Combustion of liquid hydrocarbons sprayed by air jet

. In the present work, the combustion of liquid hydrocarbons sprayed by air jet was studied using diesel fuel as an example. The dependences of nitrogen oxides and carbon monoxide concentrations in combustion products on the fuel and air flow rates were investigated using an atmospheric burner device. It is shown that while diesel fuel sprayed by air jet there is a wider range of operating parameters in comparison with steam spraying due to the flameout. Compared with atomization by air jet at room temperature, steam and heated air spraying have advantages, for these regimes the NO x concentrations are observed to be lower.


Introduction
Due to the tightening of exhaust emission standards, research into emission reduction technologies is relevant.A method of combustion of liquid hydrocarbon fuel dispersed by a heated jet of air or superheated steam is investigated in IT SB RAS [1].
A distinctive feature of this method is that it is based on the interaction of a high-speed gas jet with a fuel drop, which provides high-quality dispersion of various fuels [2] and can be used even for combustion of liquid combustible waste.At the same time, it was shown in works, that the use of superheated steam can significantly reduce the content of harmful emissions in the combustion products, compared to other types of diluents.For example, the use of superheated water steam can reduce the concentration of nitrogen oxides in exhaust gases by up to 50% at high combustion efficiency of hydrocarbon fuels [3].In works, such effect is connected with decrease in the flame temperature, due to which formation of "thermal" NOx is decreased, as well as occurring of steam gasification, which also allows maintaining high combustion efficiency of fuels.This way of combustion is one of the promising ways to utilize low-quality fuels and production wastes, which can be subject to combustion [4].
Among the different methods of emission control, dilution is widely used in modern combustion systems to reduce the peak combustion temperature and hence NOx concentrations in exhaust gases.For instance, exhaust gas recirculation method [5], staged combustion systems [6], dilution with carbon dioxide [7], MILD combustion [8] and others.
There is a growing interest to investigate the effect of different diluents and dilution factors on the environmental and thermal combustion characteristics.
In paper [9] R.G. Papagiannakis studies the effect of air inlet preheating on operation of a pilot ignited dual fuel diesel engine.It is spoken that this way in concert with EGR method allows to improve the engine efficiency (by up to 5%) and reduce CO emissions (by up to 10%).The results by Z. Xu et al. [10] are proposed that the increase in initial temperature of premixed syngas/air flames leads to the increase in the laminar burning velocity and adiabatic flame temperature.In [11] authors considered how intake air temperature (40 ℃ and 60 ℃) influence on combustion characteristics od diesel engine.It is shown that intake air preheat could reduce CO and smoke emissions, but it increases the NOx content in exhaust gases.
Most of these papers are experimental and numerical studies which discuss the effect of diluent type, concentration, temperature etc. on the process occurring in combustion systems.
However, the kinetics of combustion of liquid hydrocarbons during dilution by air and steam at different temperatures still remains poorly studied.Therefore, in present work, a study of burner performance was carried out when superheated water steam or heated air (as spraying agent) was replaced by cold compressed air to analyse the effect of steam presence and atomizer temperature.

Experimental setup 2.1 Burner device
For experimental studies a laboratory sample of atmospheric burner up to 20 kW was used (Fig. 1).
The main elements of the burner device are: the base, the body (total height of 140 mm, external diameter of 60 mm), the outlet nozzle (hole diameter of 25 mm), the spray nozzle mounted in the base centre (diameter of 0.6 mm, the jet opening angle of 17°), the fuel supply tube (the free end of the tube is installed near the orifice of injector).In the lower part of the body of the burner device there are holes for the natural inflow of atmospheric air from the environment.In the burner, the fuel is atomised directly by a high velocity jet of air or E3S Web of Conferences 459, 07006 (2023) https://doi.org/10.1051/e3sconf/202345907006XXXIX Siberian Thermophysical Seminar superheated steam flowing from the nozzle.The resulting fine gas-droplet stream is ignited by an external burner.The advantage of this atomisation method is that there is no direct contact between the fuel and the nozzle, which prevents it from clogging, coking and further malfunctioning of the burner device.

Experimental stand
The study was carried out on an experimental stand for investigation of soot-steam combustion mode of liquid hydrocarbons (Fig. 2).The setup consists of a burner device, water supply system, air supply system instead of steam, electric steam generator for superheating steam and heating air to a given temperature and fuel supply system.Stable mass consumption of fuel is set by a pump and a fuel nozzle.To control the mass consumption the fuel tank is placed to an electronic scale Acom PC-100W-10H.The water consumption is set by a plunger dosing pump and controlled by data from the electronic scales Acom PC-100W-10H, on which the water tank is installed.The air supply system consists of a reducer with a pressure gauge connected to the line of compressed air and a rotameter.The stand is also equipped with a flow calorimeter, control and measuring devices and a gas analyser for determining the composition of combustion products.

Measurements conditions and methods
Diesel was used as a fuel: density of 840 kg/m 3 , viscosity of 4.1 cSt, carbon of 84.6%, hydrogen of 13.6%, sulphur of 0.4%, and nitrogen of 0.6%.Fuel flow rate range Ff = 0.4 ÷ 2.2 kg/h.
In the experimental study, the air for atomisation was supplied at room temperature.The experimental data were compared with results obtained where heated air and superheated steam were used as spraying agent.The temperature of heated air and superheated steam were set constant to 260 C°.The air flow rate varied from 0.3 kg/h to 1.2 kg/h.The range of steam flow rate was Fv = 0.2 ÷ 1.4 kg/h.
The gas composition of diesel combustion products was determined using the Testo 350 gas analyzer.The measured components: O2 (measuring range 0-25 vol.%), CO (0-500 ppm), NO (0-300 ppm) and NO2 (0-500 ppm).The sample for analysis was taken at the outlet of the flow calorimeter, where the gas was at a near-room temperature.The flow calorimeter is a tubein-tube design, the inner channel is for gas flow and the outer channel is for coolant (water) that flows in the opposite direction, it is used to remove heat from the products of combustion.
In gas analysis of combustion products, concentration values of substances were obtained in ppm.For comparison with normative standards, the obtained values of nitrogen oxides and carbon monoxide concentrations were converted from ppm to mg/kWh according to formulas recommended in the standard for liquid fuel burners EN:267 [12]: where ρx is the gas density (2.056 kg/m 3 for NOx, 1.25 kg/m 3 for CO); [O2] is the measured O2 concentration in the gaseous combustion products, vol.%; Vd.g. is the theoretical reference volume, dry (10.46 m 3 /kg for diesel); H is the net calorific value (11.86 kWh/kg for diesel).

Results and discussion
In the experimental studies, the nitrogen oxides and carbon monoxide content in the combustion products were obtained in dependence on regime parameters.The mode maps with CO and NOx concentrations in exhaust gases in dependence on fuel flow rate (horizontal axis) and flow rate of the air at room temperature (vertical axis) are shown in Fig. 3.The boundaries of the region of the burner not operated are plotted by the results of visual observation of the flameout, which depends on the fuel to air ratio.In the regimes parameters in which the combustion is observed to be stable and the CO concentration is less than 500 ppm, the flame is predominantly blue with yellow zones (Fig. 4).In order to compare the results there are presented the CO and NOx concentrations in the combustion products of diesel sprayed by heated air (Fig. 5) and superheated steam (Fig. 6).Figures 3, 5 and 6 show that the nitrogen oxides and carbon monoxide contents in the exhaust gases depend on the operating parameters in a similar shape.However, some differences are observed.For air flow rate from 0.3 kg/h to 1.1 kg/h the range of minimum CO values is wider while heated air is used instead of air at room temperature.Also, when heated air is used as spraying agent, NOx concentrations are slightly lower and there is a wider range of fuel flow rate where NOx values do not exceed 100 mg/kWh.CO When compared to regimes where diesel fuel is sprayed with superheated steam, the NOx concentrations in the combustion products are much lower than with air.Also, the flameout zone is larger due to the extinguishing of the flame at high steam flow rates.

Conclusion
In the present work, the combustion of liquid hydrocarbons sprayed by air jet was studied using diesel fuel as an example.The dependences of nitrogen oxides and carbon monoxide concentrations in combustion products on the fuel and air flow rates were investigated using an atmospheric burner device.
While diesel fuel sprayed by air jet there is a wider range of operating parameters in comparison with steam spraying due to the flameout at high steam flow rates, which is not the case at similar air flow rates.Compared with atomization by air jet at room temperature, steam and heated air spraying have advantages, for these regimes the NOx concentrations are observed to be lower.Further research will be carried out to find out how the spraying of liquid hydrocarbons with an air jet at room temperature affects the flame temperature, heat release and other combustion characteristics.
The results obtained may be used in mathematical modeling for creating burner devices and development technologies that ensure high combustion efficiency with low emissions of harmful substances.

Fig. 1 .
Fig. 1.The scheme of an atmospheric burner device.

Fig. 3 .
Fig. 3. Dependance of CO and NOx concentrations in combustion products on the fuel and air flow rates.The ▲symbols indicate the studied modes.

Fig. 4 .
Fig. 4. The characteristic flame photo of diesel combustion in a cold air jet.The NOx content in exhaust gases is reduced when the air flow rate increases and the fuel flow rate