Combustion characteristics of stoichiometric hydrogen and oxygen mixture in water

0360-3199/92 $5.00 + 0.00 Pergamon Press Ltd. International Associationfor Hydrogen Energy.

Int. J. Hydrogen Energy, Vol. 17, No. 11, pp. 887-894, 1992. Printed in Great Britain.

COMBUSTION CHARACTERISTICS OF STOICHIOMETRIC HYDROGEN AND OXYGEN MIXTURE IN WATER T. KUMAKURA, K. HIRAOKA, M. IKAME, S. KAN and T. MORISHITA Power and Energy Engineering Division, Ship Research Institute, Ministry of Transport, Mitaka, Tokyo 181, Japan (Received for publication 24 June 1992)

Abstract--The authors have proposed a novel heat engine which is used as the power source for an undersea vessel. The engine is a kind of steam turbine, in which steam and water are directly heated by stoichiometric combustion of a hydrogen and oxygen mixture. This paper presents the characteristics of combustion in water at room temperature using a stoichiometric premix. The premix burned water using a single-hole-nozzle burner with a cylindrical hood, but some flame missing and flashback were observed. Combustion is not stable when the ratio of the length to inner diameter is less than 2.5. When the flame is observed, the combustion efficiency of the premix is almost 100% of the theoretical one in the range of equivalence ratio between 0.9 and 1.1. With an ion-current detector it is proved that the flame periodically enters the nozzle for an instant while the flame is observed.

INTRODUCTION Storage batteries are used as the power source of most undersea vessels but their performance is not sufficiently high in output power density. Therefore, various heat engines have been investigated because they possibly have better performance. In deeper water, however, the power output of these engines is reduced because of the increase in pumping work required on the exhaust gases. The authors have proposed a novel heat engine which is less influenced by depth and is named "the internal combustion steam turbine" (Fig. 1). Hydrogen with a stoichiometric ratio of oxygen is burnt in water and steam, then the combustion product, H20, does work along with the superheated steam. After the work is done, both the steam and the combustion product are condensed in a condenser and only the combustion product is pumped out of the cycle. The work in pumping it out is less influenced by depth, compared with gas phase combustion products. The thermal efficiency of a steam turbine is improved by an increase in the turbine inlet temperature, but the temperature of conventional turbines is restricted by the strength of the materials of superheaters. Much higher turbine inlet temperatures than those of of conventional turbines will be obtained by stoichiometric hydrogen/ oxygen combustion in steam, because no boiler tubes are required. As for the hydrogen/oxygen steam generator, a project of a demonstration plant for spinning reserve using an integrated hydrogen/oxygen steam generator is reported by Sternfeld and Heinrich [1]. In the realization of our concept of the new engine, it is essential that the combustion is performed as with little residue of noncondensible gases as possible. Therefore it is necessary to know well the characteristics of the combustion in water and steam.

As a preliminary step to this study, experiments were carried out on the combustion characteristics of stoichiometric H2-O2 premix in water at room temperature, where several single-hole-nozzle burners were used with a hood attached at their tip. In a previous paper, it was shown that the stoichiometric mixture could burn in water, but that flame missing or flashback was often observed [2]. In this document, stable combustion regions at equivalence ratios close to unity are presented in terms of the dimensions of the hoods and the nozzles. Furthermore, the combustion efficiency and the situations of flashback and flame missing are described. EXPERIMENTAL APPARATUS A schematic diagram of the experimental apparatus for the combustion of hydrogen and oxygen mixture in water is shown in Fig. 2. The cylindrical tank, which is stainless steel, 600 mm in diameter and 900 mm high, can hold 0.2 m s of water. The burner can move up and down so that it is easily submerged to a specified depth after ignition above the surface. The temperature of the tank water is controlled by a cooling pipe submerged in the tank. The burner can be observed through two windows on the side wall of the tank. Hydrogen and oxygen gases are supplied from gas storage cylinders to the burner through a float-type flow meter and a backfire preventer respectively. Flow rates are manually controlled with the needle valve of the flow meter. In the case of gas sampling, a conical gas collecting hood is put above the burner and a sampling probe is fixed at the exit of the collecting hood. The sampled gas is pumped to gas analyzers after the steam in the sampled gas

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Fig. 1. Internal combustion steam turbine.

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is removed with an electric gas cooler. Since much of the sampled gas is steam and is condensible, nitrogen at a specified flow rate is introduced into the collecting hood as a reference gas. The concentrations of the unburned hydrogen and the unused oxygen are measured with a gas chromatograph. Purity of the supplied gases is 99.99% for hydrogen and 99.93% for oxygen and nitrogen. A schematic diagram of the burner is shown in Fig. 3. A steel-cutting burner, which was used in our last report, was

machined to have a single-hole-nozzle attachment. Furthermore, a cylindrical hood was attached at the tip of the nozzle. The dimensions of the nozzle and the hood, made of copper, are listed in Table 1. The pressure at the nozzle inlet space, P~, was measured with semiconductor sensors and recorded with a digital storage oscilloscope. The temperature of the nozzle body at the point 1 mm radially apart from the nozzle hole was measured with a sheathed thermocouple, 0.5 mm diameter. In the nozzle hole close

COMBUSTION OF H2 AND 02 MIXTURE IN WATER

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to the exit, a pair of ion-current probes were installed to face each other across the nozzle hole. The probe is a Ktype sheathed thermocouple of 1 mm diameter, whose sheath is bared at the tip to expose the element wires, 0.15 mm diameter, to the flame. A voltage of 9.6 V was applied between the probes and the voltage fluctuations were recorded with an oscillograph. EXPERIMENTAL CONDITIONS The experiments on the combustion in water were carried out under the following conditions: (1) The jet direction of the H2-O2 gas mixture was downward. (2) The position of the nozzle was about 200 mm under the surface of the water. (3) The temperature range of the water was 25 ° - 4 5 ° C . (4) H2 flow rates (QH~) are 11, 21 and 31 1 min -I (normal). (5) The equivalence ratio is almost unity for the stability of stoichiometric combustion and is varied between 0.9 and 1.1 to obtain the relation of combustion efficiency to equivalence ratio.

METHOD OF GAS ANALYSIS The sampled gas with steam becomes a dry mixture of H2, O, and N2 after dehumidification. Nitrogen gas is used as a carrier. A gas chromatograph (the detector of which is of a thermal conductivity type) having a 2.5 m long column filled with activated charcoal powder, is used. The peak of H_, concentration appears first and that of 02 comes second in the gas chromatogram. When the H2 concentration is high, the measured concentration of 02 is affected because of the overlap of the two peaks. In order to improve the accuracy of the 02 measurement, it is necessary to separate the two peaks as much as possible. Therefore, the temperature of the column is selected at 30°C and the sampling volume of the gas is 1 ml, although the analyzing period takes a little longer, 3.5 min. The flow rate of the carrier N2 is set at 60 ml min-~ and the electric current of the thermal conductivity cell is 75 mA. Under these conditions, the error in the 02 concentration is within 0.08% for the concentrations 1% 02 and 5% H2 and is within 0.07% for the concentrations 0.5% O2 and 5% H2, while the H2 concentration has no influence on the 02 concentration.

Table 1. Dimensions of the nozzle and hood* d0 (mm) dt (mm) L (mm) *Labels refer to Fig. 3.

1.39, 1.77, 2.02, 2.35 6, 8, 10 10, 15, 20, 25, 30, 40, 60

DEFINITION OF COMBUSTION EFFICIENCY The combustion efficiency of the hydrogen was defined by the following equation: ~H: =

1 -

(Q~:/QH,_),

(1)

890

T. KUMAKURA et al.

where Q,: stands for the volumetric flow rate of supplied hydrogen and Qh_, for the volumetric flow rate of unburned hydrogen, expressing both rates in the normal state. The unburned hydrogen Qh2 and the unused oxygen QS, can be expressed by the following equations: Q[~, = c.,(Qh2 + Q6, + QN,), Q~, = co~(Q{~,_ + Q6: + QN,_),

(2) (3)

where (7,,_ and Co: stand for the volumetric concentration of the unreacted hydrogen and oxygen, respectively and QN, for the volumetric flow rate of the supplied nitrogen. From equations (2) and (3), Qh~ is expressed as follows: Qh, = (C.2QN~)(1 -

CH~ -- Co:).

(4)

By substituting equation (4) into equation (1), the combustion efficiency of the hydrogen is obtained from the equation: r/H, = 1 -- [CH:/(1 -- CH2 -- Co2)](QN:/QH2).

(5)

In the range of equivalance ratio ~ > 1, the combustion efficiency decreases with the increase of equivalence ratio, due to shortage of oxygen and the theoretical upper limit of the combustion efficiency results in the following form: r/u: = 1/¢,

(6)

where the equivalance ratio ¢b is defined as: ~k = QH,./2Qo,.

(7)

However, ~ was calculated from equation (8) with gas concentrations in order to reduce the effect of the measurement error from the flow meter on the value of ~: l/q5 = 1 -

The term "flashback" refers to the condition when the flame gets through the nozzle hole and enters the nozzle inlet chamber, thus cutting off the oxygen and hydrogen supply. Although there are several feasible reasons for the flame missing, such as blow-off due to too high a flow rate, flame missing caused by water intrusion into the hood inside or caused by the quenching effect of the nozzle when the flame comes too close to the nozzle hole, all of these are classified by the term "flame missing" here. The experiment to find the regions of stable combustion was carried out on the dimensional conditions of the nozzle and hood. The diameter and hood lengths are listed in Table 1. The experiment on a given condition was repeated three times and the results were categorized in the following four groups: (a) G 1: the combustion C 1 occurs three times out of three trials (stable). (b) G2: the combustion C 1 occurs two times out of three trials (unstable). (c) G3: the combustion C 1 occurs once out of three trials (unstable). (d) G4: the combustion C1 does not occur in these trials (unable). Figure 4 shows the results for the case of the H2 flow rate QH2 = 21 1 min -t (normal) using symbols ( o ) for GI, ( e ) for G2, ( a ) for G3, and ( x ) for G4. The regions encircled by the line including the open circles indicate stable combustion. The unstable combustion regions include the two types of combustion C 1 and C2. The unable regions are for the types of combustion C2 and C3. From Fig. 4, it is seen that the stability is very bad when the hood length to diameter ratio is less than 2.5. This fact is similar to the cases with other flow rates. Even with a longer hood, there are some regions in which stable combustion cannot be obtained at all. The stability regions are not explained generally so far.

[(CH2 -- 2Co2)/(1 - C,, - Co,)](QN~_/Q,:). (8) RESULTS

Stable combustion region

Stable combustion regions with H2-O2 stoichiometric mixture are found with respect to the dimensions of the hood and the nozzle and to the hydrogen flow rates. Here the term "stability" is defined as the condition which exists when combustion is sustained for more than 5 min after the burner is placed 200 mm under the water surface, after being ignited in the atmosphere. The results of the stability experiments are classified in the following three types of combustion: (a) CI: continuous combustion is maintained for 5 rain after the burner is placed at the specified location in the water. (b) C2: flashback or flame missing occurs within 5 min after the burner is fixed in the specified location. (c) C3: flashback or flame missing occurs before the burner reaches the specified location.

Behavior o f bubbles and pressure fluctuation

Figure 5 depicts the shapes and behavior of the bubbles generated by the flame and also the pressure fluctuations in the nozzle. The flame of the H2-O2 mixture is blue and it is recognized that it stretches out of the hood. The bubbles are generated around the flame and the behavior of them varies according to the dimensions of the nozzle and the hood or the H2 flow rate. The behavior is roughly classified into three types, A, B and C, which are explained later and also indicated in Fig. 4. The pressure fluctuations, which are not found in combustion above the water surface, have particular peaks which are associated with the three types: (a) Type A: the bubble rhythmically repeats growth and shrinkage at the tip of the hood. Particular peaks of the pressure fluctuation appear periodically and the period is almost the same as that of the bubble's growth and shrinkage. (b) Type B: the behavior of the bubble mostly looks like Type A, but suddenly groups of small bubbles come out of the main bubble like small explosions, together with a small

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explosive sound. The amplitude of the peaks becomes larger and the periodicity of the peaks is lost. (c) Type C: a lot of tiny bubbles rush out of the hood and, sometimes, a small circulatory flow of tiny bubbles appears along the hood rim. The flame length is shorter than that for Type A or B, even at the same flow rate. Violent bubble eruptions can be seen continuously, but sometimes intermittently, while very loud explosive sounds are heard. The period of the peaks becomes shorter than those for Type A and B, while the amplitudes of the peaks are almost constant.

Ion current and pressure fluctuation

In order to detect whether the flame enters the nozzle hole or not, an ion-current detector was used. An instant drop in voltage of the detector is observed frequently, with some periodicity, while the flame is held. The fact proves that, while the flame is observed, the flame often gets into the nozzle and reaches or passes the location of the ioncurrent probe, and comes back. After this flashback occurs, the voltage continuously has large fluctuations. Figure 6 shows an example of the relation between the pressure fluctuation and the ion current. When the ion current is detected

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for an instant, the pressure rises, but there are some pressure changes without the voltage drop. This, however, implies that an instantaneous pressure rise is caused when the flame comes into the nozzle for a moment. Situations of flashback and flame missing When flashback occurs, a very loud sound is heard and the temperature of the nozzle wall quickly rises. On the other hand, the intensity of sound is small when flame missing occurs. Both flashback and flame missing are observed with all types of bubble conditions A, B and C. With Type A, there is no particular indication in the bubble behavior before the flashback or the flame missing occurs. On Type B, both of them often take place with intensive eruptions of the groups of small bubbles and on Type C they occur together with some change of the violent bubble eruptions. At the moment of the flashback or the flame missing, the changes in the pressure are different from the oridinary pressure fluctuations. Figure 7 shows representative examples. At the point of flame missing, the peak becomes a bit larger than the preceding ones and then gets flatter. At the time of flashback, a large and abrupt change appears in the pressure, then the pressure rises gradually. Combustion efficiencies The combustion efficiencies were measured by varying the equivalence ratio from 1.1 to 0.9 under the three constant flow rates of hydrogen, and several dimensions of the hood and the nozzle were selected from the experiments on the stable combustion region. The relation between the combustion efficiencies and the equivalence ratio is shown in Fig. 8 for several dimensional parameters of the hood and the nozzle at a constant flow rate of hydrogen 21 1 min ~ (normal). The effect of the flow rate on the relation is shown in Fig. 9. The dashed line in the figures indicates

the theoretical combustion efficiency for the perfect reaction with 02. It can be seen from both of the figures that the combustion efficiencies are almost 100% when the flame is held. However, it is observed that in the region of equivalence ratio between 0.98 and 1.02, flame missing takes place sometimes, especially at higher flow rates. CONCLUSIONS The combustion of a hydrogen and oxygen mixture in water was studied using a premixing burner with a single nozzle and the results were concluded as follows: (1) A hood attached to the burner is effective in stabilizing the combustion. Stable combustion regions are found in terms of the dimensions of the hood and the nozzle at an equivalence ratio of about 1.02. Length/inner diameter ratios of the hood of less than 2.5 cannot keep the combustion stable. (2) The combustion efficiency of the premixed mixture is almost 100% of the theoretical one in the range of equivalence ratio between 0.9 and 1.1 when the flame is observed. (3) It was proved with the ion-current detector that the flame periodically enters the nozzle for an instant while the flame is observed. It was thus concluded that the periodic flame entrance causes an instantaneous pressure rise in the nozzle. REFERENCES 1. H. J. Sternfeld and P. Heinrich, A demonstration plant for the hydrogen/oxygen spinning reserve, Int. J. Hydrogen Energy 14, 703-716 (1989). 2. T. Kumakura et al., Combustion characteristics of stoichiometric hydrogen and oxygen mixture in water, Proc. 7th WHEC (October, 1988).