Thermal performance in the experiments and three-dimensional numerical simulations for a high temperature air combustion boiler where fuel can be efficiently combusted by high temperature preheated air (800C-1000C) are examined. For a MEET I boiler with 1MW of the thermal input already installed in Tokyo Institute of Technology. In the present paper, thermal performance of a scale-up MEET II boiler with 3MW of thermal input is examined. The boiler can combust not only natural gas but also low calorific gas (e.g. full gasification gas obtained from coal or wastes). The first test fueled by natural gas is done. The CO and NOx emissions are less than 50ppm. The heat flux to the boiler tubes (60-80kW/m 2) are very uniform. The three-dimensional calculation is examined for using syngas whose heating value is 3.94MJ/Nm 3. More than 300kW heat transfer to boiler tubes is predicted.