Coal fired chain grate boiler is retrofitted to coal bed methane boiler. The flow, combustion and NOx emission characteristics of low concentrations methane in the boiler furnace are studied by the numerical simulation and experiments. The results show that the axial velocity reaches the maximum at 0.5 m from the nozzle and declines sharply in the range of -0.4~0.4 m on the width direction. With the heat load increases, the position of highest temperature moves to the rear of furnace and the simulation and experimental results agree well. At the front of the furnace, the obvious bimodal distribution of temperature at low heat load along the width direction is found, but it is a single peak distribution at 100% heat load. At the rear of the furnace, the temperature shows bimodal distribution at all kinds of heat loads. Gas temperature at boiler furnace outlet increases linearly with the increasing of heat load but the experimental results are slightly lower than the simulated results. The NOx is mainly produced in the middle and rear of furnace with high temperature, and the position of NOx peak lags far behind that of temperature peak. With the excess air coefficient increases, the highest furnace temperature and NOx generation increases and then decreases.