Biomass combustion

Abstract

Improved combustion of biomass is achieved by injected first and second streams of biomass from a burner where the first stream of biomass has a median particle size larger than the biomass of the second stream and oxygen is injected with the first stream to provide an oxygen-enriched environment around the larger median sized particles. The oxygen-enriched environment is achieved either by injecting the oxygen directly into the first stream or by premixing the oxygen with the conveying air of the first stream.

What is claimed is: 
     
         1 . A biomass combustion method, comprising the steps of:
 injecting a first stream of fuel comprising particulate biomass, air, and oxygen from a burner into a combustion chamber;   injecting a second stream of fuel comprising particulate biomass and air from the burner into the combustion chamber, the second stream not including any oxygen apart from the air present in the second stream;   combusting the biomass, air and oxygen of the first stream in the combustion chamber; and   combusting the biomass and air of the second stream in the combustion chamber, wherein the particulate biomass in the first stream has a median particle size larger than that of the particulate biomass in the second stream.   
     
     
         2 . The biomass combustion method of  claim 1 , wherein the biomass is selected from the group consisting of wood pellets, straw, hog fuel, crushed olive stones, dried sewage sludge, wood dust, and combinations thereof. 
     
     
         3 . The biomass combustion method of  claim 1 , wherein the median particle size of the biomass of the first stream is less than 300 microns and the median particle size of the biomass of the second stream is greater than 400 microns. 
     
     
         4 . The biomass combustion method of  claim 1 , wherein the injected oxygen is no greater than 8% vol/vol of the total amount of oxidant injected from the burner. 
     
     
         5 . The biomass combustion method of  claim 1 , wherein the injected oxygen is supplied by an oxygen source selected from the group consisting of an air separation unit, a vapor swing adsorption unit, a vaporizer fed with liquefied oxygen, an oxygen pipeline, and combinations thereof. 
     
     
         6 . The biomass combustion method of  claim 1 , further comprising the step of separating a biomass feedstock into a first flow of biomass having a relatively larger median particle size and a second flow of biomass having a relatively smaller median particle size, wherein the first stream is derived from the first flow and the second stream is derived from the second flow. 
     
     
         7 . The biomass combustion method of  claim 6 , wherein:
 the first flow is fed to a first hopper;   the second flow is fed to a second hopper;   the first stream is drawn from the first hopper; and   the second stream is drawn from the second hopper.   
     
     
         8 . The biomass combustion method of  claim 6 , wherein the oxygen is premixed with the air of the first stream of biomass. 
     
     
         9 . A biomass combustion system, comprising a biomass burner, a biomass particle size separator, first and second biomass hoppers, first and second blowers, first and second fuel conduits, and a source of oxygen, wherein:
 the biomass particle size separator is adapted and configured to separate a biomass feed stock into first and second flows of biomass, the biomass in the first flow having a median particle size larger than that of the biomass in the second flow;   the first and second biomass hoppers receive the first and second flows of biomass, respectively;   the first blower is adapted and configured to direct a first stream of biomass from the first biomass hopper, conveyed with air from the first blower, to the biomass burner;   the second blower is adapted and configured to direct a second stream of biomass from the second biomass hopper, conveyed with air from the second blower, to the biomass burner;   the burner comprises a first injector receiving the first stream of biomass and a second injector receiving the second stream of biomass;   the first fuel injector receives the first stream of biomass and is adapted and configured to inject it from the burner into a combustion chamber;   the second fuel injector receives the second stream of biomass and is adapted and configured to inject it from the burner into a combustion chamber;   the burner receives oxygen from the oxygen source and is adapted and configured to inject it with the first stream of biomass injected from the burner by the first fuel injector either premixed with the air of the first stream of biomass or not premixed with the air of the first stream of biomass.   
     
     
         10 . The biomass combustion system of  claim 9 , wherein said source of oxygen is selected from group consisting of a vacuum swing adsorption system, an oxygen pipeline, a cryogenic air separation unit, and a vaporizer connected to a tank of liquid oxygen. 
     
     
         11 . The biomass combustion system of  claim 9 , wherein:
 the oxygen is injected with the first stream of biomass through injection of the oxygen by an oxygen injector disposed concentrically within the first fuel injector;   the first and second fuel injectors are annular; and   the second fuel injector is disposed concentrically around the first fuel injector.   
     
     
         12 . The biomass combustion system of  claim 9 , wherein:
 the oxygen is injected with the first stream of biomass through injection of the oxygen by a plurality of oxygen injectors radially distributed within the first fuel injector; and   the first and second fuel injectors are annular.   
     
     
         13 . The biomass combustion system of  claim 9 , wherein:
 the oxygen is injected with the first stream of biomass through injection of the oxygen by an oxygen injector disposed within the first fuel injector; and   the first fuel injector is disposed parallel and adjacent to the second fuel injector.   
     
     
         14 . A biomass-fired boiler installation, comprising the biomass combustion system of  claim 9  and a boiler, wherein the burner is oriented to inject the oxygen and first and second streams of biomass into a combustion chamber in an interior of the boiler. 
     
     
         15 . The biomass-fired boiler installation of  claim 14 , wherein there are a plurality of the burners.