CFD simulation of the combustion process of the low-emission vortex boiler

Domestic heat and power engineering needs means and methods for optimizing the existing boiler plants in order to increase their technical, economic and environmental work. The development of modern computer technology, methods of numerical modeling and specialized software greatly facilitates the solution of many emerging problems. CFD simulation allows to obtaine precise results of thermochemical and aerodynamic processes taking place in the furnace of boilers in order to optimize their operation modes and develop directions for their modernization. The paper presents the results of simulation of the combustion process of a low-emission vortex coal boiler of the model E-220/100 using the software package Ansys Fluent. A hexahedral grid with a number of 2 million cells was constructed for the chosen boiler model. A stationary problem with a two-phase flow was solved. The gaseous components are air, combustion products and volatile substances. The solid phase is coal particles at different burnup stages. The Euler-Lagrange approach was taken as a basis. Calculation of the coal particles trajectories was carried out using the Discrete Phase Model which distribution of the size particle of coal dust was accounted for using the Rosin-Rammler equation. Partially Premixed combustion model was used as the combustion model which take into account elemental composition of the fuel and heat analysis. To take turbulence into account, a two-parameter k-ε model with a standard wall function was chosen. Heat transfer by radiation was calculated using the P1-approximation of the method of spherical harmonics. The system of spatial equations was numerically solved by the control volume method using the SIMPLE algorithm of Patankar and Spaulding. Comparison of data obtained during the industrial-operational tests of low-emission vortex boilers with the results of mathematical modeling showed acceptable convergence of the tasks of this level, which confirms the adequacy of the realized mathematical model.


Introduction
Interest of using of solid fuels as the main world source of energy continues to increase steadily in developed countries. This is due to sharp changes in the price policy of oil and gas sales. The share of coal consumption of heat and power engineering increases with the end of the "gas pause" and the emerging trends towards deeper oil processing. The technical, economic and environmental performance of existing boiler have an extremely low level at present [1]. This leads to the need to find ways to optimize the operation of boiler plants. The modern level of development of mathematical modeling and special software allows to correctly understand the physicochemical processes taking place in the combustion chambers of boilers and in some cases replace the field experiment with a computer one as the cheapest one. Numerical modeling allows obtaining accurate results on thermochemical processes that are necessary for understanding the efficiency of existing combustion devices and developing directions for their modernization [2]. One of the ways to improve the technical, economic and environmental performance of boilers is to upgrade them to low-emission vortex combustion technology [3,4]. The basic principles of the construction of a vortex furnace process are set forth in the works of V.V. Pomerantsev and his school [5] and were industrially confirmed during the modernization of boilers in Russia, Poland, the US and the Czech Republic.
There are many works devoted to the numerical modeling of boiler units operating on pulverizedcoal fuel now [6,7]. However, most authors accept the results of numerical simulation without proper confirmation by field experiments. Based on this, the aim of the study was to develop a mathematical model for analyzing the processes taking place in the furnace of the low-emission vortex boiler E-220/100 using the software package for numerical simulation of Ansys Fluent.   Table 1. Thermotechnical characteristics and elemental composition of fuel.

The elemental composition
Thermotechnical characteristics The polydispersity coefficients and the coefficient characterizing the fineness of the granulometric composition are 0.794 and 0.048.

Numerical modelling
The commercial code Ansys Fluent 15.0 was used for the simulation of processes occurring in the combustion chamber of the boiler unit. A hexahedral grid with a number of 2 million cells was constructed for the internal volume of the boiler. A stationary problem with a two-phase flow was solved. The gaseous components are air, combustion products and volatile substances. The solid phase is coal particles at different burnup stages. Heat transfer and combustion in the gas phase are represented by Euler method description. Stationary spatial equations of mass balance, motion momentum, concentrations of gas components and energy for the gas mixture are used. The Lagrangian approach is used to describe the motion and heat capacity of single fuel particles and ash along their trajectories, taking into account the inverse effect of the dispersed phase on the carrier medium. Calculation of the coal particles trajectories was carried out using the Discrete Phase Model which distribution of the size particle of coal dust was accounted for using the Rosin-Rammler equation. Partially Premixed combustion model was used as the combustion model which take into account elemental composition of the fuel and heat analysis. To take turbulence into account, a two-parameter k-ɛ model with a standard wall function was chosen. Heat transfer by radiation was calculated using the P1-approximation of the method of spherical harmonics. The system of spatial equations was numerically solved by the control volume method using the SIMPLE algorithm of Patankar and Spaulding.
The boundary conditions at the inlet were set by means of mass flow and inlet temperature (  The boundary conditions at the outlet were set by the static pressure at the outlet. As a solution algorithm, we chose coupled with a second-order discretization scheme.

Comparison of experimental data and simulation results
Temperature measurements on the operating boiler were carried out using a pyrometer (testo 830-T2) through the inspection hatches.
Comparison of the experimental and calculated data showed that the discrepancies between them on average are ± 10%, which is a satisfactory result.

Conclusions
Comparison of the results of numerical simulation with experimental data obtained during industrialoperational tests of the boiler showed an acceptable convergence of the calculated and experimental data (table 4), which confirms the adequacy of the realized mathematical model. Loss of heat with mechanical underburn (%) 1.21 1.08 The experimentally obtained average heat loss value with mechanical fuel burn-down for a year of operation for three boilers upgraded to a low-emission vortex combustion scheme was 1.21%. The magnitude of the loss amounted to 1.18% for selected for comparative analysis balance experiment. Based on the simulation results, the value of this heat loss is 1.08%. The discrepancy is 8.5%, which is acceptable for tasks of this level.