Supercritical water oxidation for energy production by hydrothermal flame as internal heat source. Experimental results and energetic study

Supercritical water oxidation for energy production by hydrothermal flame as internal heat source. Experimental results and energetic study Pablo Cabeza, Joao Paulo Silva Queiroz1,, Manuel Criado, Cristina Jimenez, Maria Dolores Bermejo∗ , Fidel Mato, Maria Jose Cocero High Pressure Process Group, Dept. of Chemical Engineering and Environmental Technology - Universidad de Valladolid - Doctor Mergelina, s/n, 47011, Valladolid - Spain Abstract This work presents experimental and model results from a new configuration of a cooled wall reactor working with two outlets: an upper outlet through which a salt-free hot effluent (500 - 600 ◦C) is obtained and a lower outlet through which an effluent at subcritical temperature dissolving the precipitated salts is obtained. Different flow distributions were tested in order to find the best elimination conditions. Total organic carbon removal over 99.99% was obtained at injection temperatures as low as room temperature, when the fraction of products leaving the reactor in the upper effluent is lower than 70% of the feed flow. The performance of the reactor was tested with the oxidation of a recalcitrant compound such as ammonia, using isopropyl alcohol as co-fuel. Removals higher than 99% of N-NH+ 4were achieved in both effluents, working with temperatures near 700 ◦C. Slightly better eliminations were obtained in the bottom effluent because its residence time in the reactor is longer. The behavior of the reactor working with feeds with a high concentration of salts was also tested. Feeds containing up to 2.5% wt Na2SO4could be injected ∗Corresponding author. Phone: +34 983423166 Email addresses: [email protected] (Pablo Cabeza), [email protected] (Joao Paulo Silva Queiroz), [email protected] (Manuel Criado), [email protected] (Cristina Jimenez), [email protected] (Maria Dolores Bermejo), [email protected] (Fidel Mato), [email protected] (Maria Jose Cocero) 1Present address: Dept. of Chemical Engineering - Universidade Federal de Pernambuco - Prof. Artur de S´a, s/n, - Cidade Universit´aria - 50740-521, Recife, PE - Brazil Preprint submitted to Energy March 2, 2015 in the reactor without plugging problems and a total organic carbon removal of 99.7% was achieved in these conditions. Upper effluent always presented a concentration of salt lower than 30 ppm. Finally, a theoretical analysis of the energy recovery of the reactor working with two outlets was made. Keywords: Supercritical water oxidation, Hydrothermal flames, Renewable Energy, Reactor design 1. Introduction1 Since Franck and coworkers discovered hydrothermal flames [1] and it could2 be applied to the Supercritical Water Oxidation (SCWO), new challenges came3 up to the study of SCWO. For flammable compounds such as methane or4 methanol, hydrothermal flame can occur at temperatures as low as 400 ◦C [2].5 SCWO is the oxidation of organics in water under conditions above its critical6 point. In presence of hydrothermal flames the reaction times can be reduced to7 the order of milliseconds [3] without the production of sub-products typical of8 conventional combustion such as NOx [4] or dioxins [5].9 SCWO with a hydrothermal flame has a number of advantages over the10 flameless process. Some of these advantages permit overcoming the traditional11 challenges that make the successful and profitable commercialization of SCWO12 technology difficult. The advantages include the following [3]:13 •The reduced residence times (in the order of milliseconds) allows the con-14 struction of smaller reactors.15 •It is possible to carry out the reaction with feed injection temperatures16 near to room temperature when using vessel reactors [6, 7]. This avoids17 problems such as plugging and corrosion in a preheating system, having18 an advantage from the operational and energy integration perspective.19 •Higher operation temperatures improve the energy recovery.20 The first reactor probably working with a hydrothermal flame inside was21 the MODAR reactor, working in conditions of concentration, temperature and22 2 pressure above the ignition conditions of methanol and being able to work with23 injection temperatures of 25 ◦C and injecting the air at 220 ◦C [7]. In the24 ETH of Zurich, the direct injection of the waste into a diffusion hydrothermal25 flame generated inside the reactor was developed as a solution to avoid the26 external preheating of the waste up to supercritical conditions [8, 9]. Pr´ıkopsk´y27 and coworkers investigated the feasibility of injecting feeds with a 3%wt of28 sodium sulfate (Na2SO4) in the transpiring wall reactor (TWR) with a diffusion29 hydrothermal flame as internal heat source [10]. No plugging was observed30 during the experiments, but salt deposits were detected in the upper hot zone31 of the reactor. In a previous investigation of our research group [6], it was32 found that using a transpiring wall reactor, a premixed hydrothermal flame33 inside the reaction chamber could be maintained when injecting the feed at a34 temperature as low as 110 ◦C. Using a similar reactor, feeds with up to 4.74% wt35 Na2SO4could be injected [11]. The reactor worked without plugging, but the36 recovery of salts was only between 5% and 50%. Both research groups reported37 an increase in the temperature when salt was injected in the reactor [10, 11].38 Zhang et al. [12] studied the operational parameters of a TWR developed to39 generate thermal fluids for oil recovery. They used water-methanol as artificial40 fuel prior to treating oil exploration wastewater, and they found the limits of41 temperature of transpiring flow in order to avoid the quenching and extinction42 of hydrothermal flame.43 It has been proved that injection of cold feeds over a hydrothermal flame is44 only possible when working with vessel reactors [9, 10, 11] and it is not possible45 when working with tubular reactors [13]. This behavior was due to the low46 flame front velocities in hydrothermal flames that is lower than 0.1 m/s, in47 comparison to the higher flame front velocities at atmospheric conditions (0.4-348 m/s). This is the reason why flow velocities lower than 0.1 m/s are necessary49 to keep a stable hydrothermal flame where cold reagents can be injected [14].50 Our research group has succeeded in keep working continuously a vessel reactor51 injecting feeds at temperatures as low as 25 ◦C [15].52 Even though the most immediate application of hydrothermal flames is in53 3 the SCWO process for waste destruction, which is the most industrially devel-54 oped hydrothermal process, it is possible to move from the idea of hydrothermal55 flame as a technology for the destruction of wastes to consider it as a technol-56 ogy for the generation of clean energy, which could eventually substitute the57 actual technologies based on atmospheric combustion [16]. Supercritical water58 is already applied in energy fields through gasification processes for waste val-59 orization Facchinetti et al. [17], R¨onnlund et al. [18]. The efficiency in energy60 production by SCWO of coal and direct expansion of the effluent was compared61 to the efficiency of other conventional power plants by Bermejo et al. [19]. If62 the steam was produced at 650 ◦C and 30 MPa, efficiencies as high as 38% were63 obtained by SCWO. Efficiency was as high as 41% if the effluent was reheated64 and expanded a second time. The efficiencies at the same steam conditions for65 pulverized coal power plant and pressurized fluidized bed power plant were 3266 and 34% respectively. Comparison is more favorable using oxygen enriched air67 or even using pure oxygen as the oxidant. In this last option the cost of the oxi-68 dant must be assumed. Nevertheless, it is known that in traditional combustion69 power plants, oxygen is used to improve efficiency. Donatini et al. [20] simulated70 a power plant based on direct combustion of pulverized coal in a SCWO reactor71 with a system for CO2capture. They reported net efficiencies around 27% and72 found that the consumption of the air separation unit for oxygen production73 strongly affects the viability of the plant. A similar analysis has been made by74 Kotowicz and Michalski [21], whom have proposed several operations in order75 to increase efficiency for each step in a power plant model: air separation unit,76 boiler (SCWO reactor burning coal) and steam turbine.77 Arai et al. [22] proposed the supercritical oxidation of biomass wastes and78 other sustainable fuels with a hydrothermal flame as a clean energy source for79 reaching a sustainable society with a decentralized production based on renew-80 able resources. Augustine and Tester [3] also propose its utilization with low81 grade fuels. In general, this technology can be applied to the valorization of82 waste such as waste water treatment plant sludge, biomass or plastic wastes or83 any kind of waste with high energetic content. Basic theoretical calculations84 4 indicate that feeds with an energy content of 930 kJ/kg (roughly equivalent85 to an aqueous solution with 2% ww of hexane) can supply enough energy to86 preheat the feed from room temperature up to 400 ◦C, and to generate elec-87 tric power equivalent to that consumed by the high pressure pump and the air88 compressor [23]. A remarkable aspect about working with hydrothermal flames89 is improving energy recovery in SCWO system [19]. Hydrothermal flames allow90 new reactor designs that not only are able to inject feeds without preheating91 because of the possibility of injecting reactants at room temperature but also92 use the heat released by the flame for other purposes as the energetic integration93 of the process or for production of electricity by turbines [24]. Smith Jr. et al.94 [25] used exergy analysis to study the partial and total oxidation of methane in95 supercritical water for a heat-integrated supercritical water reactor and electri-96 cal energy production system. They assume a direct expansion of products (at97 400 ◦C) in a turbine, followed by heat recovery of the expanded stream. It was98 found that the process could be energy self-sufficient and optimum flow rates99 were calculated in order to minimize reactor heat requirements or maximize net100 electrical work. The high temperature effluent can also be used as heat source101 in other hydrothermal processes, such as liquefaction or gasification, where the102 heat recovery is a critical issue [26, 27]. In the case of waste with high con-103 centration of inorganic substances, new reactor designs able to separate these104 salts from the effluent must be developed in order to make it possible to directly105 expand the effluent in an electricity production turbine.106 The main goal of this work is the study of the behavior of new cooled wall107 reactor with the main particularity of having two outlets in order to try to keep108 the maximum heat released by the flame in a clean and high temperature flow109 leaving the reactor from the upper zone and other flow at subcritical conditions110 with the salts dissolved going out for the bottom of the reactor. In this way the111 upper/lower effluent relation was optimized taking into account the temperature112 profiles inside the reactor and the organic matter elimination in both streams.113 The performance of the reactor with recalcitrant pollutants such as ammonia114 was tested as well as the performance of the reactor with feeds containing salts.115 5 Figure 1: Diagram of SCWO facility with two outlets. A CFD model is also used to describe the behaviour of the reactor. Finally, a116 purely theoretical energy recovery study of the process with the new reactor was117 performed, including the possibility of direct expansion in hypothetical devices.118 2. Experimental119 2.1. Experimental setup120 All the experiments analyzed in this research have been carried out in the121 SCWO facility installed in the University of Valladolid. It consists of a contin-122 uous facility working with a feed flow of 22.5 L/h, and air supplied by a four123 stage compressor, with a maximum feed rate of 36 kg/h is used as the oxidant.124 The reactor consist of a pressure vessel made of AISI 316 stainless steel able to125 stand a maximum pressure of 30 MPa and a maximum wall temperature of 400126 ◦C, containing a reaction chamber made of Ni-alloy 625 where the temperature127 be as high as 700 ◦C. Waste water feed and air are previously pressurized and128 preheated with electrical resistances to the desired temperature before being129 injected by the bottom of the reactor. The reagents are conducted to the top of130 the reactor chamber by means of a tubular injector. At the outlet of the injec-131 6 tor the hydrothermal flame is formed. Cooling water, previously pressurized is132 circulating between the pressure vessel and the reaction chamber introduced by133 the top of the reactor in order to cool down the vessel at a temperature lower134 than 400 ◦C. This cooling water is entering in the reaction chamber through its135 lower part and leaving the reactor by the bottom together with a fraction of the136 products. The rest of the products leave the reactor by another outlet situated137 in the top of the reactor chamber. After leaving the reactor, both effluents are138 cooled down in the intercoolers and depressurized. The flow diagram of the fa-139 cility with two outlets is shown in Figure 1. More information about the facility140 can be found elsewhere [6, 13]. Figure 2 shows a scheme of the reactor with the141 different position of thermocouples inside the reaction chamber. The different142 temperature profiles are referred at the position of these four thermocouples.143 Each effluent (top and bottom flow) is measured with a rotameter in order to144 know the distribution of the feed flow respect the two outlets. 13 mm Feed Air Bottom Products Cooling water 945 mm 520 mm 250 mm 120 mm Top Products T1 T2 T3 T4 Figure 2: Scheme of the reactor with the flow distribution and the positions (mm) of the temperature measurement inside the reaction chamber. 145 7 2.2. Materials and experimental procedure146 The experiments analyzed in this research were performed using feeds pre-147 pared with isopropyl alcohol (IPA, 99% purity) and tap water without further148 purification. For experiments made with ammonia it was used ammonia (25%149 in mass). Synthetic waste containing salts were prepared using Na2SO4(purity150 >98%).151 Previous to the beginning of the experiment the reactor must be preheated152 electrically to 400 ◦C. The reaction is initiated by injected air and waste water153 streams preheated electrically up to a temperature higher than 400 ◦C. A few154 minutes after continuous injecting of IPA solution and air stream the hydrother-155 mal flame is ignited. At that moment a sharp increase the temperatures at the156 top of the reactor (T1 and T2) is registered. Then, the electrical heating of the157 wall of the reactor is turned off and the cooling water flow is connected. For158 keeping the maximum temperature constant in values around 600-700 ◦C till159 the desired injection temperature is reached, IPA concentration was increased as160 the injection temperature was decreased down to the selected injection temper-161 ature. After the target injection temperature is reached (from 300 ◦C till room162 temperature, around 20-30 ◦C), the upper flow and bottom flow are regulated163 opening or closing the decompression valves keeping the air and the pressure164 constant. Pressure must be stabilized around 23 MPa. Several stationary states165 with different prepared feeds and different flow up/bottom ratio are reached and166 samples of the liquid effluent are taken.167 Total Organic Carbon (TOC) and Total Nitrogen (TN) analysis of the sam-168 ples were performed with a TOC 5050 SHIMADZU Total Organic Carbon Ana-169 lyzer which uses combustion and IR analysis. The detection limit is 1 ppm. Salt170 concentration is measured using a conductimeter Basic 30 provided by Crison.171 For doing this, conductivities of solutions of known Na2SO4concentration are172 measured obtaining a linear calibration line between conductivity and Na2SO4 173 concentration. Nitrates and nitrites were characterized in the liquid effluent by174 ionic chromatography with an IC PAK A column of Waters. The detection limit175 is 1 ppm. NH3and NOxat the gas effluent were analyzed with Dr¨ager tubes176 8 detectors Lab Safety Supply CH29401 and CH31001. The NOxdetection limits177 for these tubes ranged from 0.5 to 100 ppm and the NH3detection limits ranged178 from 5 to 70 ppm (standard deviation for both tubes are between 10 and 15%).179 3. Modeling180 A CFD model was performed in order to study the internal behavior of the181 new reactor. The main elements of the reactor have been included in the model182 geometry, like the injector, the reaction chamber, and the space between the183 pressure shell and the chamber. The reactor is modeled as an axisymmetric 2D184 system. The turbulent flow dynamics is modeled by Reynolds-Averaged-Navier-185 Stokes equations, using the Realizable k-ϵturbulence model with enhanced wall186 treatment [28]. The density of the supercritical mixture is calculated by Peng-187 Robinson equation of state with Van der Waals mixing rules, and volume trans-188 lation (VTPR-EoS) [29]. The volume translation used in density calculations189 was fitted for each component that constitutes the system (H2O, O2, N2, CO2 190 and IPA), at the operation pressure of 23 MPa. The volume translation has191 not influence on enthalpy calculations, thus, specific enthalpy (and also cp) is192 given by original Peng-Robinson equation of state (PR-EoS) [30]. The ther-193 mal conductivity and the molecular viscosity of the mixture are calculated as194 a mass-fraction average of the properties of the pure components as function195 of temperature. The turbulent diffusion usually overwhelms laminar diffusion,196 and the specification of detailed laminar diffusion properties in turbulent flows197 is not necessary. Even so, laminar diffusion coefficient are estimated using the198 method of Mathur and Thodos [31].199 4. Results and discussion200 As general result, the new reactor with two outlets successfully eliminates201 organic material and provides a clean stream with high energy content. The202 injection at low temperatures (20 ◦C), far from the critical region, keeps the203 salts dissolved inside the injector, avoiding plugging and corrosion. Finally,204 9 !! "!! #!! $!! %!! &!! ! !! #!! %!! '!! (!!! !"#$% &"''% $) *+,-. %)'*+,-. /)(*+,-. 01234567*685935 (a) ! " # $ % & % ' ( !! !"#!$%&'%#())*+ ",,&-./#01%'2#3&,0#(4/56+ )*+,-.//.0123304 )*+,/.3123304 (b) Figure 9: (a) Temperature profiles for different cooling water flows. (b) TOC values in top and bottom effluents for different cooling water flows. were compared with results obtained with mixtures of ammonia and IPA tested285 in the same reactor working with only the bottom outlet [4]. Figure 10 shows286 Ammonia and TOC removal represented versus maximum temperature regis-287 tered inside the reactor. The upper effluent fraction was kept constant at values288 around 50% which means that the 50% of the feed injected (liquid) has been289 taken out by the upper outlet. It can be appreciated that temperatures higher ! " # $ % &!! $!! $#! $%! '"! '$! %!! ! "#$%&'()*+,%-./ 0%1+2%-34/ ()(*#+,-./01/,/-2 ()(*#+,-./01/,3-//-4 ()(*#+,-51,-./01/ (a) !" !# !$ !% !& '""!" %"" %$" %&" (#" (%" &"" !"#$%&'()*#+,- #.)/#+0"- )*+,-./01/,/-2 )*+,-./01/,3-//-4 )*+,-51,-./01/ (b) Figure 10: Ammonia removal (a) and TOC removal (b) vs max temperature inside the reactor for feeds with concentrations between 0.5-3% of ammonia and 9-11.5% of IPA working with 100% bottom flow and with 50% top flow. 290 than 700 ◦C are required to achieve N-NH+ 4removals over 99%. These tem-291 peratures are higher than those needed to obtain the same removal with the292 reactor working with only one outlet. Table 1 summarizes the average results293 for removal of the different experiments made with mixtures of ammonia and294 IPA. Working with two outlets, it is observed that ammonia removal is slightly295 16 Table 1: Removal results from the experiments made with different concentrations of ammonia. NH+ 4o IPAo Tmax TOC TOC N-NH+ 4N-NH+ 4N-NO− 3N-NO− 3 (%) (%) (◦C) Rem.(%) Rem.(%) Rem.(%) Rem.(%) top bottom top bottom top bottom (ppm) (ppm) 0.5 11.5 744 99.99 99.99 99.13 99.41 49 38 0.5 10.5 706 99.97 99.96 99.07 99.41 47 21 0.5 10.0 634 99.93 94.71 97.94 91.77 50 14 1.0 10.0 708 99.99 99.99 98.99 99.88 36 74 3.0 9.0 686 99.99 99.97 99.83 99.79 186 78 3.0 9.5 729 99.97 99.98 99.29 99.82 26 27 higher in the bottom effluent than in the top effluent, probably because the296 residence time for the products comprising the lower effluent is longer than the297 one of the top effluent, that it seems to be too short to have complete oxidation298 of ammonia [4]. Nitrate concentration is in general higher in the top effluent299 due to the higher temperatures. The concentrations of NOx and NH+ 4in the300 gas effluent were under the detection limit of 0.5 and 5 ppm respectively for all301 the experimental conditions tested.302 4.7. Behavior of the reactor working with high salt content feeds303 The main goal of this new design of the reactor is to obtain a top effluent304 at high temperature and free of salts, becoming this way available to be used305 in systems to produce energy. To achieve that, salts contained in the feed306 must precipitate and fall, leaving the reactor dissolved in the bottom effluent307 while the top effluent is free of salts. For this purpose, feeds with Na2SO4 308 concentrations until 2.5% (25,000 ppm) were injected in the reactor, using IPA309 as fuel to obtain reaction temperatures of 700 ◦C, and at feed flow rates of 13-14310 kg/h. In table 2 the main results of the experiments made with feed containing311 salts are summarized. Equation (6) explains how the salt recovery is calculated.312 As can be observed in table 2, it is possible to recover a top effluent almost313 free of salts (with conductivities below values of the tap water, equivalent to314 concentrations of Na2SO4lower than 30 ppm) and at temperatures over 500315 ◦C, available to be expanded in a turbine or for the production of steam at high316 17 Table 2: Main results for the experience made with feed containing 2.5% wt of Na2SO4. Ftop Fbottom TOC TOC Tmax Tbottom Na2SO4Na2SO4 (kg/h) (kg/h) top bottom (◦C) (◦C) top Recovery (ppm) (ppm) (ppm) bottom (%) 7.2 10.2 1.0 209 749 239 24 2.4 7.2 10.2 0.3 352 712 244 23 32.1 7.2 10.2 0.5 599 740 250 24 21.1 7.2 10.2 0.8 23 742 254 23 1.8 7.2 10.2 0.7 69 683 258 23 45.5 7.2 10.2 0.7 16 691 258 26 0.7 Average 0.7 211 719 251 23.8 17.3 temperature that could be also expanded in a turbine. Paying attention to the317 salt recovery at the bottom flow, it was possible to obtain an average of 17% of318 salt recovery. This recovery is higher than the obtained with the reactor working319 with only one outlet [32] (average of 10%) but it was not possible to improve320 and stabilize the recovery during long times. This fact could be interpreted as321 the possible formation of solid clusters of salts swept away by the outlet stream322 and dissolved in the cooling systems.323 4.8. Energy recovery324 In order to analyze the possibility of using the high temperature of the325 effluent of SCWO reactors to produce energy, an analysis of the options for326 generating energy was performed.327 4.8.1. Parameter calculations328 The following equations explain how the different parameters for the study were calculated. Firstly, the amount of energy released by the waste and fuel contained in the feed is calculated as shown in eq. (7). kW injected at the feed = FfeedCfuel∆Hc,fuel 3,600 (7) where ∆Hc,fuel is the enthalpy of combustion for IPA (3,750 kJ/kg). The energy consumed is due mainly to the pumping equipment (pumps and 18 compressors). The fraction of energy consumed with respect of the energy contained in the feed is calculated as shown in eq. (8). Consumption = kW Consumed kW injected at the feed ·100 (8) The energy production is calculated using Peng-Robinson Equation of State with Boston-Mathias alpha function considering a turbine with an isentropic efficiency of 72 %. The fraction of energy produced with respect to the energy introduced in the feed is calculated as shown in eq. (9). Relative production = kW produced kW injected at the feed ·100 (9) kW produced is the energy produced by direct expansion or steam expansion production. From the production and consumption is obtained the percentage of the efficiency in energy production of the system of each reactor and kind of oxidant as shown in eq. (10). Efficiency = kW produced −kW Consumed kW injected at the feed ·100 (10) Mass and energy balances were solved in Aspen Plus software considering ther-329 mal and volumetric properties calculated using Peng-Robinson Equation of330 State.331 4.8.2. Energy produced by steam expansion332 The most conventional method for electric generation is using the products333 stream as heat source for a Rankine cycle. As a guidance of feasibility of the334 process, it has been calculated the amount of steam which could be generated at335 different conditions for three small turbines commercialized by Siemens (table336 3).337 Increasing the pressure of the power cycle, increases the specific work pro-338 duced by expanding the steam. However, the total amount of steam is reduced,339 since the heat source is limited. That can be seen in figure 11, that shows the340 composite curves for the hot stream (reactor products) and three possibilities of341 19 Table 3: Characteristics of commercial steam turbines. Inlet P Inlet T Power (bar) (◦C) (kWh/(kg-steam)) SST-040 40 400 0.232 SST-050 101 500 0.272 SST-060 131 530 0.278 cold stream (water-steam), with a difference of 10 ◦C at pinch point. The heat342 source is a stream at 700 ◦C and 23 MPa, with the typical composition of SCWO343 effluent using air as oxidant. Finally, figure 12 presents the amount of steam344 that could be produced per kilogram of hot products and the net efficiency for345 each pressure level. ! " # $ % & ' ( % ! !% !"#!$%&'$!()*+, -.&/%0#1()234/, )*+,-./0 1123 $ 1123 % 1123 & Figure 11: Production of steam at three different pressure levels. !"#$# !"#%# !"#&# "'#( ")%( ")#( # #*) #*' #*+ #*$ #*% #*& # % )# )% !!"#"$%#& '()*+$,-.'()/0123#+ 40$**30$5674,8 ,-./0 .11232.435 Figure 12: Steam production and efficiency for commercial turbines. 346 Given these results, the following sections assume that the characteristics347 20 of the steam that can be produced from the heat contained in the effluent of348 the reactor are those shown in table 4. The low pressure steam presented in Table 4: Characteristics of the steam. Pressure Temperature (bar) (◦C) High Pressure Steam 46 400 Low Pressure Steam 10 180 349 table 4 could be produced with reactor effluents at temperatures up to 400 ◦C350 (typical effluent temperatures of vessel reactors like the transpiring wall reactor351 and cooled wall reactor) and the high pressure steam by the effluents up to 700352 ◦C (effluents of tubular reactors or the effluent of the new cooled wall reactor353 described in this work).354 With these assumptions, the possibilities of energy recovery for some reactor355 designs are compared.356 Comparison of the recovery energy for different reactors by steam production:357 •Tubular reactor [4, 13]358 This reactor consisted on a straight and empty tube made of Ni alloy359 C-276 with a total length of 5400 mm and a diameter of 1/4” (i.d. 3.86360 mm) giving an internal volume of 63.2 ml and it was thermally isolated.361 In this case (figure 13), the effluent is used firstly to preheat the feed until362 the injection temperature (around 400 ◦C) and the remaining heat flow is363 used to produce steam. !"!#$%&%'$()*% +'', -%*,!(). /*0&-&.)'$1 2%*,!()3*4 Figure 13: System recovery design for a tubular reactor. 364 21 •Original cooled wall reactor designed in the University of Valladolid (Val-365 ladolid, Spain) [33]366 This reactor is composed by two concentric tubes; the inner one is made367 of Inconel 625, and the outer shell is made of SS 316. Oxidation reaction368 takes place inside the inner tube (reaction chamber). In the gap between369 both tubes, the pressurized feed stream is going down and cooling the370 reaction media at the same time. In such way, the inner tube does not371 withstand any pressure at all; having the same pressure in one side than372 on the other, and the thickness of the inner tube (Inconel 625) can be373 reduced. The effluent of the original CWR (at 400 ◦C) can be used in a374 Rankine cycle as it is shown in figure 14. !" #$$% &'(%)*+, Figure 14: System recovery design for the original CWR. 375 •New cooled wall reactor design [32]376 The new reactor consists of a vertical Ni-alloy reaction chamber that is377 inside of a pressure vessel made of AISI 316 able to stand a maximum378 pressure of 30 MPa and 400 ◦C. Between the walls of the two vessels a379 stream of cold water refrigerates the reaction vessel. The reagents (feed380 and oxidant) are introduced in the reactor through a tubular injector up to381 the top of the reaction chamber. The flame is produced outside of the in-382 jector, normally at the top of the reaction chamber, where the maximum383 temperature is registered. Reaction chamber is refrigerated with room384 temperature pressurized water that flowed between the reaction chamber385 wall and the inner wall which supported the pressure, keeping the pressur-386 ized wall at temperatures lower than 400 ◦C, and entering in the reaction387 chamber by its lower part mixing with the reaction products. The prod-388 22 ucts flowed down the reactor leaving it by its lower part together with the389 cooling water. Following the idea of the original CWR, the products come !"#$%& '#()*+!* ,!!- ./(-)0*1 $((+234 "5*!/ 6("#.#1*!57 8/(-)0*2(3 Figure 15: System recovery designed for the new CWR with the configuration with one outlet. 390 out the reactor at temperatures around 325 ◦C and can produce steam to391 be expanded in a Rankine cycle (figure 15).392 Other configuration of this reactor is working with an outlet at the top of393 the reactor, thus having two outlets: one at high temperature and other394 at subcritical temperatures with the salt dissolved with the cooling water395 (figure 16). For performing the comparative energetic analysis among the !"#$%& '#()*+!*, -!!. /(**(0#12(.)3* $((+456 "7*!2 8("#9#,*!70 12(.)3*4(5 :(1#12(.)3* ;46<#9#,*!70 12(.)3*4(5 Figure 16: System recovery designed for the new CWR with the configuration with two outlets. 396 different reactors types, typical operational parameters for each reactor397 such as fuel concentration, oxidant excess over the stoichiometric amount398 (based on the average excess used in the majority of the experiments and399 the acceptable oxidant excess to oxidize nitrogen compounds), the per-400 centage of cooling water and the effluent temperature were fixed. These401 parameters are shown in table 5. In first place, the analysis was performed402 considering that the effluents are used to generate steam for a Rankine403 cycle. The results are also shown in table 5, at the last two columns.404 23 As can be observed, producing electricity through Rankine cycles present Table 5: Conditions fixed for the study of each reactor, recovering energy through a Rankine cycle. 5% excess of oxidant is assumed in all cases. Type of reactor Heat Cooling Injection Effluent Efficiency flow feed water T (◦C) T (◦C) (kW) (% of feed) Air O2 Original CWR 1,202 0 Room T 400 -16.9% 8.6% New CWR 1 outlet 1,202 35 Room T 325 -16.8% 4.8% Tubular reactor 372 0 350-400 700 -21.1% 5.7% New CWR 2 outlets 1,208 35 Room T Top 700 -8.0% 19.0% 100% Bot. 300 New CWR 2 outlets 1,208 35 Room T Top 700 -11.0% 14.0% 70% Bot. 300 405 only positive efficiencies (to be able to cover the energy consumption re-406 quired by the pumping equipment) when the system is using oxygen as407 the oxidant. This is due to the much higher consumption of air compres-408 sors compared to liquid oxygen cryogenic pumps. Actually, the energy409 required (defined in equation (8)) when using air and oxygen ascends to410 28% and 0.2%, respectively.411 4.8.3. Energy recovery with the new cooled wall reactor412 Focusing on the new CWR reactor design, a detailed analysis of the energetic413 recovery possibilities is shown above. In figure 17 it is shown another possibility414 of energy recovery for each effluent of the new CWR design, besides scheme415 shown in figure 16. It was assumed, as observed experimentally, that all the416 gases involved in the combustion leaves the reactor with the top effluent: CO2 417 produced in the reaction, N2(when air is used as oxidant) and O2from the418 oxidant mixed with the water flow, being the bottom effluent considered as419 pure water.420 Influence of the distribution flow through different parameters421 To analyze the electricity production with the new CWR reactor, the conditions422 assumed are: 5% oxidant excess; 1,208 kW of heat flow feed; flow of cooling423 24 !"#$%& '#()*+!*, -!!. /(**(0#12(.)3* $((+456 "7*!2 8("#9#,*!70 12(.)3*4(5 :(1#12(.)3* ;42!3*# !<175,4(5 Figure 17: Scheme of the direct expansion of top effluent for the recovery energy with the CWR with 2 outlets. water equivalent to 35% of feed flow; and effluent temperatures of 700 ◦C and424 300 ◦C, at top and bottom outlets, respectively. The selected percentage of425 cooling water is based on the optimal operational parameters obtained with the426 new reactor.427 (a) (b) Figure 18: Efficiency of the recovery energy of the new reactor with (a) direct expansion and (b) steam production. Influence of the kind of energy production system428 Figure 18 shows the efficiency of the new reactor obtained by direct expansion of429 the flow and steam production working with air and with oxygen. Different flow430 distributions are assumed. As can be observed, the energy produced by direct431 expansion of the flow from the reactor is bigger than the energy obtained by the432 production of steam in a Rankine cycle that could be expanded afterwards.433 25