Development of Low-Emission Bio-Fuel Boiler System With Plasma-Chemical Hybrid $\hbox {NO}_{\rm x}$ Reduction

A pilot-scale low-emission boiler system consisting of a bio-fuel boiler and a plasma-chemical hybrid NO_x removal system is investigated. This system can achieve carbon neutrality because the bio-fuel boiler uses waste vegetable oil (WVO) as one of the fuels. The plasma-chemical hybrid NO_x removal system has two processes: NO oxidation by ozone produced from plasma ozonizers and NO₂ removal using a Na₂SO₃ chemical scrubber. Test demonstrations of the system were carried out for mixed oils (mixture of heavy oil and WVO). A stable combustion was achieved for the mixed oil (20%-50% WVO). The properties of flue gas-e.g., O₂, CO₂, and NO_x -when firing mixed oils were nearly the same as those when firing heavy oil for an average flue gas flow rate of 1000 Nm³/h. The NO_x concentrations at the boiler outlet were 90-95 ppm. Furthermore, during a 300-min continuous operation when firing 20% mixed oil, a NO_x removal efficiency of more than 90% was confirmed. This is equivalent to less than 10 ppm at the scrubber outlet when the flue gas flow rate was 870 Nm³/h. In addition, CO₂ reduction when heavy oil was replaced with WVO was estimated. The system comparison is described between the plasma-chemical hybrid NO_x removal and the conventional NO_x removal.