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Use of thermal energy from waste for seawater desalination
DESALINATION ELSEVIER
Desalination 130 (2000) 137-146 www.elsevier.com/Iocate/desal
Use of thermal energy from waste for seawater desalination D. Dajnak, F.C. Lockwood* Department of Mechanical Engineering, lmperial College of Science, Technology and Medicine, London, SW7 2BX, UK Tel. +44 (171) 594-7032; Fax +44 (171) 581-5495; email:[email protected] Received 17 March 2000; accepted21 June 2000
Abstract
Municipal solid waste (MSW) has a significant calorific value. Increasingly, its thermal energy is being recovered by combusting or gasifiying MSW for electricity generation and district heating. However, many communities in arid regions have combined waste disposal and water shortage problems suggesting the use of the thermal energy inherent in MSW to desalinate seawater. This paper makes an initial assessment of this concept and concludes that useful quantities of fresh water can be so produced. Further study is required to optimize the waste to water conversion and to assess the economy of the concept relative to competing desalination energy sources. Keywords: Waste; Incineration; Energy; Desalination; Reverse osmosis
1. I n t r o d u c t i o n
Because of urbanization, population increase and new regulatory measures, many countries are faced with a waste disposal problem. Hot countries also have a water resource problem. Indeed, fresh water is becoming an ever-more precious commodity, again because of increasing population and prosperity and because the arid regions o f the world appear to be expanding. The desalination process consumes energy while waste may be transformed to produce electric and thermal energy. It is suggested that there must be
locations where it would be sensible to link the two processes, for example the many leisure communities which are developing in Egypt remote from the Nile. These communities have, because of their isolation, combined waste disposal and fresh water availability problems. Additionally, their scale is small enough to allow verification of the proposed hybrid technology for acceptable cost. It is not hard to identify other candidate sites, the typical Mediterranean island, for example. This paper makes a preliminary analysis to assess the feasibility of the waste to water concept.
*Corresponding author. 0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved PII: S0()I 1 - 9 1 6 4 ( 0 0 ) 0 0 0 8 1 - 3
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D. Dajnak, F.C. Lockwood/Desalination 130 (2000) 137-146
2. Incineration of municipal solid waste
The typical present-day per capita content of the "urban Western dustbin" just prior to the weekly collection is substantial. How much paper, cardboard, food residue, plastic, glass, tin cans, dust and so on does each one of us put out for disposal? Multiply this by the number of households of the whole community and one has some idea of the considerable quantities of domestic waste to be disposed of every week. Added to this is the waste from offices, restaurants, hotels, markets and other establishments. All of these refuse streams constitute what is generically known as municipal solid waste (MSW) [1]. In 1996, the average quantity of MSW generated by every urban inhabitant per year was 490 kg in the UK, with an EU average of 447kg [2]. On a nation-wide basis the UK, for example, disposes of around 25 Mt of household and commercial wastes each year, together with a similar quantity of industry-generated waste and much smaller quantities of specialized wastes such as solvent and hospital waste. The thermal energy of waste may be recovered by combustion, gasification, or pyrolysis. The combustion of waste, more frequently referred to as "incineration", is the process by which waste energy is converted into electricity via a boiler to raise steam and a turbine/generator set which converts this into electricity. The combustion process will form the basis of this study because it has already been applied. Gasification and pyrolysis technologies, although somewhat more complicated, have the potential to recover more of the thermal energy in a cleaner manner, but this has yet to be demonstrated. Worldwide, very little of the energy potential of waste is currently exploited. The energy content of MSW is significant. With reference again to the UK, if all its MSW energy potential were recovered by efficient combustion systems, it would equate to an electricity generating capacity of nearly 4,000MW.
The incineration of refuse with recovery of its thermal energy is therefore an attractive disposal option provided that it is effected cleanly and that the economic incentives are sufficiently appealing. Principal among the economic considerations is that incineration plant should be located in an urban area near to an energy user such as a district heating complex, an industrial company, or a desalination plant, in order to minimize transportation costs. A further economic factor relates to the possibility of splitting the energy output between the diverse consumer groups depending on seasonal and other influences.
3. Review of seawater desalination methods
The fundamental nature of the world's weather results in large expanses of arid and semi-arid lands in the middle latitudes. The population explosion has given rise to a demand for people to live in these areas, most often requiring freshwater supplies larger than local sources can produce [3] or in the case of lowlying terrain with no natural basins, more than can be stored. As a result of the development of arid regions, and also because of intensifying use of water in urban areas all over the world, fresh water is frequently not available in the quantities desired. Since about 1950, desalination has become increasingly important where local supplies of high quality freshwater are less than adequate. Desalination is generally defined as a process that removes dissolved minerals (including but not limited to salt) from seawater, brackish water, or treated wastewater. A variety of desalination processes has been developed, and in 1993 more than 15 Mm3/d of fresh water were produced by the desalination industry worldwide [4]. At present, distillation, electrodialysis and membrane techniques are favoured and are the three principal competing processes, the choice being site dependent. These technologies are summarized below.
D. Dajnak, F.C. Lockwood~Desalination 130 (2000) 137-146
1. Distillation is the most well known and longest used desalination process. The principle of all variants is that the vapour produced from the boiling saline water is free from minerals and pure water is obtained when the vapour condenses. There are two major kinds of distillation processes: multiple effect distillation (MED) and multistage flash (MSF). In MED the evaporators are in series, each evaporator of the series being termed an "effect". Vapour from one effect is used to evaporate water in the next lower pressure effect. In MSF, intake water is heated, then discharged into a chamber maintained slightly below the saturation vapour pressure of the incoming stream so that a fraction of the water content flashes into steam. The steam condenses on the exterior surface of heat transfer tubing and becomes product water. The unflashed brine enters another chamber at a lower pressure where a further portion flashes to steam. 2. Electrodialysis is a process by which the ions forming the salt are pulled out of the saline water by electrical forces and concentrated in separate compartments. The higher the salinity of the raw water, the more electrical power needed for this process. 3. Reverse osmosis (RO), also called hyperfiltration and ultrafiltration, involves forcing water molecules through a semi-permeable membrane. The salt molecules are unable to transgress the membrane, and the water molecules that do form a potable product. In contrast to thermal desalination processes with phase transition and consequent high specific energy inputs, the RO processes relies on mechanical energy (pressure) and the specific energy requirement is relatively low [5]. A further method, to date little applied, should be mentioned: Crystallization exploits the fact that the phase transition from liquid to solid produces ice crystals that contain only pure water and no mineral salts, so that fresh water is produced when they melt. A principal advantage of this method is that the low temperatures
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involved greatly reduce the corrosion problems of the "hot" techniques. Until about 1980, distillation was the preferred method for desalinating seawater. However, because of the economic attraction of hyperfiltration, by 1986 almost one-half of the contracted desalination capacity for the international market was represented by RO plants. In addition to the desalting technology, the energy requirement of a desalination installation depends on the salinity and temperature of the feed water, the quality of the water produced and other sitedependent factors. Because of these variations, it is difficult to obtain sufficient data to enable a precise comparison of the energy needs of the different desalination methods. However, it appears that for seawater desalination, the competing processes are MSF, MED and RO, while for brackish water the competing processes are RO and electrodialysis [4]. We have not been able to find in the literature performance figures for a full-scale crystallization plant.
4. Pollution issues
Waste disposal and minimization of pollution are key problems that must be addressed for cities of the future. Sulphur pollution emitted from power stations and volatile organic components from automobile exhausts form urban "smog" in the presence of sunlight in many major cities, and this fact more than any other has contributed to public awareness and concern about combustion processes. The incineration process has the potential to generate the same general pollution problems as other combustion processes: NOx, SOx, CO, CO 2, HCI, volatile organic compounds, etc. However, there are additional environmental issues specific to MSW plants such as the emissions of trace organic compounds, particularly polychlorinated dioxins and furans, and also toxic metal trace elements such as mercury, lead and cadmium. There have been one
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D. Dajnak, F. C Lockwood / Desalination 130 (2000) 137-146
or two well publicized emissions incidents occurring with old chemical plant such as the one in Seveso, in Italy, where the local inhabitants were contaminated with the potent toxicant dioxin, released into the area as a result of an industrial accident in 1976. It is therefore not surprising that the public image of incinera-tion, especially when an association is made with dioxine release, is particularly bad. However, stringent regulation and advanced pollution abatement technology have resulted in modem plants which are extremely clean and which are increasingly replacing the problematic landfill alternative. Extracting energy from waste offers clear environmental benefits, for example the reuse of materials, the reduction of fossil fuel consumption and the elimination of methane release from landfill sites. Another major advantage of incineration is the substantial bulk reduction, commonly of the order of 90% of the original volume and 60% of its weight [1]. Modern MSW systems use good combustion practices to destroy most organic compounds during the combustion process [6]. Regulatory minimum temperatures and residence times are specified to ensure the elimination ofpolycyclic chlorinated compounds and volatile organic compounds. The combustion process is followed by gas cleaning to ensure that harmful elements such as hydrochloric acid and particulates are not emitted. Although most trace elements can be controlled by using efficient particulate removal devices, carcinogenic metals such as Cd and Pb are enriched in the submicron particles which are difficult to capture by conventional flue gas cleaning equipment. Conventional cleaning equipment is also very poor in removing the volatile elements, particularly the toxic metal mercury. New techniques such as cleaning devices using sorbents have been developed. In addition to flue gas clean up, other discharges are either being eliminated or recycled, for example the ashes may be converted to usable aggregate
for products in the construction industry. Other ash treatment methods are solidification, ash melting or vitrification and extraction/recovery processes [7]. In summary, pollution control constitutes a major proportion of the cost, technological sophistication and space requirement of the modern incinerator. Older plants built before about 1985 represent older technology where the gas clean-up consisted mainly of particulate removal systems such as electrostatic precipitators. There was little public or legislative concern over acidic gas emissions, heavy metals such as cadmium and mercury, or organic micropollutants such as dioxins and furans and polycyclic aromatic hydrocarbons. Consequently, municipal waste incinerators deservedly developed a poor image as pollution sources. However, as can be seen from Table 1, the modern incinerator built in the late 1980s and 1990s is an efficient combustion plant with a gas clean-up system complying with the highest standards [8]. At the same time, the older polluting incinerator plants have largely been decommissioned as the introduction of stringent emissions legislation has been enacted throughout the world. Desalination plants produce liquid wastes that may contain all or some of the following constituents: high salt concentrations, defouling and pretreatment chemicals, and toxic metals (which are most likely to be present when the discharge water contacts certain metallic materials which may constitute the fabric of the plant). Liquid wastes may be discharged directly into the ocean, combined with other discharges (e.g., power plant cooling water or sewage treatment plant effluent), discharge into a sewer for treatment in a sewage treatment plant, or dried out and disposed of in a landfill site. Desalination plants also produce a small amount of solid waste (e.g., spent pretreatment filters and solid particles that are filtered out in the pretreatment process).
D. Dajnak, F.C Lockwood~Desalination 130 (2000) 137-146 Table 1 Emissions to the atmosphere comparing older municipal plants with typical modern plants, and citing current concentration limits for release of pollutants into the air
Emission, mg/m3
Particulates CO HCI SO2 HF NOX As, Cr, Cu, Pb, Ni, Sn Cd Hg Dioxins, ng/m3
United Kingdom
Regulation a
Older plant
Modern plant
16-2800 6-640 345-950 180-670 --0.1-50
1.4 9 11 8 0.01 370 0.05
<0.1-3.5 0.21-0.39 0.73-1215
0.001 0.1 0.005 0.1 0.02-0.1 1
30 -30 300 2 350 1
aFigures apply to MSW incinerators of greater than 1tonne/h capacity regulated under Integrated Pollution Control by the Environment agency; smaller units are dealt with by local authorities [15].
5. Energy generated from MSW In broad terms four tonnes of MSW contains as much energy as 1 tonne of coal. This fact was first recognized in 1895 when the very first facility appeared in Oldham, Lancashire, UK. Since then, the technology and efficiency of waste to energy plants have taken dramatic steps forward [9]. The conversion of MSW to energy can conserve more valuable fuels and enhance the environment by lessening the amount of waste that must be landfilled and by conserving energy and natural resources. One route to utilizing the energy value of MSW is to burn it in a steam power plant to generate electricity. As their country of residence, the UK again constitutes a convenient example for the authors (we
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apologize for this and recognize that the desalination needs of our country are negligible with fresh water on most days filling the very air!). The total amount of municipal waste generated in the UK including industry-generated waste is currently about 50 million tonnes per year, which rather amazingly is roughly just short of 1 tonne per citizen! This has an energy equivalent of 10 to 15 million tonnes of coal and represents about 5% of the entire energy needs of the country [ 10]. The average gross calorific value of mixed MSW is about 6500-10,000kJ/kg. As an example, a typical present-day incinerator burning 200,000 tonnes of MSW per year would produce 21.5MW of electricity. However, in large towns such as London and Birmingham, sufficient waste is generated to support several modern facilities, each capable o f treating 400,000 tonnes per year [11]. At present, the UK's generating capacity of electricity from municipal and industrial waste (MIW) combustion is only about l l5MW, but this is expected to rise rapidly. The technology is already mature and has been extensively demonstrated in overseas markets. For example, over 50% of the MSW in Switzerland, Belgium and Sweden is now disposed of by incineration. In Japan and Luxembourg the figure is 75%. By the year 2025, it is anticipated that the energy recovery from municipal and general industrial waste combustion in the UK could contribute an energy equivalent of about 1400MW of electricity [12]. In the US, the use of incineration to convert waste into electricity has risen from 13% to 19% in the last 10 years. Table 2 shows the high calorific value and electricity generated by 1 kg of various solid fuels such as oil, natural gas and coal. Indeed, Egypt and the other middle east countries have supplies of internal oil and natural gas. However, increasing prosperity and environmental concern have brought waste disposal problems to Europe. Many areas of Southern Europe suffer seasonal
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D. Dajnak, 1£.C. Lockwood~Desalination 130 (2000) 137-146
Table 2 Calorific values of various fuels Calorific value, Electricity MJ/kg produced, MJ/kg Coal Saw dust Eucalyptus wood Sewage sludge Oil Natural gas MSW
30 18 17.6 12.6 42.8 50.7 8.5-12.8
12 7.2 7 5 17.1 20.3 3.4-5.1
water shortage difficulties, a problem which is also exacerbated by increasing prosperity and the associated more liberal usage of water. Application of the waste-to-water technology of the present study could therefore be of direct benefit to Europe. Consequently, since the European countries such as UK still use a large quantity of coal to generate energy, it is also important to compare the calorific value of MSW with coal. A typical calorific value for MSW given as a gross calorific value of 8.5 MJ/kg by the Royal Commission on Environmental Pollution [8] is used in this table. It has been assumed that the conversion efficiency of calorific value to electrical energy is 40%, a value achievable by a well conceived incinerator.
6. Waste required to desalinate seawater Amongst the various desalination techniques referred to above, RO has been widely used in recent years, mainly due to the development of membrane technology and its low energy requirements [13]. RO plants usually have a lower energy requirement. However, it is difficult to effect a precise comparison of the energy needs of the desalination methods because of site-dependent factors such as salinity and temperature of the raw material and because
some plants are powered solely by electricity while others consume combined electricity and thermal energy. RO can be started up and shut down without a major time lag while evaporator systems involve a large heat sink resulting in extended start-up times and high transient losses. RO plants have fewer problems with corrosion and take up less surface area than distillation plants for the same amount of water production. The RO process can remove unwanted contaminants such as pesticides and bacteria. An additional consideration is that the RO technology separates the power generation and water desalination processes. Therefore, these two functions can be performed at separate locations, with each function using the technology optimized for the task [ 14]. In view of all of these considerations, we believe that RO is currently the preferred desalination technology to be linked to waste incineration. In this analysis we choose to base our calculations on electricity generation by the MSW incineration plant and the use of this electricity to power a desalination plant. However, since the RO process requires pressure, some form of direct mechanical coupling may prove more efficient. The electrical consumption of RO plants can vary from 3 to 9kWh (pumping power) per m 3 of fresh water produced. Table 3 gives the quantity of fresh water produced by RO using the electrical energy created by various solid fuels assuming a RO power consumption of 5 kWh/m 3of fresh water produced. We see, rather remarkably, that 188L of fresh water can be produced by the desalination of seawater by incinerating just 1 kg of waste. In other words, 1 tonne of waste is capable of producing 188 m 3 of fresh water. The per capita consumption of water can vary considerably depending on the wealth of the locality and a variety of cultural factors. If we again consider the UK, the yearly average of residential water consumption for domestic and service community uses is some 151m 3 per
D. Dajnak, F.C. Lockwood~Desalination 130 (2000) 137-146
Table 3 Solid fuel to water conversion coefficients using a RO system Production of fresh water by RO, litres of water per kg of fuel Coat Sawdust Eucalyptus wood Sewage sludge Oil Natural gas MSW
667 400 389 278 951 1128 188
~/x,4
Fig. 1. Overall picture of the generation of fresh water for domestic and public consumption by recycling waste.
person. By burning 490kg, the average amount generated per UK inhabitant per year, a MSW incinerator could generate 1666 MJ of electricity. Referring to Table 3, this converts to 92 m 3 of fresh water by desalination in a RO plant, or well above half the per capita consumption. An overall
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picture of the generation of fresh water for domestic and public consumption by recycling waste is seen in Fig. 1.
7. Sample MSW incinerator in London The Southeast London combined heat and power (SELCHP) facility is a modern selfcontained waste incinerator located in a residential area of London where it has received wide public acceptance. It utilizes advanced technology to recover the energy contained in raw waste. SELCHP is designed to burn over 400,000tonnes of refuse each year and has the capacity to generate a valuable 32MW of electricity. At a later stage it could also provide heat for over 7500 homes. The combustion process is strictly controlled to respect the level of emission fixed by the UK government. A flue gas treatment system removes most pollutants before ejecting clean gases into the atmosphere via a 100-m chimney. The flue gases are free from odours. Sound emission is tightly regulated to the extent that the background noise level of the locality is unaltered. The plant is architecturally pleasing and a carefully planned traffic scheme ensures no local disruption. The combustion process utilized by SELCHP produces residues amounting to around 10% of the original waste volume. Largely made up of ash, the residues are biologically inactive, as well as being virtually inert and odour free. Most importantly, the residues are consistent in quality, easily managed and have potential for recycling in construction applications. Magnetic recovery of ferrous metals from the ash residue is effected. The convenient position of the plant makes it a realistic disposal point for waste generated throughout central, northeast and southeast London. The SELCHP plant demonstrates that waste incineration can be effected in an urban area in a manner which is both clean and nonintrusive. SELCHP cost £85 million (E142
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19. Dajnak, F C Lockwood~Desalination 130 (2000) 137-146
Fig. 2. SELCHP incineratorin central London. million) to design and construct in 1994 [9]. l f a similar incinerator were built in a community near a RO desalination plant, its energy output could produce about 75 M m3/y of fresh water. Fig. 2 shows a diagram of the SELCHP incinerator in central London [9].
8. Project realization The above brief analysis is very preliminary, but it does show that the concept of waste to water is worthy of further consideration. The present study has examined the straightforward coupling of state-of-the-art technologies. However, there are no doubt ways in which the coupling itself can be improved to yield overall efficiency gain. For example, since the RO technique is pressure driven, is it necessary to employ the medium of electricity to generate the pressure? Also, we have made no provision for the recovery of low-grade heat. It is unlikely in
countries suffering from a water shortage that there will be much call for district heating! However, there may well be some way of enhancing the process yield through recovery, rather than rejection, of the low-grade energy. This lowgrade heat could possibly be used for airconditioning and refrigeration. Clearly, there is scope for innovative thinking and supporting process efficiency calculations. Once optimized routes have been identified, detailed economic and environmental evaluations would follow. In this respect it is particularly important to assess the cost of the proposed technology relative to conventional treatment of waste and the use of solar and wind energy for desalination. If after such studies the technology maintains its early promise, a pilot plant might be constructed, especially if a novel coupling scheme has been proposed. With or without this stage, a sitespecific design exercise would follow. The chosen site would in the first instance preferably be rather small and self contained. So for
D. Dajnak, F. C. Lockwood~Desalination 130 (2000) 137-146
example, one of the many developing Egyptian resort communities remote from the Nile would be a candidate, or a Mediterranean island. At this stage some o f the factors to be considered are: Who will own and operate the plant? What type of waste is available? What quality of desalinated water is required? What is its required quality? What will be the cost to end-user? The subsequent steps leading to a working plant are clear: procurement o f finance, detailed design preparation, construction and commissioning.
9. Conclusions Recovery of thermal energy from municipal waste makes good environmental sense. Waste combustion reduces the loss of space to landfill sites, it reduces the environmental impacts caused by gas and leachate formation from landfilled raw organic waste, and it contributes to reducing reliance on fossil fuel. The recovery process normally involves the generation of electricity, possibly combined with district heating. We have shown that thermal energy can be used to generate considerable quantities of fresh water, more than half the typical per capita daily need, simply by coupling the existing waste incineration and desalination technologies. An improved recovery rate is no doubt possible by innovative coupling. We have used incineration, i.e., combustion, as a basis for comparison. Pyrolysis and gasification waste transformation processes are increasingly being examined and they may provide additional economic and environmental advantages. The RO desalination process has very little air and water environmental impact and can, like modern incinerators, be located near or within population centres. Whether or not the proposed concept can compete against the separate generation of electric power and desalination from other renewables such as wind or solar energy, which
145
is intermittent of course, needs to be demonstrated. Our intention in this paper has simply been to show that the basic concept has merit and is worthy of further study.
Acknowledgements The authors would like to thank Prof. M.N. Damir, President of Alexandria University and chairman o f ADST (the Alexandria University Desalination Studies and Technology Centre) and Dr. M. Abdel Rahman for the opportunity to present and discuss this study at the International Desalination Workshop in Alexandria in 1999. The authors gratefully thank Dr. P. Costen for his scientific discussions concerning the MSW matter.
References [l] W. Hall and M. Keynes, Unit 14, Municipal Waste Management,The Open University, 1985. [2] K. Kuchta and J. Jager, 2"dInternational Symposium on Incineration and Flue Gas Treatment Technologies, Sheffield,UK, 1999. [3] E.D. Howe, Fundamentals of Water Desalination, Marcel Dekker, New York, 1974. [4] K.S. Spiegler and Y.M. EI-Sayed, A Desalination Primer, Balaban Desalination Publications, Santa Maria Imbaro, 1994. [5] H. Hams-Gunter, Saline Water Processing, VCH, Weinheim, 1990. [6] L.A. Ruth, Prog. Energy Combust. Sci., 24 (1998) 545. [7] S. Sakai, S.E. Sawell, A.J. Chandler, T.T. Eighmy, D.S. Kosson, J. Vehlow, H.A. van der Slott and O. Hjelmar, Waste Management, 16 (1996) 341. [8] P.T.William, Waste Treatmentand Disposal, Wiley, New York, 1998. [9] SELCHP, A new direction in waste disposal for London, turning waste into energy, company informationbrochure, 1994.
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[10] J. Swithenbank, V. Nasserzadeth, A. Wasantakoran, P.H. Lee and C. Swithenbank, Future integrated waste, energy and pollution management (WEP) systems exploit pyrotechnology, 2nd International Symposium on Incinerator and Flue Gas Treatment Technologies, Sheffield, UK, 1999. [11] N. Patel and I. Higham, MSW combustion: economics and projections for energy recovery to the year 2000, Energy Technology Support Unit, Harwell Laboratory, 1995.
[12] Department of Track and Industry, Energy from Waste, programme brochure, UK. [13] D. Voivontas, A. Zervos and D. Assimacopoulos, Desalination, 21 (1999) 159. [i4] I. Moch, Jr., F. Depenbrock and Y. Mussalli, Water, Air Soil Poll., 90 (1996) 231. [15] Chief Inspector's Guidance to Inspectors, Process Guidance Note IPR 5/3, Waste Disposal and Recycling, Municipal Waste Incineration.Department of the Environment, HMSO, London, 1995.
Desalination 130 (2000) 137-146 www.elsevier.com/Iocate/desal
Use of thermal energy from waste for seawater desalination D. Dajnak, F.C. Lockwood* Department of Mechanical Engineering, lmperial College of Science, Technology and Medicine, London, SW7 2BX, UK Tel. +44 (171) 594-7032; Fax +44 (171) 581-5495; email:[email protected] Received 17 March 2000; accepted21 June 2000
Abstract
Municipal solid waste (MSW) has a significant calorific value. Increasingly, its thermal energy is being recovered by combusting or gasifiying MSW for electricity generation and district heating. However, many communities in arid regions have combined waste disposal and water shortage problems suggesting the use of the thermal energy inherent in MSW to desalinate seawater. This paper makes an initial assessment of this concept and concludes that useful quantities of fresh water can be so produced. Further study is required to optimize the waste to water conversion and to assess the economy of the concept relative to competing desalination energy sources. Keywords: Waste; Incineration; Energy; Desalination; Reverse osmosis
1. I n t r o d u c t i o n
Because of urbanization, population increase and new regulatory measures, many countries are faced with a waste disposal problem. Hot countries also have a water resource problem. Indeed, fresh water is becoming an ever-more precious commodity, again because of increasing population and prosperity and because the arid regions o f the world appear to be expanding. The desalination process consumes energy while waste may be transformed to produce electric and thermal energy. It is suggested that there must be
locations where it would be sensible to link the two processes, for example the many leisure communities which are developing in Egypt remote from the Nile. These communities have, because of their isolation, combined waste disposal and fresh water availability problems. Additionally, their scale is small enough to allow verification of the proposed hybrid technology for acceptable cost. It is not hard to identify other candidate sites, the typical Mediterranean island, for example. This paper makes a preliminary analysis to assess the feasibility of the waste to water concept.
*Corresponding author. 0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved PII: S0()I 1 - 9 1 6 4 ( 0 0 ) 0 0 0 8 1 - 3
138
D. Dajnak, F.C. Lockwood/Desalination 130 (2000) 137-146
2. Incineration of municipal solid waste
The typical present-day per capita content of the "urban Western dustbin" just prior to the weekly collection is substantial. How much paper, cardboard, food residue, plastic, glass, tin cans, dust and so on does each one of us put out for disposal? Multiply this by the number of households of the whole community and one has some idea of the considerable quantities of domestic waste to be disposed of every week. Added to this is the waste from offices, restaurants, hotels, markets and other establishments. All of these refuse streams constitute what is generically known as municipal solid waste (MSW) [1]. In 1996, the average quantity of MSW generated by every urban inhabitant per year was 490 kg in the UK, with an EU average of 447kg [2]. On a nation-wide basis the UK, for example, disposes of around 25 Mt of household and commercial wastes each year, together with a similar quantity of industry-generated waste and much smaller quantities of specialized wastes such as solvent and hospital waste. The thermal energy of waste may be recovered by combustion, gasification, or pyrolysis. The combustion of waste, more frequently referred to as "incineration", is the process by which waste energy is converted into electricity via a boiler to raise steam and a turbine/generator set which converts this into electricity. The combustion process will form the basis of this study because it has already been applied. Gasification and pyrolysis technologies, although somewhat more complicated, have the potential to recover more of the thermal energy in a cleaner manner, but this has yet to be demonstrated. Worldwide, very little of the energy potential of waste is currently exploited. The energy content of MSW is significant. With reference again to the UK, if all its MSW energy potential were recovered by efficient combustion systems, it would equate to an electricity generating capacity of nearly 4,000MW.
The incineration of refuse with recovery of its thermal energy is therefore an attractive disposal option provided that it is effected cleanly and that the economic incentives are sufficiently appealing. Principal among the economic considerations is that incineration plant should be located in an urban area near to an energy user such as a district heating complex, an industrial company, or a desalination plant, in order to minimize transportation costs. A further economic factor relates to the possibility of splitting the energy output between the diverse consumer groups depending on seasonal and other influences.
3. Review of seawater desalination methods
The fundamental nature of the world's weather results in large expanses of arid and semi-arid lands in the middle latitudes. The population explosion has given rise to a demand for people to live in these areas, most often requiring freshwater supplies larger than local sources can produce [3] or in the case of lowlying terrain with no natural basins, more than can be stored. As a result of the development of arid regions, and also because of intensifying use of water in urban areas all over the world, fresh water is frequently not available in the quantities desired. Since about 1950, desalination has become increasingly important where local supplies of high quality freshwater are less than adequate. Desalination is generally defined as a process that removes dissolved minerals (including but not limited to salt) from seawater, brackish water, or treated wastewater. A variety of desalination processes has been developed, and in 1993 more than 15 Mm3/d of fresh water were produced by the desalination industry worldwide [4]. At present, distillation, electrodialysis and membrane techniques are favoured and are the three principal competing processes, the choice being site dependent. These technologies are summarized below.
D. Dajnak, F.C. Lockwood~Desalination 130 (2000) 137-146
1. Distillation is the most well known and longest used desalination process. The principle of all variants is that the vapour produced from the boiling saline water is free from minerals and pure water is obtained when the vapour condenses. There are two major kinds of distillation processes: multiple effect distillation (MED) and multistage flash (MSF). In MED the evaporators are in series, each evaporator of the series being termed an "effect". Vapour from one effect is used to evaporate water in the next lower pressure effect. In MSF, intake water is heated, then discharged into a chamber maintained slightly below the saturation vapour pressure of the incoming stream so that a fraction of the water content flashes into steam. The steam condenses on the exterior surface of heat transfer tubing and becomes product water. The unflashed brine enters another chamber at a lower pressure where a further portion flashes to steam. 2. Electrodialysis is a process by which the ions forming the salt are pulled out of the saline water by electrical forces and concentrated in separate compartments. The higher the salinity of the raw water, the more electrical power needed for this process. 3. Reverse osmosis (RO), also called hyperfiltration and ultrafiltration, involves forcing water molecules through a semi-permeable membrane. The salt molecules are unable to transgress the membrane, and the water molecules that do form a potable product. In contrast to thermal desalination processes with phase transition and consequent high specific energy inputs, the RO processes relies on mechanical energy (pressure) and the specific energy requirement is relatively low [5]. A further method, to date little applied, should be mentioned: Crystallization exploits the fact that the phase transition from liquid to solid produces ice crystals that contain only pure water and no mineral salts, so that fresh water is produced when they melt. A principal advantage of this method is that the low temperatures
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involved greatly reduce the corrosion problems of the "hot" techniques. Until about 1980, distillation was the preferred method for desalinating seawater. However, because of the economic attraction of hyperfiltration, by 1986 almost one-half of the contracted desalination capacity for the international market was represented by RO plants. In addition to the desalting technology, the energy requirement of a desalination installation depends on the salinity and temperature of the feed water, the quality of the water produced and other sitedependent factors. Because of these variations, it is difficult to obtain sufficient data to enable a precise comparison of the energy needs of the different desalination methods. However, it appears that for seawater desalination, the competing processes are MSF, MED and RO, while for brackish water the competing processes are RO and electrodialysis [4]. We have not been able to find in the literature performance figures for a full-scale crystallization plant.
4. Pollution issues
Waste disposal and minimization of pollution are key problems that must be addressed for cities of the future. Sulphur pollution emitted from power stations and volatile organic components from automobile exhausts form urban "smog" in the presence of sunlight in many major cities, and this fact more than any other has contributed to public awareness and concern about combustion processes. The incineration process has the potential to generate the same general pollution problems as other combustion processes: NOx, SOx, CO, CO 2, HCI, volatile organic compounds, etc. However, there are additional environmental issues specific to MSW plants such as the emissions of trace organic compounds, particularly polychlorinated dioxins and furans, and also toxic metal trace elements such as mercury, lead and cadmium. There have been one
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or two well publicized emissions incidents occurring with old chemical plant such as the one in Seveso, in Italy, where the local inhabitants were contaminated with the potent toxicant dioxin, released into the area as a result of an industrial accident in 1976. It is therefore not surprising that the public image of incinera-tion, especially when an association is made with dioxine release, is particularly bad. However, stringent regulation and advanced pollution abatement technology have resulted in modem plants which are extremely clean and which are increasingly replacing the problematic landfill alternative. Extracting energy from waste offers clear environmental benefits, for example the reuse of materials, the reduction of fossil fuel consumption and the elimination of methane release from landfill sites. Another major advantage of incineration is the substantial bulk reduction, commonly of the order of 90% of the original volume and 60% of its weight [1]. Modern MSW systems use good combustion practices to destroy most organic compounds during the combustion process [6]. Regulatory minimum temperatures and residence times are specified to ensure the elimination ofpolycyclic chlorinated compounds and volatile organic compounds. The combustion process is followed by gas cleaning to ensure that harmful elements such as hydrochloric acid and particulates are not emitted. Although most trace elements can be controlled by using efficient particulate removal devices, carcinogenic metals such as Cd and Pb are enriched in the submicron particles which are difficult to capture by conventional flue gas cleaning equipment. Conventional cleaning equipment is also very poor in removing the volatile elements, particularly the toxic metal mercury. New techniques such as cleaning devices using sorbents have been developed. In addition to flue gas clean up, other discharges are either being eliminated or recycled, for example the ashes may be converted to usable aggregate
for products in the construction industry. Other ash treatment methods are solidification, ash melting or vitrification and extraction/recovery processes [7]. In summary, pollution control constitutes a major proportion of the cost, technological sophistication and space requirement of the modern incinerator. Older plants built before about 1985 represent older technology where the gas clean-up consisted mainly of particulate removal systems such as electrostatic precipitators. There was little public or legislative concern over acidic gas emissions, heavy metals such as cadmium and mercury, or organic micropollutants such as dioxins and furans and polycyclic aromatic hydrocarbons. Consequently, municipal waste incinerators deservedly developed a poor image as pollution sources. However, as can be seen from Table 1, the modern incinerator built in the late 1980s and 1990s is an efficient combustion plant with a gas clean-up system complying with the highest standards [8]. At the same time, the older polluting incinerator plants have largely been decommissioned as the introduction of stringent emissions legislation has been enacted throughout the world. Desalination plants produce liquid wastes that may contain all or some of the following constituents: high salt concentrations, defouling and pretreatment chemicals, and toxic metals (which are most likely to be present when the discharge water contacts certain metallic materials which may constitute the fabric of the plant). Liquid wastes may be discharged directly into the ocean, combined with other discharges (e.g., power plant cooling water or sewage treatment plant effluent), discharge into a sewer for treatment in a sewage treatment plant, or dried out and disposed of in a landfill site. Desalination plants also produce a small amount of solid waste (e.g., spent pretreatment filters and solid particles that are filtered out in the pretreatment process).
D. Dajnak, F.C Lockwood~Desalination 130 (2000) 137-146 Table 1 Emissions to the atmosphere comparing older municipal plants with typical modern plants, and citing current concentration limits for release of pollutants into the air
Emission, mg/m3
Particulates CO HCI SO2 HF NOX As, Cr, Cu, Pb, Ni, Sn Cd Hg Dioxins, ng/m3
United Kingdom
Regulation a
Older plant
Modern plant
16-2800 6-640 345-950 180-670 --0.1-50
1.4 9 11 8 0.01 370 0.05
<0.1-3.5 0.21-0.39 0.73-1215
0.001 0.1 0.005 0.1 0.02-0.1 1
30 -30 300 2 350 1
aFigures apply to MSW incinerators of greater than 1tonne/h capacity regulated under Integrated Pollution Control by the Environment agency; smaller units are dealt with by local authorities [15].
5. Energy generated from MSW In broad terms four tonnes of MSW contains as much energy as 1 tonne of coal. This fact was first recognized in 1895 when the very first facility appeared in Oldham, Lancashire, UK. Since then, the technology and efficiency of waste to energy plants have taken dramatic steps forward [9]. The conversion of MSW to energy can conserve more valuable fuels and enhance the environment by lessening the amount of waste that must be landfilled and by conserving energy and natural resources. One route to utilizing the energy value of MSW is to burn it in a steam power plant to generate electricity. As their country of residence, the UK again constitutes a convenient example for the authors (we
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apologize for this and recognize that the desalination needs of our country are negligible with fresh water on most days filling the very air!). The total amount of municipal waste generated in the UK including industry-generated waste is currently about 50 million tonnes per year, which rather amazingly is roughly just short of 1 tonne per citizen! This has an energy equivalent of 10 to 15 million tonnes of coal and represents about 5% of the entire energy needs of the country [ 10]. The average gross calorific value of mixed MSW is about 6500-10,000kJ/kg. As an example, a typical present-day incinerator burning 200,000 tonnes of MSW per year would produce 21.5MW of electricity. However, in large towns such as London and Birmingham, sufficient waste is generated to support several modern facilities, each capable o f treating 400,000 tonnes per year [11]. At present, the UK's generating capacity of electricity from municipal and industrial waste (MIW) combustion is only about l l5MW, but this is expected to rise rapidly. The technology is already mature and has been extensively demonstrated in overseas markets. For example, over 50% of the MSW in Switzerland, Belgium and Sweden is now disposed of by incineration. In Japan and Luxembourg the figure is 75%. By the year 2025, it is anticipated that the energy recovery from municipal and general industrial waste combustion in the UK could contribute an energy equivalent of about 1400MW of electricity [12]. In the US, the use of incineration to convert waste into electricity has risen from 13% to 19% in the last 10 years. Table 2 shows the high calorific value and electricity generated by 1 kg of various solid fuels such as oil, natural gas and coal. Indeed, Egypt and the other middle east countries have supplies of internal oil and natural gas. However, increasing prosperity and environmental concern have brought waste disposal problems to Europe. Many areas of Southern Europe suffer seasonal
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D. Dajnak, 1£.C. Lockwood~Desalination 130 (2000) 137-146
Table 2 Calorific values of various fuels Calorific value, Electricity MJ/kg produced, MJ/kg Coal Saw dust Eucalyptus wood Sewage sludge Oil Natural gas MSW
30 18 17.6 12.6 42.8 50.7 8.5-12.8
12 7.2 7 5 17.1 20.3 3.4-5.1
water shortage difficulties, a problem which is also exacerbated by increasing prosperity and the associated more liberal usage of water. Application of the waste-to-water technology of the present study could therefore be of direct benefit to Europe. Consequently, since the European countries such as UK still use a large quantity of coal to generate energy, it is also important to compare the calorific value of MSW with coal. A typical calorific value for MSW given as a gross calorific value of 8.5 MJ/kg by the Royal Commission on Environmental Pollution [8] is used in this table. It has been assumed that the conversion efficiency of calorific value to electrical energy is 40%, a value achievable by a well conceived incinerator.
6. Waste required to desalinate seawater Amongst the various desalination techniques referred to above, RO has been widely used in recent years, mainly due to the development of membrane technology and its low energy requirements [13]. RO plants usually have a lower energy requirement. However, it is difficult to effect a precise comparison of the energy needs of the desalination methods because of site-dependent factors such as salinity and temperature of the raw material and because
some plants are powered solely by electricity while others consume combined electricity and thermal energy. RO can be started up and shut down without a major time lag while evaporator systems involve a large heat sink resulting in extended start-up times and high transient losses. RO plants have fewer problems with corrosion and take up less surface area than distillation plants for the same amount of water production. The RO process can remove unwanted contaminants such as pesticides and bacteria. An additional consideration is that the RO technology separates the power generation and water desalination processes. Therefore, these two functions can be performed at separate locations, with each function using the technology optimized for the task [ 14]. In view of all of these considerations, we believe that RO is currently the preferred desalination technology to be linked to waste incineration. In this analysis we choose to base our calculations on electricity generation by the MSW incineration plant and the use of this electricity to power a desalination plant. However, since the RO process requires pressure, some form of direct mechanical coupling may prove more efficient. The electrical consumption of RO plants can vary from 3 to 9kWh (pumping power) per m 3 of fresh water produced. Table 3 gives the quantity of fresh water produced by RO using the electrical energy created by various solid fuels assuming a RO power consumption of 5 kWh/m 3of fresh water produced. We see, rather remarkably, that 188L of fresh water can be produced by the desalination of seawater by incinerating just 1 kg of waste. In other words, 1 tonne of waste is capable of producing 188 m 3 of fresh water. The per capita consumption of water can vary considerably depending on the wealth of the locality and a variety of cultural factors. If we again consider the UK, the yearly average of residential water consumption for domestic and service community uses is some 151m 3 per
D. Dajnak, F.C. Lockwood~Desalination 130 (2000) 137-146
Table 3 Solid fuel to water conversion coefficients using a RO system Production of fresh water by RO, litres of water per kg of fuel Coat Sawdust Eucalyptus wood Sewage sludge Oil Natural gas MSW
667 400 389 278 951 1128 188
~/x,4
Fig. 1. Overall picture of the generation of fresh water for domestic and public consumption by recycling waste.
person. By burning 490kg, the average amount generated per UK inhabitant per year, a MSW incinerator could generate 1666 MJ of electricity. Referring to Table 3, this converts to 92 m 3 of fresh water by desalination in a RO plant, or well above half the per capita consumption. An overall
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picture of the generation of fresh water for domestic and public consumption by recycling waste is seen in Fig. 1.
7. Sample MSW incinerator in London The Southeast London combined heat and power (SELCHP) facility is a modern selfcontained waste incinerator located in a residential area of London where it has received wide public acceptance. It utilizes advanced technology to recover the energy contained in raw waste. SELCHP is designed to burn over 400,000tonnes of refuse each year and has the capacity to generate a valuable 32MW of electricity. At a later stage it could also provide heat for over 7500 homes. The combustion process is strictly controlled to respect the level of emission fixed by the UK government. A flue gas treatment system removes most pollutants before ejecting clean gases into the atmosphere via a 100-m chimney. The flue gases are free from odours. Sound emission is tightly regulated to the extent that the background noise level of the locality is unaltered. The plant is architecturally pleasing and a carefully planned traffic scheme ensures no local disruption. The combustion process utilized by SELCHP produces residues amounting to around 10% of the original waste volume. Largely made up of ash, the residues are biologically inactive, as well as being virtually inert and odour free. Most importantly, the residues are consistent in quality, easily managed and have potential for recycling in construction applications. Magnetic recovery of ferrous metals from the ash residue is effected. The convenient position of the plant makes it a realistic disposal point for waste generated throughout central, northeast and southeast London. The SELCHP plant demonstrates that waste incineration can be effected in an urban area in a manner which is both clean and nonintrusive. SELCHP cost £85 million (E142
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19. Dajnak, F C Lockwood~Desalination 130 (2000) 137-146
Fig. 2. SELCHP incineratorin central London. million) to design and construct in 1994 [9]. l f a similar incinerator were built in a community near a RO desalination plant, its energy output could produce about 75 M m3/y of fresh water. Fig. 2 shows a diagram of the SELCHP incinerator in central London [9].
8. Project realization The above brief analysis is very preliminary, but it does show that the concept of waste to water is worthy of further consideration. The present study has examined the straightforward coupling of state-of-the-art technologies. However, there are no doubt ways in which the coupling itself can be improved to yield overall efficiency gain. For example, since the RO technique is pressure driven, is it necessary to employ the medium of electricity to generate the pressure? Also, we have made no provision for the recovery of low-grade heat. It is unlikely in
countries suffering from a water shortage that there will be much call for district heating! However, there may well be some way of enhancing the process yield through recovery, rather than rejection, of the low-grade energy. This lowgrade heat could possibly be used for airconditioning and refrigeration. Clearly, there is scope for innovative thinking and supporting process efficiency calculations. Once optimized routes have been identified, detailed economic and environmental evaluations would follow. In this respect it is particularly important to assess the cost of the proposed technology relative to conventional treatment of waste and the use of solar and wind energy for desalination. If after such studies the technology maintains its early promise, a pilot plant might be constructed, especially if a novel coupling scheme has been proposed. With or without this stage, a sitespecific design exercise would follow. The chosen site would in the first instance preferably be rather small and self contained. So for
D. Dajnak, F. C. Lockwood~Desalination 130 (2000) 137-146
example, one of the many developing Egyptian resort communities remote from the Nile would be a candidate, or a Mediterranean island. At this stage some o f the factors to be considered are: Who will own and operate the plant? What type of waste is available? What quality of desalinated water is required? What is its required quality? What will be the cost to end-user? The subsequent steps leading to a working plant are clear: procurement o f finance, detailed design preparation, construction and commissioning.
9. Conclusions Recovery of thermal energy from municipal waste makes good environmental sense. Waste combustion reduces the loss of space to landfill sites, it reduces the environmental impacts caused by gas and leachate formation from landfilled raw organic waste, and it contributes to reducing reliance on fossil fuel. The recovery process normally involves the generation of electricity, possibly combined with district heating. We have shown that thermal energy can be used to generate considerable quantities of fresh water, more than half the typical per capita daily need, simply by coupling the existing waste incineration and desalination technologies. An improved recovery rate is no doubt possible by innovative coupling. We have used incineration, i.e., combustion, as a basis for comparison. Pyrolysis and gasification waste transformation processes are increasingly being examined and they may provide additional economic and environmental advantages. The RO desalination process has very little air and water environmental impact and can, like modern incinerators, be located near or within population centres. Whether or not the proposed concept can compete against the separate generation of electric power and desalination from other renewables such as wind or solar energy, which
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is intermittent of course, needs to be demonstrated. Our intention in this paper has simply been to show that the basic concept has merit and is worthy of further study.
Acknowledgements The authors would like to thank Prof. M.N. Damir, President of Alexandria University and chairman o f ADST (the Alexandria University Desalination Studies and Technology Centre) and Dr. M. Abdel Rahman for the opportunity to present and discuss this study at the International Desalination Workshop in Alexandria in 1999. The authors gratefully thank Dr. P. Costen for his scientific discussions concerning the MSW matter.
References [l] W. Hall and M. Keynes, Unit 14, Municipal Waste Management,The Open University, 1985. [2] K. Kuchta and J. Jager, 2"dInternational Symposium on Incineration and Flue Gas Treatment Technologies, Sheffield,UK, 1999. [3] E.D. Howe, Fundamentals of Water Desalination, Marcel Dekker, New York, 1974. [4] K.S. Spiegler and Y.M. EI-Sayed, A Desalination Primer, Balaban Desalination Publications, Santa Maria Imbaro, 1994. [5] H. Hams-Gunter, Saline Water Processing, VCH, Weinheim, 1990. [6] L.A. Ruth, Prog. Energy Combust. Sci., 24 (1998) 545. [7] S. Sakai, S.E. Sawell, A.J. Chandler, T.T. Eighmy, D.S. Kosson, J. Vehlow, H.A. van der Slott and O. Hjelmar, Waste Management, 16 (1996) 341. [8] P.T.William, Waste Treatmentand Disposal, Wiley, New York, 1998. [9] SELCHP, A new direction in waste disposal for London, turning waste into energy, company informationbrochure, 1994.
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[10] J. Swithenbank, V. Nasserzadeth, A. Wasantakoran, P.H. Lee and C. Swithenbank, Future integrated waste, energy and pollution management (WEP) systems exploit pyrotechnology, 2nd International Symposium on Incinerator and Flue Gas Treatment Technologies, Sheffield, UK, 1999. [11] N. Patel and I. Higham, MSW combustion: economics and projections for energy recovery to the year 2000, Energy Technology Support Unit, Harwell Laboratory, 1995.
[12] Department of Track and Industry, Energy from Waste, programme brochure, UK. [13] D. Voivontas, A. Zervos and D. Assimacopoulos, Desalination, 21 (1999) 159. [i4] I. Moch, Jr., F. Depenbrock and Y. Mussalli, Water, Air Soil Poll., 90 (1996) 231. [15] Chief Inspector's Guidance to Inspectors, Process Guidance Note IPR 5/3, Waste Disposal and Recycling, Municipal Waste Incineration.Department of the Environment, HMSO, London, 1995.