Small-Scale Incinerator for Domestic Hot Water Generation from Municipal Solid Wastes

Transcription

1 European Journal of Scientific Research ISSN X Vol.39 No.3 (2010), pp EuroJournals Publishing, Inc Small-Scale Incinerator for Domestic Hot Water Generation from Municipal Solid Wastes G. O Unachukwu National Centre for Energy Research and Development University of Nigeria Nsukka, Enugu State, Nigeria C. N. Anyanwu Corresponding Author National Centre for Energy Research and Development University of Nigeria Nsukka, Enugu State, Nigeria cnasofia@yahoo.com Tel: Abstract A solid waste incinerator, which can be used for domestic hot water generation was designed, successfully constructed and tested. A simple scrubber was also devised and coupled to the hood for removal of noxious gases and other harmful emissions from the flue gases. Performance testing of the system was carried out while the characteristics of the scrubbed flue gases were monitored using UniGas 3000 series analyzer. Although the concentration of CO was above the allowable limits, most of the results showed that noxious and greenhouse gases in the flue gases are low, an indication that the system is preferable to open air burning. It could be very useful for domestic application in urban and semi-urban settlements, especially in school and college hostels where paper and other non toxic solid wastes are generated in large quantities. Keywords: Municipal solid waste, incinerator, domestic hot water, flue gases Introduction In many developing countries, uncontrolled dumping is often used for the disposal of solid wastes. These dumps are frequently allowed to burn - either deliberately, as a means of volume reduction, or accidentally. Typical materials found in the waste which contribute to harmful emissions include certain plastics, batteries, paints, domestic chemicals, pharmaceuticals and many industrial wastes, which pose environmental hazards. However, the internationally accepted waste hierarchy (Clarke, 2002; Seadon, 2006) recommends minimization and prevention, re-use, recycling and composting as superior to energy recovery, direct incineration and land filling techniques. Although serious efforts have been made in some European countries such as Belgium and Austria where over 50% of MSW is currently recycled, it is noteworthy that many countries still resort to land filling as a major waste management technique. In large cities, mass burning of urban wastes leading to the generation of electricity is possible. However, about 100,000 tonnes of MSW is required to generate 7MW of electricity, which is enough for 10, 000 inhabitants on the average (Kassenberg and de Sutter, 2006).

2 Small-Scale Incinerator for Domestic Hot Water Generation from Municipal Solid Wastes 431 Although re-use of plastic polyethylene bags is quite common in Nigeria, recycling facilities are few and far between throughout the country. As such the proportion of waste being recycled presently is rather difficult to ascertain if not insignificant. The most common method of waste management in schools and urban areas is still incineration and land filling techniques. Controlled incineration could be an effective means of reducing waste volume since it ensures cleaner and more complete combustion of waste and lends itself well to waste disposal in areas where population density is relatively high. This is more appropriate when conceived in a waste to energy conversion system. In this case, potential pollutants can be contained within the resulting residue which, if disposed of carefully, reduces the risk of contamination of local groundwater (Anon. 2007). Waste incineration has been accepted as the most effective way of managing many wastes since it is the only disposal technique recommended for treating all types of hazardous wastes. Municipal wastes are generally classified according to their content as defined by the Interim Guide for Good Practice for Incineration at U.S Facilities (Cheremissnoff and Lagaridis, 1989; US EPA). Waste papers containing about 10 % by weight of plastic bags, coated paper, laminated paper, treated corrugated cardboard, oily rags and plastic or rubber scraps with about 10 % moisture content is quite common in urban centres in Nigeria. It is characterized by a heating value of about 8500 Btu per pound i.e MJ/kg, which makes its incineration as fuel in an energy recovery system quite attractive. Studies on waste composition in some Nigerian cities (Anon. 2007) indicate that about 25% of most urban wastes in Nigeria comprise of paper and other non-toxic materials, which can be combusted in suitably designed incinerators. Presently, these waste materials are either burnt uncontrollably to reduce volume or disposed of in landfills and these have ecological implications: the emissions from this type of uncontrolled burning can be noxious and harmful, whereas availability of sites for landfill in urban cities is rather low. Furthermore, the application of Waste-to-Energy (WTE) techniques - a solid waste management strategy that combusts wastes to generate steam or electricity and reduces the volume of municipal solid waste (MSW) that would otherwise need to be disposed of by up to percent ensures that this fraction of urban waste stream is put to valuable use for domestic water heating. This could alleviate the economic and social cost of producing hot water for domestic use while resolving the problem of waste disposal. Electricity generation using refuse-derived fuel (RDF) processed from municipal solid wastes has been realised in some parts of Europe, although this technology is more suitable for large scale waste handling facilities. As part of the solution to urban waste disposal problems in developing countries, the present work is concerned with the realization of a small-scale, simple and affordable solid waste incinerator at the National Centre for Energy Research and Development, University of Nigeria Nsukka suitable for domestic hot water generation applications Materials and Method Controlled- or starved-air incineration, often referred to as pyrolytic or hearth incineration is capable of converting most organic solid wastes into an inert ash with a weight and volume reduction of about 95% (Cheremissnoff and Lagaridis, 1989). It is essentially a two-stage combustion process whereby waste material is fed and heated with less than stoichiometric oxygen in the primary chamber leading to pyrolytic destruction of most volatile substances. Required endothermic heat for this process is provided by the oxidation of the fixed carbon content of the wastes (Cheremisinoff, 1986). In the secondary (upper) chamber, more air is introduced resulting in more complete combustion, which yields more Carbon (IV) Oxide and water vapour in the stack gases. Removal of the stack gases takes place through the hood after cleaning to reduce the amounts of harmful gases The Incinerator Cold water from the storage tank A is made to pass through a bank of tubes situated just above the combustion section of the incinerator such that the direct heat from waste combustion is utilized to

3 432 G. O Unachukwu and C. N. Anyanwu raise the temperature of the water contained in the tubes to about boiling point (Fig.1). Condensing water flows by gravity into the hot water storage tank B, which is lagged with 2 mm thick polyurethane materials. Flue gases exiting through the chimney undergo scrubbing before exiting finally into the atmosphere Design Considerations Taking economic factors into account namely; the quantity of waste required to operate a continuous incinerator system in addition to its capital cost, it becomes clear that such a system does not match the needs of a residential unit in a typical Nigerian urban setting. A batch process was found to be more flexible in terms of the above mentioned considerations since it is affordable and can easily be operated by a hotel, school hostel, family unit as well as a number of households living in the same quarters. Once refuse is combusted and used to produce hot water, generated hot water can be stored and used for domestic washing and food processing applications. This strategy could resolve the waste problem while reducing over-dependence on electricity, kerosene and fuel wood for domestic hot water generation Description of Water Tank and Combustion Chamber Typically, the average amount of solid refuse generated in most Nigerian cities ranges between kg/capita/day (Anon. 2007). Taking about 25% of this waste (non-toxic component) for an average family size of six persons it is possible to obtain nearly 2 kg of combustible waste every two days. This has an energy equivalent of about 30 MJ, which is capable of generating about 142 litres of water when stored at C assuming 10 % heat losses only (taking feed water temperature as 30 0 C, Cp water as 4220 J/kg.K (Lienhard and Lienhard, 2005) as shown in eqn. 1. This scenario becomes even more attractive when considering refuse generation from students hostels and other similar locations. 3 Q 30x10 m = = = kg (1) CpΔθ 4.22x50 where, m = mass of water (kg) Q = Heat energy (kj), Cp =Specific heat capacity (kj/kg) Δθ =Temperature difference (K). General household wastes comprising mainly of papers, polyethylene bags etc. were used as basis for the present design. The average calorific value of the aforementioned waste lies in the region of MJ/kg. Hence a lower average of 15 MJ/kg was assumed as design value of C f. To determine the amount of heat generated from the combustion of the waste, several trials were made to ascertain the approximate firing rate R of the waste using semi-confined containers to mimic the behaviour of the combustion chamber. On the average, it took about 8-13 minutes (10 minutes average) to completely burn 4kg of waste, equivalent to a firing rate (R) of Mg 4 2 R = = = x10 Kg/s (2) T 10x60 Hence q = RC f = kw (3) Where, Mg = Mass of waste (kg), T = Time (s), q = Heat power kw, C f = Calorific value (KJ/kg).

4 Small-Scale Incinerator for Domestic Hot Water Generation from Municipal Solid Wastes 433 Supplied for a period of 10 minutes, this quantity of heat energy is capable of vapourizing about 21 litres of water at Celcius, considering 10% heat losses and a feed water temperature of 30 0 Celcius (taking l, the latent heat of vapourization of water as 2260 KJ/kg). i.e. Q 0.9* *10*60 m = = = 21.13kg 21litres (ρ=1000kg/m 3 ) (4) C pδθ + l 4.22* The above considerations served as a guide in choosing the storage tank sizes needed for the present work. Thus, two cylindrical drums of about 200 litres (D = 0.7m, h = 0.52m) capacity each were used as storage for both feed cold water and heated water respectively. The hot water tank is well lagged with glass wool, which serves as insulation material and enclosed in a rectangular casing. The detailed drawings of the incinerator system layout and exploded parts are as shown in Figs. 1, 2 and 3 below. Figure 1: Incinerator -System Layout Figure 2: Exploded Parts

5 434 G. O Unachukwu and C. N. Anyanwu 2.4. The Combustion Chamber The primary combustion chamber of a conventional solid waste incinerator is usually lined with refractory material such as ceramics capable of resisting the high temperature in the chamber while reflecting most of the heat back toward the chamber. However, in the present work only the insulation material made of glass wool was employed to reduce heat losses by radiation and conduction from the combustion chamber. The chamber is equipped with a combustion grate to support the waste material as well as primary and secondary air inlets Sizing of Insulation Material To reduce heat losses from the burning waste it was necessary to insulate the combustion chamber with properly selected and sized material. The insulation material is glass wool with thermal conductivity k=0.03 w/mk. Assuming the average temperature of the combustion chamber is 300 o C whereas the outer wall temperature is 30 o C, then applying Fourier s heat transfer equation to Fig. 3 while following the method described by Maloney (1999): dq dt = ka (5) dt dx or θ1 θ4 q = (6) x1 x2 x3 + + k1a1 k2a2 k3a3 Where, dq/dt = q = heat power (flux), x 1 = x 3 = thickness of steel plate (0.002 m) x 2 = thickness of insulator, m. k 1 = k 3 = heat transfer coefficient of steel (45 w/mk) k 2 = heat transfer coefficient of glass wool (0.03 w/mk) A 1 = A 3 = Area of steel plate, m 2 A 2 = Area of glass wool insulator, m 2 θ 1 = flame temperature (300 o C) θ 2 = temperature at the inner metal/insulator boundary ( o C) θ 3 = temperature at the outer metal/insulator boundary ( o C) θ 4 = outer wall temperature (30 o C). Figure 3: Temperature Profile of the Wall of Combustion Chamber θ 1 θ 2 θ 3 θ 4 X 1 X 2 X 3

6 Small-Scale Incinerator for Domestic Hot Water Generation from Municipal Solid Wastes 435 Obviously, θ 1 θ4 2x1 x = 2 k2a2 (7) q k1a1 The quantity of heat power, q was obtained from the initial considerations and substituted in eq.6 such that x 2 = 2 mm was found to be adequate glass wool insulation for the present design Heat Transfer Tubes Water Tube Diameter and Tube Bank Arrangement Each bundle of heat transfer tubes consists of steel pipes with a heat transfer coefficient, k = 45.0 W/mK and a total surface area of m 2. Sizing of the water tubes was carried out considering the heat transfer coefficients. According to Knudsen et al. (1999), the heat transfer coefficient from tube surface to fluid inside the tube, h 1 is given by ( θ ) Vmax h1 = (8) 0.4 D Whereas in the case of heat transfer from combustion gases to the tube, the transfer coefficient, h 2 is given by 0.4 D h = ( θ ) V (9) max where, D = internal diameter of the water tube, m θ = average temperature of the hot water inside the tube and the tube wall, K Vmax = maximum hot water flow rate inside the tube, m 3 /s Assuming a maximum flow rate of 5 litres/s, water temperature of 86 0 C (359k) and tube wall temperature of C (573 k), then θ = = 466k (10) 2 The heat transfer coefficients h 1 and h 2 were computed for different values of the internal diameter, D and the trade off value of D was obtained graphically as the meeting point of the two curves. Thus, D = 1.4 cm (i.e m) The centre-to-centre distance was taken as 4 cm leading to a view factor of 0.9. The bank of tubes is shown in Fig. 3 below. Heated water is allowed to rise through the tube bank and finally escape as vapour through a header into the hot water storage tank.

7 436 G. O Unachukwu and C. N. Anyanwu Figure 4: Bank of Heat Transfer Tubes 2.7. Flue Gas Chimney and Scrubber Stack Gas Chimney Knudsen et al. (1999) recommend that the dimension of the stack gas chimney is dependent upon the flow characteristics of the effluent gases through the chimney according to the following equation: 24000W N Re = (11) D( T ) Where, N Re = Modified Reynold s Number W = Flow rate of stack gases, lb/hr; D = Chimney diameter, in T 1 = Average stack temperature, degrees F. From volumetric estimates based on experimental trials carried out using targeted wastes, it was established that 1 kg of waste yields approximately kg of stack gases. Then, since 4 kg of waste required about 10 minutes to burn completely, it follows that a typical gas flow rate of about 0.24 kg/hr (about 0.53 lb/hr) is possible. Assuming average stack temperature of about 225 degrees Celcius (437 degrees F), the following relationship can be obtain between the Reynold s number and chimney diameter: N Re D = (11) Then, iterations based on values of these parameters as presented in Nomographs (Fig in Knudsen et al. (1999)) yielded the following results: Reynold s number N Re = 5 Diameter of chimney = 2in (0.05m) Friction factor, f = According to established practices, chimney diameter should not exceed 8 % of its height. Thus, d 0.08 L From where the minimum height for the stack of about 62.5 cm is deduced leading to adoption of a design height of 70 cm including the rain cap. To estimate the actual draft, it was necessary to obtain both the theoretical draft as well as the losses occurring in the chimney. Theoretical draft, Pt was estimated using the relationship described by Woodruff et al. (2004) as well as Perry and Green (1984):

8 Small-Scale Incinerator for Domestic Hot Water Generation from Municipal Solid Wastes ΔP t = 0.256LP (12) T T1 Where, L = Stack height, ft (2.310) P = Barometric pressure, in. Hg ( 14.4) T = Ambient temperature, degrees R ( 546) T 1 = Average stack temperature, degrees R ( 897). The result obtained using equation 12 is in inches and had to be converted to mm of H 2 0. Hence, a theoretical draft of mm H 2 O was calculated. Draft loss is given by the equation: 2 u fl ρ g Δ P = 1 + f (13) 2g D 5. 2 Where, u = stack exit velocity, ft./s ( assuming ρ gas of 1.20 kg/m 3 ) g = dimensional constant (32.17) f = friction factor (0.025) L = stack height, ft (2.310) D = stack diameter, ft. (0.165) ρg = average density of stack gases, lb./cu. ft.(0.075) Again the obtained result was converted from inches to mm. This gave rise to draft losses of about mm water (5.35 x 10-6 in.), which is negligible compared to the theoretical draft. The actual draft for the chimney is therefore, about mm water Performance Testing Performance evaluation of the realised system entails incinerating typical college hostel wastes in the system and determining the quantity and temperature of the generated hot water as well as the chemical composition of the scrubbed flue gases leaving the system. Solid waste used for performance evaluation of the incinerator was collected around the University of Nigeria where the experiments were also carried out. 5.0 kg of the solid waste comprising mostly of papers, cartons and polyethene sachets commonly used as packaging for water in Nigeria was fed into the combustion chamber and ignited while the feed storage tank is filled with cold water initially at 28 o C. The waste was allowed to burn completely whereas some escaping flue gases were trapped in a sample bottle during the process. At the end of the combustion process, the quantity and temperature of the water collected in the hot water storage tank were measured, while collected stack gas was analysed using a UniGas 3000 gas analyser (product of E Instruments Group LLC USA) in order to determine the quantities of its constituent gases Results and Discussion The average temperature of the stored, generated, hot water was found to be between 75 and 82 degrees C, whereas the composition of the flue gases were as presented in Table 1 below. This indicates that the system can be used for waste disposal, especially in college hostels and urban settlements, where large quantity of waste is often generated. It is evident that the concentration of CO in the flue gases is well above the standard requirement stipulated by the Nigerian Federal Environmental Protection Agency, FEPA (Orubu, 2004), showing that the O 2 provided in combustion chamber could be less than the stoichiometric value. Also, the CO 2 and NOx concentrations are above the stipulated values according to FEPA standards, an indication that the scrubbing system, which was a metal gauze filter needs to be replaced with a more efficient filter, preferably impregnated with chemical reagents. However, the NO 2 concentration was well below the allowable limit. The

9 438 G. O Unachukwu and C. N. Anyanwu concentration of dioxins, HCl and other harmful chemicals were not quantified due to lack of monitoring instrument. However, since further work on the system is envisaged, these important parameters would be determined before the system can be recommended for large scale application. Table 1: Concentration of Stack Gases Parameter Value FEPA Standard* O 2 (%) 20.5 NA CO (%) 0.05 (500 ppm) 10 ppm CO 2 (%) 0.3 NA NO (%) (8 ppm) NA NO 2 (%) 0 NA NOx (%) (8 ppm) ppm Air Temperature (degrees C) 32.5 Efficiency (%) 79.8 * Cited from Orubu (2004) 5.0, Conclusions Waste heat from a simple solid waste incinerator was successfully applied for hot water generation at the University of Nigeria Nsukka. The energy recovery incinerator was designed and constructed using available construction materials making its maintenance easy. Typical University hostel waste materials were combusted to generate hot water during the performance testing of the device. Test results indicated that the system can be used for waste disposal and energy recovery in form of hot water although the flue gases are not safe for the environment since the concentrations of certain harmful gases were above the recommended standards, an indication that further work is required both in the combustion chamber and the scrubbing system before it can be recommended for large scale application.

10 Small-Scale Incinerator for Domestic Hot Water Generation from Municipal Solid Wastes 439 References [1] Anon. (2007). Solid Waste Management in Sub-Saharan African Emergencies. Waste Management in Emergencies Group. Accessed on 10 th April, [2] Cheremisinoff, P. N and A. A. Lagaridis. Incineration for Pathological Waste Disposal, Ch. 7 in Thermal Treatment of Hazardous wastes, Vol. 1 of Encyclopaedia of Environmental Control Technology, Gulf Publ. Coy. Houston, Texas [3] Cheremisinoff, P. N. Special Report: Treatment of Hazardous Wastes, Pollution Engineering, Nov [4] Clarke, M. J. (2002).Introduction to Waste Prevention and Recycling. Paper No In Proc. Air and Waste Management Assoc. Annual Meeting. Baltimore MD June 23-27, P.17. [5] Kassenberg, A. and R. de Sutter (2006). Strategic Evaluation on Environment and Risk Prevention under Structural and Cohesion Funds for the Period national Evaluation Report for Poland. [6] Knudsen, J. G; Hottel, H. C; Serafim, A. F; Wankat, P.C and Knaebel, K. S (1999). Heat and Mass Transfer, Ch. 5 in Perry, K. H; Green, D. W and Maloney, J. O. (Ed). Chemical Engineers Handbook, 7 th Ed. McGraw-Hill Companies Inc. New York, [7] Lienhard J. H (IV) and Lienhard J. H (V). A Heat Transfer Textbook, 3 rd Edition. Phlogiston Press, Cambridge Massachusetts, [8] Maloney J. O. Conversion Factors and Mathematical Symbols. Ch. 1 in Perry, K. H; Green, D. W and Maloney, J. O. (Ed). Chemical Engineers Handbook, 7 th Ed. McGraw-Hill Companies Inc [9] Orubu, C. O. (2004). Using Transportation Control Measures and Economic Instruments to Reduce Air Pollution due to Automobile Emissions. Journal of Social Science 8 (3) Pp [10] Perry, R. H. and D. W. green. Perry s Chemical Engineers Handbook, 6t Edition. 1984, Page McGraw Hill Book Coy. ISBN [11] Seadon, J.K.(2006). Integrated Waste Management Looking beyond the Solid waste Horizon. Waste Mgt. Vol. 26 (12). Pp [12] U.S. Environmental Protection Agency, Office of Solid Wastes: EPA Guide for Infectious Waste Management, Washington D. C [13] Woodruff, E. B., H. B. Lammers and T. B. Lammers. Steam Plant Operation, 8 th Edition. McGraw Hill Professional ISBN