Biomass　(straw　and　wood)　co-firing　with　coal　was　investigated　in　a　drop　tube　furnace.　Emission　of　NO　and　CO　along　the　height　of　the　furnace　was　monitored　by　a　gas　analyzer　on　line,　and　the　content　of　unburned　carbon　(UBC)　in　fly　ash　was　also　tested.　The　effects　of　fuel　type,　temperature,　co-firing　and　air　staging　ratio　were　considered.　Results　showed　that　along　the　height　of　the　furnace,　NO　concentration　increased　first　and　then　decreased　slowly　for　all　the　fuels,　and　NO　emission　from　straw　or　wood　combustion　only　accounted　for　about　1/3　or　1/2　of　that　from　coal　combustion　respectively.　In　the　present　temperature　range　of　900-1100　degrees　C,　the　increasing　in　temperature　increased　NO　emission　but　decreased　UBC　in　some　degree,　while　the　effect　on　CO　emission　was　little.　Under　the　condition　of　biomass　co-firing,　NO　emission,　CO　emission　and　UBC　were　significantly　reduced　more　with　the　increasing　in　biomass　co-firing　ratio.　Moreover,　results　of　air　staging　combustion　showed　that　for　coal　combustion　air　staging　notably　reduced　NO　emission　while　increased　UBC　and　CO　emission,　but　for　biomass　combustion　the　effect　of　air　staging　was　less　significant.　Further　test　of　air　staging　on　biomass　co-firing　indicated　that　only　under　the　co-firing　ratio　of　0.4　could　air　staging　play　a　great　influence　on　NO　emission　and　combustion　efficiency.　This　present　study　could　provide　a　guide　for　the　optimization　of　biomass　co-firing　in　industrial　furnaces.　(C)　2014　Published　by　Elsevier　Ltd.