The use of low-calorific value gases is a way to gaseous fuels supply diversification. LCV gases present an application potential in various technologies e.g. reburning process. The experimental investigation of the reburning process in small scale coal-fired boiler using sewage sludge-derived syngas was carried out. The influence of the LCV gas stream on the nitrogen oxides emission reduction was analyzed. The emission reduction reached almost 30% with 11% gas share in thermal input.