Low-temperature swirl method of burning combustible waste

. Burning combustible industrial waste increases the efficiency of using raw materials while at the same time solving the issues of protecting the environment from pollution by eliminating waste dumps. The paper deals with the low-temperature swirl method of waste incineration of microbiological production  hydrolytic lignin. A combustion device has been developed that allows using hydrolytic lignin as a fuel for the production of electrical energy and heat without illumination of the torch and with high efficiency and reduced emissions of gaseous pollutants into the atmosphere. On the basis of the developed model of the boiler TP-35U, a quantitative estimate of the level of nitrogen oxides was made when introducing the low-temperature swirl method. The results of the calculations show the advantages of low-temperature swirl combustion of hydrolytic gaseous pollutants formed are made using the developed mathematical model of the combustion process and a computer program.


Introduction
An increase in the consumption of wood raw materials entails the formation of a significant amount of bark, chips, sawdust unsuitable for processing, and necessitates their disposal or storage. In full, this applies to hydrolytic lignin -waste processing plant materials at hydrolysis and biochemical plants [1]. Despite the wide potential applications, hydrolyzed lignin in enterprises is used in obviously insufficient quantities, and is burdensome waste.
The most appropriate and economically feasible at the present time is the direct combustion of hydrolytic lignin in boiler furnaces for the production of electrical energy and heat [2,3]. The greatest problems with burning hydrolytic lignin occur in the combustion chamber of the boiler. Due to the low calorific value (1600 kcal/kg) and high humidity (Table 1), additional torch lighting with gas or fuel oil is required to burn lignin. The presence of illumination leads to the melting of the mineral part and intensive slagging of the heating surfaces, which makes impossible reliable operation of the boiler plant.
The low-temperature swirl combustion method eliminates the listed drawbacks, and also provides reduced emissions of toxic gaseous pollutants into the atmosphere.

Principles and organization of low-temperature swirl furnace process
The concept of the low-temperature swirl combustion (LTS) method was proposed in the late 1960s by Professor V.V. Pomerantsev. Orange as an alternative to direct-flow torch ( Fig. 1, a). The LTS method is based on the principle of organizing the multiple circulation of relatively large fuel particles with their periodic return to the zones with the initial oxygen concentration [4][5][6][7] (Fig. 1, b).  combustion technology [8,9]. In the case of replacement of boiler equipment that has developed its life, the use of the LTS method allows to install new boilers in existing building cells (Fig. 2), which drastically reduces the cost of building a new building [10,11].

Fig. 2.
The layout of the boiler with low-temperature swirl furnace in the boiler room.
In addition, the low-temperature swirl method is a technological method of protecting the environment from harmful emissions of nitrogen oxides and sulfur oxides [12][13][14][15][16], implemented at the stage of fuel combustion by the design of the combustion device and operating characteristics of its operation.

Object and research methods
The object of the study is the boiler TP-35U (produced by the Belgorod boiler plant) of one of the enterprises of the microbiological industry and its reconstruction with the LTSmethod of combustion. The subject of research is the combustion of hydrolytic lignin in the LTS-furnace, the generation and conversion of gaseous flue gas pollutants.
The reconstruction of the cat TP-35U with the transfer to low-temperature swirl combustion of hydrolytic lignin involves changing the geometry of the combustion chamber (while maintaining the external dimensions), and its installation in gas-tight design. In the lower part of the front and rear firebox, slopes of a "cold" funnel is forming. In the middle part of the firebox of the front wall panel form a front aerodynamic protrusion, on the lower generator of which two direct-flow burners are installed. Secondary air is supplied to the lower part of the burners. To create a swirl zone in the mouth of the furnace funnel mounted bottom air nozzle. On the rear screen are the nozzles of the tertiary blast. To reduce costs, the fuel supply system is performed in the millfree version. Equipment in front of the boiler includes fuel bunkers and feeders for metering and supplying fuel to the burners of the boiler. A general view of a boiler installation with an LTS-boiler TP-35U for the millfree combustion of hydrolytic lignin is shown in Fig. 3 Calculations of the burning of hydrolytic lignin in the LTS-furnace of the boiler TP-35U and determining the amount of gaseous pollutants formed are made using the developed mathematical model of the combustion process and a computer program. The vectors of gas-air flow rates (Fig. 4) were located using the Ansys Fluent software package at the nodal points of the elementary cells into which the combustion chamber was divided.
Calculations of particle trajectories in a known velocity field were carried out by numerically solving the Meshchersky equation, written in the projection on the axis of a Cartesian coordinate system, taking into account the influence of two main forces: gravity and aerodynamic drag: The combustion model of hydrolytic lignin in the LTS furnace uses the theory of "reduced film" proposed by V.V. Pomerantsev and S.M. Shestakov et al. [20]. Combustion of large coke particles of highly moist fuel is described by a set of chemical reactions (thermal effect in kJ/mol) typical for the "double burning" boundary layer scheme (the case of "wet" gasification): The combustion process is described by a system of nonlinear differential equations of diffusion and kinetics: taking into account the oxidation and reduction reactions occurring on the surface of the particle and the reactions occurring within its boundary layer (Fig. 5).
The loss of mass and particle size is calculated by the expressions:   where Mc = 12 kg/kmol is the molar mass of carbon; m = /6 3 eqс is the mass of the spherical particle, kg; f =  2 eq -is the area of the outer surface, m 2 . When calculating the combustion of coke carbon, the "shrinking particle" model was used provided that its density remains unchanged with the introduction of a coefficient K r taking into account the relative content of coke in the working mass of the fuel: Fields of concentrations of the main reactive gas components (O2, CO2, H2O) were taken as typical for LTS furnaces, Fig. 6. . 6. TP-35U, %: a  oxygen; b  carbon dioxide; c  water vapor.
The particle size distribution of the initial fuel was described by the Rosin-Rammler-Bennet dependance [20,21]: where b and n are experimental coefficients. The distribution of NO concentrations along the section of the furnace in a known gas flow velocity field was determined by numerical solution (the "anti-flow" scheme) of the differential mass transfer equation, in the presence of a source term (NO generation zone) for each of the unit cells [22]: where СNO -mass concentration of nitrogen oxides; w -gas flow rate; JNO -the intensity of generation of nitrogen oxides (power source NO [21]): DNO -the average effective diffusion coefficient of NO in a mixture of flue gases, determined by the Wilk dependence, and the mutual diffusion coefficients of substances under real conditions according to Winkelman: where D012  coefficient of mutual diffusion of substances at P0 = 101.3 kPa, T0 = 273 K.
The amount of nitrogen oxides decomposed on the surface of burning carbon particles is calculated from the balance of reaction No. 5 of system (2).

Calculation results, their analysis and discussion
Calculations of the combustion process of hydrolytic lignin in the low-temperature swirl furnace of the TP-35U boiler were carried out for fuel (Table 1)   The trajectories of the reacting particles of hydrolytic lignin are shown in Figure 8. The particles of lignin, dried during the multiple circulation process, are evenly distributed across the width of the furnace and sufficiently fill the volume of the lower swirl zone, where the maximum concentrations of nitrogen oxides formed during the release and combustion of volatile substances are located. After the combustion of volatile, almost the entire mass of fuel continues to circulate in the lower swirl zone, where reaction No. 5 of the system (2) decomposes nitrogen oxides on the surface of burning particles [23]. As a result, molecular nitrogen and carbon monoxide are formed, which, in turn, burns down in   In the middle (above the burners) and top (at the exit window level) parts of the furnace, the generation of nitrogen oxides practically does not occur, and their concentration to the output is at a level of 150 mg/nm 3 (near the screens) to 300 mg/nm 3 (along the axis of the furnace) torch), Fig. 9.
According to the requirements of the Ministry of Energy, emissions of nitrogen oxides when burning hydrolyzed lignin should not exceed 470 mg/nm 3 . Thus, the implementation of the LTS-method when burning lignin makes it possible to reduce emissions of NOx by a factor of 1.5 to 2 relative to the established standards. This allows considering LTS-burning as a low-cost technological method of protecting the environment from harmful emissions during operation of power boilers.

Final provisions and conclusions
The results of the work confirm the high technical, economic and environmental performance of a low-temperature swirl boiler designed to burn hydrolyzed lignin. As applied to the boiler with a steam generating capacity of D = 35 t/h (9.72 kg/s), it is shown that its conversion to lignin combustion by the LTS method allows, with minimal capital expenditures for reconstruction, to increase the efficiency factor of operation of the boiler to 89 %, reduce emissions of nitrogen oxides by 1.5...2 times and reach a level lower than that established by regulatory documents. In addition, the development of lignin dumps contributes to the release of territories and the improvement of the state of the environment as a whole.