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Mild Combustion of Industrial Hydrogen-Containing Byproducts Marco Derudi,* Alessandro Villani, and Renato Rota Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”/CIIRCO, Politecnico di Milano, Via Mancinelli 7, 20131 Milano, Italy
In this work, a laboratory-scale burner has been used to investigate the sustainability of mild combustion with the coke oven gas (COG), an industrial byproduct mainly constituted by methane and hydrogen (CH4/ H2 40/60% by volume). Operating conditions for stable mild combustion have been identified and operating parameters maps have been deduced for this hydrogen-containing fuel, evidencing low NOx and CO emissions. With respect to the well-established, clean mild combustion of methane, mild combustion of coke oven gas required larger jet velocity but allowed for using lower average furnace temperatures. Also demonstrated was the ability of hydrogen to lead to completion the hydrocarbon oxidation in very dilute conditions such as those created by the mild technology. These findings, together with the potentiality of mild conditions for soot depression and destruction, open the possibility of using hydrogen as a doping agent for burning, with this technology, industrial wastes, and low-calorific dirty fuels. 1. Introduction In the coke-making process, coal is fed into a series of ovens; at high temperature, a pyrolysis process is carried out to obtain coke, but several gaseous byproducts, such as the coke oven gas (COG), are also formed. For economical reasons, particular importance is devoted to the recovery and the utilization of these byproducts. Because the COG is mainly constituted by methane and hydrogen, it is a highly flammable and a high-calorific waste gas that could be used as an energetic source, together with natural gas or as an alternative nonconventional fuel, for the coke oven heating, as well as for heating furnaces in rolling mills or in other high-temperature operations. Because the utilization of energetic sources is strictly connected to their environmental impact, many studies have been focused on technologies, such as the high-temperature air combustion (HiTAC),1 that could improve the utilization of fossil fuels, in terms of substantial energy savings and environmental issues. The HiTAC technology, also called mild combustion2-4 or flameless oxidation (FO),5,6 offers the potential for high thermal efficiencies and a significant reduction of airborne emissions, in particular CO and NOx. The essential principles of mild combustion are both a high preheating of the combustion air and a massive recycle of burnt gases within the combustion chamber. It could be done by means of separated or coflowing high-velocity jets for the injection of fuel and combustion air into the burner.7,8 These jets create a low-pressure zone near the burner nozzle, thus inducing a massive recycle of exhausts into the reactants prior to the combustion reactions take place. The exhaust gases entrainment raises the temperature of the fresh reactants and increases the inert content of the fuel and air mixture; as a result, the oxygen concentration in the combustion air quickly decreases, leading to an increase of the characteristic reaction time that becomes comparable with the characteristic mixing time, which, on the contrary, is lowered by the high turbulence due to the reactants jets. Such a condition allows for obtaining the disappearance of the flame front, which means the elimination of the temperature peak of the conventional diffusion flame, leading to a fuel oxidation spread in the whole volume of the combustion * Corresponding author. Tel.: + 39 0223993109. Fax: + 39 0223993180. E-mail:
[email protected].
chamber and, consequently, to a flattening of the temperature profiles inside the furnace. Mild combustion occurs in a diffuse reaction zone, so the maximum temperature rise due to the oxidation results everywhere minimal; thus, when operating at low-temperature, it is possible to obtain a significant reduction of NOx emissions produced via the thermal route. Moreover, a lower maximum temperature than that found in a traditional burner offers the opportunity of preheating reactants flows, increasing the global yield of the process. Despite the fact that it is well-known that it is possible to realize stable mild combustion conditions when a standard fuel, such as methane or natural gas, is used,4-10 less information is available concerning the sustainability of mild combustion technology for nonconventional fuels,11 such as industrial byproducts and gasification fuels. The possibility to use byproducts containing a large amount of hydrogen, as the COG, allows for producing energy and heat with a very low pollutants emission. Consequently, this work has been mainly focused on the possibility to use the experimental results obtained in a laboratory-scale burner to identify the main design parameters for burning nonconventional gaseous fuels with the mild combustion technology. In particular, the case of the industrial coke oven “waste” gas (CH4/H2 40/60% by volume) has been considered. 2. Experimental Section 2.1. Experimental Burner. The laboratory-scale equipment has been extensively described in detail elsewhere.4,9 For this reason, only the main features of the burner, sketched in Figure 1, are here summarized. It is basically a quartz reactor made by two sections: the combustion chamber and the preheating section for the combustion air. The top of the combustion chamber is closed with a quartz plate provided by holes that host the thermocouples and the gas-sampling line. To prevent heat losses, this plate was covered with several layers of quartz wool. Because of the small scale of the apparatus (the internal diameter of the cylindrical chamber is 5 cm, while the chamber height is 35 cm), a configuration with a single high-velocity nozzle, with an inner diameter of 3 mm, located on the bottom of the combustion chamber has been chosen. With reference to Figure 1, the fuel (point A) enters
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Figure 2. Detail of the single high-velocity nozzle used to feed reactants inside the combustion chamber.
Figure 1. Laboratory-scale mild burner layout.
the nozzle perpendicularly, through a capillary pipe; for this reason, inside the nozzle body, fuel and combustion air are partly premixed before they enter the furnace, as shown in Figure 2, but no fuel oxidation occurs because of the small diameter of the pipe and the short residence time. The single nozzle is used to create a high-velocity jet that entrains a large amount of exhaust from the combustion chamber, inducing a fast and strong dilution of combustion air; in addition to the internal hydrodynamic recycle, it is also possible to simulate an external burnt-gas recycle by vitiating the combustion air with an inert gas, such as nitrogen (point B). Moreover, the bottom part of the preheating section is provided by a secondary inlet for the combustion air (point C), which is used during the system startup4 to preheat the furnace with a conventional stabilized diffusive flame before switching to mild conditions. The mild combustion regime could not be sustained in a “cold” combustor, so it is necessary to reach a high furnace temperature in order to overcome this fundamental limitation; as a consequence, the burner is initially operated with a stable flame combustion to heat up the system. Both sections of the burner are located in refractory insulated electrical ovens. In particular, the lower one is used to simulate the performances of a regenerative industrial heat exchanger, preheating the combustion air up to 1300 °C, while the upper one is used to minimize heat losses from the combustion chamber. For this reason, the upper oven temperature has been usually kept at a temperature lower than that of the furnace, namely, 200 °C below the average value continuously detected by three thermocouples hosted into the furnace at top, middle, and bottom (close to the nozzle tip) heights, respectively. Concerning pollutants emissions, the gases are sampled, dried, and analyzed by means of an on-line gas analyzer (Horiba PG-250), which measures the concentrations of NOx, O2, CO, and CO2.
2.2. Fundamental Parameters of Mild Combustion. The experimental setup, which has been previously described, allowed for studying independently those parameters that are relevant for mild conditions achievement: combustion chamber temperature, dilution ratio, and jet velocity. In particular, as evidenced by several researchers,5,12 the dilution ratio inside the combustion chamber, KV, defined as the ratio between the recycled exhausts and the incoming air and fuel flow rates, is a key parameter for the definition of mild burner working conditions. While in real-size burners only the internal hydrodynamic recycle exists and the KV factor is completely defined by the system geometry and the jet flow rate constituted by the reactants, in the laboratory-scale burner of the present work, it is possible to change the KV value by feeding a secondary air stream around the jet as well as some inert gas together with the combustion air. When a secondary air stream around the jet is fed to the burner, the jet entrains preheated combustion air instead of exhaust gases and the overall KV value is lower than the internal dilution ratio, R, determined by the jet velocity. However, when some inert gas (reproducing an external recirculation of exhaust gases) is fed together with the combustion air, the overall KV value is larger than the internal dilution ratio, R. As a consequence of several calculations performed with a general-purpose code for computational fluid dynamics,4 it was found that the maximum value of the recycle factor imposed by the jet presence in the chamber, R, is equal to about 5 for all the conditions investigated, in terms of both jet composition and velocity, evidencing a negligible modification of the main recirculation position. It is possible to correlate the maximum values of parameters KV and R for the laboratory-scale burner by using a new definition of the dilution ratio that accounts for both internal and external recycles as well as the presence of a secondary air inlet (which flow rate is lower than the entrained one),
KV )
(I/A)(1 + R) R - S/A + 1 + S/A (1 + F/A)(1 + S/A)
(1)
where the flow rates of incoming primary (A) and secondary (S) air, inert gas (I), and fuel (F) have been considered. As expected, when neither secondary air nor inert gas are fed to the laboratory-scale burner, the previous relation leads to KV ) R. Using the same approach, the oxygen concentration within the combustion region can be expressed as a function of the
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Figure 3. T vs KV diagram sketching boundaries of peculiar regions identified in this work and the experimental test procedure.
flow rates considered in the generalized definition of KV (eq 1), as follows:
O2 (%) )
21(1 + S/A) (1 + F/A + I/A)(1 + R)
(2)
2.3. Mild Combustion Regime. Since the burner could not be operated directly in mild conditions, it is necessary to identify the boundaries of the mild combustion regime; for this reason, two of the relevant parameters previously discussed, the minimum average furnace temperature and the minimum dilution ratio (KV) required to sustain mild combustion, should be defined. In order to highlight the influence of those parameters on the combustion characteristic of the investigated fuel, T vs KV diagrams, complemented by the jet velocity data, can be drawn. The more usual representations reported in literature4,5 identify different regimes of stable and unstable flame combustion and a flameless oxidation region. In this work, because of several peculiar features evidenced by the use of hydrogen-containing fuels, a revised version of the T vs KV diagrams has been proposed, as shown for the sake of example in Figure 3; in particular, this diagram summarizes five main regimes: a clean mild combustion region, where mild conditions can be easily sustained without any significant pollutant emission; a mixed region, where low-emission mild conditions can be achieved by suitably selecting some key operating parameters, such as the combustion air temperature; a nocombustion or extinction zone, where both the combustion efficiency and its sustainability are compromised, as evidenced by the rise of CO emissions; a thermal region, labeled as “Thermal NOx”, where mild conditions can be sustained but NOx emissions become significant; and finally, a traditional flame combustion region. The laboratory-scale burner allowed for obtaining such diagrams for a given fuel through a simple experimental procedure able to identify the aforementioned boundaries. The test procedure, which is roughly sketched on the operating map of Figure 3, requires the preliminary achievement of a high temperature in the combustion chamber through a traditional flame combustion, as previously mentioned, and then mild conditions can be realized by increasing the dilution ratio. The main steps of this procedure are the following: (a) Ignition and preheating: A secondary air stream is provided around the nozzle to stabilize a diffusion flame over its tip, allowing for the combustion chamber preheating, while a small quantity of air is fed directly through the burner nozzle (step 1). (b) Transition: Once a hot environment is obtained, it is possible to switch from conventional combustion to mild combustion. The secondary air flow rate is progressively
decreased, increasing at the same time the primary combustion air flow rate through the nozzle, which produces a highmomentum jet that increases the entrainment of exhaust gases; behind a given KV threshold, namely, the vertical boundary of the mild combustion region, the system enters the mild regime (step 2). (c) Cooling or diluting: at a constant KV, the preheating furnace temperature is reduced to cool the combustion chamber (step 3), enabling the identification of the lower horizontal threshold of the mild combustion region. In addition, a similar result can be obtained by replacing a reactants fraction with an inert stream, simulating a further dilution of the reaction environment coupled with a reduction of the furnace thermal input (step 4). On the contrary, the upper horizontal boundary can be defined by reversing step 3 and increasing the preheating furnace temperature. Finally, a criterion to qualitatively and quantitatively discriminate among flame, mild, and no-combustion regimes should be defined. The mild regime is usually identified by flame disappearing, reduction of temperature gradients inside the furnace, and a decrease of noise; this means a strong abatement of NOx and CO emissions, coupled with a still-high fuel conversion. 3. Results and Discussion As previously mentioned, this work focuses on an industrial gaseous byproduct, the so-called COG. It is mainly constituted by methane and hydrogen, so in this work, a binary mixture, with a composition CH4/H2 40/60% by volume, has been used to reproduce a typical composition of the coke oven gas. The possibility of achieving mild conditions for this fuel has been assessed, and the influence of the hydrogen presence within the fuel, with reference to the case of methane, for which the behavior has been discussed in a previous paper,4 has been evaluated. In order to define the operating maps of the investigated fuels, a lot of experimental conditions have been tested: depending on the run, for both methane and COG, the input thermal power was varied in the range 0.2-0.3 kW, adopting the same procedure when KV was changed to simulate an external recirculation of exhausts, while the equivalence ratios of the fuels were in the range 0.94-0.97. It is worth noting that the equivalence ratio, in the case of COG, must be estimated considering the mixture composition of the fuel; for this reason, the mole fractions of methane, hydrogen, and air were considered to compute an effective fuel equivalence ratio, as defined by El-Sherif13 for methane-air flames doped with a hydrogen addition. The effective fuel equivalence ratio is based on the assumption that the oxygen is available first to oxidize the hydrogen, and it represents the remainder that reacts with the hydrocarbon fuel. For a pure fuel, the effective fuel equivalence ratio is equal to the traditional one. From preliminary tests, no significant differences have been observed when hydrogen is added to methane with respect to thermal homogenization of the furnace and CO emissions during the transition toward mild conditions, as shown for the sake of example in Figure 4, where different thermal profiles are reported for various dilution ratios and for different positions inside the furnace. It is interesting to notice that, close to the transition from flame to mild combustion, the temperature differences in the combustion chamber are definitely reduced, similarly to what happens for methane mild combustion; in particular, it was possible to reduce temperature gradients of about 350 °C, detected during the stable flame combustion (∆Tflame), until 70-80 °C, operating the burner in mild condi-
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Figure 4. Thermal profiles within the reaction chamber during a test carried out using COG as fuel; open and gray symbols represent the temperatures detected at the top and at the bottom of the chamber, respectively, while black symbols represent the average furnace temperature that is computed from measures collected at three different positions within the combustion chamber.
Figure 5. NOx trends found in typical experiments with CH4 (full symbols) and COG (empty symbols), respectively.
tions (∆Tmild). Moreover, the distributed oxidation of the fuel, inducing a thermal homogenization of the chamber, causes a uniform heat generation in the whole reaction zone. Concerning the formation of CO, in mild conditions, no emissions of CO were practically detected for the COG. On the other hand, as was found also for methane, in the transition between flame and mild conditions, an increase of CO emissions has been evidenced even in the presence of a hydrogencontaining fuel. This behavior is not surprising, since the larger reactivity of hydrogen with respect to methane did not influence the CO production, which is due to the worsening of the combustion characteristics in the transition region; in this case, when the secondary air flow rate is reduced and the jet momentum is increased by a higher flow rate of the primary air, a progressive liftoff of the flame has been found, showing also some intermittencies and instabilities of the high-temperature flame, which lead to the formation of small amounts of CO. On the contrary, it is possible to highlight a difference between NOx emissions profiles detected during experimental tests carried out with COG and methane. As evidenced by two representative profiles (concerning tests performed with an air preheating of 1100 °C) reported in Figure 5, the combustion of CH4 in flame mode (low KV values) leads to a high NOx production, and the transition toward mild conditions is clearly characterized by a sudden decrease of the NOx concentration, which allows for identifying the vertical boundary of the mild region, while COG shows a less sharp reduction of NOx emissions as a function of the dilution ratio. This could be partly ascribed to the high flame stability induced by the hydrogen presence that leads to a more difficult thermal homogenization in the combustion chamber, which is achieved slowly and at a larger KV value with respect to the case of methane. For this
Figure 6. Comparison between the operating mild combustion maps of methane and COG.
reason, a different jet-flow velocity is required to obtain mild combustion conditions when a high reactive fuel as hydrogen is present. COG requires a jet velocity larger than about 75 m/s to realize a marked decrease of NOx emissions and the possibility to attain mild conditions, while for methane, a jet velocity equal to about 50 m/s has been used. The smooth transition evidenced by COG and, in particular, the absence of a sudden reduction of NOx does not permit a clear identification of the mild combustion boundaries. Since the relevant issue of a mild combustion process is a very low emission of pollutants, a “clean mild” condition has been defined, imposing to the criterion previously mentioned for the identification of mild conditions an additional requirement concerning a maximum pollutants concentration within the exhausts gases: NOx below 30 ppm and CO below 50 ppm. As a consequence, to point out when mild combustion conditions are achieved while operating the burner with a hydrogencontaining fuel, such as COG, the main pollutants emissions (namely, NOx and CO) have been carefully monitored (all pollutants concentrations here reported are normalized on a dry basis at 3% O2 and depurated by the dilution effects due to any inert stream injection). With the reference to the operating map of Figure 3, the boundaries of a clean mild combustion zone have been defined considering the region where only clean mild conditions were fulfilled. In addition to a smooth transition toward mild conditions, it was also observed that hydrogen-containing fuels, particularly when a high hydrogen content is present, did not present a clear flame disappearing; this leads occasionally to the achievement of clean mild conditions outside the region identified as clean mild combustion. For this reason, the transition region, where clean mild conditions are sometimes observed depending on the experimental conditions, has been identified as the “mixed zone”. All the results of the experiments carried out have been used to define the operating map for the COG (CH4/H2 40/60% by vol.). Both mixed-zone and clean mild region boundaries have been identified, as reported in Figure 6, where these issues are compared with those found for methane. In order to highlight the differences between the investigated fuels, experimental points have been omitted from the diagram; moreover, data concerning methane mild combustion, which have been extensively discussed in a previous paper,4 have been reanalyzed considering the new constraints imposed by the “clean mild” criterion and the new representation proposed in this work. This comparison confirms that the hydrogen presence requires a strong reactants dilution in the combustion chamber to sustain the clean mild combustion condition, which is achieved slowly and at a larger KV value with respect to the case of methane. Hydrogen induces a significant change in the lower KV
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threshold, that is dominated by the fuel reactivity. Because a furnace provided by a single nozzle has been used for the experiments, fuel and air are partly premixed before they enter the combustion chamber; this geometry represents the worst case in terms of the possibility to realize mild combustion of highly reactive fuels, such as COG, because the combustion could start before the complete dilution of the reactants with the entrained exhausts is met. The characteristic reaction time of the fuel is lowered in the presence of hydrogen, and this effect should compensate reducing the mixing time to obtain stable mild conditions; this requires a high-momentum jet of reactants, with a velocity > 75 m/s. Hydrogen also has several positive effects on mild combustion sustainability; its low self-ignition temperature and its wide flammability range,14 coupled with the tendency to produce a large amount of H radicals, allow for extending the clean mild boundaries toward very dilute environments and also allow for operation with lower average furnace temperatures. Hydrogen induces a clear reduction of the lower temperature threshold thanks to the fundamental role it plays in the chain-branching reactions sequence:15
H + O2 f O + OH O + H2 f H + OH H2 + OH f H2O + H
(3)
and/or control radicals reactions responsible for the formation of NOx, but sometimes NOx emissions higher than those expected according to the detected temperatures have been recorded. This means that this pathway can be at most partially responsible for these emissions in small regions where temperature is much higher than the average value. For this reason, another pathway, originally proposed by Bozzelli and Dean,17 could contribute to the nitrogen oxides formation. This route forms NO through the oxidation of NNH radicals. Several studies, involving different combustion systems,18-20 have confirmed and suggested that significant amounts of NOx can be produced during combustion from N2 via NNH, following this reactions sequence:
N2 + H ) NNH
O + H2O f OH + OH
NNH + O f NO + NH
Below the lower temperature threshold, the combustion efficiency decreases, leading to an excessive formation of CO, with a subsequent reduction of the CO2 yield. The chainbranching sequence is also active in determining the practical absence of an upper KV boundary for the COG; this effect is really significant, because for methane and other hydrocarbons it is possible to identify an upper KV value that determines the progressive worsening of the fuel conversion, resulting in high CO emissions and a possible slip of unreacted fuel from the furnace outlet. Experimental investigations highlighted that, for COG, which means for high hydrogen amounts, less oxygen is required to sustain a complete fuel oxidation, thus allowing a clean mild combustion also for high-diluted environments; it is worth noticing that, in such conditions, clean mild combustion still produced very low CO emissions and a high fuel conversion has been always obtained, as ensured by the on-line monitoring of the exhaust composition. This behavior suggests that hydrogen could probably be used to dope low-calorific fuels with the aim to burn them in a more efficient way, enhancing the full oxidation of hydrocarbons in very dilute streams and preventing the formation of other pollutants, namely, unburnt hydrocarbons or carbonaceous particles. Only the upper temperature threshold, which is close to 1100 °C, did not seem influenced by the hydrogen presence within the fuel mixture; the NOx production in this region could be ascribed mainly to the thermal-NOx route (Zeldovich mechanism),15,16
O + N2 f NO + N N + O2 f NO + O
Figure 7. Effect of the preheating temperature on NOx emissions and dilution ratios for the COG.
(4)
whose overall rate strongly depends on the temperature and only to a lesser extent on the oxygen concentration. According to the Zeldovich pathway, the furnace temperature could activate
NH + O2 f NO + OH
(5)
NH + O f NO + H The NNH route of NOx formation is influenced by O and H radicals concentrations; this explains why this pathway could be relevant even at low or moderate temperatures, also in dilute environments. Moreover, H radicals play a key role in the initiation step of the NNH intermediate mechanism, so it is possible to assume that, in systems with an appreciable hydrogen content, this pathway contributes markedly to NOx formation; therefore, the smooth reduction observed in the NOx trends and the relatively high NOx concentrations found in many tests carried out with COG, within the clean mild combustion regime, could be ascribed to this mechanism. Finally, in the higher-temperature region, it was observed that the transition toward the clean mild condition, particularly when COG is used, leads to “hot spots” inside the furnace, with values of the temperature higher than the average furnace temperature. In particular, this behavior is evidenced during tests carried out with a significant preheating of the combustion air, namely, above 900 °C; this slightly increases NOx emissions, resulting in a bended boundary in the upper part of the mixed zone. The air preheating, as evidenced in Figure 7, strongly affects the NOx formation, influencing the minimum dilution ratio, KV, that is required to obtain clean mild combustion conditions. The preheating temperature influences the average furnace temperature that is reached within the combustion chamber; subsequently, this temperature directly affects both the reactivity of the system in terms of combustion stability and the pollutants formation. Experimental results evidenced that a lower reactants preheating practically reduces the rate of the fuel oxidation, increasing the characteristic combustion time. With reference to the COG, this means that hydrogen, when a lower preheating
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of the combustion air is used, did not start to produce H radicals that react with oxygen immediately, close to the nozzle tip, but the radical pool formation and the fuel oxidation are driven far from the feeding point, toward a position closer to the recirculation zone. As a consequence, in order to reach mild combustion conditions, it is no longer necessary to artificially increase KV by means of an external dilution of the reactants to detach the flame stabilized on the nozzle tip. In such a way, highly reactive fuels, such as COG or other hydrogen-containing fuels, can be forced “to behave like methane”, producing very low NOx emissions and obtaining a stable mild condition at low KV values. This similarity could be evidenced by the comparison of the NOx trend obtained for COG with a preheating temperature of 650 °C, reported in Figure 7, with the one shown for methane in Figure 5, which has been obtained by preheating the combustion air at 1100 °C. 4. Conclusions In this study, using a laboratory-scale burner provided with a single high-velocity nozzle, it has been shown that the combustion of hydrogen-containing industrial byproducts, such as the coke oven gas (COG), can be efficiently carried out using the mild combustion technology, thus confirming the high NOx and CO reduction potential of the mild combustion. With respect to the well-established clean mild combustion of methane, COG required larger jet velocity but allowed for sustaining mild conditions at lower average furnace temperatures. Since a single-nozzle configuration of the burner has been used, this represents the worst possibility in terms of the feasibility of mild combustion, because when operating the burner with highly reactive fuels such as H2-containing mixtures, combustion could start before complete dilution of the reactants with the entrained exhausts is achieved. However, partly premixed burners that are able to meet the requirements suggested by this study can realize a stable mild combustion of H2-hybrid fuels, while for burners with a nonpremixed configuration, that means separated high-velocity nozzles for air and fuel; these requirements are probably conservative. Apart from the burner geometry, experimental findings evidenced that, with COG, a lower amount of oxygen is required to sustain a complete fuel oxidation, thus allowing a clean mild combustion also in highly diluted environments; this ability suggests that hydrogen could be used as a doping agent for low-calorific fuels with the aim to burn them in a mild combustor in a more efficient way, enhancing the full oxidation of hydrocarbons and preventing the formation of other products of environmental concern, namely, unburnt hydrocarbons and soot.
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ReceiVed for reView December 31, 2006 ReVised manuscript receiVed May 15, 2007 Accepted May 17, 2007 IE061701T
