Process and apparatus for producing thermal and electric energy

Abstract

The invention relates to a process and apparatus for the production of thermal and electric energy in a pulp mill, in particular a sulfate pulp mill, according to which process and waste liquor of the cellulose pulp cooking liquor is concentrated, and this concentrated liquor is burned in the recovery boiler in the presence of biogenic fuels, in which case the thermal energy is recovered from the flue gases from the combustion, and this thermal energy is, when so desired, converted to electric energy. According to the invention, the biogenic fuel used is wood bark or similar waste wood, which is dried to a moisture content below 30%, whereafter it is gasified to fuel gas, which is fed into the recovery boiler. The process according to the invention makes the acquisition of a separate bark boiler in pulp mills completely unnecessary, which significantly lowers the investment costs. The process simplifies the apparatus, its operation and maintenance, and improves energy efficiency.

1 . A process for the production of thermal and electric energy in a pulp mill, according to which process the waste liquor of the cellulose pulp digestion liquor is concentrated, and this concentrated liquor is burned in the recovery boiler in the presence of biogenic fuels, the thermal energy of the flue gases obtained from the burning being recovered and, optionally, being converted to electric energy, wherein the biogenic fuel used is bark or other similar wood waste, which is dried to a moisture content below 30%, whereafter it is gasified to produce a fuel gas that is fed into the recovery boiler.  
   
   
       2 . The process according to  claim 1 , wherein the bark or bark residue is dried to a moisture content below 20%, whereafter it is gasified, and at least 40% by volume of the gas thus produced is fed into the recovery boiler.  
   
   
       3 . The process according to  claim 1  or  2 ,  claim 1  wherein ash is separated from the fuel gas before it is fed into the recovery boiler.  
   
   
       4 . The process according to  claim 1 , wherein the heat produced using the fuel gas is used for superheating the soda recovery boiler steam in a superheating chamber separate from the flue gases of the soda recovery boiler.  
   
   
       5 . The process according to  claim 1 , wherein the waste wood is bark waste, bark-containing sludge, fiber sludge, branch or fiber reject from classification or surplus sludge from biologic wastewater treatment or logging waste, fire-wood separately harvested from the forest, surplus wood from wood processing, or other wood material suitable for burning.  
   
   
       6 . The process according to  claim 1 , wherein for the drying of the bark or corresponding waste wood there are used gases substantially having a temperature below 200° C.  
   
   
       7 . The process according to  claim 6 , wherein there is used steam or flue gas having a temperature below 180° C.  
   
   
       8 . The process according to  claim 1 , wherein as the energy for the drying of the solid fuel there is used steam having a pressure of 0.1 . . . 100 bar, preferably 2 . . . 14 bar.  
   
   
       9 . A process for producing thermal or electric energy in a sulfate pulp mill, according to which process 
 the wood material used for pulp production is in part digested in the cooking liquor to separate the fibers from each other,    the digested wood material is extracted as black liquor from the fiber material,    the black liquor is concentrated by evaporation, and    the concentrated liquor is burned in the soda recovery boiler to regenerate the cooking chemicals and to produce heat and electricity by using biogenic fuels, wherein    the solid biogenic fuel is brought into a gaseous form,    the formed ash is separated, and    a significant proportion of the gas is burned in the same boiler, equipped with heat recovery, as the concentrated liquor.    
   
   
       10 . The process according to  claim 9 , wherein the biogenic fuel to be gasified is wood and/or wood bark and/or bark-containing sludge and/or fiber sludge and/or branch or fiber reject from classification and/or surplus sludge from biologic wastewater treatment.  
   
   
       11 . The process according to  claim 9 , wherein the biogenic fuel to be gasified is logging waste collected from the forest and/or firewood separately harvested from the forest and/or surplus wood from wood processing and/or other wood material suitable for burning.  
   
   
       12 . The process according to  claim 9 , wherein the biogenic fuel to be gasified is peat.  
   
   
       13 . The process according to  claim 9 , wherein the solid fuel to be brought into a gaseous form is dried before the gasification to a moisture content of 5 . . . 0.40%, preferably 10 . . . 15%.  
   
   
       14 . The process according to  claim 9 , wherein the solid fuel is dried using the heat remaining after the actual heat recovery in the flue gas formed in the combustion chamber, by bringing the flue gas into direct contact with the solid fuel to be dried.  
   
   
       15 . The process according to  claim 9 , wherein the solid fuel is dried using as energy steam at a pressure level of 0.1 . . . 100 bar or, however, preferably at a pressure level that is the distribution pressure of the mill's bled steam or back-pressure steam network, preferably at a level of 2 . . . 0.14 bar.  
   
   
       16 . The process according to  claim 9 , wherein the solid fuel is dried using as energy the surplus heat that is present at the pulp mill in, for example, various warm waters and expansion steams.  
   
   
       17 . The process according to  claim 9 , wherein the combustion chamber is in the direction of the flow of flue gases divided into two parts, in the first of which there is burned the fuel that has been rendered gaseous, the heat produced therefrom being used to a significant degree for the superheating of steam, and in the second part there is burned the concentrated liquor, the heat released therefrom being used primarily for the vaporization of the boiler water.  
   
   
       18 . The process according to  claim 9 , wherein a portion of the solid biogenic fuel that has been brought into a gaseous form is, after the separation of ash, burned in the lime sludge reburning kiln and/or other units where its use replaces the use of fossil fuels.  
   
   
       19 . An apparatus for producing, from wood bark, a biogenic fuel gas to be fed into the recovery boiler of a pulp mill, the apparatus being connected to the feed unit of the recovery boiler, wherein it comprises as a combination 
 a bark-drying unit having feed means for the bark to be dried and outlet means for the bark that has been dried, and    a dried-bark gasifier for producing fuel gas from the bark, the apparatus having feed means for bark and outlet means for fuel gas, the feed means of the gasifier being connected to the outlet means of the drying unit and the gas outlet means being connected to the feed unit of the recovery boiler to feed into the boiler the fuel gas produced from the bark by gasification.    
   
   
       20 . The apparatus according to  claim 19 , wherein the drying unit comprises at least two separate dryers, which are arranged as a dryer cascade, the outlet means of the dryer subsequent in the series being connected to the feed means of the gasifier.  
   
   
       21 . The apparatus according to  claim 20 , wherein between the first and the second dryer there is arranged a pretreatment unit for treating the bark obtained from the first drier before it is fed into the second drier, the pretreatment unit having a feed unit connected to the outlet means of the first drier and an outlet unit connected to the feed means of the second dryer.  
   
   
       22 . The apparatus according to  claim 21 , wherein the pretreatment unit comprises a grinder.  
   
   
       23 . The apparatus according to  claim 19 , wherein flue gases and/or steam are used for the drying in at least in one of the dryers.  
   
   
       24 . The apparatus according to  claim 20 , wherein the first drier comprises a bed dryer.  
   
   
       25 . The apparatus according to  claim 20 , wherein the second dryer comprises a fluid-bed drier.  
   
   
       26 . The apparatus according to  claim 19 , wherein the gasifier is a fluid-bed boiler having an ebullating or rotary bed.  
   
   
       27 . The apparatus according to  claim 19 , wherein the outlet means of the gasifier is connected to a gas purification unit to separate impurities from the fuel gas before it is fed into the recovery boiler.