A Pilot-Scale Hybrid Microwave-Assisted Tunnel Kiln for Energy-Efficient Porcelain Firing: Design, Performance, and Process Development

This work reports the development process of a 12 m continuous hybrid microwave–gas tunnel kiln for white-glazed porcelain firing. The kiln combines 40 conventional 2.45 GHz magnetrons with 10 natural-gas burners and was developed through the GreenWave and CerWave projects. The experimental program was production-oriented rather than statistically designed. Firing cycles from 3.5 to 8 h were evaluated, with 7 h identified as the principal validated condition for white-glazed porcelain based on industrial visual-acceptance criteria. The maximum individual saving was 12.4%, while the average saving was 5.5%. Shorter cycles showed process-development potential but produced colour deviations and, for the shortest times, visible surface defects. Hybrid firing reduced specific energy consumption relative to equivalent gas-only firing, with a maximum individual saving of 14.7% for 5 h firing cycles, yet a fraction exhibited yellowish colour. Energy-cost sensitivity analysis indicates that economic attractiveness depends strongly on the electricity-to-natural-gas price ratio; using the 7 h average energy data, the break-even ratio is approximately 1.3 on an equivalent-energy basis. The study also identifies engineering challenges involving refractory shrinkage, wagon sealing/interlocking, microwave leakage, thermal management and scale-up. Direct temperature measurement of porcelain bodies and a complete techno-economic or environmental assessment were outside the scope of the study.