MODELLING OF AEROBIC REACTORS FOR LANDFILL METHANE OXIDATION.

Transcription

1 Proceeding of the 13 th International Conference on Environmental Science and Technology Athen, Greece, 5-7 September 13 MODELLING OF AEROBIC REACTORS FOR LANDFILL METHANE OXIDATION. I.M. MANCINI, S. MASI, P. ZIRPOLI, C. AMODEO and D. CANIANI Univerity Of Bailicata, School Of Engineering, Viale Dell ateneo Lucano, Potenza, Italy Donatella.Caniani@Uniba.It EXTENDED ABSTRACT Landfill ga i produced by anaerobic degradation of organic wate. Landfill are one of the principal anthropogenic ource of atmopheric methane, a trong greenhoue ga. At the preent, abatement technique of landfill bioga conit in the energy recovery for the production of electrical energy, when the percentage of methane i in the order of 4-5% v/v. In thi cae, the complete combution and the ubequent functioning of the engine for the production of energy i enured. For percentage of the order of 3% v/v, the extracted bioga i conveyed to a ytem of ga flare which enure the complete thermal oxidation before entering into the atmophere. In all cae of low production of landfill ga or low methane concentration (mall landfill or landfill in the terminal phae of tabilization), the combution of bioga i difficult. In uch condition the bioga produced i often directly emitted into the atmophere. Technical pecification for the operation of ga flare indicate a minimum flow of 5 Nm 3 /h and a methane concentration of 3% v/v. A flow of thi ize i equivalent to an annual emiion of approximately 3 ton of CO eq. It i however known that methane can be metabolized by pecific CH 4-reducing microorganim. The aim of thi work i the evaluation of the efficiency of an aerobic bioreactor for the oxidation of methane, through the application of a mathematical model repreentative of the biological oxidation proce, by implementing a calculation algorithm. The developed mathematical model decribe the evolution of the phenomenon of methane oxidation. It i able to evaluate the efficiency of the ytem under varying operating condition with the aim of optimizing the performance of the "biofilter". Literature data have been ued in order to build the model and to drawing up the equation that decribe the proce. Through the implementation of the model in the MATLAB oftware, good reult on the performance of thi ytem were obtained. The factor that motly affect the efficiency of the proce of methane oxidation and that actually regulate the entire proce have been highlighted in thi work. The reult obtained from the mathematical model howed that the biofilter ytem i imple to implement and manage and allow the achievement of high efficiency of methane oxidation. Keyword: Landfill ga, methane oxidation, methanotroph microorganim, biofilter, methane emiion. 1. INTRODUCTION Landfill mut be equipped with ytem capable of reducing the impact during both phae of management and pot-management. A erie of control and abatement ytem mut be provided to minimize the environmental impact that may derive from landfill. The procee of decompoition of organic ubtance occur by mean of anaerobic bacteria, which are preent in landfill, and lead to the production of leachate and landfill ga (LFG). Their diffuion in the environment would caue pollution of oil, CEST13_358

2 water (urface and groundwater) and atmophere. Landfill ga, which i mainly compoed by methane and carbon dioxide, i the final product of the anaerobic degradation of organic wate. The production of bioga continue until the organic material ha not been completely degraded and can alo continue for decade. CH 4 and CO are claified a greenhoue gae (GHG). The global atmopheric concentration of CH 4 ha increaed from a pre-indutrial value of about 715 to 173 part per billion (ppb) in the early 199, and wa 1774 ppb in 5 (IPCC, 7). The atmopheric concentration of CH 4 in 5 exceeded the natural range of the lat 65. year (3 to 79 ppb) a determined from ice core (IPCC 7). Current atmopheric methane level are due to continuing anthropogenic emiion. The current contribution of methane to climate change i 18% of the total radiative balance of all long-lived greenhoue gae, LLGHG (Forter et al., 7). Many countrie, in recent year, have recognized the contribution of landfill ga emiion to the global climate change due to the greenhoue effect. Conequently mot of the indutrialized countrie provide by law the extraction and treatment of landfill ga, epecially in recently contructed intallation. At the ame time, cientific reearch ha focued it attention on the development of new technologie that are able to balance the cot/benefit ratio and, at the ame time, to adapt themelve to different cenario. The conventional method mot known and mot ued for the treatment of landfill ga i the method of flaring. Alternatively, for large torage ite, the flaring operation i replaced with a recovery and reue, often at the ame ite, of the methane preent in bioga. Thi technique lead to a benefit from an environmental and economic point of view and energy aving. Conventional treatment, uch a flaring, for mall or old landfill are not applicable from a technical and economic point of view. Traditional ytem, in fact, cannot be applied to large landfill in the firt phae of cultivation and tail, when the concentration of methane are too low. In recent decade, coniderable reearch and teting laboratory have been conducted with the aim to identify a olution to the problem of inapplicability of traditional ytem. A good olution can be repreented by the biological filtration. The biofilter i a biological reactor claified a "immobilized bioma". Structurally it conit of a metal caing (biocontainer) containing a filling material, on which the methanotrophic bioma can develop. The biocontainer i connected to the landfill ga extraction network. It i neceary to promote the contact between landfill ga and microorganim in order to enure the functioning of the ytem. In fact, only in particular condition, the methanotrophic microorganim have growth rate uch a to allow the achievement of high overall efficiencie of the ytem. Structurally, there are two different type of biofilter, aid up-flow and down-flow, depending on the input poition of the landfill ga flow. The bioga upply to the biofilter can be "active" or "paive." In active ytem, bioga i extracted from the landfill and ent to the biofilter through a conventional extraction ytem. Active ytem generally operate at contant flow and enure contant temperature. Paive ytem, on the contrary, are baed on the preure difference between the atmophere and the landfill body. It follow that, the flow i ignificantly variable a well a the temperature and the humidity. Typically, thee ytem operate at ambient temperature (Scheutz et al. 9). The cot for invetment and operation of biofiltration decreae with the increae of the biofilter ize. The average cot of invetment of a biofilter of 1,, and 4 m 3 are 18, 15 and 11 USD per m 3, repectively. The annual operational cot, including both fixed and variable cot, are 6,, 17 USD per m 3, repectively (Mele et al. 5). The aim of thi work i the evaluation of the efficiency of an aerobic bioreactor for the oxidation of methane, through the application of a mathematical model repreentative of the biological oxidation proce. The working group of the author ha alo produced a full-cale prototype of the biofilter for the evaluation of the influence of macro-parameter on proce efficiency. In thi paper we report the reult of numerical modeling. CEST13_358

3 . MATERIALS AND METHODS.1 The proce of CH 4 oxidation in the biofilter The filling material of the biofilter mut provide optimum condition for methanotrophic microorganim. Furthermore, a large pecific urface area for ma exchange i required, i.e. the material had to be fine-grained. In addition, the clogging of the material, e.g. due to the production of exopolymeric ubtance (EPS), hould be ignificantly reduced. In order to match thee requirement, a filter material generally conit of a mixture of equal volume of wate compot, peat, and pruce wood fiber (Streee at al., 3). Methanotrophic bacteria (or methanotroph) are a ubet of a phyiological group of bacteria known a methylotroph. They are unique in their ability to utilize CH 4 a a ource of carbon and energy. The complete pathway for the microbial oxidation of CH 4 to CO by methanotroph conit in intermediate tep for oxidation of CH 4 to methanol, followed by oxidation of methanol to formaldehyde (CHOH) and the ubequent oxidation of formaldehyde to formate (CHOOH) and the finally oxidation of formaldehyde to CO. In recent year everal tudie have been done on the ubject and mainly two type of methanotroph have been ditinguihed: type I and type II (Scheutz et al. 9). The methanotroph type I ue a particular enzyme defined pmmo. Mot of methanotroph type I are not able to fix nitrogen (N ). The methanotroph type II, intead, are able to fix nitrogen by uing a oluble enzyme defined MMO. Only type I methanotroph grow at low temperature (3 1 C), but both type grow at C (Börjeon et al. 4). It mut be aid, however, that it i not poible a 1% converion of the methane in microbial bioma (Scheutz et al. 9). The aerobic microbial oxidation of methane occur in the biophere, wherever CH 4 and O are preent at the ame time. Aerobic CH 4 oxidation proceed according to the following overall reaction: CH O CO H O heat 4 1 G 78 kj mol CH 4 In many tudie (Scheutz et al. 9, De Vicher et al. 1 etc.), author propoe to adopt the Michaeli-Menten kinetic of the firt-order with two limiting factor: rmax CCH C 4 o r (1) K C K C Where: r = CH 4 oxidation rate [mol/(m 3 *h)]; r max = maximum CH 4 oxidation rate [mol/(m 3 *h)]; K m = Michaeli Menten (or half-aturation) contant [mol/m 3 ]; C = CH 4 concentration [mol/m 3 ]; C O = O concentration [mol/m 3 ]. m. Environmental factor influencing the proce The biological oxidation of methane i a proce that occur normally in the cover layer of landfill. In thee cae oxidation i controlled by a number of environmental factor: oil texture, temperature, oil moiture content, CH 4 and O upply, nutrient, etc. Environmental condition are very important for the effective CH 4 oxidation rate. Temperature ha a deep effect on all biological procee, including CH 4 oxidation activity. Moiture i an eential factor for micro-organim to utain their activity a it i the tranport medium for nutrient upply and alo for removal of reidual metabolic compound (Park et al. ). Too much moiture may low down gaeou tranport procee in the oil becaue molecular diffuion in water i about 1 time lower than in the air (Bender et al. 1995). Another important factor that affect the proce of methane oxidation within a biofilter i the preence of exopolymeric ubtance (EPS). EPS are high molecular weight compound that conit mainly of polyaccharide and m o CEST13_358

4 are produced by many bacteria, including methanotroph. Accumulation of EPS reduce, intrinically, the poroity of the ytem by preventing, or at leat reducing, the gaeou ubtrate diffuivity or even creating preferential channel for the gaeou flow. The regular bioma wahing i ued to overcome the problem (Scheutz et al., 9). A in mot of the chemical reaction, the ph play an important role for the ucce of the proce: the optimal ph value for the growth of methanotrophic i between 5,5 and 8,5. Due to the change of the atmopheric preure, paive landfill ventilation caue periodically the revere of the ga flux. Thi phenomenon can caue the air flow from the biofilter into the landfill during period of high atmopheric preure (Gebert et al.,1). Thi mean that the methanotrophic population of the biofilter i regularly deprived of methane, ometime for longer period, with conequent lowering of the methan oxidation rate (Gebert et al., 3)..3 The developed mathematical model The implifying aumption for the realization of the mathematical model provide that the biofilter i a Plug Flow Reactor (PFR) (Figure 1). The flow condition are a follow: complete mixing in the tranvere direction; abence of mixing in the longitudinal direction (i.e. along the z direction); abence of gradient of temperature in the ytem. Figure 1 - Schematization of the reactor decribed by the mathematical model According to the principle of the ma conervation: IN OUT ± GENERATION = ACCUMULATION () the input term indicate the methane ma flow rate which enter inide the control volume: IN W TOT. y CH 4 where W TOT. i the ma flow rate [mol*h -1 ] and y i the methane mole fraction. W TOT. can be calculated with the following equation: Q PTOT mol WTOT T R h where: Q = flow rate entering expreed in [Nm 3 /h]; P TOT = atmopheric preure = 1 [atm]; T = temperature in Normal Condition = 73,15 [K] R = ga contant = 8.*1-5 [(m 3 *atm)/(mol * K)] The out term will be: OUT W TOT. y CH 4 z zdz CEST13_358

5 For the generation term it mut be conidered the diffuion proce that occur within the ytem (Figure ). Figure Schematization of the tranport phenomena through the biofilm The methane flow, N, which effectively reache the bioma can be indicated with the N k a C k a C C (3) following expreion: g g k g : methane tranport coefficient [m/h]; a : pecific urface area [m /m 3 ]; ΔC: concentration difference [mol/m 3 ]. dnch The accumulation term ACC 4, auming that the ytem i in teady tate dt condition. Replacing the above term in the equation (), we obtain: W y W y k a C y y dv TOT z TOT CH zdz g TOT CH CH Now if we define: Q = flow rate entering in the biofilter [m 3 /h]; S = effective biofilter urface [m ]; r = CH 4 oxidation rate [mol/(m 3 *h)]; C TOT. = total concentration = (P TOT.)/(R*T) [mol/m 3 ]; r MAX = CH 4 maximum oxidation rate [mol/(m 3 *h)]; K = Michaeli Menten or half-aturation contant [ad.] and recalling that the apex indicate what actually come to the bioma, we can obtain the end ytem of equation relative to the variation of the methane concentration along the biofilter height: dych k a 4 g y y S dz Q r y y kg a C rmax y rch 4 K y CH 4 TOT Finally, we defined the degree of methane converion (x ) uch a: ych, y 4 CH 4 x % CH 1 4 y,.4 Model parameter The firt parameter i the tranport coefficient k g. The term (k g*a) that appear in the equation (3) i defined tranport volumetric coefficient, [h -1 ]. A uggeted by Seongyup CEST13_358

6 et al. (8), we conidered that the numerical value of thi coefficient i a function of the volumetric flow rate of ga, according to the following mathematical relationhip: a 69.4 Q / S. k g 46 The relationhip between r MAX and T wa determined according with the tudy of Cella Mazzariol et al. (9): r r r MAX MAX MAX per T 5C E - 5 T per T 3 8C.17E - 3T 8.5E - T 3.48E - 1 per 5 T 8C The parameter K, i.e. the Michaeli Menten (or half-aturation) contant, i alo function of the temperature. The trend of K a a function of temperature wa determined according with the data preent in the work of De Vicher et al. (1): K K K E - 7 T.5171 per T 5C per T 3 9.6E - 6 T 35C 3.54E - 6 T 6.4E - 4 per 5 T 35C After aigning the numeric value to the remaining parameter, uch a vacuum degree (ε=.5) and effective biofilter urface (S=16.5 m ), we tarted the numerical imulation with the MATLAB oftware (The MathWork). For the technological unit implemented the calculation parameter aume the following value: filter effective height: H =.65 m; landfill ga flow rate: Q = 5 Nm 3 /h; inide temperature: T = 5 C; methane mole fraction in landfill ga flow: y =.5; total concentration of methane preent in the inflow: C TOT. = mol/m RESULTS AND DISCUSSION The reult of the imulation allow u to repreent graphically the efficiency of the biofilter a a function of the height (Figure 3). The ytem loe quickly efficiency approaching the condition of limiting ubtrate (CH 4), a we expected from the model of Michaeli-Menten. It i evident that the complete removal of methane by a biological ytem i not obtainable, a oppoed to a thermal ytem. In real application, the phyiological lo of efficiency i compenated by the wider operating range a compared to thermal ytem which are le flexible. The methane mole fraction decreae by a rate equal to 36.8%, for the ytem conidered in thi work (H=.65 m) Biofilter 4 height (m) Degree of methane converion- x (%) Figure 3 Variation of the degree of methane converion x a a function of the biofilter height The obtained reult can help evaluating the ytem repone to the variation of important parameter that regulate the proce of methane oxidation. Different operating condition are repreented in the following table: CEST13_358

7 Biofilter height (m) Biofilter height (m) Table 1 Different operating condition of the ytem CASE H (m) T( C) Q(Nm 3 /h) y C TOT.(mol/m 3 ) Variable parameter Variable parameter Variable parameter The following figure how the graphic repreentation of the reult for each operating condition (Figure 4 and 5). Q = 1 Q = 5 Q = 35 Q = 5 Q = 75 Q = 1 Biofilter height (m) Degree of methane converion - x (%) y=.5 y=.1 y=. y=.35 y=.5 Biofilter height (m) Degree of methane converion - x (%) Figure 4 A) Cae 1: Variation of the degree of methane converion x a a function of the biofilter height by varying the flow. B) Cae : Variation of the degree of methane converion x a a function of the biofilter height by varying the methane mole fraction T = 1 C T = 15 C T = C T = 3 C T = 5 C Degree of methane converion - x (%) T = 5 C T = 6 C T = 7 C T = 75 C Degree of methane converion - x (%) Figure 5 Cae 3: Variation of the degree of methane converion x a a function of the biofilter height by varying the temperature: A) from -3 C to 5 C, B) from 5 C to 79 C. The reult demontrate that the increae of the flow rate value involve a worening of the oxidative proce in term of methane converion. Alo the increae of the methane concentration in the gaeou flow involve a worening of the converion yield. Finally, the biological procee have a trong dependence by the temperature change, that affect the kinetic parameter of the biological reaction. Temperature i a parameter that greatly influence the proce of methane oxidation. In detail we have that, for low temperature, the degree of methane converion i an increaing function of T: there i an improvement of the performance of the proce. Intead, over 5 C (value correponding to the maximum of the converion) the increae of the temperature begin to have the oppoite effect, i.e. it reult in a decreae of the converion in the ytem. When the biofilter i integrated into the landfill cover ytem and not externally heated, it temperature regime in general follow the change of the ambient temperature, according to the thermal conductivity of the choen filter material (Gebert et al., 6). Through thi analyi, therefore, we can conclude that the temperature range in the biofilter hould CEST13_358

8 tand between 4 C and 6 C, in order to optimize the treatment ytem of the methane. The temperature control i eay but expenive. The reduction i alway poible by operating on the ratio air/landfill ga, while the increae of the temperature can be made through a heat exchanger, but it i economically very expenive unle it ha a flow reidual heat (e.g. other ytem uing bioga). For thee reaon, a trong inulation of the unit i eential in order to avoid the temperature drop in winter period. In thee cae, a layer inulation equivalent of more than 5 cm, with a thermal conductivity equal to.34 [W/(m*K)], i required. 4. CONCLUSIONS The propoed biological ytem i not antagonitic to the energy production ytem from landfill ga, but fall into the category in which the reduced production of landfill ga (mall landfill and/or landfill that have been cloed for many year) doe not allow the intallation of energy recovery ytem. The biological ytem ha proved to be an economic ytem (eae of implementation and management) with good performance. Moreover, the reult of the imulation how that the landfill ga treatment through biological ytem, in the above-mentioned condition, have yield of methane abatement higher than 7%. The working group of the author i working on a full-cale biofiltration ytem in the landfill of the municipality of Venoa (Potenza - Italy). The technical dicuion about the firt experimental reult obtained will be potponed in the next paper. 5. REFERENCES Bender, M. & Conrad, R. (1995) Effect of concentration and oil condition on the induction of oxidation activity. Soil Biology and Biochemitry, 7, Börjeon, G., Sundh, I. & Svenon, B. (4) - Microbial oxidation of at different temperature in landfill cover oil. FEMS Microbiology Ecology, 48, Cella Mazzariol P.P., G.F. Galanzino and C. Ubbiali (Entorga Italia SRL) Landfill methane degradation to reduce greenhoue gae emiion. Cella Mazzariol P.P., G.f. Galanzino, C. Ubbiali, D. Mainero, G.L. Soldi and E. Magnano Treatment of landfill bioga uing the Geko ytem.- Accepted paper Sardinia Sympoium 9. De Vicher, A., Schipper M. & Van Cleemput O. (1) Short-term kinetic repone of enhanced methane oxidation in landfill cover oil to environmental factor. Biology and Fertility of Soil, 33, Forter, P., Ramawamy, V., Artaxo, P., Bernten, T., Fahey, D.W., et al. (7) Change in atmopheric conituent and in radiative forcing. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marqui, M., Averyt, K.B., Tignor, M. & Miller, H.L. (ed.): Climate Change 7: The Phyical Science Bai. Contribution of Working Group I to the Fourth Aement Report of the Intergovernmental Panel on Climate Change. Cambridge Univerity Pre, Cambridge, UK and New York, NY, USA. Gebert, J. & Gröngröft, A. (6) Performance of a paively vented field-cale biofilter for the microbial oxidation of landfill methane. Wate Management, 6, Gebert, J., Gröngröft, A. & Miehlich, G. (3) Kinetic of microbial landfill methane oxidation in biofilter. Wate Management, 3, Haubrich, R. & Widmann, R. (6) Evaluation of aerated biofilter ytem for microbial methane oxidation of poor landfill ga. Wate Management, 6, Mele, R.W. & Van der Werf, A. (5) Biofiltration for mitigation of methane emiion from animal hubandry Environmental Science and Technology, 39, IPCC (7) Summary for policymaker. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marqui, M., Averyt, K.B., Tignor, M. & Miller, H.L. (ed.): Climate Change 7: The Phyical Science Bai. Contribution of Working Group I to the Fourth Aement Report of the Intergovernmental Panel on Climate Change. Cambridge Univerity Pre, Cambridge, UK and New York, NY, USA. Park, S., Brown, K.W. & Thoma, J.C. () The effect of variou environmental and deign parameter on methane oxidation in a model biofilter. Wate Management and Reearch,, Scheutz C., Kjelden P., Bogner J.E., De Vicher A., Gebert J., Hilger H. A., Huber-Humer M., Spoka K. (9) Microbial methane oxidation procee and technologie for mitigation of landfill ga emiion. Wate Management and Reearch, 7, Seongyup K., Dehue M. A. Determination of ma tranfer coefficient for packing material ued in biofilter and biotrickling filter for air pollution control. Experimental reult. Chemical Engineering Science 63 (8) Streee J. & Stegman, R. (3) Microbial oxidation of from old landfill in biofilter. Wate Management, 3, CEST13_358