ANALYSIS OF EMISSIONS DURING THE COMBUSTION OF PELLETS IN BOILER UNITS

The object of research is the technological process of burning pellets in boilers. Using solid fuel boilers on pellets for heat production, they are trying to reduce dependence on hydrocarbon energy resources. Often such boilers are installed for autonomous heating of schools and other communal facilities. One of the most problematic areas is the analysis of the environmental impact when using pellets as a fuel for municipal boiler houses in urban areas. Boilers operating on solid fuels are characterized by a larger range of pollutants, as well as large values of concentrations emitted through the chimney. In the course of the research, instrumental measurement tools were used, which made it possible to obtain the value of the concentrations of pollutants in the emissions from the chimneys of boilers. In particular, the average maximum concentrations of the main pollutants were: nitrogen dioxide – 271.78 mg/m 3 , carbon emissions – 1935.44 mg/m 3 , sulfur dioxin – 13.37 mg/m 3 , suspended solids, undifferentiated in composition (dust, ash) – 93.2 mg/m 3 . Using the data of instrumental measurements according to the method of calculating the concentrations of harmful substances in the atmospheric air, a map of the dispersion of emissions was created. Surface concentration fields were plotted on the scatter map, which makes it possible to compare the obtained values with hygienic normative atmospheric air. This makes it possible to determine the distance where the highest value of pollutants from the emission source is recorded. By combining instrumental measurement methods and calculation methods, the volumes and nomenclature of emissions were determined. This made it possible to determine the amount of pollutants per unit mass of burned pellets in the boiler. Using the conducted studies and theoretical calculations for various types of fuel in the production of heat, this will allow assessing their impact on air pollution and reducing the risk of harm to human health.


Introduction
In the early 70s of the last century, the German inventor Rudolf Gunerman, who moved to the United States in 1949, patented his invention of pellets, which literally means «granules» in English. So, to the traditional fuel sources: firewood, coal, peat, oil refining and gas products, which have long been known to mankind and existed as minerals, a fundamentally new artificially created fuel source was added -pellets made from waste wood, agriculture. The energy content of 1 kg of wood pellets corresponds to approximately 0.5 liters of diesel fuel.
In the early nineties of the last century, first in the northern countries: Canada, Sweden, and Finland, then in the European countries of middle latitudes: the Czech Republic, Slovakia, Poland, Germany, and then the Asian countries began the industrial production of pellets. As a raw material for pellets, wood waste was used, as well as agricultural products: husks, corn roots, straw, willow bushes, grass, etc.
The main advantages of pellets are: -lower ash content compared to wood and coal; -made from natural raw materials; -high density provides more heat production; -convenience and hygiene in use [1,2]. Pellets are used in gas generating and solid fuel boilers. The rapid growth in the consumption of pellets is especially noticeable in the last 10-15 years, when oil and gas prices have risen sharply and environmental requirements for fuel-burning equipment have significantly increased. In Ukraine, the production and use of pellets has been actively developing in the last 5-8 years. Pellet boilers have become actively used as a replacement for expensive natural gas, for example, in communal boiler houses equipped with autonomous heating systems, especially often in: -school institutions; -kindergartens; -residential ACOAB (association of co-owners of an apartment building); -budgetary institutions; -shops; -post offices in the countryside, etc.
In the works of researchers, considerable attention is paid to the technology of pellet production, types of raw materials for the production of pellets, methods of growing, TECHNOLOGY AUDIT AND PRODUCTION RESERVES -№ 4/3(66), 2022 ISSN 2664-9969 transporting, and storing biomass [3][4][5]. At the same time, the direct impact of the operation of boilers using pellets on the environment, the analysis of emissions, their distribution, especially in dense urban areas, buildings of different heights, are not fully carried out and without the use of modern measuring instruments. Therefore, it is important to study the effect of pollutant emissions from the operation of boilers using pellets on air pollution.
Thus, the technological process of burning pellets in boilers was chosen as the object of research.
The aim of research is to determine the volume of emissions and the range of pollutants in the technological process of using a typical pellet boiler for a school institution with a communal boiler capacity of up to 500 kW.

Research methodology
To study the emissions of pollutants from boilers using pellets as fuel, a standard design of an autonomous boiler house was taken for most schools, ACOAB, budgetary institutions, shops, etc. using two solid fuel boilers with a capacity of up to 100 kW each. The above combination of two solid fuel boilers is typical and common throughout the country.
The initial data for research are as follows: For heat supply during the heating period, two boilers are installed in the boiler room of a typical municipal institution: There are no primary measures to reduce pollutant emissions. Exhaust gas cleaning is not provided. Emissions of pollutants are formed as a result of the processes of combustion of the specified fuel in boiler units. Emissions of pollutants depend only on the combustion process, which is due to the design of the boilers. Tables 1, 2 show the technical data on boilers, raw materials, time and mode of operation.
Instrumental measurements of sources of emissions from boiler units were carried out using highly sensitive measuring instruments: gas analyzers, meters of velocity, temperature and volumetric flow rates of gas and dust flows according to DSTU 8725:2017, DSTU 8726:2017, DSTU 8812:2018 and DSTU 8826:2019 [6][7][8][9]. Table 3 shows the technical characteristics of a portable highly sensitive gas analyzer of the OKSI-5M type (Ukraine).
The time for the gas analyzer to enter the operating mode is not more than 60 s Devices of the OKSI series are designed for environmental and thermal measurements of the volume concentration of oxygen O 2 , CO, NO, NO 2 and SO 2 in flue gases and air, flue gas temperature (T). The device has the ability to store up to 250 measurement results. Communication with a computer is carried out via RS 232. Gas analyzers are portable automatic microprocessor devices of continuous operation and are produced for the needs of the national economy. Scope of gas analyzers: maintenance, environmental control, repair and debugging of fuel-burning equipment [10].
The instrumental values of the measurements (averaged over 90 days of boiler operation in winter) according to the approved measurement methods, pollutant values and other parameters (temperature, pressure, flow rate) of the gas and dust flow in emissions from boiler units when using pellets are given in Table 4.  Table 4 for sources of formation No. 1, 2, the concentration of pollutants is given, reduced to normal conditions -6 % oxygen (solid fuel). The calculation of emissions of harmful substances emitted into the atmosphere during the combustion of fossil fuels is carried out according to [11].

Research results and discussion
The gross emission of the j-th pollutant E j entering the atmosphere with the flue gases of the power plant over the time interval P is determined by the formula: where E ji -the gross emission of the j-th pollutant during the combustion of the i-th fuel over a period of time P, t; k ji -the emission index of the j-th pollutant for the i-th fuel, g/GJ; B i -the consumption of the i-th fuel over the time interval P, t; (Q r i ) i -the lower working calorific value of the i-th fuel, MJ/kg.
The mass elemental composition (%) of wood pellets is filled in accordance with the protocol for laboratory testing of pellets ( Table 5).
The specific emission index determined on the basis of measurements of pollutant concentrations (reduced to the standard oxygen content O 2 = 6 %) is given in Table 6.
The definition of the gross emission of a pollutant according to the formula (1) is given in Table 7.
The calculations were performed in accordance with the methodology for boiler No. 1 ALTEP DUO UNI PLUS «KT-2E-N» and boiler No. 2 ALTEP DUO UNI PLUS «KT-2E-N». Summary calculated data on the level of emissions of pollutants from two boiler units using pellets are given in Table 8.    Table 9 shows data on the amount of pollutant emissions per ton of burned pellets. Using the software package «EOL+», which implements the «Method of calculating the concentrations in the atmospheric air of harmful substances contained in the emissions of enterprises. OND-86», which regulates the calculation of scattering and the determination of surface concentrations [12]. The resulting concentration fields make it possible to estimate the levels of impact on atmospheric air pollution. The calculation was carried out at background concentrations, and for all substances a value of 0.4 MPC was taken. Fig. 1 shows the dispersion zones of pollutants, the values are presented in fractions of MPC (maximum allowable concentration) for better comparison with standard MPC values.  . .