Formation mechanisms and methods for calculating pollutant emissions from natural gas combustion depending on the burner emission class Текст научной статьи по специальности «Строительство и архитектура»

https://doi.org/10.21122/1029-7448-2019-62-6-565-582 UDC 532.5 + 621.181.7

Formation Mechanisms and Methods for Calculating Pollutant Emissions from Natural Gas Combustion Depending on the Burner Emission Class

Yu. Р. Yarmolchick1-1

'-Belarusian National Technical University (Minsk, Republic of Belarus)

© Белорусский национальный технический университет, 2019 Belarusian National Technical University, 2019

Abstract. The combustion of hydrocarbon fuels in the chambers of heat generating plants is one of the main sources of pollutant emissions. Environmental standards and rules that limit emissions are becoming more stringent and their implementation requires the introduction of advanced technologies and equipment. The main device in combustion systems are blow burners, the design of which largely determines the level of emission. The article considers factors that intensify the formation of normalized pollutants, provides global chemical reactions, various types of mechanisms, and kinetic schemes. Based on the analysis of modern methods for reducing harmful emissions, the most effective design solutions for mixing devices, nozzles and systems for distributing the flow of fuel and air supplied to combustion are determined. A comparative analysis of the methods and conditions for determining the emission class of the burner device is carried out depending on the selected units of measure, the coefficient of excess air (oxygen concentration in flue gases), air humidity and the initial composition of natural gas using examples of EU and EAC standards. The methodology for calculating the emissions of nitrogen oxides depending on the measurement conditions is given. The conversion factors for the values of pollutant emissions from the accepted units in the EU (mg/(kW-h)) into the units indicated according to the EAC environmental rules (mg/m3) taking into account the respectively normalized coefficient of excess air are obtained. As a result of the calculations, the types of burners were determined by emission classes corresponding to the applicable environmental standards and rules in the Republic of Belarus, depending on the heat output of the boiler plants.

Keywords: environmental standards, excess air coefficient, mixing device, flame head, conversion factors, concentration of nitrogen oxides

For citation: Yarmolchick Yu. Р. (2019) Formation Mechanisms and Methods for Calculating Pollutant Emissions from Natural Gas Combustion Depending on the Burner Emission Class. Energetika. Proc. CIS Higher Educ. Inst. and Power Eng. Assoc. 62 (6), 565-582. https://doi.org/ 10.21122/1029-7448-2019-62-6-565-582

Механизмы образования и методика расчета выбросов загрязняющих веществ при сжигании природного газа в зависимости от эмиссионного класса горелок

Ю. П. Ярмольчик1-1

'^Белорусский национальный технический университет (Минск, Республика Беларусь)

Реферат. Сжигание углеводородного топлива в камерах сгорания теплогенерирующих установок - один из основных источников выбросов загрязняющих веществ. Экологические

Адрес для переписки Address for correspondence

Ярмольчик Юрий Петрович Yarmolchick Yury P. Белорусский национальный технический университет Bekrusian National Technical University

просп. Независимости, 65/2, 65/2 Nezavisimosty Ave.,

220013, г. Минск, Республика Беларусь 220013, Minsk, Republic of Belarus

Тел.: +375 17 293-92-16 Tel.: +375 17 293-92-16

[email protected] [email protected]

нормы и правила, ограничивающие выбросы, становятся все более жесткими, и их соблюдение требует внедрения передовых технологий и оборудования. Основным устройством в системах сжигания являются дутьевые горелки, от конструкции которых во многом зависит уровень эмиссии. В статье рассмотрены факторы, интенсифицирующие образование нормированных загрязняющих веществ, приведены глобальные химические реакции, различные типы механизмов и кинетические схемы. На основе анализа современных методов снижения вредных выбросов определены наиболее эффективные конструкторские решения смесительных устройств, насадок и систем распределения потоков топлива и воздуха, подаваемого на горение. Проведен сравнительный анализ методов и условий определения эмиссионного класса горелочного устройства в зависимости от выбранных единиц измерения, коэффициента избытка воздуха (концентрации кислорода в дымовых газах), влажности воздуха и исходного состава природного газа на примерах стандартов ЕС и ЕАС. Приведена методика расчета выбросов оксидов азота в зависимости от условий измерения. Получены коэффициенты пересчета значений выбросов загрязняющих веществ из принятых единиц в ЕС (мг/(кВт-ч)) в единицы, указанные по экологическим правилам ЕАС (мг/м3) с учетом соответственно нормируемого коэффициента избытка воздуха. В результате расчетов определены типы горелок по эмиссионным классам, соответствующим действующим экологическим нормам и правилам в Республике Беларусь в зависимости от тепловой мощности котельных установок.

Ключевые слова: экологические нормы, коэффициент избытка воздуха, смесительное устройство, пламенная голова, коэффициенты пересчета, концентрация оксидов азота

Для цитирования: Ярмольчик, Ю. П. Механизмы образования и методика расчета выбросов загрязняющих веществ при сжигании природного газа в зависимости от эмиссионного класса горелок / Ю. П. Ярмольчик // Энергетика. Изв. высш. учеб. заведений и энерг. объединений СНГ. 2019. Т. 62, № 6. С. 565-582. https://doi.org/10.21122/1029-7448-2019-62-6-565-582

Introduction

With the coming into force in the Republic of Belarus since-October 1, 2017 of environmental norms and rules of EcoNiP 17.01.06-001-2017 "Environmental protection and nature management. Environmental safety requirements" [1], the issue of limiting harmful emissions not only in existing boiler houses, the emission standards of which are set slightly higher than for newly built ones, became acute, and a reduction in existing indicators can be resolved by installing condensing heat exchangers at the exit from the boilers and improving heat and mass transfer processes in boiler furnaces [2], but also for newly designed boiler plants, emission standards for which are significantly tightened.

In this regard, for manufacturers of boilers, the task is to optimally design newly manufactured plants, and for design organizations - a quality choice of equipment offered on the market. The complexity of the solution of this problem is primarily due to the fact that the manufacturers of burner devices uniquely determine the emission class of their products based on the measured values when burning the corresponding types of fuel in once-through furnaces significantly exceeding the size of the free flame, having an extremely low aerodyne-mic drag and almost complete absence of reverse flows [3]. Boilers with similar combustion chambers, usually referred to as single-pass or span, due to the need to retrofit a particular system of utilizing the heat of the flue gases to increase the overall efficiency of the installation [4], are rarely used and mainly as energy

ones. In the market of heating and industrial boilers, currently mainly boilers with two-way reversible and three-way continuous combustion chambers are offered. An additional difficulty in unambiguously determining the level of emissions in accordance with [1] is the discrepancy in the accepted conditions and units for measuring the concentration of pollutants in the EU [3] - the main producer of burner devices, as well as incomplete compliance of the chemical composition and, as a result, the composition of flue gases with standard types of fuel in the EU and EAC. The proposed simplified conversion methods, for example, described in [5, 6], can lead to design inaccuracies and, as a result, to errors in choosing the optimal designed equipment. For more accurate calculations, it is necessary to apply a combined technique, taking into account the described factors.

Main part

Before considering the features of the effect of the emission class of burners on the formation of harmful emissions, it is necessary to determine the actual composition of the flue gases and which pollutants should be determined as the object of study. According to [1], when burning gaseous fuels, the following issues are standardized: carbon oxide (CO), nitrogen oxides - in terms of nitrogen dioxide (NOx), sulfur dioxide (SO2); and when burning liquid fuel, the same substances plus solid particles.

The factors that intensify the formation of each of the normalized pollutants are considered below.

Carbon monoxide is formed, primarily and almost exclusively, by the combustion of fossil fuels due to the incomplete oxidation of hydrocarbon molecules [7]. To reduce its amount in flue gases, the flow rate of air entering the combustion is increased. It is with a decrease in the formation of this gas that such a concept as the coefficient of excess air is associated. However, the desire of many installers of gas burner devices to completely get rid of CO by increasing the volume of air supplied to the combustion leads to an overestimation of the excess air coefficient and, as a result, to a decrease in the technical efficiency of the heat generating unit and to some extent an increase in the emission of another normalized pollutant - nitrogen oxides. We also note that, according to experimental studies [8], the process of converting hydrocarbon fuels to the final products of combustion of H2O and CO2 is divided into two stages: the first is the oxidation of hydrocarbons to CO - the rate of processes in a high-temperature medium (above 600 °C) is very high; the second is slow: oxidation of CO to CO2. Proceeding from this, the qualitative oxidation of CO depends not only on temperature, but to a large extent on the time spent in the high-temperature zone. This conclusion is extremely important when comparing processes occurring in reversible and continuous combustion chambers. Under conditions of reverse flue gas flows, not only does the average molecular path inside the furnace increase, but their speed also slows down due to cross-border turbulent interpenetrations of multidirectional peripheral flows, viz. a flow of a bur-

ning air-fuel mixture and a return flow of combustion products [9]. As a result, this leads to a significantly longer (almost 2 times) period of time for the oxidation of CO directly in the furnace compared to through-passage combustion chambers. During commissioning, which consists in fixing certain fuel-air ratios in an adjustable power range, presented both by the boiler manufacturer and the burner manufacturer for each type of device, such a mechanism, as a result, can significantly reduce the excess air required for CO oxidation to normalized values. In this case, the coefficient of excess air will weakly depend on changes in the power of the heat generating unit. In the passage furnaces, where the time allocated for CO oxidation is almost proportional to the flow rate, the excess air coefficient will increase with increasing power for a given furnace size. However, on the other hand, modern boilers, unlike earlier ones, have so-called "long furnaces" and CO oxidation to normalized values occurs already when the amount of 02 in the exhaust gases is ~(3—3.5) %, which corresponds to a quite acceptable excess air coefficient up to 1.2. It should be also noted that in the table. E10 [1] for boiler plants with a rated capacity of more than 0.1 MW commissioned on January 1, 2019, the standards for carbon monoxide emissions are not standardized at all - up to a plant capacity till 25 MW.

Nitrogen oxides and, above all, nitrogen monoxide during the combustion of fuels containing a small amount of bound nitrogen, are formed mainly in the high temperature zone of ~1850 °C according to the so-called "Zeldovich mechanism" [10]:

N2 + O ^ NO + N;

2 (1) N2 + O2 ^ NO + O,

which subsequently [11] was added by the reaction

N + OH ^ NO + H. (2)

Together these reactions are usually called the "extended Zeldovich mechanism". However, it was noted that the experimentally measured concentrations of NOx in the exhaust gases exceed those calculated by the Zeldovich mechanism. An explanation of the additional mechanism of the formation of nitrogen oxides is associated with the presence of the CH radical in the initial combustion zone, which reacts with molecular nitrogen [12]:

CH + N2 ^ NCN + H. (3)

These reactions are called by the name of their discoverer "Fenimore mechanism" or, in association with their occurrence almost exclusively in the initial combustion zone, - "fast mechanism". Currently, it is believed that NO is formed from NCN in a number of subsequent reactions involving various radicals [13]. Given the individual reactions defined in [11-13] for the general picture of the process, we present a generalized kinetic diagram of the formation of NO by the fast mechanism (Fig. 1).

Fig. 1. The scheme of reactions for NO formation via the prompt-NO mechanism

Since reactions according to the "fast mechanism" occur in the initial combustion zone, to determine their contribution to the total NOX concentration in the flue gases, the simplest and most realistic approach is to partially exclude the influence of the thermal mechanism with subsequent measurements of the actual NOX values. The organization of poor mixtures burning almost completely eliminates the thermal mechanism, especially when burning of natural gas. Therefore, using this method, it is possible to determine the concentration of nitrogen monoxide formed by the "fast mechanism", and it is widely used in gas turbine plants [14]. For its implementation, a pre-prepared poor fuel-air mixture (with a significant excess of the excess air coefficient) is fed into the combustion chamber. Moreover, due to excess ballast gas, the temperature of the combustion products does not reach the values required for the reactions of the Zeldovich mechanism. However, to reduce the total concentration of nitrogen oxides, it is precisely the part formed by the "fast mechanism" that is least affected by the applied external devices that organize mixing cooling flows and/or flame separation, due to their occurrence exclusively in the initial combustion zone. In addition to the Zeldovich and Fenimore mechanisms described above, a low-temperature mechanism [15] related to the decomposition of nitrogen-containing fuel components can make a significant contribution to the total concentration of NOX in flue gases. But the types of fuel that are commonly used with blast burners - natural gas and light liquid fuels - usually have a very small amount of nitrogen in their composition, and this mechanism seems to be important in the pyrolysis and direct combustion of solid fuels and secondary fuels of chemical plants. In any case, the design of the burner devices cannot significantly reduce the formation of nitrogen oxides obtained as a result of the action of fast and low-temperature mechanisms. For these mechanisms, the most effective way to reduce NOX in exhaust gases seems to be their purification. For this, the most widespread means are: selective catalytic [16] and selective non-catalytic [17] recovery, the implementation of which requires additional sophisticated equipment and significant capital costs. Thus, in order to ensure normalized NOX values in the flue gases of heating and industrial boilers when working with blow burners, it is first necessary to consider the factors affecting the rate of reactions according to the Zeldovich mechanism, since this is the only mechanism that

can be influenced by optimizing internal flows in the combustion chamber, and also due to the fact that the reactions proceeding along this path make the most significant contribution to the formation of nitrogen oxides in the high-temperature zone. In order to do this, we divide this task into two, viz. determining effective factors for the design of burner devices, on the one hand, and the design of combustion chambers, on the other, and then combine them to determine a qualitative assessment of the mutually affecting processes and characteristics. The first part of this task will be considered in the present article.

Blow gas burner devices according to the generation of pollutant emissions are divided into three emission classes [3, 18]: 1st class - CO < 120 mg/(kW-h), NOX < 170 mg/(kW-h); 2nd class - CO < 80 mg/(kW-h), NOX < 120 mg/(kW-h); 3rd class - CO < 60 mg/(kW-h), NOX < 80 mg/(kW-h).

Currently approved methods of reducing emissions from the combustion of hydrocarbon fuel are:

1) minimizing the coefficient of excess air to ensure complete combustion of fuel;

2) the introduction of cooling flows into the combustion zone (recirculation of part of the flue gases; injection of steam, water, etc.);

3) two-stage combustion of fuel (creation of a primary and secondary flame);

4) distribution of fuel to the periphery of the flame (creating a group of peripheral flames with the smallest volume of flame nuclei);

5) reducing the temperature of the heating of the air entering the combustion.

The last method is a regime-technological one and cannot be implemented

solely due to the design of the burner. Briefly considered are the remaining methods.

1. A low coefficient of excess air can be ensured by high-quality mixing of the fuel with the flow of air forced into the combustion. For this, various kinds of mixing devices are used, as a rule, twisting and dividing the gas-air mixture flow into a number of smaller flows. This approach allows one to obtain high-quality combustion with a slight excess of air (^ ~ (1.15-1.17)) and practically

solve the problem of reducing CO emissions to minimum values. Such a burner design may well provide emissions in the 2nd emission class (Fig. 2). However, to reduce the generation of NO molecules to guaranteed values of the 3rd emission class, this method is not enough.

2. The introduction of additional flows of external media (steam, water) is associated with an additional complication of the entire system and, as a consequence, a significant increase in the cost not only of the burner device and the external cooling medium supply system, its significant complexity and the growth not only of capital (equipment

Fig. 2. Mixer head of the 2nd emission class burner. Gas burner MG3 designed by Enertech GmbH Division Giersch (Germany)

cost), but also operating costs. As a result, the main direction of application of this method is the creation of recirculating flows of flue gases into the combustion zone. Such flows can be created by additional external caps on the flame tube of the burner (Fig. 3) or by a device for exiting the air-fuel flow from the flame tube (for example, narrowing the flow with an external ring using the Coande effect [19]) (Fig. 4).

3. The creation of a two-stage flame not only significantly complicates the combustion system, but also reduces the range of regulation of the power of the burner in conditions of stable complete combustion of fuel. And yet the generation of interdependent flames requires additional sensors and an interdependent regulation system. Given that modern requirements for heat generators include a wide range of power modulation, this method can be primarily effective in systems with stable heat consumption, which significantly limits its application. However, it should be noted that the stepped flames is an effective method for simultaneous combustion of several types of fuel in multi-fuel burners [20-22].

Fig. 3. Various types of caps on the flame tube of the burner to create flue gas recirculation. Photo from the test laboratory of Enertech GmbH Division Giersch (Germany)

Fig. 4. Flame head of a burner of the 3rd emission class with a nozzle providing recirculation of flue gases with the formation of the Coande effect. Gas burner MG3-LN designed by Enertech GmbH Division Giersch (Germany)

4. The method of distributing the hottest combustion zones (nuclei) to the periphery of the flame is one of the most effective ones and widely used in modern burner devices. For its implementation, it is enough to divide the fuel flow into several independent jets directed to the periphery of the flame (Fig. 5).

Fig. 5. Flame head and distribution nozzles of the 3rd emission class burner with gas flow fission system. Gas burner MG10-LN designed by Enertech GmbH Division Giersch (Germany)

Thus, the main methods for providing the 3rd emission class are reduced to the use of burner designs according to the 2nd and 3rd of the described methods, or a combination thereof.

It should be noted that burner manufacturers according to the EN DIN standard [3] determine the quantitative values of the concentration of pollutants at the outlet of the combustion chamber (in mg/(kW-h)) for dry gases with air humidity entering the combustion d = 10 g/kg [23]. These units (mg/(kW-h)) are selected because, when used, the amount of pollutants refers to the unit of generated heat energy and, in this case, it does not matter during stoichiometric or non-stoichiometric combustion measurements are made, i. e. the amount of 02 in the flue gas is not required. At first glance, such an approach seems correct. Indeed, with an increase in excess air, the volume of emissions increases, but the average temperature of the exhaust gases decreases. The heat capacity of CO2 [24] and H20 [25] - the main components of the flue gases - varies slightly in the temperature range of stable combustion, and the amount of heat, defined as the product of the average temperature and volume, remains close to unchanged for different values of the coefficient of excess air. In the Republic of Belarus and the CIS countries, emission indicators are determined (in mg/m3). Such units are directly dependent on the volume of emissions. As a result, the problem arises of unambiguous conversion of these units. For this, it is necessary to set an additional value - either by the coefficient of excess air X, or by the volume concentration of oxygen in the flue gases KV . Because high-quality combustion of the

O2

air-fuel mixture by modern blast burners with the lowest heat loss with flue gases is possible at X ~ (1.15-1.20), which corresponds to an oxygen concentration of KVq ~ (2.8-3.5) %, then the burner manufacturers have established a convenient conversion rule for the average whole fixed value KVo = 3 % [5]. But for complete uniqueness, it is also required to have a coefficient of direct conversion of mg/(kW-h) to mg/m3. For this, the inverse coefficient f is usually indicated, the value of which varies according to different sources, for example, f = 1.001 [5] or f = 1.164 [26] for natural gas of class E (H) and f = 1.018 for natural gas of class L (LL) [5] at KV = 3 % or f = 0.857 at KV = 0 % [27]. The conditions