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Show SOLID WASTE FUEL CASE STUDY Low NOx Sander Dust Cofiring over a Bark Grate Boiler: A large number of industrial wood processes produce wood waste of various types and sizes. If the facility processes raw logs, then they generate a large amount of bark which is also called hog fuel. This bark fuel must be either consumed or disposed. Bark fuel has both good heating value and fair combustion characteristics for firing on a grate in a boiler for steam and electricity generation. Sander dust is also a waste product with excellent combustion potential as it has very fine sawdust like particle sizes. However, facilities that produce plywood or oriented-strand board use glues and resins that contain high amounts of nitrogen resulting in high fuel bound nitrogen (FBN) in the sander dust. Sander dust F B N values now range from 1 % to 6 % with the largest F B N being observed with plants that are using new faster drying resins. Mitigation of N O x generation, or conversion of F B N to N O x , in these sander dust burners becomes of paramount concern in today's regulatory environment. This application of solid waste combustion is where sander dust is fired over a bark fired grate in a boiler. The burner chosen for this application is a Dual Air Zone (DAZ) burner equipped with a scroll to inject the dilute phase convected sander dust, see Figure 7. The boiler's furnace consists of a bark fired primary combustion zone in the lower furnace with two auxiliary burners firing sander dust in the upper furnace. The sander dust contains up to 2 % F B N . The fuel split design specifies a maximum Figure 7: DAZ Burner with Sander Dust Scroll sander dust firinS rate of 5 0 % of the heat input to the boiler at any condition. The baseline N O x from the bark fired on the grate is 4 0 % of the total N O x allowed to be produced with the sander dust firing. The expected N O x emissions from unstaged combustion of the sander dust at normal excess air levels is over 2 0 0 % of the allowable N O x emissions for the boiler with both fuels firing. Hence, a staged combustion solution is investigated where overfire air ports will be added above the sander dust burners and Computational Fluid Dynamics (CFD) modeling has been requested to establish the proper location and size of these ports. The objectives of the staging for the boiler are to fire the sander dust burners at sub-stoichiometric oxygen levels and inhibit the conversion of F B N to N O x. To simplify the combustion aspects of the CFD problem, a gaseous methane reaction mechanism is used instead of the far more complex wood-burning reaction. Particulate combustion of the sander dust is evaluated with kinetics modeling once C F D has defined the time, temperature, composition path of the particles in the boiler. The potential N O x reduction is evaluated in much the same method. In addition, only the radiative section of the furnace is included in the computational grid. The goal of the C F D study is to achieve complete 6 |