Abstract A pyrolysis combustion technology (PCT) was developed for high-efficiency and environment-friendly chain grate boilers (CGBs). The realization of the PCT in a CGB requires that extremely large and widely sized coal particles should be first pyrolyzed in a semi-fluidized state before being transported into the combustion chamber of the boiler. This article was devoted first to investigating the fluidization of 0–40 mm coal particles in order to demonstrate the technical feasibility of the PCT. In succession, through mixing 0–10 mm and 10–20 mm coal particles in different proportions, multiple pseudo binary mixtures were prepared and then fluidized to clarify the effect of particle size distribution. With raw steam coal used as the feedstock, the superficial gas velocity of about 2.0 m/s may be suitable for stable operation of the fluidized-bed pyrolyzer in the CGB with the PCT. In the fluidization of widely sized coal particles, approximately half of the coal mass is segregated into the bottom section of the bed, though about 15% of 10–20 mm large particles are broken into 0–10 mm small particles because of particle attrition. The experimental results illustrate that an advanced CGB with the PCT has a high adaptability for various coals with different size distributions.