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Evaluation of burnout performance of biomass wastes in a rocket-engine-based incinerator
Fuel 143 (2015) 308–317
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Evaluation of burnout performance of biomass wastes in a rocket-engine-based incinerator Jin Woo Son, Chae Hoon Sohn ⇑ Department of Mechanical Engineering, Sejong University, Seoul 143-747, Republic of Korea
h i g h l i g h t s Burning performance of biomass wastes is evaluated numerically in the RBI chamber. The correlation is found between burnout ratio and a similarity variable. A single-valued parameter is provided for prediction of burning rate of biomass. Burning performance in the RBI is still high in burning of biomass wastes.
a r t i c l e
i n f o
Article history: Received 20 August 2014 Received in revised form 13 November 2014 Accepted 21 November 2014 Available online 2 December 2014 Keywords: Burnout performance Biomass wastes Rocket-engine-based incinerator Injectors Mobility
a b s t r a c t Burning performance of biomass wastes has been evaluated numerically in the chamber of a rocketengine-based incinerator (RBI), which was suggested as a device of a solid-particle incinerator for the purposes of both high-performance burnout and mobility. Especially, the incineration is fitted better for disposal of hazardous animal carcass than its burial. For high burning performance, the chamber of a RBI has the shape of a rocket combustor and in-chamber swirl flow is formed by peripheral injectors. First, the chamber is optimized to maximize burning rate because biomass has broader ranges of elements content and thermal properties than coal, leading to degraded burning of biomass. The optimal ratio of chamber diameter to length and the optimal deflection angle of the injector are found. Next, four kinds of fuels or wastes are burned and their burning rates are evaluated with the optimal chamber. Wood has the highest burning rate of them, and solid wastes and animal carcass have lower one than coal, but their rates are still absolutely high in this chamber. Burning rates of solid wastes and animal carcass decrease abruptly as particle diameter increases over 1 mm. The proposed chamber with the RBI concept has been verified to attain higher burnout performance of biomass wastes than a conventional incinerator by a factor of 5. And, pressure and burnout ratio are increased additionally when the 2nd chamber is attached downstream. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Incineration is one of methods to dispose of a wide variety of wastes, especially, solid wastes produced at homes, offices, agricultural farms, animal feedlots, and industrial factories. These wastes may cause environmental problems of soil and groundwater contamination, leading to endangering human health if they are not managed properly or are left for a long time without disposal. Incineration has the advantage of land-fill and ocean-dumping of wastes. Its main advantages are significant decrease in both volume and mass by more than 60% [1,2] as well as reduced biological
⇑ Corresponding author. E-mail address: [email protected] (C.H. Sohn). http://dx.doi.org/10.1016/j.fuel.2014.11.069 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
reactivity of the wastes and energy production, which is an extra benefit from incineration of wastes. Incineration technologies have gained long interest because human activities have continued to generate wastes and burn out them in places near the waste sources [2–4]. Although incineration still has problems of pollutant emission and ash disposal [5], it is a sole alternative to the other methods. The problems need to be solved technically by studies and should be minimized through the appropriate pre-processes of solid wastes before burning and post-processes of combustion products from the incinerator after burning [6,7]. There are two typical technologies to burn out or incinerate solid wastes. One is fluidized-bed combustion technology [3] and the other is pulverized-coal-firing technology [8–11]. Especially, the two previous works [10,11] were focused on numerical
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modelling with radiative heat transfer in pulverized coal firing. The fluidized-bed incinerator is a well-known burning system for wastes and biomass fuels [3,4,12] and the latter is relevant to industrial boilers for coal burning in power-plants. For highperformance burning in the latter technology, tangentially fired combustion [13,14] has been adopted and significance of swirl flow in a chamber was emphasized. More recently, co-firing biomass fuels with coal is considered a promising technology in both aspects of power generation and environmental issues. It is being studied based on interest in using cost-effective waste-derived fuels with fluidized-bed boilers [12,15] and to investigate adaptability of the existent coal-fired system for burning of biomass [16–18]. Much effort to convert wastes to fuels for power generation is being made. This is a recent trend in researches on municipal solid waste (MSW) incineration. However, there are also produced hazardous biomass wastes from sick or diseased animals of livestock such as cows, pigs, duck, goose, and chicken. For example, animals contaminated with the transmissible spongiform encephalopathy agent proved to be responsible for the fatal Bovine Spongiform Encephalopathy (BSE) symptom [19]. Accordingly, the diseased animals are considered to be inevitably slaughtered or killed to prevent the risky agent from spreading to the other animals. The slaughtered animals would be quite harmful wastes in the end and should be disposed of and isolated from the residential district as soon as possible. The most conventional method to dispose of them has been the burial to the underground like land-fill. But, it can cause finally a serious problem of the agent’s spreading through soil and underground water even if they are bagged with a cover upon the burial. The problem would be more concerned when huge amount of animals are diseased and should be slaughtered on the spot. In this situation, the burial is not effective and an alternative to it is required. However, there has been neither proposed a feasible alternative nor found a notable study yet. In this regard, this study suggests incineration or burnout of harmful biomass wastes as an alternative to the burial. Incineration has disadvantage of relatively greater expense than the other methods because it requires burning facilities such as supply system of wastes, incinerator or chamber, and drain system with emission control. In addition, pulverization of biomass and animal carcass is required for effective burning before its feeding to the incinerator. This pre-process is practically difficult and costs much. To overcome this handicap, a high-performance incinerator would fit best with the economic constraint [20]. Two requirements to be met upon burning harmful wastes are high burning rate for promptness and mobility of the incinerator for in situ burnout. The latter requirement is needed for accessibility to the narrow space as well as instant burning on the spot without collecting the harmful wastes from their sources. This study investigates feasibility for high burning performance with a compact incinerator when biomass wastes are burned out for disposal.
2. Design concept of an incinerator for high burning rate and mobility Since the key component of burning facilities is a chamber, this study is focused only on burning in a combustion chamber. Regarding compactness of a chamber, the conventional chambers used in fluidized bed or pulverized coal boilers are not satisfactory. Recently, a rocket-engine-based-incinerator (RBI) with a new concept was suggested to satisfy this requirement [21], where combustion technologies from both conventional incinerators and rocket engines were combined to attain high burning rate. They are tangential firing and burning with high energy density [13,14,21]. In the previous work [21], burning performance of the
309
RBI was evaluated numerically to be over 270 kg/h m3 in burning coal particles, which showed feasibility that the RBI could make burning rate remarkably higher by a factor of 10 than conventional boilers. Although the previous results were from burning of coal particles, the concept of a RBI is still applicable to the present study burning biomass wastes because physical, chemical, and fuel properties of coal are the baselines of those for any kinds of solid fuels [22,23]. Generally, biomass is comparable to a low-rank coal in terms of fuel properties. But, biomass has broad ranges of its properties in terms of density, particle size, content of elements such as carbon, oxygen, hydrogen, sulfur, nitrogen, and heating value [20]. Relative to coal, biomass generally has less carbon, more oxygen, lower heating value, higher moisture content, and lower density as shown in Table 1. Accordingly, several problems are predicted in burning biomass. The high moisture and ash contents in biomass fuels can cause ignition problem. The ash content can cause fouling and slagging problems as well. Because of their lower heating values, biomass burning might be accompanied by flame stability problem and leads to low burning rate. It is anticipated that blending biomass with high-quality coal will reduce flame instability as well as corrosion [23]. In this study, the RBI concept is applied to the incinerator burning biomass wastes as well as coal with the assumption that the incinerator fit for coal burning is still working effectively in burning the other solid particles of biomass. Accordingly, biomass wastes need to be pulverized or pelletized before burning [24]. The chamber shape of a RBI is shown in Fig. 1. It follows the shape of a rocket combustor with high energy density and employs vortex or swirl flow [21,25] for enough residence time. It consists of two chambers, which are connected through a narrow throat. It is expected that most of fuel is burned in the 1st chamber and additional burning is made in the 2nd chamber. Dimensional sizes of the 1st and the 2nd chamber with the baseline design configuration are shown in Fig. 1. Two chambers are connected with each other by a nozzle throat shown in Fig. 2. The throat has the geometries to generate high-speed and strong tangential flow, called cyclone, in the flow passage and thereby, swirl flow is formed at the inlet of the 2nd chamber. The geometry of the nozzle throat is not studied in detail in this work. Two kinds of injectors, each of which has 4 injectors, are installed around the 1st chamber to inject solid fuel and air and to generate swirl flow of fuel–air mixture in the chamber. In the previous work [21], flow and combustion of fuel and air only in the 1st chamber was studied and it was reported that design parameters of deflection and incline angles of the injectors, hd and hi, respectively could control swirl flow and high burning rate would be realizable with a RBI. The present chamber of a RBI aims at a target of burning performance which is higher by an order of magnitude than conventional incinerators. For quantitative design practice, the same target of 270 kg/h m3 as in the previous work [21] is selected here, for which chamber volume and fuel supply rate are chosen to be 27 103 m3 or 27 l and 2.24 g/s, respectively. It corresponds to biomass supply or disposal rate of 8 kg/h and the chamber can burn out it at the target rate if it attains higher burnout ratio [21] than 90%. 3. Numerical method 3.1. Numerical methods The steady-state three-dimensional governing equations of continuity, momentum, energy, and species equations are adopted to simulate a reacting flow field in a chamber. The equations can be found in the literatures [21,26] and omitted here. To simulate turbulent flow, Reynolds-averaged Navier–Stokes (RANS) equations, of which form is well known on the basis of any k–e turbulent
310 Table 1 Properties of various [18,23,27,33,34].
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fuels/wastes
adopted
Coal
Wood
Solid wastes
Animal carcass
Ultimate analysis (% dry ash free) C content (wt% of dry fuel) 89.3 H content 5.0 O content 3.4 N content 1.5 S content 0.8
50 6.0 42.4 0.3 1.3
51.2 6.1 38.4 4.0 0.3
39.1 6.7 48.3 4.7 1.2
Proximate analysis (% dry ash free) Fixed carbon 85.1 Volatile 6.9 Ash 8.0 Moisture 0.0
21.9 77.6 0.5 0.0
12.0 70.0 8.0 10.0
8.4 40.3 43.8 7.5
7600
4397
4000
2389
1550 0.330
700 0.173
2500 0.349
600 0.309
1680
2310
2093
3140
Fuels/wastes
Physical properties Higher heating value, Q (kcal/kg) Density, q (kg/m3) Thermal conductivity, k (W/m-K) Cp (J/kg-K)
for
numerical
simulations
Fig. 2. Geometry of the nozzle throat of a RBI (inlet diameter: 13.5 mm).
Fig. 1. Geometry and computational grids of a RBI (rocket-engine-basedincinerator).
models [26], are solved simultaneously with the aid of appropriate numerical schemes. All simulations in this study are conducted by using a general purpose CFD code, FLUENT [27]. Of various
turbulent flow models, the realizable k–e turbulent model [28,29] is adopted here. Radiative heat transfer is considered with the P 1 model adopted [27,30], where solid-phase and gas-phase radiations are modelled. The radiation model is cost-effective and can account for particulate effects such as scattering, emission, and exchange of radiation between gas and particulates. The governing equations are discretized in space by finite difference scheme with the 2nd order accuracy and the discretized equations are solved with appropriate boundary conditions for the domain with unstructured grids. In numerical simulations, four solid fuels of coal, wood, municipal solid wastes, and animal carcass are chosen for burnout. Physical, chemical, and fuel properties of them can be found in the literatures [18,23,27,31–34], but they still have much ambiguity in each property. Although properties for each of them cannot be specified clearly by a single value, typical representative properties for each are adopted here for numerical simulations and listed in Table 1. For simulation of coal combustion, devolatilization and char reactions are to be considered. For devolatilization, the two competing rates model is adopted, where two distinct rates are employed at high and low temperatures, respectively [35]. After the devolatilization, char and ash are left. The char is reacted with gases. For the heterogeneous surface reaction rate model of char, the multiple surface reaction model [27] is adopted, which is similar to wall surface reaction model. Char reactions with gases are included as a sub-set in a multi-step chemistry adopted in this work. For simulation of flame evolution more accurately, an 8-step multiple chemical mechanism for solid- and gas-phase reactions of coal [27,36–41] is employed for a simplified chemistry for coal and biomass instead of a global one- or two-step chemistry. The multi-step chemistry is shown in Table 2, which is the same as that adopted in the previous work [21] for consistency. It is known that adoption of a multi-step chemistry has the advantage of improved
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J.W. Son, C.H. Sohn / Fuel 143 (2015) 308–317 Table 2 Chemical kinetic parameters for coal char reactions and gas reactions [37]. No.
Reaction step
Pre-exponential factor, A
Activation energy, Ea (J/kmol)
References
1 2 3 4 5 6 7 8
Volatiles + 2.207O2 ? 0.1CO2 + 4.408H2 C(S) + 0.5O2 ? CO C(S) + CO2 ? 2CO C(S) + H2O ? H2 + CO H2 + 0.5O2 ? H2O CO + H2O ? CO2 + H2 CO2 + H2 ? CO + H2O CO + 0.5O2 ? CO2
2.119e+11 1.36e+06 6.78e+04 8.55e+04 6.8e+15 2.75e+10 2.65e02 2.2e+12
2.027e+08 1.30e+08 1.63e+08 1.40e+08 1.68e+08 8.38e+07 3.96e+03 1.67e+08
[27] [38]
resolution of the flame envelope zone in terms of the temperature distribution and species concentrations, relative to the one- and two-step reaction schemes [36]. The reaction steps with Nos. 2–4 are for char reactions. And, a finite-rate/eddy dissipation model is adopted for consideration of turbulent combustion accompanying chemical reactions applied [28]. As boundary conditions for the simulations, fuel and air are injected at the rates of 0.56 g/s and 2.9 g/s, respectively, through each primary injector of which diameter is 10 mm. Total rate of fuel supply is 2.24 g/s. The injection velocity of air is about 31 m/s. And, through each secondary injector, air only is injected at the rate of 4.4 g/s and its velocity is about 45 m/s. Overall equivalence ratio, u is 0.81, which corresponds to air-excess ratio of 1.2. The temperatures of fuel and air at the inlets are 400 K and 650 K, respectively. Constant temperature of 500 K is applied at the chamber wall and atmospheric pressure is assigned at the chamber exit. Because wood, solid wastes, and animal carcass have lower heating values than coal, ignition failure and unstable combustion might take place. To facilitate chemical reactions of coal/biomass, sufficiently high temperature air of 2000 K is initially distributed in all the computational domain, i.e., in the chamber, which plays a role as an initial guess for steady-state solutions. Convergence criterion that residual of each normalized governing equation should be smaller than 104 is adopted for steady-state solutions. Polyhedral grids are adopted here and typically, the numbers of grids were chosen to be about 500,000 and 1,320,000 without and with the 2nd chamber, respectively, after grid-dependency check. To quantify burning efficiency of fuel in the chamber, burnout ratio, Br, is introduced and defined as
Br ð%Þ
mi mf 100; mi
ð1Þ
where mi and mf denote initial combustible mass of fuel before burning and its final mass after burning, respectively. Final mass, mf, is calculated by unburned combustible mass at the chamber exit. With a fuel supply rate fixed, burnout ratio or burning efficiency has the meaning of burning rate or burnout performance. 3.2. Design parameters for optimization of the chamber Although the 1st chamber is proven to offer high burning performance of coal [21], the chamber should be improved for enhanced burning of biomass because biomass burning is in more trouble than coal. There are various design parameters affecting burning in the chamber. In this study, only the 1st chamber is optimized in terms of geometry because it is dominant over the 2nd chamber. Major geometric parameters of the 1st chamber are shown in Fig. 3, which are the diameter, D, and the lengths, L and hi’s, shaping the chamber. The other parameters are deflection and incline angles of the injectors, hd and hi, respectively, which have nothing to do with the chamber geometry, but will affect significantly internal flow in the chamber. For mobility of the
[39] [40] [41]
Fig. 3. Fixed and variable design parameters of the 1st chamber (L and D: variable, the others: fixed).
incinerator, the chambers should be compact and the total volume is fixed to be 27 l as mentioned in the preceding section. As in the previous work, where only the 1st chamber was considered, volume of 6.7 l is assigned to the 1st chamber and the other of 20.3 l is for the nozzle throat and the 2nd chamber. When the 1st chamber is optimized, its volume is fixed and some of design parameters are variable. Of 8 parameters in all, the three parameters of D, L, and hd are selected to be variable and the other five parameters are fixed. The fixed values for the latter parameters are from the baseline design configuration of the 1st chamber adopted in the previous work [21]. Numerical analyses are carried out with the variable parameters over the wide range of each. The fixed values and the ranges of the variables are listed in Table 3. With the chamber volume fixed, 6.7 l, the parameters of D and L are adjusted to maximize burning performance of coal. A basic assumption is made that the increase in burning rate of coal leads to increased burning of the other fuels. Considering the importance of swirl flow formed in a chamber, the deflection angle is made variable, but the incline angle is set
Table 3 Fixed and variable geometric parameters of the 1st chamber for optimization. Incline angle of the secondary injectors Fixed design parameters 5° Deflection angle of the primary injectors
Volume (m3)
h2/h1
h3/h1
h4 (m)
0.0067
0.316
0.474
0.01
D (m)
L (m)
Variable design parameters 5–75° (step size: 10°) 0.2–0.27
0.14–0.25
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constant because of its slight impact on swirl flow. Deflection angle, hd, changes widely, ranging from 5° to 75° with the step size of 10°. For geometric similarity of the optimal shape, a nondimensionalized parameter of Z is introduced, which is defined as Z = D/L, the ratio of chamber diameter to length. The parameter, Z, affects both residence time of fuel or wastes and tangential fire ball in the chamber. With a high value of Z, the chamber has relatively large diameter and relatively short length, leading to enhanced formation of swirl flow and tangential fire ball, but reduced residence time of fuel. On the other hand, a low value of Z increases residence time and weakens swirl flow. Accordingly, any value of Z provides two competing factors in terms of burning rate. 4. Results and discussions 4.1. Optimization of the chamber When dimensional sizes of the 1st chamber are given, the controlling parameter for high burning rate is swirl number as reported in the previous work [21]. During the optimization, both of Z and hd are changed as listed in Table 3 and burnout ratios of coal are calculated from numerical results at their particular values. Fig. 4 shows dependences of burnout ratio on the deflection angle and the ratio of diameter to length, Z = D/L, with coal particle diameter, d = 300 lm. It is seen that burnout ratio is affected significantly by deflection angle, but a little by the ratio, Z. It increases as deflection angle is reduced until the angle reaches 5°. If it decreases further, burnout ratio decreases abruptly and flame is not stably sustained with hd 0. At larger deflection angles than 5°, burnout ratio decreases with deflection angle. It can be explained as follows. When coal and air are injected with higher deflection angle, their mixing zone becomes closer to the cold chamber wall and chemical reaction takes place near the wall. Then, the rate of heat loss to the wall will be increased. This is verified by heat flux to the wall calculated over wide ranges of Z and hd shown for coal fuel in Fig. 5. As shown in Figs. 4 and 5, deflection angle increases wall heat flux and decreases burnout ratio. That is, higher rate of heat loss to the wall, induced by large deflection angle, causes lower burnout ratio. From Fig. 4, it is seen that the highest burnout ratio of 98.7% is attained at Z = 1.0 and hd = 5°,
Fig. 5. Dependences of wall heat flux in burning coal on the deflection angle of the injectors and the ratio of chamber diameter to length (average coal diameter: 300 lm).
which is the optimal point, and one of local minima is found at Z = 0.8 and hd = 55°, where Br is 88.4%. Fig. 6 shows temperature fields for coal burning at these two points of (a) the local minimum and (b) the optimal. The hightemperature zone is formed off the chamber wall at the optimal point as shown in Fig. 6b. It means reduced heat loss to the wall. And, the peak temperature is higher for the optimal condition than for the local minimum because the reaction zone is formed farther from the wall and heat loss is reduced more for the optimal condition. This is confirmed strongly by Fig. 7, which shows a reaction rate fields at these two points of (a) the local minimum and (b) the optimal. At the optimal point, relative to the local minimum point, the reaction zone penetrates more into the chamber center and becomes broad. But, at the local minimum point, chemical reaction is confined to the narrow zone near the wall, leading to low burning rate of coal. From the numerical results, the optimal point to maximize burning rate has been found, which is characterized by low heat loss to the wall and broad reaction zone. Biomass burning will be done with this optimized chamber in the later section. 4.2. Biomass burning in the optimized chamber of a RBI
Fig. 4. Dependences of burnout ratio of coal on the deflection angle of the injectors and the ratio of chamber diameter to length (average coal diameter: 300 lm).
To evaluate burnout ratios of four kinds of fuels listed in Table 1, they are burned with several particle diameters in the optimized chamber found in the preceding section. Temperature fields are shown in Fig. 8 with particle diameter, d = 500 lm. Coal has the broadest reaction zone and the highest temperature in the reaction zone. And, wood, solid waste, and animal carcass are the next in a descending order. For animal carcass, the reaction zone is confined to a narrow zone near the chamber wall and the lowest temperature in the zone is observed. These phenomena can be confirmed by reactions rates of volatiles and char reactions. The reaction rates of the step Nos. 1–4 are seen in Fig. 9 for each fuel. They are volume-averaged values in the whole domain of the chamber. As predicted, the slowest reaction is the reaction step No. 3. For volatiles reaction, wood has the highest and solid waste, animal carcass, and coal are the next in a descending order with smaller diameters than 1000 lm. For char reactions, coal does the highest and wood, solid waste, and animal carcass are the next irrespective of the reaction step.
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Fig. 6. Temperature fields in burning coal for (a) a condition of low burnout ratio of 88.4% with Z = 0.8 and hd = 55°, and (b) the optimal condition with Z = 1.0 and hd = 5° (average coal diameter: 300 lm).
Fig. 7. Reaction rate fields of the step, C + CO2 ? 2CO, in burning coal for (a) a condition of low burnout ratio of 88.4% with Z = 0.8 and hd = 55°, and (b) the optimal condition with Z = 1.0 and hd = 5° (coal average diameter: 300 lm).
Their burnout ratios are shown in Fig. 10. Wood and animal carcass have the burnout ratio of nearly 100% with fine particles. Animal carcass has the worst properties in terms of combustion and leads to low reaction rates followed by low flame temperature in the chamber, which has been confirmed by the results in Figs. 8 and 9. Nevertheless, it has the highest burnout ratio. Because animal carcass has the smallest amount of combustibles, which occupy only 48.7% of total mass. Accordingly, most of the combustible mass in animal carcass can be burned out although the overall reaction rate is low. On the other hand, wood has the largest combustible mass (99.5%) of them and lower reactivity than coal, but it shows higher burnout ratio close to 100% than that of coal with smaller particle diameters. Wood has volatiles of 77.6% and char of 21.9% and coal has 6.9% and 85.1%, respectively. Burnout ratio of char is relatively low to that of volatiles when much char is burned. Accordingly, coal, which is occupied mostly by char, has a little lower burnout ratio than wood. As shown in Fig. 10a, these phenomena observed in animal carcass are changed when particles with larger diameters than 700 lm are burned. Low reaction rates of anima carcass become effective with large diameters. That is, large diameter particles of animal carcass are not burned completely because of low reaction rates, i.e., insufficient chemical reaction, and its burnout ratio falls down abruptly. This phenomenon is also observed in solid waste. From these results, it is found that the combustible mass, its constituents, and particle diameter affect burnout ratio significantly.
To consider different combustibles content in each fuel, burnout ratio needs to be modified by multiplication of the combustibles (fixed carbon and volatile) content ratio, i.e., (combustibles content of each fuel)/(combustibles content of wood). Wood has the highest content of combustibles. The relative or normalized burnout ratio is denoted by Br_rel. There are found relative burnout ratios of wood, coal, solid wastes, and animal carcass in a descending order as shown in Fig. 10b. Wood can be almost burned out, but animal carcass has a half of burnout ratio of wood, around 50%. Burnout ratio decreases as particle diameter increases. This decrement in Br is magnified in burning solid wastes and animal carcass, which would be caused by their lower reaction rates than those of wood and coal. As expected, animal carcass has the lowest burnout ratio around 50% relative to wood, but it is worth noting that the chamber of a RBI can still work with animal carcass at a high burning rate if it is supplied to the chamber with smaller diameters than 1000 lm. Accordingly, burning performance of the chamber should be compromised with economic load to pulverize biomass into fine particles. The reaction zones in burning wood and coal develop near the injectors and spread to the center and downstream, but in burning animal carcass, their development is retarded and they are formed near the contraction part of the chamber more downstream as shown in Fig. 8. Retarded formation of reaction zones results in insufficient chemical reaction, leading to low temperature in the chamber. The peak temperature in burning animal carcass was lower than that for coal by 200–900 K.
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Fig. 8. Temperature fields in the first chamber for (a) coal, (b) wood, (c) solid waste, and (d) animal carcass with particle diameter, d = 500 lm.
Fig. 9. Reaction rates of the volatiles and char reactions for each fuel for several particle diameters: (a) reaction rates for the reaction step No. 1, (b) reaction rates for the reaction step No. 2, (c) reaction rates for the reaction step No. 3, and (d) reaction rates for the reaction step No. 4.
It has been found that burnout ratio is affected by various parameters such as fuel type, particle diameter, and thermal and flow conditions. It would be a complex function of the parameters
and it is difficult to express explicitly correlation of burnout ratio with them. Accordingly, it will be useful to introduce a singlevalued independent parameter physically, e.g., Damköhler number,
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315
Fig. 11. Burnout ratios of all the fuels expressed by Damköhler number.
Fig. 10. Burning performances of the RBI: (a) burnout ratios and (b) relative burnout ratios as a function of particle diameter for each fuel.
Da, to express all the burnout-ratio data irrespective of fuel type and particle diameter. By dimensional analysis, it can be defined here in a form,
Da ¼
atres 2
Ad
q ; Q
the nozzle throat and then, it is accelerated locally to supersonic flow with Mach number higher than 1.0. And then, finally, it is decelerated to subsonic flow in the 2nd chamber. It is seen that the present throat plays a significant role in both flow acceleration and tangential-flow generation although it is not tuned or optimized. The maximum Mach No. in burning coal reaches about 1.4 while it does only 1.1 in burning animal carcass. As discussed in the preceding section, the former case has higher pressure and burning rate when the fuel is burned only in the 1st chamber. Fig. 13 shows pressures in the 1st chamber and burnout ratios in burning coal and animal carcass when they are burned with and without the 2nd chamber attached. It is seen than burnout ratio and chamber pressure are increased significantly by adding the 2nd chamber because residence time of fuel is increased and additional reactions occur in the 2nd chamber. It is worth noting that increment in burnout ratio induced by the 2nd chamber is magnified more as particle diameter increases. Accordingly, the 2nd chamber with a nozzle throat can be a viable solution to enhance relatively low burning rate of fuel with large diameters, which is difficult to burn. On the other hand, this result means that the 2nd chamber is required necessarily for high performance when fuel or wastes with larger diameters than 1000 lm.
ð2Þ
where a, tres, Q, A, and q denote thermal diffusivity of fuel, residence time of the mixture in the chamber, higher heating value, frequency factor, and reaction rate, respectively. For the last two parameters, the third reaction step in Table 2 was selected because it is the rate-limiting step of char reactions in the reaction zones. Burnout ratios expressed by Da are shown in Fig. 11, where it is seen that all the data have good similarity. Burnout ratio increases with Da and it levels off at higher Damköhler number than 4 1010. The correlation between burnout ratio and the suggested Damköhler number is useful in predicting burning rate of any kinds of fuel. 4.3. Effects of the 2nd chamber and nozzle throat on burning rate To enhance the burning rate of animal carcass with large particle diameters, the 2nd chamber should be attached to the 1st chamber as shown in Fig. 1, which leads to a complete shape of a rocket combustor. For this purpose, a nozzle throat connecting the 2nd chamber with the 1st chamber is installed between two chambers. As aforementioned, the throat should generate highspeed and strong tangential flow in the passage. Reactive flow fields are calculated with the 2nd chamber with all the fuels and Mach number fields in burning coal and animal carcass are demonstrated near the nozzle throat in Fig. 12. In the contraction part of the 1st chamber, flow is accelerated and sonic speed is attained at
Fig. 12. Mach number fields in burning coal and animal carcass near the nozzle throat between the 1st and the 2nd chambers (particle diameter: 700 lm).
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Damköhler number was provided and it will be useful for the chamber design. For further improvement of burning performance, optimization of the nozzle throat and the 2nd chamber should be done and experimental validations are required before engineering applications of a RBI. They will be interesting subjects for future works. In terms of economy, the incineration still has cost problems in pre-process of pulverization to make suitable feeding. When animal carcass with a disease germ is burned out, in-situ burnout of carcass at its origin is absolutely required for prevention of the surroundings from a secondary infection. Considering these two competing factors, losses or costs for pre- and post-process can be offset by the gain. Of course, economic consideration should be made more rigorously based on quantitative data on required costs and benefit in the future. Fig. 13. Relative burnout ratios and chamber pressures in burning coal and animal carcass with and without the 2nd chamber.
Acknowledgments 5. Conclusion Burning performance of biomass wastes has been evaluated numerically in the chamber of a rocket-engine-based incinerator (RBI), which was suggested as a device of a solid-particle incinerator for the purposes of both high-performance burnout and mobility. Especially, hazardous animal carcass would be disposed of with this chamber. First, optimization of the 1st chamber was made over wide ranges of the ratio of chamber diameter to length and deflection angle of the injector with volume of the chamber fixed. It is more difficult to burn biomass than coal in terms of fuel properties. Accordingly, the chamber of a RBI should be optimized to keep high performance even in burning biomass. The chamber was optimized by exhaustive numerical computations and the optimal point with the maximum burnout ratio was found to be at the chamber diameter-to-length ratio of 1.0 and deflection angle of 5°. Low heat loss to the chamber wall and broad reaction zone of the fuel/air mixture lead to high burnout ratio in the chamber. Accordingly, these two factors should be considered to increase burnout performance. Next, with the optimized chamber, various fuels or wastes such as coal, wood, municipal solid wastes, and animal carcass were burned. Burnout of biomass wastes has been examined numerically by adopting approximate properties of biomass. Numerical results showed that burnout ratio of animal carcass would be as low as a half of that of coal, but it is still 5 times higher than that of conventional incinerators. Relative burnout ratios of all of fuels have the same order of magnitude irrespective of fuel. But, it should be verified experimentally in the future. And then, a specific biomass or animal carcass would be selected for intensive works in terms of burning rate. Lastly, the 2nd chamber is attached to the 1st chamber through the nozzle throat to make additional increase in burning rate, especially when large diameter particles are burned. With this complete chamber, pressure in the 1st chamber is increased further and sonic speed is attained at the nozzle throat, resulting in strong tangential flow in the 2nd chamber. In the end, burnout ratio is increased significantly with the 2nd chamber attached. The present study verified the feasibility of a RBI for high burning rate of biomass wastes for fast burnout and a compact size for mobility or accessibility. Accordingly, the proposed chamber of a RBI will be a viable incinerator for burning out biomass including harmful animal carcass. A single-valued parameter of a similarity variable was found from characteristic time scales eligible for a RBI after numerical data were gathered for various fuels and particle diameters. The correlation between burnout ratio and
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean government (MEST) (Grant No. 2013-023030) and also supported by Advanced Research Center Program (No. 2013073861) through the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP) contracted through Next Generation Space Propulsion Research Center at Seoul National University.
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Evaluation of burnout performance of biomass wastes in a rocket-engine-based incinerator Jin Woo Son, Chae Hoon Sohn ⇑ Department of Mechanical Engineering, Sejong University, Seoul 143-747, Republic of Korea
h i g h l i g h t s Burning performance of biomass wastes is evaluated numerically in the RBI chamber. The correlation is found between burnout ratio and a similarity variable. A single-valued parameter is provided for prediction of burning rate of biomass. Burning performance in the RBI is still high in burning of biomass wastes.
a r t i c l e
i n f o
Article history: Received 20 August 2014 Received in revised form 13 November 2014 Accepted 21 November 2014 Available online 2 December 2014 Keywords: Burnout performance Biomass wastes Rocket-engine-based incinerator Injectors Mobility
a b s t r a c t Burning performance of biomass wastes has been evaluated numerically in the chamber of a rocketengine-based incinerator (RBI), which was suggested as a device of a solid-particle incinerator for the purposes of both high-performance burnout and mobility. Especially, the incineration is fitted better for disposal of hazardous animal carcass than its burial. For high burning performance, the chamber of a RBI has the shape of a rocket combustor and in-chamber swirl flow is formed by peripheral injectors. First, the chamber is optimized to maximize burning rate because biomass has broader ranges of elements content and thermal properties than coal, leading to degraded burning of biomass. The optimal ratio of chamber diameter to length and the optimal deflection angle of the injector are found. Next, four kinds of fuels or wastes are burned and their burning rates are evaluated with the optimal chamber. Wood has the highest burning rate of them, and solid wastes and animal carcass have lower one than coal, but their rates are still absolutely high in this chamber. Burning rates of solid wastes and animal carcass decrease abruptly as particle diameter increases over 1 mm. The proposed chamber with the RBI concept has been verified to attain higher burnout performance of biomass wastes than a conventional incinerator by a factor of 5. And, pressure and burnout ratio are increased additionally when the 2nd chamber is attached downstream. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Incineration is one of methods to dispose of a wide variety of wastes, especially, solid wastes produced at homes, offices, agricultural farms, animal feedlots, and industrial factories. These wastes may cause environmental problems of soil and groundwater contamination, leading to endangering human health if they are not managed properly or are left for a long time without disposal. Incineration has the advantage of land-fill and ocean-dumping of wastes. Its main advantages are significant decrease in both volume and mass by more than 60% [1,2] as well as reduced biological
⇑ Corresponding author. E-mail address: [email protected] (C.H. Sohn). http://dx.doi.org/10.1016/j.fuel.2014.11.069 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
reactivity of the wastes and energy production, which is an extra benefit from incineration of wastes. Incineration technologies have gained long interest because human activities have continued to generate wastes and burn out them in places near the waste sources [2–4]. Although incineration still has problems of pollutant emission and ash disposal [5], it is a sole alternative to the other methods. The problems need to be solved technically by studies and should be minimized through the appropriate pre-processes of solid wastes before burning and post-processes of combustion products from the incinerator after burning [6,7]. There are two typical technologies to burn out or incinerate solid wastes. One is fluidized-bed combustion technology [3] and the other is pulverized-coal-firing technology [8–11]. Especially, the two previous works [10,11] were focused on numerical
J.W. Son, C.H. Sohn / Fuel 143 (2015) 308–317
modelling with radiative heat transfer in pulverized coal firing. The fluidized-bed incinerator is a well-known burning system for wastes and biomass fuels [3,4,12] and the latter is relevant to industrial boilers for coal burning in power-plants. For highperformance burning in the latter technology, tangentially fired combustion [13,14] has been adopted and significance of swirl flow in a chamber was emphasized. More recently, co-firing biomass fuels with coal is considered a promising technology in both aspects of power generation and environmental issues. It is being studied based on interest in using cost-effective waste-derived fuels with fluidized-bed boilers [12,15] and to investigate adaptability of the existent coal-fired system for burning of biomass [16–18]. Much effort to convert wastes to fuels for power generation is being made. This is a recent trend in researches on municipal solid waste (MSW) incineration. However, there are also produced hazardous biomass wastes from sick or diseased animals of livestock such as cows, pigs, duck, goose, and chicken. For example, animals contaminated with the transmissible spongiform encephalopathy agent proved to be responsible for the fatal Bovine Spongiform Encephalopathy (BSE) symptom [19]. Accordingly, the diseased animals are considered to be inevitably slaughtered or killed to prevent the risky agent from spreading to the other animals. The slaughtered animals would be quite harmful wastes in the end and should be disposed of and isolated from the residential district as soon as possible. The most conventional method to dispose of them has been the burial to the underground like land-fill. But, it can cause finally a serious problem of the agent’s spreading through soil and underground water even if they are bagged with a cover upon the burial. The problem would be more concerned when huge amount of animals are diseased and should be slaughtered on the spot. In this situation, the burial is not effective and an alternative to it is required. However, there has been neither proposed a feasible alternative nor found a notable study yet. In this regard, this study suggests incineration or burnout of harmful biomass wastes as an alternative to the burial. Incineration has disadvantage of relatively greater expense than the other methods because it requires burning facilities such as supply system of wastes, incinerator or chamber, and drain system with emission control. In addition, pulverization of biomass and animal carcass is required for effective burning before its feeding to the incinerator. This pre-process is practically difficult and costs much. To overcome this handicap, a high-performance incinerator would fit best with the economic constraint [20]. Two requirements to be met upon burning harmful wastes are high burning rate for promptness and mobility of the incinerator for in situ burnout. The latter requirement is needed for accessibility to the narrow space as well as instant burning on the spot without collecting the harmful wastes from their sources. This study investigates feasibility for high burning performance with a compact incinerator when biomass wastes are burned out for disposal.
2. Design concept of an incinerator for high burning rate and mobility Since the key component of burning facilities is a chamber, this study is focused only on burning in a combustion chamber. Regarding compactness of a chamber, the conventional chambers used in fluidized bed or pulverized coal boilers are not satisfactory. Recently, a rocket-engine-based-incinerator (RBI) with a new concept was suggested to satisfy this requirement [21], where combustion technologies from both conventional incinerators and rocket engines were combined to attain high burning rate. They are tangential firing and burning with high energy density [13,14,21]. In the previous work [21], burning performance of the
309
RBI was evaluated numerically to be over 270 kg/h m3 in burning coal particles, which showed feasibility that the RBI could make burning rate remarkably higher by a factor of 10 than conventional boilers. Although the previous results were from burning of coal particles, the concept of a RBI is still applicable to the present study burning biomass wastes because physical, chemical, and fuel properties of coal are the baselines of those for any kinds of solid fuels [22,23]. Generally, biomass is comparable to a low-rank coal in terms of fuel properties. But, biomass has broad ranges of its properties in terms of density, particle size, content of elements such as carbon, oxygen, hydrogen, sulfur, nitrogen, and heating value [20]. Relative to coal, biomass generally has less carbon, more oxygen, lower heating value, higher moisture content, and lower density as shown in Table 1. Accordingly, several problems are predicted in burning biomass. The high moisture and ash contents in biomass fuels can cause ignition problem. The ash content can cause fouling and slagging problems as well. Because of their lower heating values, biomass burning might be accompanied by flame stability problem and leads to low burning rate. It is anticipated that blending biomass with high-quality coal will reduce flame instability as well as corrosion [23]. In this study, the RBI concept is applied to the incinerator burning biomass wastes as well as coal with the assumption that the incinerator fit for coal burning is still working effectively in burning the other solid particles of biomass. Accordingly, biomass wastes need to be pulverized or pelletized before burning [24]. The chamber shape of a RBI is shown in Fig. 1. It follows the shape of a rocket combustor with high energy density and employs vortex or swirl flow [21,25] for enough residence time. It consists of two chambers, which are connected through a narrow throat. It is expected that most of fuel is burned in the 1st chamber and additional burning is made in the 2nd chamber. Dimensional sizes of the 1st and the 2nd chamber with the baseline design configuration are shown in Fig. 1. Two chambers are connected with each other by a nozzle throat shown in Fig. 2. The throat has the geometries to generate high-speed and strong tangential flow, called cyclone, in the flow passage and thereby, swirl flow is formed at the inlet of the 2nd chamber. The geometry of the nozzle throat is not studied in detail in this work. Two kinds of injectors, each of which has 4 injectors, are installed around the 1st chamber to inject solid fuel and air and to generate swirl flow of fuel–air mixture in the chamber. In the previous work [21], flow and combustion of fuel and air only in the 1st chamber was studied and it was reported that design parameters of deflection and incline angles of the injectors, hd and hi, respectively could control swirl flow and high burning rate would be realizable with a RBI. The present chamber of a RBI aims at a target of burning performance which is higher by an order of magnitude than conventional incinerators. For quantitative design practice, the same target of 270 kg/h m3 as in the previous work [21] is selected here, for which chamber volume and fuel supply rate are chosen to be 27 103 m3 or 27 l and 2.24 g/s, respectively. It corresponds to biomass supply or disposal rate of 8 kg/h and the chamber can burn out it at the target rate if it attains higher burnout ratio [21] than 90%. 3. Numerical method 3.1. Numerical methods The steady-state three-dimensional governing equations of continuity, momentum, energy, and species equations are adopted to simulate a reacting flow field in a chamber. The equations can be found in the literatures [21,26] and omitted here. To simulate turbulent flow, Reynolds-averaged Navier–Stokes (RANS) equations, of which form is well known on the basis of any k–e turbulent
310 Table 1 Properties of various [18,23,27,33,34].
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fuels/wastes
adopted
Coal
Wood
Solid wastes
Animal carcass
Ultimate analysis (% dry ash free) C content (wt% of dry fuel) 89.3 H content 5.0 O content 3.4 N content 1.5 S content 0.8
50 6.0 42.4 0.3 1.3
51.2 6.1 38.4 4.0 0.3
39.1 6.7 48.3 4.7 1.2
Proximate analysis (% dry ash free) Fixed carbon 85.1 Volatile 6.9 Ash 8.0 Moisture 0.0
21.9 77.6 0.5 0.0
12.0 70.0 8.0 10.0
8.4 40.3 43.8 7.5
7600
4397
4000
2389
1550 0.330
700 0.173
2500 0.349
600 0.309
1680
2310
2093
3140
Fuels/wastes
Physical properties Higher heating value, Q (kcal/kg) Density, q (kg/m3) Thermal conductivity, k (W/m-K) Cp (J/kg-K)
for
numerical
simulations
Fig. 2. Geometry of the nozzle throat of a RBI (inlet diameter: 13.5 mm).
Fig. 1. Geometry and computational grids of a RBI (rocket-engine-basedincinerator).
models [26], are solved simultaneously with the aid of appropriate numerical schemes. All simulations in this study are conducted by using a general purpose CFD code, FLUENT [27]. Of various
turbulent flow models, the realizable k–e turbulent model [28,29] is adopted here. Radiative heat transfer is considered with the P 1 model adopted [27,30], where solid-phase and gas-phase radiations are modelled. The radiation model is cost-effective and can account for particulate effects such as scattering, emission, and exchange of radiation between gas and particulates. The governing equations are discretized in space by finite difference scheme with the 2nd order accuracy and the discretized equations are solved with appropriate boundary conditions for the domain with unstructured grids. In numerical simulations, four solid fuels of coal, wood, municipal solid wastes, and animal carcass are chosen for burnout. Physical, chemical, and fuel properties of them can be found in the literatures [18,23,27,31–34], but they still have much ambiguity in each property. Although properties for each of them cannot be specified clearly by a single value, typical representative properties for each are adopted here for numerical simulations and listed in Table 1. For simulation of coal combustion, devolatilization and char reactions are to be considered. For devolatilization, the two competing rates model is adopted, where two distinct rates are employed at high and low temperatures, respectively [35]. After the devolatilization, char and ash are left. The char is reacted with gases. For the heterogeneous surface reaction rate model of char, the multiple surface reaction model [27] is adopted, which is similar to wall surface reaction model. Char reactions with gases are included as a sub-set in a multi-step chemistry adopted in this work. For simulation of flame evolution more accurately, an 8-step multiple chemical mechanism for solid- and gas-phase reactions of coal [27,36–41] is employed for a simplified chemistry for coal and biomass instead of a global one- or two-step chemistry. The multi-step chemistry is shown in Table 2, which is the same as that adopted in the previous work [21] for consistency. It is known that adoption of a multi-step chemistry has the advantage of improved
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J.W. Son, C.H. Sohn / Fuel 143 (2015) 308–317 Table 2 Chemical kinetic parameters for coal char reactions and gas reactions [37]. No.
Reaction step
Pre-exponential factor, A
Activation energy, Ea (J/kmol)
References
1 2 3 4 5 6 7 8
Volatiles + 2.207O2 ? 0.1CO2 + 4.408H2 C(S) + 0.5O2 ? CO C(S) + CO2 ? 2CO C(S) + H2O ? H2 + CO H2 + 0.5O2 ? H2O CO + H2O ? CO2 + H2 CO2 + H2 ? CO + H2O CO + 0.5O2 ? CO2
2.119e+11 1.36e+06 6.78e+04 8.55e+04 6.8e+15 2.75e+10 2.65e02 2.2e+12
2.027e+08 1.30e+08 1.63e+08 1.40e+08 1.68e+08 8.38e+07 3.96e+03 1.67e+08
[27] [38]
resolution of the flame envelope zone in terms of the temperature distribution and species concentrations, relative to the one- and two-step reaction schemes [36]. The reaction steps with Nos. 2–4 are for char reactions. And, a finite-rate/eddy dissipation model is adopted for consideration of turbulent combustion accompanying chemical reactions applied [28]. As boundary conditions for the simulations, fuel and air are injected at the rates of 0.56 g/s and 2.9 g/s, respectively, through each primary injector of which diameter is 10 mm. Total rate of fuel supply is 2.24 g/s. The injection velocity of air is about 31 m/s. And, through each secondary injector, air only is injected at the rate of 4.4 g/s and its velocity is about 45 m/s. Overall equivalence ratio, u is 0.81, which corresponds to air-excess ratio of 1.2. The temperatures of fuel and air at the inlets are 400 K and 650 K, respectively. Constant temperature of 500 K is applied at the chamber wall and atmospheric pressure is assigned at the chamber exit. Because wood, solid wastes, and animal carcass have lower heating values than coal, ignition failure and unstable combustion might take place. To facilitate chemical reactions of coal/biomass, sufficiently high temperature air of 2000 K is initially distributed in all the computational domain, i.e., in the chamber, which plays a role as an initial guess for steady-state solutions. Convergence criterion that residual of each normalized governing equation should be smaller than 104 is adopted for steady-state solutions. Polyhedral grids are adopted here and typically, the numbers of grids were chosen to be about 500,000 and 1,320,000 without and with the 2nd chamber, respectively, after grid-dependency check. To quantify burning efficiency of fuel in the chamber, burnout ratio, Br, is introduced and defined as
Br ð%Þ
mi mf 100; mi
ð1Þ
where mi and mf denote initial combustible mass of fuel before burning and its final mass after burning, respectively. Final mass, mf, is calculated by unburned combustible mass at the chamber exit. With a fuel supply rate fixed, burnout ratio or burning efficiency has the meaning of burning rate or burnout performance. 3.2. Design parameters for optimization of the chamber Although the 1st chamber is proven to offer high burning performance of coal [21], the chamber should be improved for enhanced burning of biomass because biomass burning is in more trouble than coal. There are various design parameters affecting burning in the chamber. In this study, only the 1st chamber is optimized in terms of geometry because it is dominant over the 2nd chamber. Major geometric parameters of the 1st chamber are shown in Fig. 3, which are the diameter, D, and the lengths, L and hi’s, shaping the chamber. The other parameters are deflection and incline angles of the injectors, hd and hi, respectively, which have nothing to do with the chamber geometry, but will affect significantly internal flow in the chamber. For mobility of the
[39] [40] [41]
Fig. 3. Fixed and variable design parameters of the 1st chamber (L and D: variable, the others: fixed).
incinerator, the chambers should be compact and the total volume is fixed to be 27 l as mentioned in the preceding section. As in the previous work, where only the 1st chamber was considered, volume of 6.7 l is assigned to the 1st chamber and the other of 20.3 l is for the nozzle throat and the 2nd chamber. When the 1st chamber is optimized, its volume is fixed and some of design parameters are variable. Of 8 parameters in all, the three parameters of D, L, and hd are selected to be variable and the other five parameters are fixed. The fixed values for the latter parameters are from the baseline design configuration of the 1st chamber adopted in the previous work [21]. Numerical analyses are carried out with the variable parameters over the wide range of each. The fixed values and the ranges of the variables are listed in Table 3. With the chamber volume fixed, 6.7 l, the parameters of D and L are adjusted to maximize burning performance of coal. A basic assumption is made that the increase in burning rate of coal leads to increased burning of the other fuels. Considering the importance of swirl flow formed in a chamber, the deflection angle is made variable, but the incline angle is set
Table 3 Fixed and variable geometric parameters of the 1st chamber for optimization. Incline angle of the secondary injectors Fixed design parameters 5° Deflection angle of the primary injectors
Volume (m3)
h2/h1
h3/h1
h4 (m)
0.0067
0.316
0.474
0.01
D (m)
L (m)
Variable design parameters 5–75° (step size: 10°) 0.2–0.27
0.14–0.25
312
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constant because of its slight impact on swirl flow. Deflection angle, hd, changes widely, ranging from 5° to 75° with the step size of 10°. For geometric similarity of the optimal shape, a nondimensionalized parameter of Z is introduced, which is defined as Z = D/L, the ratio of chamber diameter to length. The parameter, Z, affects both residence time of fuel or wastes and tangential fire ball in the chamber. With a high value of Z, the chamber has relatively large diameter and relatively short length, leading to enhanced formation of swirl flow and tangential fire ball, but reduced residence time of fuel. On the other hand, a low value of Z increases residence time and weakens swirl flow. Accordingly, any value of Z provides two competing factors in terms of burning rate. 4. Results and discussions 4.1. Optimization of the chamber When dimensional sizes of the 1st chamber are given, the controlling parameter for high burning rate is swirl number as reported in the previous work [21]. During the optimization, both of Z and hd are changed as listed in Table 3 and burnout ratios of coal are calculated from numerical results at their particular values. Fig. 4 shows dependences of burnout ratio on the deflection angle and the ratio of diameter to length, Z = D/L, with coal particle diameter, d = 300 lm. It is seen that burnout ratio is affected significantly by deflection angle, but a little by the ratio, Z. It increases as deflection angle is reduced until the angle reaches 5°. If it decreases further, burnout ratio decreases abruptly and flame is not stably sustained with hd 0. At larger deflection angles than 5°, burnout ratio decreases with deflection angle. It can be explained as follows. When coal and air are injected with higher deflection angle, their mixing zone becomes closer to the cold chamber wall and chemical reaction takes place near the wall. Then, the rate of heat loss to the wall will be increased. This is verified by heat flux to the wall calculated over wide ranges of Z and hd shown for coal fuel in Fig. 5. As shown in Figs. 4 and 5, deflection angle increases wall heat flux and decreases burnout ratio. That is, higher rate of heat loss to the wall, induced by large deflection angle, causes lower burnout ratio. From Fig. 4, it is seen that the highest burnout ratio of 98.7% is attained at Z = 1.0 and hd = 5°,
Fig. 5. Dependences of wall heat flux in burning coal on the deflection angle of the injectors and the ratio of chamber diameter to length (average coal diameter: 300 lm).
which is the optimal point, and one of local minima is found at Z = 0.8 and hd = 55°, where Br is 88.4%. Fig. 6 shows temperature fields for coal burning at these two points of (a) the local minimum and (b) the optimal. The hightemperature zone is formed off the chamber wall at the optimal point as shown in Fig. 6b. It means reduced heat loss to the wall. And, the peak temperature is higher for the optimal condition than for the local minimum because the reaction zone is formed farther from the wall and heat loss is reduced more for the optimal condition. This is confirmed strongly by Fig. 7, which shows a reaction rate fields at these two points of (a) the local minimum and (b) the optimal. At the optimal point, relative to the local minimum point, the reaction zone penetrates more into the chamber center and becomes broad. But, at the local minimum point, chemical reaction is confined to the narrow zone near the wall, leading to low burning rate of coal. From the numerical results, the optimal point to maximize burning rate has been found, which is characterized by low heat loss to the wall and broad reaction zone. Biomass burning will be done with this optimized chamber in the later section. 4.2. Biomass burning in the optimized chamber of a RBI
Fig. 4. Dependences of burnout ratio of coal on the deflection angle of the injectors and the ratio of chamber diameter to length (average coal diameter: 300 lm).
To evaluate burnout ratios of four kinds of fuels listed in Table 1, they are burned with several particle diameters in the optimized chamber found in the preceding section. Temperature fields are shown in Fig. 8 with particle diameter, d = 500 lm. Coal has the broadest reaction zone and the highest temperature in the reaction zone. And, wood, solid waste, and animal carcass are the next in a descending order. For animal carcass, the reaction zone is confined to a narrow zone near the chamber wall and the lowest temperature in the zone is observed. These phenomena can be confirmed by reactions rates of volatiles and char reactions. The reaction rates of the step Nos. 1–4 are seen in Fig. 9 for each fuel. They are volume-averaged values in the whole domain of the chamber. As predicted, the slowest reaction is the reaction step No. 3. For volatiles reaction, wood has the highest and solid waste, animal carcass, and coal are the next in a descending order with smaller diameters than 1000 lm. For char reactions, coal does the highest and wood, solid waste, and animal carcass are the next irrespective of the reaction step.
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Fig. 6. Temperature fields in burning coal for (a) a condition of low burnout ratio of 88.4% with Z = 0.8 and hd = 55°, and (b) the optimal condition with Z = 1.0 and hd = 5° (average coal diameter: 300 lm).
Fig. 7. Reaction rate fields of the step, C + CO2 ? 2CO, in burning coal for (a) a condition of low burnout ratio of 88.4% with Z = 0.8 and hd = 55°, and (b) the optimal condition with Z = 1.0 and hd = 5° (coal average diameter: 300 lm).
Their burnout ratios are shown in Fig. 10. Wood and animal carcass have the burnout ratio of nearly 100% with fine particles. Animal carcass has the worst properties in terms of combustion and leads to low reaction rates followed by low flame temperature in the chamber, which has been confirmed by the results in Figs. 8 and 9. Nevertheless, it has the highest burnout ratio. Because animal carcass has the smallest amount of combustibles, which occupy only 48.7% of total mass. Accordingly, most of the combustible mass in animal carcass can be burned out although the overall reaction rate is low. On the other hand, wood has the largest combustible mass (99.5%) of them and lower reactivity than coal, but it shows higher burnout ratio close to 100% than that of coal with smaller particle diameters. Wood has volatiles of 77.6% and char of 21.9% and coal has 6.9% and 85.1%, respectively. Burnout ratio of char is relatively low to that of volatiles when much char is burned. Accordingly, coal, which is occupied mostly by char, has a little lower burnout ratio than wood. As shown in Fig. 10a, these phenomena observed in animal carcass are changed when particles with larger diameters than 700 lm are burned. Low reaction rates of anima carcass become effective with large diameters. That is, large diameter particles of animal carcass are not burned completely because of low reaction rates, i.e., insufficient chemical reaction, and its burnout ratio falls down abruptly. This phenomenon is also observed in solid waste. From these results, it is found that the combustible mass, its constituents, and particle diameter affect burnout ratio significantly.
To consider different combustibles content in each fuel, burnout ratio needs to be modified by multiplication of the combustibles (fixed carbon and volatile) content ratio, i.e., (combustibles content of each fuel)/(combustibles content of wood). Wood has the highest content of combustibles. The relative or normalized burnout ratio is denoted by Br_rel. There are found relative burnout ratios of wood, coal, solid wastes, and animal carcass in a descending order as shown in Fig. 10b. Wood can be almost burned out, but animal carcass has a half of burnout ratio of wood, around 50%. Burnout ratio decreases as particle diameter increases. This decrement in Br is magnified in burning solid wastes and animal carcass, which would be caused by their lower reaction rates than those of wood and coal. As expected, animal carcass has the lowest burnout ratio around 50% relative to wood, but it is worth noting that the chamber of a RBI can still work with animal carcass at a high burning rate if it is supplied to the chamber with smaller diameters than 1000 lm. Accordingly, burning performance of the chamber should be compromised with economic load to pulverize biomass into fine particles. The reaction zones in burning wood and coal develop near the injectors and spread to the center and downstream, but in burning animal carcass, their development is retarded and they are formed near the contraction part of the chamber more downstream as shown in Fig. 8. Retarded formation of reaction zones results in insufficient chemical reaction, leading to low temperature in the chamber. The peak temperature in burning animal carcass was lower than that for coal by 200–900 K.
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Fig. 8. Temperature fields in the first chamber for (a) coal, (b) wood, (c) solid waste, and (d) animal carcass with particle diameter, d = 500 lm.
Fig. 9. Reaction rates of the volatiles and char reactions for each fuel for several particle diameters: (a) reaction rates for the reaction step No. 1, (b) reaction rates for the reaction step No. 2, (c) reaction rates for the reaction step No. 3, and (d) reaction rates for the reaction step No. 4.
It has been found that burnout ratio is affected by various parameters such as fuel type, particle diameter, and thermal and flow conditions. It would be a complex function of the parameters
and it is difficult to express explicitly correlation of burnout ratio with them. Accordingly, it will be useful to introduce a singlevalued independent parameter physically, e.g., Damköhler number,
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Fig. 11. Burnout ratios of all the fuels expressed by Damköhler number.
Fig. 10. Burning performances of the RBI: (a) burnout ratios and (b) relative burnout ratios as a function of particle diameter for each fuel.
Da, to express all the burnout-ratio data irrespective of fuel type and particle diameter. By dimensional analysis, it can be defined here in a form,
Da ¼
atres 2
Ad
q ; Q
the nozzle throat and then, it is accelerated locally to supersonic flow with Mach number higher than 1.0. And then, finally, it is decelerated to subsonic flow in the 2nd chamber. It is seen that the present throat plays a significant role in both flow acceleration and tangential-flow generation although it is not tuned or optimized. The maximum Mach No. in burning coal reaches about 1.4 while it does only 1.1 in burning animal carcass. As discussed in the preceding section, the former case has higher pressure and burning rate when the fuel is burned only in the 1st chamber. Fig. 13 shows pressures in the 1st chamber and burnout ratios in burning coal and animal carcass when they are burned with and without the 2nd chamber attached. It is seen than burnout ratio and chamber pressure are increased significantly by adding the 2nd chamber because residence time of fuel is increased and additional reactions occur in the 2nd chamber. It is worth noting that increment in burnout ratio induced by the 2nd chamber is magnified more as particle diameter increases. Accordingly, the 2nd chamber with a nozzle throat can be a viable solution to enhance relatively low burning rate of fuel with large diameters, which is difficult to burn. On the other hand, this result means that the 2nd chamber is required necessarily for high performance when fuel or wastes with larger diameters than 1000 lm.
ð2Þ
where a, tres, Q, A, and q denote thermal diffusivity of fuel, residence time of the mixture in the chamber, higher heating value, frequency factor, and reaction rate, respectively. For the last two parameters, the third reaction step in Table 2 was selected because it is the rate-limiting step of char reactions in the reaction zones. Burnout ratios expressed by Da are shown in Fig. 11, where it is seen that all the data have good similarity. Burnout ratio increases with Da and it levels off at higher Damköhler number than 4 1010. The correlation between burnout ratio and the suggested Damköhler number is useful in predicting burning rate of any kinds of fuel. 4.3. Effects of the 2nd chamber and nozzle throat on burning rate To enhance the burning rate of animal carcass with large particle diameters, the 2nd chamber should be attached to the 1st chamber as shown in Fig. 1, which leads to a complete shape of a rocket combustor. For this purpose, a nozzle throat connecting the 2nd chamber with the 1st chamber is installed between two chambers. As aforementioned, the throat should generate highspeed and strong tangential flow in the passage. Reactive flow fields are calculated with the 2nd chamber with all the fuels and Mach number fields in burning coal and animal carcass are demonstrated near the nozzle throat in Fig. 12. In the contraction part of the 1st chamber, flow is accelerated and sonic speed is attained at
Fig. 12. Mach number fields in burning coal and animal carcass near the nozzle throat between the 1st and the 2nd chambers (particle diameter: 700 lm).
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Damköhler number was provided and it will be useful for the chamber design. For further improvement of burning performance, optimization of the nozzle throat and the 2nd chamber should be done and experimental validations are required before engineering applications of a RBI. They will be interesting subjects for future works. In terms of economy, the incineration still has cost problems in pre-process of pulverization to make suitable feeding. When animal carcass with a disease germ is burned out, in-situ burnout of carcass at its origin is absolutely required for prevention of the surroundings from a secondary infection. Considering these two competing factors, losses or costs for pre- and post-process can be offset by the gain. Of course, economic consideration should be made more rigorously based on quantitative data on required costs and benefit in the future. Fig. 13. Relative burnout ratios and chamber pressures in burning coal and animal carcass with and without the 2nd chamber.
Acknowledgments 5. Conclusion Burning performance of biomass wastes has been evaluated numerically in the chamber of a rocket-engine-based incinerator (RBI), which was suggested as a device of a solid-particle incinerator for the purposes of both high-performance burnout and mobility. Especially, hazardous animal carcass would be disposed of with this chamber. First, optimization of the 1st chamber was made over wide ranges of the ratio of chamber diameter to length and deflection angle of the injector with volume of the chamber fixed. It is more difficult to burn biomass than coal in terms of fuel properties. Accordingly, the chamber of a RBI should be optimized to keep high performance even in burning biomass. The chamber was optimized by exhaustive numerical computations and the optimal point with the maximum burnout ratio was found to be at the chamber diameter-to-length ratio of 1.0 and deflection angle of 5°. Low heat loss to the chamber wall and broad reaction zone of the fuel/air mixture lead to high burnout ratio in the chamber. Accordingly, these two factors should be considered to increase burnout performance. Next, with the optimized chamber, various fuels or wastes such as coal, wood, municipal solid wastes, and animal carcass were burned. Burnout of biomass wastes has been examined numerically by adopting approximate properties of biomass. Numerical results showed that burnout ratio of animal carcass would be as low as a half of that of coal, but it is still 5 times higher than that of conventional incinerators. Relative burnout ratios of all of fuels have the same order of magnitude irrespective of fuel. But, it should be verified experimentally in the future. And then, a specific biomass or animal carcass would be selected for intensive works in terms of burning rate. Lastly, the 2nd chamber is attached to the 1st chamber through the nozzle throat to make additional increase in burning rate, especially when large diameter particles are burned. With this complete chamber, pressure in the 1st chamber is increased further and sonic speed is attained at the nozzle throat, resulting in strong tangential flow in the 2nd chamber. In the end, burnout ratio is increased significantly with the 2nd chamber attached. The present study verified the feasibility of a RBI for high burning rate of biomass wastes for fast burnout and a compact size for mobility or accessibility. Accordingly, the proposed chamber of a RBI will be a viable incinerator for burning out biomass including harmful animal carcass. A single-valued parameter of a similarity variable was found from characteristic time scales eligible for a RBI after numerical data were gathered for various fuels and particle diameters. The correlation between burnout ratio and
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean government (MEST) (Grant No. 2013-023030) and also supported by Advanced Research Center Program (No. 2013073861) through the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP) contracted through Next Generation Space Propulsion Research Center at Seoul National University.
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