Biomass combustion burner, biomass-mixed fired boiler, and biomass fuel combustion method

A method for burning biomass fuel, comprising:

• burning biomass fuel using a biomass combustion burner,
• the biomass combustion burner comprising:
• a biomass fuel jet nozzle having a bent section and a fuel jet port that jets biomass fuel conveyed by primary air;
• a secondary air nozzle having a secondary air jet port that surrounds an opening in the fuel jet port, the secondary air nozzle jetting secondary air;
• and
• a tertiary air nozzle having a tertiary air jet port that surrounds the secondary air jet port, the tertiary air nozzle jetting a swirl flow of tertiary air, wherein
• the biomass combustion burner includes a swirler disposed at a center inside the biomass fuel jet nozzle, the swirler changing a biomass fuel stream drifted by the bent section into an axially whirling swirl flow to thereby make a fuel concentration lower on a pipe axis side and higher on an outer circumferential portion side, and includes a degree-of-swirl adjusting plate disposed on a pipe inner wall immediately upstream of the fuel jet port, the degree-of-swirl adjusting plate reducing a degree of swirl of the fuel stream that jets from the fuel jet port, thereby optimizing a fuel concentration distribution,
• the fuel jet port has a funnel-shape that widens the biomass fuel stream outwardly,
• the secondary air jet port has a funnel-shape that widens a secondary air stream outwardly,
• the secondary air jet port is disposed upstream of the fuel jet port,
• an opening in the secondary air jet port and an opening in the tertiary air jet port are adjusted to reduce an amount of the secondary air and a buffer stream is thereby formed between the fuel stream and a tertiary air stream,
• the biomass fuel is burned by supplying the biomass combustion burner with a fuel stream that contains woody biomass fuel exhibiting a grain size distribution of 2 mm or under, the woody biomass fuel being conveyed by the primary air such that fuel conveying air including the primary air has an A/C value that is:
• not less than 0.8, and
• not greater than proportional distribution between A/C 2.5 in a quantity of fuel of burner rating and A/C 1.5 in a quantity of fuel of 60% of the burner rating, and
• the A/C value is a ratio of fuel conveying air flow rate (Nm3/h) to fuel (kg/h), the fuel conveying air being equal to the primary air.