SUSTAINABLE LANDFILLS A TECHNIQUE FOR EXTRACTING HEAT TO PROLONG SERVICE-LIFE OF GEOMEMBRANE LINERS

Transcription

1 SUSTAINABLE LANDFILLS A TECHNIQUE FOR EXTRACTING HEAT TO PROLONG SERVICE-LIFE OF GEOMEMBRANE LINERS R. Kerry Rowe GeoEngineering Centre at Queen s-rmc, Queen s University, Kingston, Ontario, Canada Andrew Pollard, Andrew Chong, Eric Chisholm, Rebecca Toda, and Caroline Tomson Department of Mechanical and Materials Engineering, Queen s University, Kingston, Ontario, Canada ABSTRACT The results of a preliminary investigation of the feasibility of both controlling liner temperature and extracting heat for onsite use are reported. The proposed design solution involves the installation of an active coolant horizontal pipe array in a sand protection layer above the gravel drainage layer of the leachate collection system. It is shown that under yearlyaveraged conditions, the proposed design will reduce the liner temperature from 45ºC to 29ºC, thus increasing the service life of the liner by hundreds of years. RÉSUMÉ Les résultats d'une recherche préliminaire relative à la faisabilité concomitante de la mesure de la température de la barrière d'étanchéité et de l'extraction de chaleur pour une utilisation in-situ sont présentés. La conception proposée comporte l'installation d'une rangée de tubes horizontaux contenant un liquide réfrigérant intégrée à une couche de sable protectrice située au-dessus de la couche drainante granulaire constituant le système de collecte de lixiviat. On montre que dans des conditions idéales moyennées, la circulation d'un fluide réfrigérant de température initiale égale à 10ºC pourrait réduire la température de la barrière d'étanchéité de 45ºC à 29ºC, augmentant de ce fait la durée de vie de la barrière d'étanchéité de plusieurs centaines d'années. 1 INTRODUCTION Recent research has demonstrated that the heat generated by the biodegradation of the organic component of municipal solid waste (MSW) can increase the temperature at the top of the (primary) landfill liner to 30-40ºC after about a decade under typical conditions (see Rowe, 2005; Figure 1). These high temperatures can be reached in about 5-10 years (Brune et al., 1991; Rowe, 2005) depending on the rate of waste placement. By increasing the availability of moisture, such as when the landfill is operated as a bio-reactor, the liner temperature can increase to 40-45ºC in about 5 years (Koerner and Koerner, 2006). Increased landfill temperatures can also lead to enhanced degradation of the performance of secondary liners. Unpublished measurements indicate that the temperature of the secondary geomembrane (GM) in double composite liner systems involving just a GM and geosynthetic clay liner may only be 3ºC or less below that of the primary GM (Legge, pers. comm.). This is consistent with theoretical modeling conducted by Rowe and Hoor (2007) that suggested ~1ºC difference assuming no cooling is induced by the leak detection layer. If there is a compacted clay liner (CCL) or foundation layer as part of the primary liner, then the added thermal resistance will lead to a reduction in the increase in temperature on the secondary GM. This reduction will depend primarily on the thickness of the clay liner/foundation layer. Rowe and Hoor (2007) showed that for a steady state 40 C increase in temperature on a primary GM underlain by a 0.75m thick CCL, there would be a 30 C increase in temperature on the secondary liner. Germany USA Wet Cell USA Dry Cell Canada Japan Figure 1: Some observed temperatures at the base of landfills (US data: Koerner and Koerner, 2006; Canadian data: Rowe, 2005; Japanese data: Yoshida and Rowe, 2003; German data: Klein et al after Rowe 2007) It has been demonstrated (Rowe, 2005; 2007) that the service life of a GM liner decreases with an increase in liner temperature. As noted in Table 1, an increase in temperature from 30 o C to 40 o C would reduce the service life of the geomembrane by 50 to 75 years (from years to only years). 1310

2 Table 1: Estimated service lives for an HDPE geomembrane for a MSW landfill (updated from Rowe 2005) Temperature (ºC) Estimated Service Life Range (years) The objective of this paper is to use hydraulic and heat transfer modelling to explore a possible method for extending the service life of the GM liner, extracting heat generated by the MSW from above the liner, and then making use of this energy through the application of established heat exchanger and thermodynamic technology. energy at time of exit. A cooling tower could be used to dissipate excess heat to return the coolant to ambient temperature before being recirculated. 3 LAYOUT AND MECHANICAL CONSIDERATIONS Figure 3 is a schematic view of the horizontal pipe array that would run through the sand layer below the MSW. Basic heat exchanger principles dictate that the cooling is most efficient if the hot and cold pipes are alternated as shown in Figure 3. The pipe system returns hightemperature coolant to the heat transfer equipment (pump, heat exchanger, and air coil) housed in a maintenance building. A cooling tower could be connected through an outside wall to these components. 2 THE CONCEPT The design involves the installation of a heat exchanger system within a MSW landfill. Its main component is a horizontal pipe array buried within a sand protection layer, above the gravel drainage layer of the leachate collection system (Figure 2). Coolant circulated through these pipes would absorb the heat generated by biodegradation processes. MSW Horizontal Pipe Array Compacted Clay Prepared Subgrade Sand Protection Layer Geotextile Leachate Collection System Geomembrane (GM) Figure 2: Schematic cross-section of the design examined. Without regard to cost, the preliminary design examined in this study considers 160mm diameter stainless steel 316 pipes at 3m centres in the sand protection layer. These pipes would run the length of each landfill cell and circulate water, although other fluid coolants could be considered. The flow would be controlled by a fluid pump system and several control valves. Once outside the landfill, the hot coolant would pass through either an air coil to re-heat return air, or a heat exchanger to pre-heat domestic hot water, depending on the desired use of the Figure 3: Schematic plan view of proposed design concept 3.1 Selection of Pipe Material and Dimensions This study considered coolant pipes made of stainless steel 316, selected for its good heat transfer properties and well-documented corrosion resistance (Schweitzer, 2004). SS 316 has previously been used in sewage treatment and desalination facilities, and is recommended for use with organic acids, pickled sauces, and liqueurs (Korb, 1987). In order to ensure that the pipes would remain usable for the lifespan of the system, a pipe wall thickness of 30mm was recommended. This thickness, coupled with a Teflon coating (resistant up to 204ºC) on the inside surface of the pipe to protect against erosion/ corrosion due to the coolant flow, would allow the piping system to last approximately fifty years (Schweitzer, 2004). The possibility of stress corrosion cracking (SCC) in the pipes was investigated, although this is only likely in an environment of less than 0.1% humidity (Korb, 1987), and thus was considered highly unlikely in landfill applications due to expected near-saturated conditions in the sand layer. The minimum inside pipe diameter required to transmit the coolant was calculated to be 90mm, giving an outside diameter of 150mm. Thus a standard 160mm diameter pipe was selected. 3.2 Coolant Treated water was selected as the coolant based on considerations of availability, cost, long term maintenance, and potential risks in the event of a leak. An average coolant temperature of 12 C was assumed based on average weather conditions in Kingston, ON. 1311

3 4 MODELING STRATEGY A two-stage numerical modeling strategy was adopted to assess the potential efficacy of the introduction of the cooling system (Section 2) on the temperature of a primary liner. This strategy began with a 2D model, which was then extended to a 3D model. The pipes were assumed to be arranged in pairs with a pipe introducing coolant and an adjacent return pipe leading to the surface for heat extraction. The pairs of pipes were alternated such that the inlet pipe of one pair was adjacent to the outlet of the next, and vice versa. To reduce the size of the cross-section that required modeling, periodic boundary conditions were specified at the sides of the model in FLUENT and only one coolant loop (e.g. one inlet and one outlet) was analyzed numerically. 4.1 Profile of the Landfill Layers The profile of the model landfill layers involved MSW, a 0.5m sand protection layer into which the pipes were inserted for heat extraction, a 0.3m gravel leachate collection layer, a 0.75m thick compacted clay liner, and a 3m thick soil attenuation layer above the aquifer. The 1.5mm HDPE thick geomembrane above the clay liner was assumed to have a negligible impact on heat transfer. The 160mm diameter heat extraction pipes were installed with centres 100 mm above the bottom of the sand protection layer, with the remainder of the sand layer providing protection from heavy machinery above this layer. The typical range of properties and the values used in the analysis are given in Table 2. Values used in the analysis were selected to represent reasonable values. The effect of variation around these values will be examined in a subsequent paper. Table 2: Ranges of typical values and properties adopted in this study. MSW Layer Range Layer thickness (m) (W/mK) Density (kg/m 3 ) Thermal conductivity Specific heat capacity (J/kgK) Used Sand Used Range Gravel 2242 Used Clay Range Liner Used Sub- Used Soil 4.2 Two-Dimensional Computational Model of Pipe System ANSYS s FLUENT was first used for a two-dimensional analysis to assess the effect of the problem s geometry and component placement on the expected thermal distribution and liner temperature in a typical crosssection of the landfill. Since it was a 2D analysis, heat transfer along the pipe was not considered. It can be anticipated that there would be minor thermal gradients between cross-sections as the fluid transferred heat from the landfill into and along the pipe. This limitation of the 2D was addressed using FLUENT in 3D and it was found that the coolant temperature increased by 10.7ºC along one coolant loop of about 400m of pipe, or about 0.027ºC/m for a coolant flow rate of 18kg/min. Hence, the simplification of the 2D analysis does not involve much error between cross sections. Additionally, 2D thermal effects were considered only for landfill locations away from the edge of the landfill. The 2D modeling assumed that the outlet temperature would be equal to the sand layer near the exit to maximize the amount of heat extracted per unit volume of coolant. Thus, the inlet pipe face was held at 12ºC and the outlet pipe was allowed to assume the temperature of its surroundings. 4.3 Three-Dimensional Modeling of the Pipe System The assumptions adopted in the 3D modeling were largely the same as in the 2D modeling. The primary purposes of the 3D model were (a) to observe heat transfer along the direction of fluid flow in the pipes, (b) to allow the temperature to increase in the pipe as heat was absorbed from the waste, and (c) to examine the effect of coolant flow rate on maximum pipe temperature (i.e. at the outlet of the coolant loop). The inlet pipe temperature was taken to be 12ºC, as noted earlier. The pipe length for entry to exit from the sand layer was taken to be 403m for the modeling discussed herein (i.e. a 200m long cell was examined with inlet and outlet 3m apart). The effect of varying this distance is subject to further investigation. 4.4 Modeling of the Heat Source The only source of heat was assumed to be the biodegradation of the MSW. Seasonal effects were ignored by using annual averaged temperatures. Finally, the radial temperature gradient of the coolant within the pipes was considered to be negligible for the 2D crosssection. For the modeling conducted herein the target liner temperature in the absence of any cooling layer was selected to be 45ºC (based on Koerner and Koerner, 2006). This was achieved by modeling the MSW waste in three sub-sections, with the middle section held at a constant temperature that would result in a geomembrane temperature of 45 o C without any cooling. Heat was dissipated from this section both upward to the surface (held at annual average ambient temperature) and downward through the waste, liner system, and underlying soil to an underlying aquifer. Groundwater in the aquifer was assumed be at a constant specified temperature of 10ºC (i.e. groundwater flow was considered to be fast enough to dissipate any heat reaching the aquifer). 5 RESULTS 5.1 2D Results Figure 4 shows a contour plot of the 2D temperature distribution for the case being examined. The 2D model reported a range of temperatures across the GM liner from 18.2ºC directly below the inlet pipe to a maximum of 1312

4 28.9 C below the outlet pipe, with an average liner temperature of 25.6ºC. These can be compared to 45ºC in the absence of a cooling system. Based on data presented by Rowe (2005), this reduction in temperature would substantially increase the service life of the geomembrane (see Table 1). It should be noted that this also represents a worst case for cooling with this pipe array since no attempt was made in the 2D analysis to cool the soil at the outlet pipe. 5.2 System Parameter Variation Figure 5 shows the liner temperature distribution for two other cases, without changing the distance of the pipes from the top of the sand layer. Doubling the thickness of the sand layer from 0.5m to 1m normalized the temperature distribution on the liner and decreased its maximum temperature by 1.2ºC to 27.6ºC. Increasing pipe diameter by 20%, however, had negligible effect on maximum liner temperature (less than 0.1ºC). Pipe wall thickness was increased as well, but also had essentially no effect on liner temperature. These results suggest that alterations to pipe dimensions have little effect on the thermal performance of the base design. coolant inlet Figure 4: Contour plot of temperature distribution for 2D analysis (inlet pipe temperature of 12ºC) 5.3 3D Results coolant outlet The 3D analysis yielded the distribution of temperature on the GM liner shown in Figure 6. With a coolant flow rate of 18kg/min (0.3kg/s), the minimum and maximum temperatures on the liner were encountered, respectively, below the inlet pipe, and between the coolant pipes (middle of the model) where the GM liner is furthest from a coolant pipe. The 3D analysis gave minimum and maximum geomembrane temperatures of 25.8ºC and 30.1ºC respectively, and an average temperature of 29.4ºC. These temperatures are in reasonable agreement with the values from the 2D analysis. 5.4 Optimal Coolant Exchange Rate MSW sand gravel geomembrane compacted clay One of the motivations for performing the 3D analysis was to estimate a coolant mass flow rate that could be used for specification of pumping equipment. Based on preliminary results, a coolant-loop mass flow rate of soil aquifer (10ºC) Temperature (ºC) Position (m) Base Case Double Sand Pipes +20% Figure 5: Temperature distribution on the geomembrane liner surface for (a) base case with 0.5m sand layer (b) a 1m thick sand layer, and (c) larger diameter pipes approximately 12kg/min (0.2 kg/s) was required to maximize heat extracted from the sand layer per unit mass of coolant. However, such a low flow rate yields higher temperatures on the GM liner (a maximum of 32.1ºC versus 30.1ºC with 18kg/min, which was used to get Figure 6). The 12 kg/min flow rate, which maximizes heat extraction, was obtained by iteratively adjusting coolant inlet velocity until the coolant outlet temperature was 27 C. This temperature was considered to be the coolant exit temperature that corresponded to maximized heat extraction per unit coolant based on the 2D model. The 3D model revealed that the warmest point on the GM liner still occurred between the pipes, as with the 18 kg/min flow rate. 6 DISCUSSION The results from this preliminary feasibility study of controlling landfill liner temperature and taking advantage of the extracted heat suggest that it is feasible to control liner temperature and substantially extend the service life of composite primary and, where present, secondary liners. The study assumed annual average thermal conditions for the cooling of the coolant. Future work will examine the effects of seasonal variations in temperature on the performance of the system. Limited attention was paid to optimizing the coolant flow rate. One could select a flow rate that maximized the heat extracted per unit mass of coolant. Increasing the flow rate through the system above this level would maintain the pipe array and sand layer (and thus the GM liner) at a lower overall temperature thereby increasing the geomembrane service life, but at the cost of a lower outlet coolant temperature and therefore less useful heat energy. A flow rate that minimized the liner temperature would have the location of the maximum temperature on the geomembrane midway between pipes where the GM is furthest from the influence of a cold pipe

5 coolant inlet pipe coolant outlet pipe The landfill system was modeled in FLUENT with boundary conditions and material heat transfer properties set to be as similar as possible to those expected in the field. With the pipes running water as a coolant through the sand layer, the average temperature of the HDPE liner was reduced from 45ºC to 29ºC. Considering the median values at a given temperature in Table 1, this reduction corresponds to an increase in service life of the primary geomembrane by about 200 years, from 65 years to 270 years. The temperature of the return coolant to the external heat management system depends on the flow rate of the coolant. It is unlikely to be sufficient to provide highenough quality energy to be used for generation of electricity, although it could still be used in a heat exchanger, air coil, or cooling tower. Figure 6. Temperature distribution on the geomembrane liner surface as calculated from the 3D model (coolant inlet temperature was 12ºC and outlet temperature was 27ºC). A more detailed examination of the effect of flow rate, pipe spacing, and pipe distance from the GM liner will be undertaken in a subsequent study, with the objective of optimizing the arrangement for both cost and control of liner temperature to an acceptable value for ensuring good long-term performance. The piping examined here was stainless steel which, while effective, is very expensive. A much more costeffective alternative would be to use high density polyethylene (HDPE) pipes. This alternative will be examined as the design concept is further developed. 7 CONCLUSIONS The heat generated by the biodegradation of the organic component of municipal solid waste can substantially reduce the service life of composite liners used to prevent leachate from escaping to the environment. The results of a preliminary exploration of one possible method of reducing the temperature on landfill liners, by extracting heat from above the liner and using the energy extracted for heating adjacent structures, have been reported. The proposed design concept involves the installation of an active coolant horizontal pipe array in a sand protection layer above the gravel drainage layer of the leachate collection system. The design examined has 160mm diameter stainless steel 316 pipes at 3m centers in the sand protection layer. These pipes run the length of each landfill cell. Preliminary cost estimates examined for the stainless steel system suggest that cost would likely be excessive for most applications. However, the cost could be substantially reduced by the use of high density polyethylene (HDPE) pipes instead of stainless steel. Studies are currently underway to assess the effect of using HDPE pipes and the effect on performance of increasing the spacing between pipes. It is anticipated that this work will provide additional insights regarding feasible means of extracting heat from MSW landfills and extending the service life of landfill liners. ACKNOWLEDGEMENTS Funding from the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. REFERENCES Brune, M., Ramke, H.G., Collins, H., and Hanert, H.H Incrustations process in drainage systems of sanitary landfills, 3rd Int. Landfill Symp., Cagliari: Klein R., Baumann T., Kahapka E. & Niessner R Temperature development in a modern municipal solid waste incineration (MSWI) bottom ash landfill with regard to sustainable waste management, Journal of Hazardous Materials, B83: Koerner, G. R., Koerner R. M Long-term temperature monitoring of geomembranes at dry and wet landfills, Geotextiles and Geomembranes, 24(1): Korb, L ASM Handbook Volume 13: Corrosion: Fundamentals, Testing and Protection Rowe, R.K Long-term performance of contaminant barrier systems, 45th Rankine Lecture, Geotechnique, 55(9): Rowe R.K Advances and Remaining Challenges for Geosynthetics in Geoenvironmental Engineering Applications, 23rd Manual Rocha Lecture, Soils and Rocks, 1(1)(In press) Rowe, R.K and Hoor, A Temperature of secondary liners in municipal solid waste landfills, Geosynthetics 2007, Proc. Environmental Conference, Washington,

6 Rowe, R.K., Quigley, R.M., Brachman, R.W.I., Booker, J.R Barrier Systems for Waste Disposal Facilities, Taylor & Francis Books Ltd (E & FN Spon) London, 587p Schweitzer, P.A Corrosion Resistance Tables: Metals, Nonmetals, Coatings, Mortars, Plastics, Elastomers and Linings and Fabrics, Marcel Dekker Ltd. Yoshida, H., Rowe, R. K Consideration of landfill liner temperature, Proc. 9th. Int. Landfill Sym., Cagliari, Italy (CD-ROM). 1315