The invention provides a method and a system for injection of a solution of a compound into a gas at elevated temperature, evaporating the solution and decomposing the compound in the gas at the elevated temperature, comprising injecting the solution into an injection channel being provided into at least a portion of an outer channel and being surrounded by and spaced apart from the outer channel, passing the gas along space between inner wall of the outer channel and outer wall of the injection channel and along inner space of the injection channel, evaporating the solution in and on inner wall of the injection channel and decomposing the compound in and on the inner wall of the injection channel and in the gas. The solution can be an aqueous solution comprising urea or ammonia, the gas is an exhaust gas from combustion and the injection channel typically is a cylindrical channel concentrically installed in the outer channel.