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Flow Distribution Bark Fuel Chips and Carryover NOx Comparison

Bark boilers are used in pulp and paper mills to burn bark and other waste fuel and to produce process steam and electrical power. Most of the problems associated with bark boilers are related to inadequate mixing between the combustible gases and air. Many existing bark boilers require more than 8%-10% of exit O2 in the flue gas to compensate for the poor mixing conditions, and to maintain acceptable levels of CO at the boiler exit. An optimized air system should not have more than about 4% of exit O2. The increased flue gas flow requires higher capacity induced and forced draft fans, and results in higher upward flow velocities. The upward flow of the flue gas entrains airborne particulate and carries it into the upper sections of a boiler. This particulate carryover accumulates in economizer hopper from which it has to be removed and disposed of.

Boiler Service

We offer a comprehensive analysis and optimization of your boiler that will:

Reduction of NOx Emissions from Bark Boilers

PSL models NOx emission by computing the creation and evolution of NOx from fuel (fuel-NOx), and from high temperature zones where atmospheric nitrogen may undergo oxidation (thermal-NOx). The NOx model allows to analyze typical operational conditions of a boiler, and to propose modifications of the firing practices with the ultimate goal of reducing the NOx emissions. Various aspects are analyzed, including splitting of the air between undergrate air, overfire air, and burners cooling air. Special attention is given to obtaining good mixing conditions in the middle section of the furnace. PSL works to ensure that your boiler can meet today's EPA emissions requirements. In most cases older boilers can be upgraded to the today's standards at relatively low cost. Performing process simulation in advanced, can be used to check the proposed modifications to the boiler in advance, giving high assurance of successful boiler upgrade. The decision of the mill management to modernize a bark boiler would be supported by the results from the computer model.

Traditional Way

Boiler designs and modifications were based on experience and simple physical modeling. Because of the complexity of the turbulent gas flow and combustion in a boiler, such an approach is frequently insufficient and has not produced desired effect. Many pulp and paper companies have spent hundreds of thousands of dollars on modifications that have not fulfilled their expectations or the contractor's promises. The advent of high-speed, cost effective computing has produced a new and powerful analysis tool: process modeling. We can simulate and predict in advance the outcome of any boiler modification, supporting the mill managers decisions concerning boiler operations and air/fuel delivery system retrofits.

PSL Service Delivery Method

We work closely with the boiler operators to collect the information necessary to set up and run the bark boiler model. When all the necessary data is collected the model is set up and a baseline case is computed.  PSL uses our process modeling software to simulate gas flow and gas phase combustion.  The results of the computer simulation are presented as graphs showing flow fields, gas temperature, fuel and carryover trajectories, fuel distribution on the floor/grate, and other relevant information.   Computer animations of the flow field and carryover may also be included. Simulation results allow for a thorough understanding of the boiler operation, and provide the basis for optimization of the existing air and fuel system.  In some cases it may be sufficient to improve the boiler's operating practices, in other cases, it may be beneficial to retrofit the boiler with a modern overfire air system.  The decision depends on the severity of the operational problems and the budget available for the boiler upgrade.  Any of these changes can be evaluated in advance by our model, helping to minimize risks in the decision making process.