Microsoft Word - 26jonassen.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 40, 2014 A publication of The Italian Association of Chemical Engineering www.aidic.it/cet Guest Editor: Renato Del Rosso Copyright © 2014, AIDIC Servizi S.r.l., ISBN 978-88-95608-31-0; ISSN 2283-9216 Evaluation of Landfill Surface Emissions Laura Capellia*, Selena Sironia, Renato Del Rossoa, Enrico Magnanob aPolitecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”, piazza Leonardo da Vinci 32, 20133 Milano, Italy bEmendo S.r.l., via Rocca Grue 17, 15050 Sarezzano (Al), Italy *laura.capelli@polimi.it This study was started with the aim of evaluating different sampling techniques in order to develop a suitable and scientifically sound methodology for measuring the emissions of LFG from landfill surfaces. For this purpose, the work involved a bibliographical and theoretical study aiming to design new sampling equipment, as well as an experimental phase consisting in repeated measurement campaigns for the evaluation and comparison of such equipment on a sample site, i.e. a MSW landfill located in Northern Italy. Two sampling methods were designed and realized: a “flux chamber” and a “static box”. The results of the first sampling campaigns show a good agreement between the two sampling techniques, but further studies will be needed in order to validate those results and study the influence of the meteorological conditions on the measured LFG fluxes. 1. Introduction Landfills are known to be an important source of environmental pollution. Landfill gas (LFG) represents the major source of atmospheric pollution related to municipal solid waste (MSW) landfills (US EPA, 2008). LFG is the gaseous product of the anaerobic microbial decomposition of the organic matter of the waste, and it is composed for over 99% by CH4 (typically in the range of 40-70%) and CO2 (30-60%) (El Fadel et al., 1997), thus being classified as a greenhouse gas. Indeed, LFG is estimated to account for approximately 3-19% of annual anthropogenic emissions of CH4, although there remains significant uncertainty associated with these estimates (Park and Shin, 2001). CH4 and CO2 are odourless, however, the presence of trace compounds (typically below 1%) having low odour thresholds, such as H2S, organic sulphur compounds (Kim et al., 2004) and VOCs (Davoli et al., 2003) gives the LFG a characteristic, highly concentrated and unpleasant odour. As a matter of fact, in many cases, offensive odours represent the major cause of populations’ worries and complaints against landfills, and are therefore often the limiting factor to their exercise, or to the design and realization of new plants. For these reasons, it is extremely important to be able to quantify LFG emissions into the atmosphere, in order to evaluate their environmental impact on the territory, and to prevent citizens from the exposure to odours or to potentially harmful pollutants by applying suitable control strategies (Palmiotto et al., 2014). A critical aspect associated with LFG emissions into the atmosphere is that they are variable, depending on different parameters, such as mainly atmospheric pressure, terrain humidity, but also temperature, wind speed and precipitations. These phenomenon is well known to field experts, but there are still few studies on that, and none of them up to now defines a clear cause-effect correlation (Czepiel et al., 2003; Rachor et al., 2013). The first step towards the evaluation of landfills environmental impact is the quantification of the LFG emitted. Two ways are available for this purpose: either by using mathematical models or by conducting specific sampling campaigns on site. Different models exist which allow to estimate the LFG produced by the landfill body as a function of the amount of yearly landfilled waste, its biodegradability, and rainfall. The LFG emitted may then be calculated as the difference between the LFG produced and the LFG extracted. The main drawback of such models is that they are generally very sensitive to the input parameters, thus making them unsuitable for environmental impact assessment purposes. DOI: 10.3303/CET1440032 Please cite this article as: Capelli L., Sironi S., Del Rosso R., Magnano E., 2014, Evaluation of landfill surface emissions, Chemical Engineering Transactions, 40, 187-192 DOI: 10.3303/CET1440032 187 For this reason, it is preferable to base such studies on periodical site-specific sampling campaigns, which are more congruent with the real emissions. Unfortunately, despite the existence of guidelines and scientific studies indicating different methods for measuring the landfill gas emitted from a landfill surface (Mosher et al., 1999; Park and Shin, 2001; Sironi et al., 2005; Rachor et al., 2013), such methods are disagreeing and give not reproducible results. The aim of this study was to evaluate different sampling techniques in order to develop a suitable and scientifically sound methodology for measuring the emissions of LFG from landfill surfaces. For this purpose, the work was organized in two phases. The first phase included a bibliographical and theoretical study aiming to design new sampling equipment; whereas the second, experimental phase consisted in repeated measurement campaigns for the evaluation and comparison of such equipment on a sample site. 2. Materials and methods 2.1 The studied site The site where the experimental campaigns were conducted is a MSW landfill located in Northern Italy, active since 1993, and with a surface of about 250’000 m2. The landfill is divided in 6 parcels: 5 are exhausted and currently in post-management, while the cultivation of the 6th parcel started in 2006. 2.2 Preliminary study of the landfill historical emission data The studied landfill is very active from the point of view of environmental control: as far as diffuse emissions of LFG are concerned, these are measured regularly every 6 months by Emendo S.r.l. The technique adopted by Emendo S.r.l. is the one described in the “Guidance on monitoring landfill gas surface emission” by the UK EA. This method is based on the use of a so called “flux box”, which, despite its name, is a closed, non-fluxed (i.e. static) box at whose interior it is possible to measure an increase of the CH4 concentration over time until a stationary condition is reached. From this trend it is then possible to calculate the CH4 flux in mg/m 2/s. Measurements shall be repeated on a sufficient number of points, which is determined by the following equation: = 6 + 0.15√ Where n is the number of points and S the monitored surface (m2). The emission of the whole landfill is calculated as the average of the flux data measured on the different sampling points, multiplied by the landfill surface. Hotspots have to be considered, as well. The advantage of this method is that it is simple and repeatable, although the fact that the sampling box used is closed may create some overpressures, thereby altering the CH4 measurement (Rachor et al., 2013). The flux box used in this case is in polypropylene, it has a volume of 0.1104 m3 and a base surface of 0.01104 m2. CH4 concentration was measured by means of a portable FID (GasTech by Crowcon). As indicated by the English guideline, the landfill was divided in areas with similar characteristics, from which the LFG emissions should be comparable. The area considered for this study was named DEA-1 (Diffuse Emission Area) and includes all the areas of the landfill having a final cover, giving a surface of 102’277 m2, and 54 sampling points. Based on the analysis of the historical data measured by Emendo S.r.l. it was possible to identify some particularly significant emission points of the studied landfill. 2.3 Application of LandGem model Before starting the experimental campaigns on the landfill surface, it was decided to make a preliminary evaluation by applying a model for the evaluation of the CH4 generation. The model used was the LandGem by US EPA, which can be downloaded free from their website (www.epa.gov) as an Excel file. The model is based on a first order decay model: = 1.3 − Where QCH4 is the methane generation rate at time t (m 3/y), L0 the methane generation potential (m 3/Mg), R the annual refuse acceptance during active life (Mg/y), k the methane generation constant (1/y), c the time since landfill closure (y), and t the time since initial refuse placement (y). Table 1: Annual refuse acceptance relevant to the studied landfill Based on the site-specific data, L0 and k were set equal to 100 m 3/Mg and 0.03 1/y, respectively. These values were then varied in order to analyse the model sensitivity to the input parameters. The annual refuse acceptance is reported in Table 1. Year 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 Refuse acceptance (Mg/y) 272886 332873 374407 479973 430213 374061 340583 318513 273783 274977 250028 Year 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 Refuse acceptance (Mg/y) 240814 240814 101044 101044 290103 193552 151624 173000 82274 153915 0 188 2.4 Field tests with Static Box by Polimi The design of our “static box” was inspired by a study by Rachor et al. (2013), who used a specific kind of static chamber altered with respect to traditional chambers by addition of a 3-m open tube in order to prevent pressure differences with the atmosphere. The chamber by Rachor et al. was an aluminium cylinder with a height of 50 cm and a base area of 0.12 m2. Because of the low CH4 flux expected based on literature studies (< 1 L/h/m2) and the CH4 diffusion coefficient in air, which makes the CH4 concentration likely to be not uniform along the height of the chamber, it was decided to reduce our static box height to 10 cm. Moreover, in order to have a sufficient volume to measure the CH4 concentration with the portable FID GasTech by Crowcon, which sucks about 0.85 L/min, and to cover a representative portion of the landfill surface, our “static chamber” was designed as a parallelepiped with a quadratic base (50 cm x 50 cm), thus having a total volume of 25 L. The chamber was realized in steel and it was equipped with a 3-m open TeflonTM tube (Figure 1, left). Once the chamber is positioned on the landfill surface, the CH4 is measured inside the chamber at regular time intervals (about 3 mins): The CH4 flux relevant to the sampled point is then calculated as follows: = Where QCH4 is the CH4 flux (mg/m 2/s), Vsb the volume of the static box (m 3), Ssb the base surface of the static box (m2) and dc/dt the CH4 concentration variation over time (mg/m 3/s). dc/dt can be calculated as: = ∙ ∑ ∙ − ∑ ∙ ∑∙ ∑ − ∑ Where n is the number of measurements, t the times of the measurements (s) and c the measured CH4 concentrations (mg/m3). Figure 1: Detail of the “static box” with the TeflonTM open tube to prevent pressure differences with the atmosphere (left), and experimental apparatus for “flux chamber” sampling (right) 2.5 Field tests with Flux Chamber by Polimi The second sampling equipment that was tested is a “flux chamber”, which was designed based on the flux chamber sampling system described in the US EPA guideline “Measurement of gaseous emission rates from landfill surfaces using an emission isolation flux chamber” (1986). This chamber was designed with the logic of being fluxed with neutral air. It consists of a semi-sphere in plexiglass, having a 50-cm diameter and a volume of about 30 L. The top of the semi-sphere is provided with two TeflonTM tubes: one is connected to an air bottle for the neutral air inlet, whereas the other one is connected to the FID when performing the measurement. The air from the air bottle passes through a flow-meter before entering the flux chamber, which allows to regulate the inlet neutral air flow. The inlet neutral air flow was varied in a rather wide range, comprised between 40 L/h and 300 L/h, in order to verify how it affects the measured CH4 concentration (Figure 1, right). Based on the theory, the product between measured CH4 concentration and inlet air flow should be constant. Indeed, the mass balance on the flux chamber gives: + = Which can be also expressed also as: ∙ 0 + ∙ = ∙ , Where Qair is the neutral air inlet flow coming from the bottle and regulated by the flow-meter, QLFG is the LFG flow from the landfill surface, and Qout is the flux chamber outlet flow, which can be considered equal to the inlet air flow, given that QLFG is much lower (by about two orders of magnitude). In this case, a CH4 concentration (CCH4) equal to 50% was considered. 189 Once the flux chamber is positioned on the landfill surface, it is necessary to wait a time at least equal to one chamber exchange time ( = ⁄ ). After this time, the FID is connected to the flux chamber outlet tube and the CH4 concentration is measured. The advantage of this sampling method is that the measured CH4 concentration is constant, being a function of the inlet air flow and the LFG flow from the landfill surface. The drawback is that it requires longer sampling times (from 10 to 50 mins depending on the inlet air flow) and the usage of neutral air bottles. 3. Results and discussion 3.1 Results of the LandGem application The result obtained by the application of the LandGem model, by considering the actual refuse acceptance (Table 1) and a CH4 concentration in LFG of 50%, and by setting the input parameters L0 and k equal to 100 m3/Mg and 0.03 1/y, respectively, gives a LFG generation in 2014 of 28530 Mg/y (Figure 2), corresponding to 2608 m3/h. Figure 2: Results of the application of the LandGem model (in Mg/y) setting L0 and k equal to 100 m 3/Mg and 0.03 1/y, respectively. In order to evaluate the model sensitivity towards the input parameters L0 and k, these were varied between 80 m3/Mg and 120 m3/Mg, and 0.02 1/y and 0.04 1/y, respectively, which are all reasonable values for the studied landfill (Table 2). Table 2: Results of the LandGem model obtained by varying the input parameters L0 and k k (1/y) L0 (m3/Mg) LFG generation rate (106 m3/y) LFG generation rate (m3/h) CH4 generation rate (Mg/y) 0.02 80 13.69 1562 4565 0.02 100 17.11 1953 5706 0.02 120 20.53 2343 6848 0.03 80 18.27 2086 6096 0.03 100 22.84 2608 7620 0.03 120 27.41 3129 9144 0.04 80 21.76 2484 7260 0.04 100 27.21 3106 9075 0.04 120 32.65 3727 10890 It is possible to observe that, despite the input parameters L0 and k were varied within a reasonable range if considering the studied landfill site-specific data relevant to the rainfall of the area and the organic content of the accepted waste, which affect the values of k and L0, respectively, significantly different results were obtained. Indeed, the LFG generation rate estimated by the model varies from a minimum of 1562 m3/h to a maximum 3727 m3/h. 190 Such variations may be acceptable if the model is used as an instrument for previsional estimations for the design of the LFG collection system, but it may lead to unacceptable errors in the case of environmental impact assessment purposes. Indeed, the quantity of LFG emitted into the atmosphere can be calculated as the difference of the LFG generated and the LFG collected. In the case of the studied landfill, where the LFG is burned in co-generation motors for the production of electricity, the amount of collected LFG is easily obtained from the motors datasheets, and it is equal to 2200 m3/h. According to the values reported in Table 2, this means that, for the studied landfill, the collection efficiency would range from a minimum of 59% to a maximum of 140%, thus giving that the amount of LFG emitted is between +1500 m3/h and -700 m3/h, with an average emission rate of 400 m3/h. 3.2 Field tests with flux chamber and static box Table 3 shows the results of the measurements conducted on the studied landfill by means of our “flux chamber”. It is possible to observe that, within the same day, the LFG emission rates measured at different inlet air flow rates are comparable to each other, as expected by theory. It is also possible to observe that there are significant differences in the LFG fluxes measured in different days: for instance, the 21 May extremely low CH4 concentration were measured (always <10 ppm) with respect to the other days. From a first analysis of the meteorological data, the LFG fluxes seem to be related to the trend of the atmospheric pressure of the 48 h prior to sampling, giving that low LFG fluxes are measured when the atmospheric pressure is higher than the average pressure relevant to the studied area, but more measurements will have to be carried out in order to allow to have sufficient data to study such correlations. Table 3: Results of the measurements with “flux chamber” Date Inlet air flow (L/h) CCH4 (ppm) QCH4 (m3/m2/s) QLFG (L/m2/h) QLFG (Mg/y) 05/03/2014 100 280 3.96E-08 2.85E-01 342.23 250 110 3.89E-08 2.80E-01 336.12 300 130 5.52E-08 3.97E-01 476.68 500 60 4.24E-08 3.06E-01 366.68 18/04/2014 60 110 9.34E-09 6.72E-02 80.67 150 70 1.49E-08 1.07E-01 128.34 300 40 1.70E-08 1.22E-01 146.67 24/04/2014 60 19 1.61E-09 1.16E-02 13.93 100 20 2.83E-09 2.04E-02 24.45 150 25 5.31E-09 3.82E-02 45.83 200 33 9.34E-09 6.72E-02 80.67 400 23 1.30E-08 9.37E-02 112.45 12/05/2014 30 26 1.10E-09 7.95E-03 9.53 80 20 2.26E-09 1.63E-02 19.56 300 14 5.94E-09 4.28E-02 51.33 19/05/2014 80 10 1.13E-09 8.15E-03 9.78 100 7 9.90E-10 7.13E-03 8.56 21/05/2014 60 7 5.94E-10 4.28E-03 5.13 80 5 5.66E-10 4.07E-03 4.89 100 2 2.83E-10 2.04E-03 2.44 200 1 2.83E-10 2.04E-03 2.44 10/06/2014 60 27 2.29E-09 1.65E-02 19.8 80 21 2.38E-09 1.71E-02 20.53 100 19 2.69E-09 1.94E-02 23.22 200 11 3.11E-09 2.24E-02 26.89 Table 4 compares the LFG specific fluxes (L/m2/h) measured with the two different sampling methods tested: “flux chamber” vs. “static box”. Of course, these are only preliminary results that will have to be verified and confirmed by the execution of other sampling campaigns, but it seems that there is a good agreement between the two sampling methods. 191 Table 4: Comparison of LFG specific fluxes measured by means of “flux chamber” and “static box” Date QLFG "flux chamber" (L/m2/h) QLFG "static box" (L/m2/h) 05/03/2014 3,17E-01 2,80E-01 18/04/2014 9,88E-02 7,95E-01 24/04/2014 4,62E-02 _ 12/05/2014 2,23E-02 _ 19/05/2014 7,64E-03 _ 21/05/2014 3,11E-03 _ 10/06/2014 1,88E-02 3,51E-02 4. Conclusions A “flux chamber” and a “static box” were designed and developed based on literature studies and other considerations deriving from our Laboratory’s experience in the field of environmental sampling for measuring LFG fluxes from landfill surfaces. The preliminary results of the measurement campaigns show a good agreement between those two measurement methods, but more data is needed in order to verify these results. Future studies should focus on the correlation between the measured LFG fluxes and the meteorological conditions, such as atmospheric pressure that seems to have a direct influence on the LFG emission rate. This study also proves that models for the estimation of LFG generation are unsuitable for environmental impact assessment purposes, because they may lead to unacceptable errors due to their sensitivity to the input parameters. References Czepiel P.M., Shorter J.H., Mosher B., Allwine E., McManus J.B., Harriss R.C., Kolb C.E., Lamb B.K., 2003, The influence of atmospheric pressure on landfill methane emissions, Waste Manage. 23, 593- 598. Davoli E., Gangai M.L., Morselli L., Tonelli D., 2003, Characterisation of odorants emissions from landfills by SPME and GC/MS. Chemosphere 51, 357-368. El-Fadel M., Findikakis A.N., Leckie J.O., 1997, Environmental impacts of solid waste landfilling, J. Environ. Manage., 50, 1-25. Kim K.-H., Choia Y.J,, Jeona E.C., Sunwoo Y., 2004, Characterization of malodorous sulfur compounds in landfill gas, Atmos. Environ. 39, 1103-1112. Mosher B.W., Czepiel P.M., Harriss R.C., Shorter J.H., Kolb C.E., McManus J.B., Allwine E., Lamb B.K., 1999, Methane emissions at nine landfill sites in the Northeastern United States, Environ. Sci. Technol. 33, 2088-2094. Palmiotto M., Fattore E., Paiano V., Celeste G., Colombo A., Davoli E., 2014, Influence of a municipal solid waste landfill in the surrounding environment: Toxicological risk and odor nuisance effects, Environment International 68, 16-24. Park J.-W., Shin H.-C., 2001, Surface emissions of landfill gas from solid waste landfill, Atmos. Environ. 35, 3445-3451. Rachor I.M., Gebert J., Gröngröft A., Pfeiffer E.-M., 2013, Variability of methane emissions from an old landfill over different time-scales, Eur. J. of Soil Sci. 64, 16-26. Sironi S., Capelli L., Céntola P., Del Rosso R., Il Grande M., 2005, Odour Emission Factors for the Assessment and Prediction of Italian MSW Landfills Odour Impact, Atmos. Environ. 39, 5387-5394. US EPA (US Environmental Protection Agency), 2008, AP 42, Fifth Edition, Compilation of Air Pollutant Emission Factors, Volume 1, Chapter 2.4 (draft): Municipal Solid Waste Landfills accessed 31.01.2014 192 Controfacciata.pdf Pagina vuota Pagina vuota Pagina vuota