Experimental investigations on indirect blending lignite with bituminous coal were carried out on an ultra supercritical 660 MW unit.On the basis of pulverizing system test and lignite co-combustion test

the operation safety and economy of the system after co-firing lignite was evaluated.The results show that

after the medium speed roller mill was retrofitted from grinding single lignite to single lignite coal

to ensure the same lignite particle production

the particle size will increase a little.In term of raw lignite with moisture mass ratio of 30%

the heating process in mill can guarantee the stability and safety of coal feeding in the primary air pipeline

in which no additional heating device was required.After the resistance inside the coal mill increased

the primary air inlet pressure and its outlet temperature(55℃to 65℃)in mill are two factors to ensure the operation safety.Blending lignite affects little on the boiler thermal efficiency.When the price difference between lignite and bituminous coal reaches about 50 yuan

the operation cost in the power plant will be reduced effectively.