Energy recovery from ventilation air methane via reverse-flow reactors

Energy 92 (2015) 13e23

Contents lists available at ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

Energy recovery from ventilation air methane via reverse-flow reactors Krzysztof Gosiewski*, Anna Pawlaczyk, Manfred Jaschik Institute of Chemical Engineering, Polish Academy of Sciences, ul. Bałtycka 5, 44-100 Gliwice, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 December 2014 Received in revised form 26 May 2015 Accepted 1 June 2015 Available online 26 June 2015

Nearly 70% of the methane released from hard coal seams, as the so-called Ventilation Air Methane, is emitted to the atmosphere with the air discharged by the mine ventilation system. Therefore, utilization of this emission, especially with a rational heat recovery becomes an important challenge for hard coal mines. The paper proposes combustion in Thermal Flow Reversal Reactors, currently as the most promising and technically advanced method of solving this problem. The operating principle of such reactors is briefly described with a short review of the current literature on the subject, particularly focussing on aspects of heat recovery. A progress report of research and development activities, carried out in the recent years in the Institute of Chemical Engineering, Polish Academy of Sciences in Gliwice, Poland, has been given. This part provides a brief overview of kinetic studies on thermal combustion, results of experiments carried out on a research and demonstration plant, discussion of computer simulations as well as preliminary analysis of the possibilities of the process intensification. The article draws attention to the possibility of thermal asymmetry formation in the flow reversal reactors. The ways of the process control to prevent asymmetry are also discussed. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Reverse-flow reactors Thermal combustion (VAM) Ventilation air methane Energy recovery

1. Introduction When coal is mined large amounts of methane are released from hard coal seams. There are three streams of gas containing methane which is discharged from the coal: gas drained from a seam before mining (60e95 vol.% CH4), gas drained from work areas of the mine (30e95 vol.% CH4) and emitted with mine ventilation air (0.1e1.0 vol.% CH4) [1], as the so-called VAM (Ventilation Air Methane). In Poland due to safety regulations CH4 concentration of VAM is lower and according to current limitations it should not exceed 0.75 vol.%. Poland's State Mining Authority estimates [2] that in 2013 domestic coal seams discharged totally 847.8 million m3 of methane, from which only 276.6 million m3 (i.e. about 33%) as a gas with a higher concentration drained from the seam could be effectively used for energy production. Remaining amount of methane 571.2 million m3 (appr. 67%) is emitted to the atmosphere as the VAM, with an average concentration of 0.55 vol.% CH4 (acc. [3]). In other countries e.g. in the US [1] it can

* Corresponding author. E-mail address: [email protected] (K. Gosiewski). http://dx.doi.org/10.1016/j.energy.2015.06.004 0360-5442/© 2015 Elsevier Ltd. All rights reserved.

reach or even slightly exceed 1 vol.%. In 2009 the global VAM emission was roughly [4] 28.7 billion m3 of methane. That causes that the gaseous fuel is wasted unnecessarily, simultaneously contributing to greenhouse effect, since methane as GHG has a GWP (global warming potential) at least over 20 times higher than CO2. Therefore, combustion even without a heat recovery could be ecologically or economically attractive, but energy recovery significantly improves the profitability of VAM utilization. Thus, the present paper is focused on possibilities of energy recovery. Interest in this problem appeared nearly 20 years ago. Su et al. [5] discuss and compare several methods of VAM utilization and divide them into two basic categories: ancillary uses and principl uses. For the ancillary uses, ventilation air is used to substitute ambient air in various combustion processes, including gas turbines, internal combustion engines and coal-fired power stations. For the principal uses, methane in ventilation air is a primary fuel. Methods of ancillary uses seem to be the easiest and the cheapest. However, they are hardly feasible in practice because of large flow rates of ventilation air, at least of the order of hundreds of thousands of cubic meters of air per hour. It is difficult to find or build up a power station which needs such large amounts of air for combustion in the vicinity of a ventilation shaft. Such plant would have a power of several hundred MWs.

14

K. Gosiewski et al. / Energy 92 (2015) 13e23

As the principal uses in Ref. [5] the following technologies are discussed:

applications to the Patent Offices in the USA, Canada, Australia, China, Ukraine and Russia have been submitted.









2. Principle of flow reversal reactors operation

CFRR (Catalytic Flow Reversal Reactor), TFRR (Thermal Flow Reversal Reactor), CMR (Catalytic Monolith Combustor), Catalytic lean burn gas turbine, Recuperative gas turbine, Methane concentrator.

The last listed i.e. the methane concentrator can be applied in two ways either to enable direct combustion, when methane may be concentrated to directly combustible gas or to facilitate another principale method of VAM utilization by some concentration increase. A concentrator could be used to enrich methane in mine ventilation air to levels that meet the requirements of lean-burn methane utilization technologies, such as CFRR or TFRR. Methods of increasing the methane concentration in ventilation air are nowadays investigated but there is no information about the costeffective industrial applications. Since the concentration of VAM is not very high, utilization of this source by the direct combustion in a conventional process is possible only with huge amount of additional fuel (e.g. as in the ancillary use). However, such a lean air-methane mixture can be not only autothermally combusted without the additional fuel but also, for a little higher CH4 concentrations, even with energy recovery. Then, a special combustion technology e.g. FRR (Flow Reversal Reactor): either catalytic e CFRR or thermal e TFRR should be applied. These methods have specific advantages but also drawbacks. Thus, only CFRR and TFRR are seriously considered for industrial application, therefore only these two options will be taken into account in this paper. The idea of using the FRRs for VAM combustion is not quite new. At present there are even rather large industrial installations working according to this technology, especially in TFRR option [6]. Institute of Chemical Engineering of the Polish Academy of Sciences (ICE-PAS) has been engaged in the problems of VAM combustion since 2001. The first studies focused on the CFRRs, and then since 2007 they have been directed to the TFRRs. Significant progress in these studies was achieved during the project [7] when for combustion of low concentrated methane-air mixtures there was built a research and demonstration plant. In 2011 a Letter of Intent was signed and cooperation with the Kompania We˛glowa S.A. (Coal Mining Company) was initiated. In the following year a Consortium Agreement on the research and implementation associated with the combustion of VAM was formed. Moreover, in 2011, the method developed in ICE-PAS has been registered as an invention under the PCT procedure [8], and in 2013,

2.1. General remarks Generally, FRR [9] may be used not only for exothermic reactions but also for endothermic ones [10]. However, in industrial applications they are used most often for exothermic reactions, usually for combustion and, in such a case, they belong to the category of the socalled autothermal reactors [11]. FRR is a chemical reactor, integrated with a common device with regenerative heat exchanger what may allow maintaining the process without any additional fuel. Regenerative heat exchange is realized by periodic reversals of flow direction through the reactor packing. Autothermicity is ensured when the amount of heat generated by combustion is at least equal to the heat losses of the system. If it is higher, a surplus of heat can be utilized outside. An excellent review of various concepts of the autothermal reactors is given in Ref. [11]. The FRR is a technology that allows burning diluted flammable gaseous pollutants, without additional source of fuel. Therefore, it can also be used to combust the methane contained in ventilation air. The two types of such reactors are known: CFRR (Catalytic Flow Reversal Reactor) and TFRR (Thermal Flow Reversal Reactor). Examples of both types of FRRs are shown in Fig. 1. The structure of them is similar. The main difference is that in CFRR except for heat regenerating inert material, additional layers of catalyst are applied. Simultaneously in the flow reversal reactor the two phenomena occur: regenerative heat exchange in the bed and chemical exothermic reaction which gives additionally generation of the reaction heat. Heat for regeneration is stored in the inert (i.e. catalytically inactive) bed and in CFRR the heat is partially accumulated also in the catalyst layers. Heat regeneration in this case €m is not achieved by rotating drum, as it is realized in the Ljungstro rotary heat regenerator, well-known in boiler technology, but by cyclic switch of the flow direction. In Fig. 1 the alternative flow directions are shown either by green or blue arrows. The flow reversal reactors have been increasingly used in practice, especially in catalytic oxidation of various organic compounds that produce too little heat to sustain autothermicity in a traditional stationary reactor with external e.g. shell & tube heat exchanger, and also in processes in which efficient heat recovery of the exothermal reactions is of crucial importance. The relevant literature is abundant; it will be only mentioned here a monograph of Matros [9] or e.g. extensive papers [12,13], which contain both a general problem description, theoretical background and applications of the concept. Lit. [13] quotes as much as 174 references. The idea of the reverseflow combustion is also not new, since widely cited in the

Fig. 1. General idea of flow reversal reactors: (a) Catalytic flow reversal reactor e CFRR, (b) Thermal flow reversal reactor e TFRR.

K. Gosiewski et al. / Energy 92 (2015) 13e23

literature the first patent of Cottrell [14], describing an apparatus such as a flow reversal reactor, was registered as early as in 1938. From the newer literature a Review paper [11] is worth mentioning, as it contains 62 relevant references. It is also worth recalling a monograph [15] in which a discussion of the reverse-flow reactors constitutes a substantial part. The concept of using CFRR for VAM combustion has similarly a long history and is quoted e.g. in Refs. [5,16] as a feasible option to this end. First attempt to VAM combustion in CFRR was supposedly done in Boreskov Institute of Catalysis [17]. At present the most advanced in development of catalytic reversal technology of lean air-methane mixtures combustion is CANMET from Canada [18,19]. Recently CANMET has developed and tested a CFRR design termed CH4MIN and in cooperation with the firm Sindicatum Sustainable Resources from Singapore will develop commercial process in a coal mine in Shanxi province, China. Non-catalytic oxidation in TFRRs is now frequently regarded as an attractive option (cf. [5,16]). MEGTEC Company has built, combusting VAM, electrical power generation plant on commercial scale (cf. [20]) at the West Cliff Colliery in New South Wales (Australia) e called the VOCSIDIZER™. The plant has been put into operation in April 2007. It is processing 250,000 m3/h of ventilation air (0.9% CH4) to generate 6 MWe of electrical power. The relatively high concentration is maintained, however, by combining coal mine drainage gas with the mine's ventilation air flow. Similar unit VAMOC™, but slightly different from that developed by MEGTEC, with ventilation air capacity of 5100 m3/h was designed and built at JWR (Jim Walker Resources)'s Mine No. 7 in Brookwood, Alabama [1] by Canadian firm Biothermica, teamed with JWR (Jim Walker Resources) Technologies. Looking at industrial productivity of these installations it can be concluded that the TFRR technology is much better suited for real coal mines applications than the CFRR one.

15

Currently in coal mines the high concentrated drainage gas (30 to even ca. 90 vol.% CH4) is entirely and effectively combusted in internal combustion engines. Therefore combining this gas with VAM in order to increase its concentration for combustion in FRRs in the mines is regarded as unattractive. This is probably the reason why the method of reversal combustion of VAM develops slowly in the world. Experimental results reported below show that the heat recovery in TFRR is feasible with no need of combining VAM with the coal mine drainage gas if only VAM concentration is higher than 0.4 vol.%. In the ICE-PAS both methods of VAM utilization (i.e. CFRR and TFRR) were taken into account, however, this paper presents only the TFRR studies, carried out in the recent years, which will be discussed in more details. 2.2. Heat recovery systems: “central cooling” vs. “hot gas withdrawal” their advantages and drawbacks When amount of heat generated by reactions is high enough then some part of heat can be withdrawn from the reactor to be utilized outside without loss of autothermicity. Matros [13] presents examples of various arrangements of heat removal from FRRs. Basically there are two systems of heat withdrawal from FRR for heat utilization: either through built-in heat exchanger (e.g. boiler), named as central cooling (shown in Fig. 2 a&b) or using the hot gas withdrawal (terminology cf. [21]) from central part of the reactor to outside (Fig. 2 c&d). Essence of the central cooling is that the gas after cooling returns to the reactor. Centrally located cooling heat exchanger (or exchangers) can obviously be also placed outside the reactor as shown e.g. in Ref. [13]. Generally in such a case, heat exchange analysis does not change much, except for possibly larger heat losses (being a drawback), but more convenient construction (being an advantage) is obtained. Nevertheless, it is

Fig. 2. Examples of various types of flow reversal reactors with heat recovery in steam boiler: (a) CFRR or (b) TFRR with central cooling and (c) CFRR or (d) TFRR with hot gas withdrawal.

16

K. Gosiewski et al. / Energy 92 (2015) 13e23

important that the gas returns to the reactor, to be finally combusted, while in hot gas withdrawal system, the withdrawn stream is usually discharged directly to the atmosphere. On the other hand the withdrawn gas can be cooled more deeply than in the central cooling method. Methane combustion is highly exothermic where the reaction rate rapidly grows with temperature. Therefore in FRRs, only very narrow combustion zone moves back and forth along the reactor bed. However, without difficulty, the reactor can be designed so, that full combustion is achieved in input Section before the gas reaches centre of the reactor. Thus, this withdrawn part of the gas after cooling flows out from the exchanger directly to the atmosphere without a problem. Both heat recovery systems may be applied in the CFRR as well as in the TFRR. More options of arrangement of the FRR with a heat recovery unit can be found in literature e.g. in Ref. [13], but they may have only minor meaning. Not only amount of the heat recovered but utility of produced energy carrier should have an important meaning. The most valuable is high-pressure steam or even better electrical energy production. To this end, as the recovery heat exchanger, use of the steam boiler is preferred. The central cooling option, with the highpressure steam boiler built into the reactor is hardly feasible, since professional steam boiler consists of a couple of separate parts: economizer, evaporator and superheater. Putting it all inside the chemical reactor is very difficult, not speaking about possibility of future overhaul or maintenance. Therefore this option from the practical point of view is feasible only when hot water or a low grade heat carrier will be produced. The process thermal asymmetry may appear in both discussed heat recovery systems, and in both types of reactors: the CFRR and the TFRR. A weak point of the central cooling system is a discontinuity in the middle of the temperature profile (temperature drop of DT, shown by the arrow in Fig. 3(a)) causing that the system is more prone to asymmetry in the reactor temperature profiles. This feature illustrated here as an example is described in more details in Ref. [22]. Earlier, the possibility of similar asymmetry in a cooled FRR was indicated in Refs. [23e27], and multiplicity and bifurcation analysis were also discussed therein. When the hot gas withdrawal is applied the asymmetry may also appear, but as it was observed it happens only for rather low fuel concentration or very high amount of the recovered heat. An example of such experimentally measured profiles in TFRR with 17% hot gas withdrawal is shown in Fig. 3(b), where for inlet concentration of CH4 1 vol.% only slight asymmetry was formed. It was observed in multiple experiments

with TFRR that “escape” into the asymmetry happens incidentally without apparent reason. In situation when due to the asymmetry only one reactor Section is hot, then it becomes very unfavourable for the heat recovery and for the reactor run. Simulations of the two heat recovery systems applied for CFRR with Pd catalyst [22] have shown that the hot gas withdrawal system guarantees more favourable symmetry than the central cooling. Authors from the University of Alberta, and the company's CANMET also confirm [28] that “this method avoids the sudden temperature discontinuity observed when a heat exchanger is used inside the reactor” and temperature profile remains smooth and continuous. Therefore in the present paper only the hot gas withdrawal option will be taken into account because this system guarantees higher heat recovery efficiency and more stable symmetry of the temperature profiles in the FRR. However, special control algorithms should be used to protect against the asymmetry of temperature, if it began to form. Generally these algorithms should provide different half-cycle times for subsequent reversals in order to move the asymmetric profile back to the centre of the reactor. This could be done by many ways e.g. see Patent Cooperation Treaty applications [8,29]. The only advantage of the central cooling over the hot gas withdrawal is that the gas flows back to the reactor after leaving the heat exchanger. In this way if the gas still contains unburned components, combustion may be continued again after cooling. On the other hand, combustion should be completed in the first half (Section) after the reactor inlet in the hot gas withdrawal systems. However, nearly full combustion in the first Section of the reactor is practically easy to realize. The central cooling is unfortunately much more vulnerable for creation of strong asymmetries of the temperature profiles, what is serious drawback of the heat recovery method. 2.3. Catalytic or thermal FRR: which is better choice? A problem of advantages and drawbacks of the both solutions: catalytic-CFRR or thermal-TFRR was discussed in more details in Ref. [30]. Generally CFRR might be advisable when VAM concentration is so low that rational heat recovery is impossible or unprofitable. On the other hand, temperature which could appear in the CFRR is too high for low-cost catalysts. Therefore, in such a reactor only expensive noble metal catalysts may be used. A practical and very important problem of finding a cheap catalyst for methane combustion remains still unsolved. As claimed in Ref. [30] when the heat recovery is seriously taken into account the TFRR is better and moreover, a much cheaper solution.

Fig. 3. Examples of asymmetric temperature profiles in reverse-flow reactors during a single half-cycle in cyclic steady state. (a) 3D plot of computer simulated profiles in CFRR with Pd catalyst for combustion of 1 vol.% CH4 with heat utilization by central cooling causing discontinuity of DT ¼ 120 K by temperature drop in middle of profile. (b) 2D experimentally measured profiles TFRR for non-catalytic combustion of 1 vol.% CH4 on research & demonstration plant (described in Section 3.2) with heat utilization by 17% (ca. 70 m3/h) hot gas withdrawal.

K. Gosiewski et al. / Energy 92 (2015) 13e23

Matros & Bunimovich in Ref. [31] claim that CFRR requires adiabatic temperature increase DTad > ca. 15  C to be autothermal. A similar threshold for TFRR is 50e90  C acc. to [31] or 45e70  C acc. to [32]. Estimated adiabatic temperature increase for methane combustion is 265  C per 1.0 vol.% CH4. It means that VAM combustion in the CFRR should be autothermal for inlet CH4 concentrations above ~0.06 vol.% while the TFRR requires at least ~0.19 vol.%, what generally agrees with the conclusions reported in Ref. [5]. Thus advantage of CFRR over TFRR for very low concentrations seems to be obvious. When significant heat recovery is taken into account, i.e. practically for concentrations above ca. 0.4 vol.%, the maximum temperature in CFRR seems to be too high for any low-cost catalyst cf. [33]. When concentration approaches 1 vol.%, even if Pd catalyst is used its temperature will be close to the permissible limit. Temperatures in the TFRR are approximately of ca. 300  C higher than in CFRR [30], therefore maximum temperature may reach or even sometimes exceed 1000  C. This is obviously beneficial when efficient heat recovery and quality of the produced heat carrier are taken into account (cf. [34]). On the other hand, a maximum temperature, significantly higher in TFRR than in CFRR, may be considered as disadvantage, due to the risk of NOx emission appearance [32,35]. Sometimes, as an argument against TFRR more expensive construction materials are mentioned. However, experiments carried out on the TFRR research & demonstration plant (see Section 3.2) revealed that maximum temperature actually did not exceed 1100  C therefore both these possible drawbacks occurred to be insignificant. An analysis of the influence of temperature in the reactor on the amount of recovered heat (cf. [34]) has shown that the higher temperature, the higher heat recovery coefficient (Eq. (2)). Thus significantly higher temperatures which appear in the TFRR are an important advantage, not a drawback.

3. R&D studies on the thermal combustion of lean methaneair mixture e short review of own results 3.1. Kinetic studies Mathematical modelling and design of the thermal reactors requires a reliable and relatively simple kinetic model of the combustion. Although a number of detailed descriptions of the homogeneous combustion kinetics are available in the literature (including those which take into account complex freeeradical reactions), their practical usefulness is doubtful. For this reason an attempt to obtain a simple, one- or two-stage model, which directly describes a global kinetics but neglects intermediates and radicals was undertaken (cf. [36e38]). However, a special attention was paid to a possible mechanism of carbon monoxide formation as the only one intermediate product that may appear on the outlet. The studies were done both for combustion in the free space and for the process taking place over various honeycomb monolith packings. The three following reactions were investigated [36,38] to find a simple kinetic model, but reliable enough for practical purposes: Methane direct oxidation to carbon dioxide: CH4 þ2O2 /CO2 þ2H2 O (I) Methane oxidation to carbon monoxide: 2CH4 þ3O2 /2CO þ 4H2 O (II) Oxidation of carbon monoxide to dioxide: 2CO þ O2 /2CO2 (III) Due to experimentally observed, CO presence as an intermediate product, methane combustion was studied according to the three reaction mechanisms:

17

 Parallel mechanism of the reactions (I) and (II),  Consecutive mechanism of the reactions (II) and next (III),  Consecutive-parallel mechanism which joins consecutive mechanism of (II) and (III) in parallel with the reaction (I). The consecutive mechanism occurred to be the most suitable and accurate [37,39], therefore it was implemented into a model used in simulations. The kinetic parameters presented in Refs. [36e39] were determined for the following Arrhenius type kinetic equation form, where the reaction rates rhom; j are defined relative to the total monolith volume (not to the volume of the monolith channels, only):

rhom; j ¼
ε



Ej dC ¼ k0;j exp  C aj dt RT

(1)

where: C ¼ CCH4 for the reactions (I) (direct oxidation CH4 to CO2) and (II) (oxidation CH4 to CO), C ¼ CCO for the reaction (III) (oxidation CO to CO2). Detailed description of estimation of the kinetic parameters is given in Ref. [37]. Kinetic experiments were carried out in a laboratory bench tubular oven filled with cylindrical samples of various monoliths. Reactor was symmetrically placed in an oven. A scheme and photographs of the experimental setup are shown in Fig. 4. The kinetic study enabled us to formulate a hypothesis how thermal methane combustion takes place in a monolith bed. The results of the study suggest that combustion of a lean methane-air mixture takes place with a larger or smaller share at the surface of the solid bed phase (heterogeneous combustion at the surface e with radicals generation at the monolith wall) and also in the free space of the monolith channels (purely homogeneous combustion). This allows supposing that in the temperatures close to initiation of the reaction, combustion runs mostly at the surface, while in higher temperatures reaction runs mostly in the free space of the monolith channels. This is attested by growing values of Ej, k0,j with the temperature increase. Systematized results of the performed kinetic studies, with a unified notation can be found in Ref. [39]. 3.2. Research and demonstration plant A research project on lean methane-air mixture combustion in the TFRR was carried out in the ICE-PAS [7]. Within this project a research & demonstration plant was built to carry out experiments as a base for future industrial development. Overall concept of the plant is similar as in Fig. 2(d). For the convenience of implementation of the reversal valves, the both parts filled with inert packing are not arranged in a vertical layout (one over another), but in a horizontal arrangement (side by side). Such an arrangement is used very often in practical embodiments of FRRs. The TFRR installation, shown in Fig. 5, consisted of 2 vertical ceramic monolith beds arranged in Sections I and II connected by a duct at the top. Part of the hot gas could have been withdrawn from this duct to the hot gas cooler, which enabled to estimate the feasible amount of the heat to be recovered. Reversal valves are located close to the bottom part of the reactor. The study, besides the plant experiments, comprised thermal combustion kinetic studies (see Section 3.1 and [36,37]) and mathematical model simulations (see Section 3.3 and [40]). In the experiments the concentration of methane was adjusted from 0.1 to 1 vol.% by mixing natural gas with air, to simulate various VAM concentrations. Electric heaters mounted at the top of the TFRR were used only for preheating the

18

K. Gosiewski et al. / Energy 92 (2015) 13e23

Fig. 4. Scheme and photographs of experimental setup for kinetic study of non-catalytic thermal methane oxidation.

monoliths to enable start of normal operation. After every change of the plant parameters, due to the large heat capacity of the equipment, a time of at least a couple of hours was necessary to obtain the so-called CSS (Cyclic Steady State). For CSS definition see Refs. [40e42]. The experiments were carried out in nine series of experiments, each lasting 5 consecutive days, 24 h a day. Gas feed was varied from 200 to 400 m3STP /h. Averaged experimental results for a gas feed of ca. 400 m3STP /h are presented in Table 1. For methane concentrations equal or higher than 0.22 vol.% operation was autothermal, but reasonable heat recovery is effective only for concentrations above 0.4 vol.% (marked with bold characters in Table 1). As the experimental results were very promising, construction of an industrial pilot plant is planned as a next step. The heat recovery coefficient is calculated as the ratio of the heat recovered (Qrec) to the total heat generated by methane combustion (Qgen):

Heat recovery coefficient ¼

Qrec Qgen

(2)

Nowadays, there are many hard coal mines which operate deep underground (800 m or deeper). In such a case the mine has throughout the year a real demand for air conditioning, necessary during mining from these deep coal seams. Therefore use of the recovered heat to produce the so-called icy chill water appears to be a right solution, since it allows using the recovered heat at the spot in the mine. In Poland the first such air conditioning system [43] was launched as early as in 2000, at the coal seam 853 m deep wek”. A co-generation system was powered in the Coal Mine “Pnio by the heat of combustion of methane from the coal mine drainage gas, and not using the heat of VAM combustion. The cooling capacity was 5.0 MWcool. Cooling capacity for an average mine can be thus estimated as approx. 4 to 6 MWcool. As can be seen in Table 1 the heat recovered from an average ventilation shaft, when emits VAM in concentration of 0.4e0.5 CH4 vol.% it may be also utilized

Fig. 5. General view of research & demonstration plant (a) with main TFRR elements indicated by arrows and flowsheet of plant (b).

K. Gosiewski et al. / Energy 92 (2015) 13e23

19

Table 1 Averaged results of experiments carried out on research & demonstration TFRR (Summary of results for flow rates ca. 400 m3STP /h). VAM

Half-cycle period

CH4 conversion

Hot gas withdrawal flow rate

Withdrawn hot gas temp.

Heat recovery coefficient

Expected heat recoverya

vol. %

s

%

m3STP /h



%

MWt

0.1 0.22 0.35 0.43 0.53 0.77 1.03

10 20 20 90 90 120 240

86.4 85.7 90.7 96.2 96.1 96.1

0 0 9.2 14.2 39.6 69.6

e e 865 884 908 950

0 0 14 19 36 50

0 0 4.3 7.2 20.2 36.7

C Reactor extinguishes

Options with a reasonable heat recovery are marked with bold characters. a Recalculated for an average ventilation shaft flow rate of 720 000 m3STP /h.

for such air conditioning in full. If necessary, only a small addition of methane from mine drainage gas might be needed, but only for cases when excessive VAM concentration drop appears. The temperature of the hot gas withdrawn from VAM combustion in TFRR and directed to utilization is high enough (from 865 to 950  C) what allows using the heat for the co-generation air conditioning purposes rationally. ICE-PAS collaborates with one of Poland's leading hard coal producer the Coal Mining Company, which proposed this way of using the VAM combustion heat. Because an internal demand for heat in coal mines is limited and possibilities of selling the heat carriers are also negligible, the use of the recovered heat for production of the air conditioning chill seems to be the right solution. Effectiveness of energy production can be high only when co-generation system is applied. CHP (Combined Heat and Power) system significantly increases overall efficiency. The air conditioning system should contain both, absorption and vapour compression chillers to use the recovered heat efficiently. The absorption chiller consuming the heat, and compression chiller fed with electrical energy enable to utilize the both forms of the recovered energy by the heat recovery system. Performed measurements have shown that for average inlet concentrations of CH4 at the reactor inlet up to 1 vol.% and maximum temperatures in the TFRR up to about 1100  C, the nitrogen oxides (NOx) do not appear in the exhaust gases. It is presumed that the combustion in the monolith channels, due to low Reynolds number, can be flameless, what makes the process completely not prone to the creation of NOx (0 ppm). 3.3. Computer simulations The mathematical model of TFRR which revealed satisfactory agreement with the results of plant measurements will not be discussed here (for details see Ref. [40]), therefore only its short description is given below. FRR constitutes an object, which works in a permanent unsteady state caused by cyclic flow direction reversals, permanently disturbing the process. Therefore, use of dynamic analysis is necessary in order to predict such object's behaviour. Nevertheless, the so-called CSS (cyclic steady state) can be defined [41], when for consecutive flowereversal cycles the process becomes repeatable. To properly predict the reactor behaviour in the CSS, which may appear for various process parameters (flow rates, fuel concentrations, reactor geometry etc.), suitable for practical purposes, mathematical model appears to be a very useful tool for the design of such units. Both the kinetic studies recalled in Section 3.1 and experiments carried out on the plant, briefly described in Section 3.2, were used to elaborate an appropriate TFRR model and to carry out its identification (i.e. validation) [40]. Due to a very large heat capacity of a concrete refractory lining of the reactor wall, a dynamic

mathematical model was developed in two options: with and without heat accumulation in the reactor walls. Due to fairly large size of the TFRR and rather uniform temperature distribution in the cross section, it could have been expected that even if wall heat capacity is large, then the wall influence on simulation results should not be very significant. To check this, the both versions of the model were investigated. The models used for simulations include parabolic partial equations, describing dynamic heat and molar balances. For brevity, given below equations for the both model options are coupled together. Extensions that describe influence of the wall are marked in bold fonts. Heat balance for the gas phase:

  vTg vTg v2 Tg ¼
ng $cg $ þ ε$leff $ 2 þ a$av $ Ts
Tg ε$rg $cg $ vt vz vz nr   X   DHj rhom;j þ εrad $s$av $ Ts4
Tg4
j¼1

where:

leff ¼

(3)

ng $cg $dh ; Peg

Peg ¼

udh Deff

(4)

Heat balance for the solid phase (packing):

ð1
εÞ$rs $cs $

  vTs v2 Ts ¼ ð1
εÞ$ls $ 2
a$av $ Ts
Tg vt vz  
εrad $s$av $ Ts4
Tg4 radial heat outflow through the wall




zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{ 4hw ðT s
T out Þ Dr

(5)

Optional variable Tout in Eq. (5) which appears in the term “radial heat outflow through the wall” depends on the model option, as follows: Tout ¼ Tw for model with wall Tout ¼ Tsurr for model without wall Heat balance for the wall (valid only for model with wall):

 vT  v2 T   w w ¼ lw $ Dr $Lw þL2w $ 2 rw $c w $ Dr $Lw þL2w $ vt vz þDr $½hw $ðT s
T w Þ
hsurr $ðT w
T surr Þ

(6)

20

K. Gosiewski et al. / Energy 92 (2015) 13e23

Molar balances for the ith component: nr   X vy v2 y vy rhom;j $ni;j ε$rg $ i ¼ ε$rg $Deff $ 2i
ng $ i þ vt vz vz j¼1

(7)

Eq. (6) (given in bold type) is solved only for the model with heat accumulation in the wall. Appropriate boundary and initial conditions as well some more detailed description of the model may be found in Ref. [40], where also results of the model validation are given. Simulated temperature profiles shown in Fig. 6, were calculated for parameters of the demonstration plant working at a total gas flow rate ca. 400 m3/h, and at two inlet CH4 concentrations, but without the hot gas withdrawal from the reactor. These simulations were performed taking into account the effect of reactor wall. As can be seen in Fig. 6 B for low inlet CH4 concentration of 0.3 vol.% TFRR is more prone to asymmetry in the temperature profile than for the higher (0.97 vol.% in Fig. 6 A). Generally the concordance in Ref. [40] between the simulation results and the measured profiles is acceptable for practical purposes. Moreover, it appeared that difference between results obtained from the model with or without influence of the TFRR wall is negligible. 3.4. Analysis of process intensification opportunities During the R&D project [7] experiments were conducted at flow rates of lean air-methane mixture up to ca. 400 m3STP /h (i.e. with apparent flow velocity ca. 0.55 mSTP/s). Experiments and simulation results suggested that the process might probably be intensified by increasing the flow rate of at least ca. 50% of the previous value. An attempt of the process intensification was done. Change the main blower into more efficient one allowed getting the flow rates up to 570 m3STP /h (i.e. ca. 0.78 mSTP/s). Table 2 shows results of introductory test of the research & demonstration TFRR that aimed to increase the flow velocity, being realized by the blower replacement. New results were intentionally carried out only for concentrations below 0.5 vol.% in order to keep similarity with concentrations occurring in the mine, where industrial pilot plant had to be built. Results of experiments done for

higher flow rates are in Table 2 highlighted in yellow and are compared with chosen previous results obtained for lower flows. The test revealed that initial increase in flow of ca. 40% did not spoil methane conversion in the TFRR. Further process intensification is possible only by reduction of flow resistance of the plant. Therefore it is now planned to reconstruct the plant comprising: - Reconstruction of upper part of the reactor in order to improve its aerodynamics, - Replacement of honeycomb monoliths with channels 3  3 mm for blocks with wider channels 5  5 mm. After the above-mentioned reconstruction, it is expected that productivity of the reactor can be increased of about 50% or even more in relation to that obtained within the project [7]. Computer simulations confirmed that such intensification should not spoil the reactor performance. 4. VAM combustion e control problems in TFRR Control of FRR seems to be relatively easy since there is a number of process parameters, that can be used as a controlling (i.e. independent manipulated) variables [21] to get desired output effect. However, the matter is not so simple due to the fact that, as it was mentioned in Ref. [22], in a cooled FRR a thermal asymmetry may appear. This phenomenon was also noted earlier in Refs. [23e27], where multiplicity and bifurcation analyses were also carried out. It was observed in multiple experiments with TFRR that “escape” into an asymmetry happens incidentally for no apparent reason. Thus, it may be presumed that this effect is caused by multiplicity of CSS. The simplest idea of temperature regime control in a FRR is to find a proper moment of the flow reversal, which ensures that the reaction does not extinguish (goes off), and simultaneously time of current half-cycle length will not be too short. Generally, there are 3 simple control methods realizing this idea:
Control with constant and manually adjusted half-cycle time,
Control in which each half-cycle lasts until temperature difference DT between two arbitrarily selected points in input and

Fig. 6. Selected model validation results for option: with influence of reactor wall and heat exchange coefficients hw ¼ 1 and hsurr ¼ 3 [W m
2 K
1] for inlet concentrations and halfcycle time tc: 0.97 vol.%, 240 s (A) and 0.3 vol.%, 20 s (B).

K. Gosiewski et al. / Energy 92 (2015) 13e23

21

Table 2 Comparison of research & demonstration TFRR results with lower [7] and higher (i.e. increased) flow rates.

Concentration

Flow rate

Hot gas temperature in centre of reactor

Reversal halfcycle

CH4 conversion

Apparent linear velocity (STP)

[vol. %]

[m3/h] / (m3STP/h)

[°C]

[s]

[%]

m/s

0.42

401 / (351)

863

90

89.7

~ 0.48

0.42

570 / (541)

939

120

97.5

~ 0.74

0.3

407 / (401)

906

~40

96.2

~ 0.55

0.27

580 / (565)

927

~50

93.6

~ 0.78

0.22

407 / (375)

840

20

87.4

~ 0.51

0.26

586 / (567)

875

60

93.4

~ 0.78

output Sections does not reach a desired setpoint value (i.e. until DT < DTsetpoint),
Control in which each half-cycle lasts until a temperature Toutlet in arbitrarily selected point of current outlet Section will not fall to a chosen temperature setpoint value (i.e. until Toutlet > Tsetpoint). The above control methods may be effective enough until adverse events do not appear, especially these connected with asymmetry or other undesired disturbances e.g. these related to the CSS multiplicities. In such circumstances some more sophisticated control algorithms are necessary. The literature related to FRR control problems is not scarce (e.g. Refs. [44e47]) but most of them are of an academic character, trying to find a control algorithm based on a computer model. They usually take as the objective a protection of the catalyst in CFRR against overheating. There is no catalyst in the TFRR, which has to be protected, thus incidental temperature increase is not so dangerous. On the other hand if the thermal asymmetry is too large, then it may spoil or at least seriously disturb the heat recovery. European Patent [29] of MEGTEC proposes an algorithm, which is intended to protect against formation of asymmetry at work of TFRR. The algorithm proposed in Ref. [29] is based on a polynomial formula, derived from mathematical model of reactor. However, the patent description does not indicate how far the general is use of this formula, since the polynomial contains fixed numerical values of its coefficients. The PCT Patent application of ICE-PAS [8] describes another, and in some respect simpler algorithm, symmetrizing temperature profiles along the reactor length, when the asymmetry begins to form.

c)

d)

e)

f)

mines than CFRR, in particular when rational combustion heat recovery is taken into account. TFRR systems may work stable and autothermally when concentration of VAM is higher than 0.2 vol.%, Moreover, stable heat recovery may be achieved when inlet CH4 concentration exceeds 0.4 vol.% Thus, the need of addition of the mine drainage gas to increase CH4 concentration is not always necessary. Experiments and computer simulations have confirmed information from earlier literature data [23e27] that thermal asymmetry may appear in a cooled FRR. Our studies revealed that the reactor with the central cooling system is more prone to such asymmetry than the hot gas withdrawal heat recovery configuration. Nevertheless an automatic control system should include algorithms enabling to restore thermal symmetry of the FRR when the asymmetry is formed. Nowadays, there are many hard coal mines which operate deep underground; therefore they have throughout the year a real need for air conditioning, necessary during mining from these coal seams. Use of the recovered heat to produce the so-called icy chill water appears to be a right solution. It allows using the recovered heat of VAM combustion in technically and economically rational way, moreover at the spot in the coal mine, since the air-conditioning is indispensable for workplace health and safety. When a ventilation shaft emits VAM at a concentration of about 0.4e0.5 CH4 vol.% and next, the released methane is combusted and the heat is recovered, than it can substantially completely cover the energy demand for air conditioning of deep coal seams in a mine.

5. Summary and conclusions Acknowledgements The objective of the VAM combustion is either reducing the environmental impact of this very powerful GHG (greenhouse gas) emission, or energy recovery, or both these aims. Methane as a GHG has a GWP (global warming potential) at least over 20 times higher than CO2. Therefore combustion even without the heat recovery could be ecologically and economically attractive. The principal conclusions of the present study follow: a) Our studies and information from the literature suggest that TFRR technology is the most promising solution for combustion of VAM in large industrial installations. b) Analysis of problem CFRR vs. TFRR [30] indicates that in the current state of the art of catalysts the TFRR solutions are better adapted to the conditions of VAM combustion in coal

Studies on thermal combustion of coal mine ventilation air methane, described in paper have been carried out since 2007 and besides continuous statutory research funding by Polish Academy of Sciences were financially supported by various institutions: Polish Ministry of Science and Higher Education (Project No. R 14 020 02 and Grant No. 42/PMPP/U/6-03.10/E-86/2010), Operational Programme Innovative Economy (Grant No. UDA-POIG.01.03.0224-044/11-00) and European Institute of Innovation and Technology (EIT) by KIC InnoEnergy Framework (Knowledge and Innovation Community) within SECoal Project. Therefore, authors gratefully acknowledge support from these all sponsors. Moreover, substantive fruitful cooperation with Kompania We˛glowa S.A. (Coal Mining Company) is also gratefully acknowledged.

22

K. Gosiewski et al. / Energy 92 (2015) 13e23

Nomenclature

References

aj av cs cg C ¼ CCH4

[1] U.S. Underground Coal Mine Ventilation Air Methane Exhaust Characterization. Coal mine ventilation air methane exhaust characterization. Coalbed methane outreach program: U.S. EPA. 2010. p. 1e16. [2] Report: security situation in mining industry in 2013 (edited in Polish: Stan  stwa pracy w go  rnictwie w 2013 roku). State Mining Authority bezpieczen _ rniczy). 2014. (Wyzszy Urza˛ d Go [3] Assessment of worldwide market potential for oxidizing coal mine ventilation air methane: EPA. 2003. [4] Global VAM Project Opportunities. Conference global VAM project Opportuw. nities, Krako [5] Su S, Beath A, Guo H, Mallett C. An assessment of mine methane mitigation and utilization technologies. Prog Energy Combust Sci 2005;31: 123e70. €llstrand A. In: MEGTEC Systems. Conference: proven technology for VAM [6] Ka Abatement e Fourth Session of Ad Hoc Group of Experts on coal mine methane. Geneva, October 2008; 2008.  czyk M, Wojdyła A, Warmuzin  ski K, [7] Gosiewski K, Jaschik M, Pawlaczyk A, Tan et al. Research&Development Project Grant No R14 020 02: “Thermal coal mine ventilation air methane combustion in a flow reversal device with heat regeneration and heat recovery” (in Polish). Gliwice: Inst Chem Eng Pol Acad Sci 2010:1e192.  czyk M, Giełzak K, Wojdyła A, et al. [8] Gosiewski K, Jaschik M, Pawlaczyk A, Tan Method for utilization of low-concentration gas mixtures of combustible gas and air with stable heat energy recovery and flow reversal device for implementation of method. In: PCT, editor. Patent Cooperation Treaty; 2011. PL: _ Chem. PAN, Katalizator Sp. z o.o. Assignee: Inst. Inz. [9] Matros YS. Catalytic processes under unsteady state conditions. Amsterdam: Elsevier Science BV; 1989. [10] Eigenberger G, Kolios G, Nieken U. Thermal pattern formation and process intensification in chemical reaction engineering. Chem Eng Sci 2007;62: 4825e41. [11] Kolios G, Frauhammer J, Eigenberger G. Review: autothermal fixed-bed reactor concepts. Chem Eng Sci 2000;55:5945e67. [12] Matros YS, Noskov AS, Chumachenko VA, Goldman OV. Theory and application of unsteady state detoxication of effluent gases from sulfur dioxide, nitrogen oxides and organic compounds. Chem Eng Sci 1988;43:2061e6. [13] Matros Yu Sh, Bunimovich GA. Reverse-flow operation in fixed bed catalytic reactors. Catal Rev Sci Eng 1996;38(1):1e68. [14] Cottrell FG. Purifying gases and apparatus therefor. US Patent Office. USA1938. [15] Silveston PL, Hudgins RR. Periodic operation of chemical reactors. Amsterdam e Boston e Heidelberg e London e New York e Oxford Paris e San Diego e San Francisco e Singapore e Sydney e Tokyo. Elsevier; 2013. [16] Technical and Economic Assessment. Mitigation of methane emissions from coal mine ventilation air. In: EPA, editor. Coalbed Methane Outreach Program; 2000. p. 96. [17] Gogin LL, Matros LL, Ivanov AG. Ekologiia i Kataliz [Ecology and catalysis] (in Russian). Novosibirsk: Izd: Nauka; 1990.  F, Trottier R. Report: elimination of dilute methane [18] Sapoundjiev H, Aube emissions from underground mine and oil and natural gas production sectors. CANMET Nat Resour Can 1999. 1999-13 (TR-J).  F. Heat recovery from lean industrial emis[19] Sapoundjiev H, Trottier R, Aube sions environmental and economic benefits of CFRR technology. Greenhouse gas control technologies. Elsevier Science Ltd; 1999. p. 805e10. [20] Somers JM, Schultz HL. Thermal oxidation of coal mine ventilation air methane. In: Wallace, editor. 12th US/North American Mine Ventilation Symposium. Reno, Nevada USA; 2008. p. 301e5. [21] Nieken U, Kolios G, Eigenberger G. Control of ignited steady state in autothermal fixed-bed reactor for catalytic combustion. Chem Eng Sci 1994;49:5507e18. [22] Gosiewski K, Warmuzinski K. Effect of mode of heat withdrawal on asymmetry of temperature profiles in reverse-flow reactors. Catalytic combustion of methane as a test case. Chem Eng Sci 2007;62:2679e89. [23] Rehacek J, Kubicek M, Marek M. Modelling of a tubular catalytic reactor with flow reversal. Chem Eng Sci 1992;47:2897e902. [24] Khinast J, Luss D. Mapping regions with different bifurcation diagrams of a reverse-flow reactor. AIChE J 1997;43:2034e47. [25] Khinast J, Gurumoorthy A, Luss D. Complex dynamic features of a cooled reverse-flow reactor. AIChE J 1998;44:1128e40. [26] Khinast J, Jeong YO, Luss D. Dependence of cooled reverse-flow reactor dynamics on model. AIChE J 1999;45:299e309. [27] Salinger AG, Eigenberger G. direct calculations of periodic states of reverse flow reactor e II. Multipl Instab Chem Eng Sci. 1996;51:4915e22. [28] Kushwaha A, Poirier M, Hayes RE, Sapoundjiev H. Heat extraction from a flow reversal reactor in lean methane combustion. Trans IChemE Part A Chem Eng Res Des 2005;83(A2):205e13. [29] Tesar M, Ruhl A, Zagar S. Determination of supplemental fuel requirement and instantaneous control thereof involving regenerative thermal oxidation. In: Office EP, editor. European Patent Office: MEGTEC Systems Inc.; 2001. [30] Gosiewski K, Pawlaczyk A. Catalytic or thermal reversed flow combustion of coal mine ventilation air methane: what is better choice and when? Chem Eng J 2014;238:78e85. [31] Matros YS, Bunimovich GA. Control of volatile organic compounds by catalytic reverse process. Ind Eng Chem Res 1995;34:1630e40.

exponent for concentration term in general kinetic Eq. (1) specific surface area, m
1 solid phase specific heat, J kg
1 K
1 gas phase specific heat, J mol
1 K
1 or CCO substrate concentration in Eq. (1) i.e. of CH4 or CO respectively, mol m
3 dh hydraulic diameter of monolith channel, m Dr reactor diameter or square size, m Deff effective mass dispersion coefficient, m2 s
1 Ej activation energy for j-th reaction, J mol
1 hw, hsurr heat transfer coefficient to wall and to surroundings respectively, W m
2 K
1 DH heat of reaction, J mol
1 k0,j pre-exponential factor in general kinetic Eq. (1) Lw wall thickness, m ng molar flux of gas, mol m
2 s
1 nr number of reactions, e Peg Peclet number for mass dispersion Qgen, Qrec heat power generated or recovered, Wth rhom, j j-th homogeneous reaction rate, mol m
3 s
1 monolith R universal gas constant, J mol
1 K
1 t time, s T temperature, K or  C Toutlet temperature in arbitrarily selected point of current outlet Section, K or  C DTad adiabatic temperature increase of reaction inlet =ðc r Þ, K ¼ ð
DHÞCCH g g 4 DT difference of temperatures chosen for control, K or  C u linear gas velocity, m s
1 yi mole fraction of i eth component, e z axial coordinate, m Greek letters a heat transfer coefficient, W m
2 K
1 ε void fraction coefficient, e εrad emissivity of radiation, e s StefaneBoltzmann constant, W m
2 K
4 l thermal conductivity, W m
1 K
1 leff effective thermal conductivity of gas phase, W m
1 K
1 r density, kg m
3 for solid or mol m
3 for gas ni,j stoichiometric coefficient of component i-th in reaction jth, e Subscripts and superscripts g gas i, j number of component i-th or reaction j-th (respectively) out at the reactor outlet s solid setpoint adjusted setpoint value for control system surr surroundings w wall

Acronyms CFRR Catalytic Flow Reversal Reactor CSS Cyclic Steady State R&D Research & Development FRR Flow Reversal Reactor ICE-PAS Institute of Chemical Engineering, Polish Academy of Sciences TFRR Thermal Flow Reversal Reactor VAM Ventilation Air Methane

K. Gosiewski et al. / Energy 92 (2015) 13e23 [32] Matros Yu Sh, Bunimovich GA, Patterson SE, Meyer SF. Is it economically feasible to use heterogeneous catalysts for VOC control in regenerative oxidizers? Catal Today 1996;27:307e13. [33] Gosiewski K, Matros YS, Warmuzinski K, Jaschik M, Tanczyk M. Homogeneous vs. catalytic combustion of lean methane-air mixtures in reverse-flow reactors. Chem Eng Sci 2008;63:5010e9. [34] Gosiewski K. Efficiency of heat recovery versus maximum catalyst temperature in reverse-flow combustion of methane. Chem Eng J 2005;107(19e25). [35] Mallett C, Su S. Progress in developing ventilation air methane mitigation and utilization technologies third international methane and nitrous oxide mitigation conference. Beijing. 2003. [36] Gosiewski K, Pawlaczyk A, Warmuzinski K, Jaschik M. A study on thermal combustion of lean methaneeair mixtures: simplified reaction mechanism and kinetic equations. Chem Eng J 2009;154:9e16. [37] Pawlaczyk A, Gosiewski K. Simplified kinetic model for thermal combustion of lean methane e air mixtures in a wide range of temperatures. Int J Chem React Eng 2013;11(1):1e11. [38] Pawlaczyk A. Description of homogeneous combustion process of low concentrated methane-air mixtures in monolith bed and assessment of its applicability for modeling of reverse flow reactors. PhD thesis. Gliwice: Polish Academy of Sciences; 2013. [39] Pawlaczyk A, Gosiewski K. Combustion of lean methaneeair mixtures in monolith beds: kinetic studies in low and high temperatures. Chem Eng J 2015: 8. http://dx.doi.org/10.1016/j.cej.2015.02.081.

23

[40] Gosiewski K, Pawlaczyk A, Jaschik M. Thermal combustion of lean methaneeair mixtures: flow reversal research and demonstration reactor model and its validation. Chem Eng J 2012;207e208:76e84. [41] Unger J, Kolios G, Eigenberger G. On efficient simulation and analysis of regenerative process in cyclic operation. Comput Chem Eng 1997;21: S167e72. [42] Gosiewski K. Effective approach to cyclic steady state in catalytic reverse-flow combustion of methane. Chem Eng Sci 2004;59:4095e101. [43] Gatnar K. Układy energetyczne wykorzystuja˛ ce metan z odmetanowania  JSW S.A. jako element lokalnego rynku energii. Polityka Energ kopaln 2007;10(Zeszyt specjalny 2):515e24. [44] Budman H, Kzyonsek M, Silverston P. Control of a nonadiabatic packed bed reactor under periodic flow reversal. Can J Chem Eng 1996;74:751e9. [45] Dufour P, Couenne F, Toure Y. Model predictive control of a catalytic reverse flow reactor. Control systems technology. IEEE Trans 2003;11(5): 705e14. [46] Edouard D, Dufour P, Hammouri H. Observer based multivariable control of a catalytic reverse flow reactor: comparison between LQR and MPC approaches. Comput Chem Eng 2005;29(4):851e65. [47] Balaji S, Fuxman A, Lakshminarayanan S, Forbes JF, Hayes RE. Repetitive model predictive control of a reverse flow reactor. Chem Eng Sci 2007;62(8): 2154e67.