## 1 Introduction
At the same time, the biochar yield decreased with increasing temperature, especially during microwave heating since the decomposition of coffee husks into volatile substances was promoted by high temperature and energy supply, which slowed down the carbonization process [16]. With microwave heating, the liquid yield first increased and then decreased, and with electrical heating, it continuously decreased with increasing temperature. During microwave pyrolysis, the irradiation power significantly affects the yield of products. The higher the power, the higher the heating rate of the raw material. In [17], waste tires were pyrolyzed in a microwave reactor at various powers, and the reaction temperature in the experiment was set to 500 \({}^{\circ}\)C with a retention time of 20 min. The high microwave power allowed the raw material surface temperature to reach about 500 \({}^{\circ}\)C in a short time, resulting in a longer reaction time than at lower microwave power. The biogas yield increased with increasing microwave power. The yield of bio-oil also increased and reached a maximum at 560 W; it was about 46 wt% and then decreased. Moreover, biochar yield tended to decrease with increasing microwave power. In [18], rice straw, rice husks, corn straw, sugar cane cake, sugar cane peel, coffee ground waste, and bamboo leaves were used for microwave pyrolysis. Before pyrolysis, the components were air-dried, milled, and sieved. The moisture content of the biomass was 5-10 wt%. Microwave pyrolysis was carried out in a single-mode microwave oven at a frequency of 2.45 GHz. The biomass was added to a quartz crucible and then placed inside a quartz tube located opposite the magnetron. After purging with an inert gas, the power source was turned on at the specified microwave power level for 30 min. Then, the power was turned off. For all agricultural residues, the maximum temperature increased with increasing microwave power. The maximum temperatures (320, 420, and 530 \({}^{\circ}\)C) were reached at certain microwave powers (300, 400 and 500 W), regardless of which feedstock was pyrolyzed. During microwave pyrolysis of typical agricultural waste with a power of 300 W, approximately half of all pyrolysis products were in liquid form, while 26-31 wt% and 18-26 wt% were solid and non-condensable gas, respectively. With an increase in the microwave power from 300 W to 500 W, the gas yield increased by 12-15 wt%, but the yield of both solid and liquid decreased by 7-9 and 3-8 wt%, respectively [18]. [PERSON] et al. [18] found that a higher level of microwave power can contribute to the formation of non-condensable gas, which is due to the self-gasification of the char [19]. [PERSON] et al. [20] studied the effect of microwave power radiation on the characteristics of the pyrolysis of waste tires. The setup consisted of a microwave oven with a maximum output power of 900 W, a quartz reactor, and a product collection unit. The pyrolysis time was 30 min. As the power increased, the yield of solid products decreased from 48.85% at 9 W/g to 43% at 15 W/g. With a further increase in specific power to 24 W/g, the solids yield continued to decrease. Final temperatures were 415 \({}^{\circ}\)C, 498 \({}^{\circ}\)C and 574 \({}^{\circ}\)C at 9 W/g, 15 W/g and 24 W/g, respectively. Liquid yield increased from 37% at 9 W/g to 45% at 15 W/g. The increase in liquid yield indicated that the temperature increased at higher power densities, resulting in the formation of more oily substances. With a further increase in specific power, the liquid yield decreased since some of the oil molecules continued to decompose into lighter gaseous molecules through secondary reactions [20,21].