Analytical study on the Heat-Transfer Characteristics of a Fluidized Bed Reactor Heated by Multi-Stage Resistance

Highlights

• A fluidized bed occupied with multi-stage resistance heaters was utilized innovatively.

• Heat transfer relationships of the dense phase and the freeboard were analyzed and established.

• The model inversion proved the order of input parameters affecting the internal heat transfer.

• Distribution characteristics along axial direction were obtained by heat transfer simulation.

Abstract

A fluidized bed heated by multi-stage resistance heaters was utilized innovatively to realize independent heating of different inside zones. A steady-state physical and mathematical model was developed to study the effect of dense phase, freeboard, fluidized carrier gas on temperature distribution in the fluidized bed. The results showed that compared with the experimental values, the differences between the calculated temperature, heat transfer coefficient and fluid velocity at the outlet of fluidized bed by the model were within 15%, 17%, and 14% respectively, indicating that the model had high reliability. The model inversion illustrated that the heat transfer coefficient at the outlet of the fluidized bed increased gradually in the order of the fluidized carrier gas temperature, dense phase temperature, fluidized carrier gas velocity and freeboard temperature. The heat transfer simulation took the height of the dense phase and the freeboard as the input parameters of the model by hundreds of iteration, the calculated results confirmed again that the existence of particles in the dense phase increased its heat transfer efficiency with the gas and the wall, ultimately making the dense phase as the main heat transfer zone.

Introduction

Fluidized beds are widely used in many industrial fields such as combustion, gasification, metallurgy and catalytic cracking for their large gas-solid contact surface area, high reaction rate, safe and reliable properties [1], [2], [3]. So these special conditions put forward high requirements on the choice of fluidized bed heat transfer mode, heat and mass transfer efficiency. The key to the design of the fluidized bed is the fast heating of internal packing and reactants to achieve fast thermochemical reactions [4]. Currently, there are mainly two heating methods to provide a stable reaction environment and a continuous heat source for fluidized beds, which are gas heating and electric heating.

Gas heating mainly used combustible gases, such as liquefied petroleum gas or combustible by-products of thermochemical reactions, to heat the wall of the fluidized bed. Generally, the gas heating method was mainly used to introduce the combustible gas into the combustion chamber to combust, which was separated from the reaction zone by the wall. So the combustion chamber and the reaction zone of the fluidized bed together constituted an annular reactor. Since both the annular zone and the cylindrical zone of ​​the annular reactor can be used as the combustion chamber or the reaction zone, there were two specific implementation methods for the gas heating method. One method was to use the annular zone as the reaction zone and the cylindrical zone as the combustion chamber, respectively [3]. Yang et al. used a built-in conical spouted fluidized bed as a combustion chamber and the reaction zone as an annular zone for the biomass steam-gasification process [5]. However, the biggest problem in the actual operation of this method was that it was difficult to control the internal combustion chamber, resulting in a large temperature gradient in the annular reaction zone, and serious uneven distribution of raw materials was likely to occur in the annular reaction zone, which seriously affected the hydromechanical properties and increased the inconvenience of packing in the reaction zone [6,7]. Another method was to use the annular zone as the combustion chamber and the inner zone as the reaction zone. Meanwhile, a large number of studies found that the best form of heat transfer in an annular reactor was to transfer heat from the outside to the inside. Bai et al. designed a double tube reactor for efficient heat transfer, whose outer tube (fixed bed) and inner tube (fluidized bed) were filled with Pall rings and quartz sand, respectively, and studied its heat transfer characteristics [2]. Zhao et al. developed a rolling circulating fluidized bed with an annular chamber to study the Dynamic control method of particle distribution uniformity [8]. In addition, Skopec et al. used fuel ash as the solid fuel for chemical cycle combustion in an oxyfuel bubbling fluidized bed combustor, and used experimental data to obtain the optimal reaction temperature range [9]. Bubnovich et al. designed a new type of annular reactor with the novelty geometry, cylindrical annular space for the combustion of lean methane/air mixtures and performed numerical simulations of combustion characteristics [10].

Electric heating was the method that the periphery of the fluidized bed was equipped with electric heaters, which directly heated the wall of the fluidized bed or the fluidized carrier gas by resistance or inductance elements, and was easy to control the temperature of the wall and gas. It had been widely used in laboratory-scale fluidized bed experimental research. However, it consumed more energy than gas heating and the heat dissipation of the whole process was serious, especially the combustible by-products of thermochemical reactions cannot be fully utilized, so it was especially not suitable for pilot-scale fluidized bed [1,3].

From the above literatures, many researchers tried to explore reasonable heating methods for fluidized bed. In general, although gas heating can rationally use the combustible by-products of thermochemical reactions to provide thermal energy and reduce energy consumption, it was difficult to provide stable thermal energy output and control the reaction temperature precisely [11,12]. Electric heating can easily control the temperature range, but its energy consumption was relatively large, and the combustible by-products of thermochemical reactions can hardly be used. Therefore, we think that the heating method of the fluidized bed should satisfy the reaction temperature, reduce the input and consumption of energy, and use the combustible by-products of thermochemical reactions as much as possible.

Comparatively speaking, the gas heating method is difficult to control the temperature of each heating position for its unstable thermal energy supply [13,14]. Although the electric heating method consumes much energy to some extent, the heating temperature range can be controlled by changing the power of the electric heater, and the fluidized bed can be equipped with multiple electric heaters for different zones to reduce the consumption of electric energy as much as possible. Therefore, we put forward a new electric heating method by multi-stage resistance for the different zones of fluidized bed to realize the above requirements.

The purpose of this paper was to study the heat transfer characteristics of the fluidized bed reactor heated by multi-stage resistance. First, a novel heating mode of electric heating by multi-stage resistance applied in the traditional fluidized bed reactor was designed and the principle and implementation were described. Then, a first attempt was made to establish a physical model of heat transfer inside the fluidized bed under this heating mode, whose rationality was verified by experimental data. Finally, based on this model combined with classical heat transfer and energy equations, the internal heat transfer characteristics of the fluidized bed was analyzed, and the mathematical relationships between temperature, heat transfer coefficient and fluid velocity distribution were obtained, so as to understand the heat transfer mechanism of each parameter more comprehensively. The results provided a new solution for the precise control of the reaction temperature with less energy consumption, which was of great significance for the actual operation and the expansion of the fluidized bed production scale.