is reported in the production of different nano-structured powders, which can be used in the coating of boiler tubes, in various aggressive environments, and these powders are not available commercially at low cost in the market[28].
Thermal spray technology has been applied for decades with great success for the on-site coatings, by spray gun. For the thermal spray techniques powders, rods and wires can be used as feedstock materials and the properties of coatings depend upon the type of feed stock materials. The various thermal spray techniques used are High velocity oxygen fuel spraying (HVOF), High velocity flame spraying (HVFS), Flame spraying, Arc spraying, Detonation gun spraying, Continuous detonation spraying, Atmospheric plasma spraying, Twin wire arc spraying, Low pressure plasma spraying or vacuum plasma spraying and Controlled atmosphere plasma spraying, These techniques can be used to spray the corresponding nano-structured coatings. Nano-structured (or nanocrystalline) materials are characterized by a micro structural length scale in the1–200nm regime and for thermal spray techniques the spherical powder particles, with dimensions ranging from 10 to 50μm, are required, which is achieved by agglomeration procedure[28].
Nano-structured (or nano-crystalline) materials have achieved much attention in recent years due to their outstanding properties like having more hardness, strength and corrosion resistance as compared to those of micron-size counterparts. It is found that particle size strongly influences the particle thermal history as small particles rapidly heat up and also rapidly cool down. It also affects the interlamellar adhesion of the splats hence influencing the mechanical properties of the coating [26- 31].
He and Schoemung [28] studied the properties of Cr3C2 - 25(Ni20Cr), Inconel 78 and Inconel 625 nano-structured coatings as compared to conventional coatings. These conventional coatings are used for erosion-corrosion resistance in many applications like power plants. It was found that the nano-structured coating of Cr3C2 - 25(Ni20Cr) exhibits a 20.5% increase in micro hardness of 1020 HV300 as compared with the corresponding conventional coating of 846 HV300. The increase in micro hardness value has also been reported for nanocrystalline Inconel 718. Also the nano-structured coating of Cr3C2 -25(Ni20Cr )exhibits a scratch resistance of 100μm that is twice that of the conventional coating of 50μm.
The coefficient of friction was observed to be 0.216 for nano-structured coatings as compared to 0.495 for conventional coatings. Also the nano-structured Cr3C2 –NiCr coating possesses higher apparent fracture toughness relative to that of the conventional coating.
So, it is found that there is improvement in the properties of nano-structured coatings as compared to the conventional coatings, There is also need to study these coatings at high temperature and in the aggressive corrosive environment like power plants and especially WTE plants in which conventional coatings still do not show the required results against the erosion-corrosion wear of the boiler tubes at high temperature.
It is difficult to handle and process powders composed of nano-sized particles in the range of 100 nm. Nano-scaled particles can easily distribute in air, penetrate the human skin or pass through the respiratory tract and the lungs, finally entering the blood circuit, and can cause serious health problem. The risks of these materials to health is not yet fully examined and well understood [29].
During nano-powder coating by thermal spray process, the feed stock material should exhibit a nano-sized structure to get the properties of a nano-structured coating after the spray process[29]. But a general problem is that during the melting process in the thermal spray process, the agglomerated particles lose their nano-structure, so it is not yet clear if there is any benefit of using nano-structured feed stock materials as compared to conventional powders. Also due to decreasing particle size, the fluidization of the spray powder gets more and more challenging, so it is difficult to decrease the coating thickness below a value in the range of 30 μm by thermal spray process and a reduction of particle sizes in the nano range of below 5 μm needs improvement in powder feeding technique [26].
Also the processing of a nano-powder by means of the standard thermal spray procedure, first of all requires the agglomeration of the nano-powders, in order to form spray particles with appropriate grain size that are suitable for a standard powder feeding device. These powder qualities are yet expensive and not available in the market for every combination of materials [26, 29].
Gadow and Killinger [26, 29] proposed a new Highvelocity suspension flame spraying (HVSFS) technique to spray micron, submicron or nano-particles with hypersonic speed to form thin and dense coating layers. For this purpose, the powder is dispersed in aqueous or organic solvent and fed axially into the combustion chamber of a modified High-Velocity Oxyfuel (HVOF) spray torch. This technique needs to be tested for the coatings in the aggressive environment of WTE plants.
The most recent development work aims at a nanoparticle- based approach of a sealant to close the porosity of thermal spray coatings. In this method nanoparticles are produced by the sol–gel route and the sol is directly applied to the coatings. After gelation the coating is heat-treated at 700 °C so that the nano-particles sinter together to form a continuous layer, which can maintain the protective effect of the coating under the conditions of waste incineration. These coatings are not used much on a large industrial scale. But these high quality composite coatings consisting of a ductile and a brittle phase has high potential to improve the corrosion resistance even in the almost hopeless case of high temperature corrosion [27].
Recently, a new thermal spray process, known as cold gas-dynamic spray or simply cold spray process has been introduced to produce metal, alloy, and composite coatings with superior qualities. The cold spray process was developed in the mid-1980s at the Institute of Theoretical and Applied Mechanics of the Russian Academy of Science in Novosibirsk by Papyrin and colleagues [32]. A U.S. patent on the cold spray technology was issued in 1994[40] and a European one in 1995[33]. Cold spray process uses high velocity rather than high temperature to produce coatings, and thereby avoid/minimize many deleterious high-temperature reactions, which are characteristics of typical thermal sprayed coatings[34,35].
In India, cold spray coatings are nowadays used in very less industrial applications and they are not in use at higher temperature. Therefore, it is foreseen as a viable solution to develop the different coatings using cold spray process, to the boiler tubes used in energy generation systems and to hot sections components in other applications, and to study their oxidation and hot corrosion resistance behaviour according to Indian environmental conditions[36]. .
After studying the literature critically it is found that there is very rapid surface degradation and hence failure of tubes of WTE plants in short duration due to aggressive nature of the environment in the plants and at high temperature above than 5000C.
The following conclusions are derived from this study: