1 Introduction
Microwave heating has the advantages of high heating rate, ease of start-up and shut-down, and low thermal inertia. Therefore, it is widely used in biomass treatment and conversion such as drying, roasting, pyrolysis, gasification and carbonization [1-5]. However, few studies on industrial scale microwave device for biomass conversion were reported [6-8]. One main reason is the lack of knowledge about the interaction between microwaves and various biomass materials [9]. Therefore, the improvement in microwave device is necessary.
In recent years, microwave-assisted biomass pyrolysis has attracted much attention. The non-premixing, i.e., the feedstock is directly poured on the heated microwave absorbent bed for pyrolysis reactions is one of methods adopted for microwave pyrolysis [10]. Based on the non-premixing method, our group designed a microwave-assisted reactor coupled with atomization feeding and used it in the pyrolysis of methyl ricinoleate [11-14]. We have achieved good results in a small-scale microwave reactor and now focus on the device scale-up. In fact, this reactor can work continuously and efficiently for the pyrolysis reactions, which can be used in the pyrolysis of biomass and oils, therefore it has the potential of industrial application, and the scale-up process is of great significance.
The surface of SiC microwave absorbent bed is the main site of pyrolysis reactions in non-premixing microwave pyrolysis reactor, in consequence, the uniformity and stability of the temperature distribution for the bed surface without feeding is important to the scale-up. Due to the limited penetration depth of microwave and the existence of heat transfer, the temperature distribution on the bed surface has particularly significance to the pyrolysis process in a large-scale microwave device. Some reports on temperature distribution under microwave heating through the simulation method have been found [15-17]. However, the temperature distribution of the entire bed surface was rarely studied experimentally since the accurate temperature measurement in microwave devices is complex especially for large-scale devices. Infrared thermal imager can be used to determine the surface temperature nondestructively based on contactless temperature measurement technology. With the development of infrared thermal imaging technology, the distribution of surface temperature under microwave heating was monitored and studied by infrared thermography [19-21]. The infrared thermal imager has characteristics of spatial visualization of two-dimensional temperature distribution and large temperature measurement area [18]. Therefore, it can be used in the measurement of temperature distribution in large-scale microwave devices.
For the scale-up of microwave pyrolysis reactor, a promising route is to use multiple low-power magnetrons rather than single large-power magnetron, which has the advantages of low equipment cost and ease of maintenance [9]. Thus, it is necessary to investigate the effect of multiple magnetrons position on the temperature distribution of the bed surface in the microwave-assisted pyrolysis large-scale reactor. However, the influence of magnetron position on microwave heating was previously studied mostly by simulation [22].
In this work, a non-premixing microwave pyrolysis large-scale reactor equipped with ten 1 kW magnetrons was developed. The influence of magnetron position on the temperature distribution of the SiC microwave absorbent bed without feeding was studied using the infrared thermography.