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Unformatted text preview: CHE 633--Combustion Priciples Assignment 5 Due Wednesday, February 15, 2012 Problem 1: You are running an oxy-fired process heater burning methane at stoichiometric conditions. The cost of the air separation increases with the oxygen purity. With pure oxygen you produce no NOx, which is good since removing NOx is expensive. Clearly there is a tradeoff: high oxygen purity high cost; high purity low NOx; high purity higher temperatures higher NOx. To understand this tradeoff, plot the moles of equilibrium NO versus moles of reactant N2 when burning 1 mole of methane and 2 moles of oxygen. (That is, for 1 mole of methane, 2 moles of oxygen, vary the reactant N2 and plot the moles of product NO versus reactant N2). On the same graph, plot the ratio of (product moles of NO/reactant moles of N2) and Tad/10000 versus the reactant moles of N2. Include enough points to clearly resolve the shape of the curve and include points n_N2=0.1 and n_N2=7.52. Discuss (that is, learn from) the results. Problem 2: For methane/air combustion and beginning with (adiabatic) equilibrium products: (a) Plot the NO mole fraction (ppmv) as a function of time for an equivalence ratio of 1.0. (b) Compute a characteristic timescale as the time to reach the equilibrium NO using the initial rate of NO formation. Use a PFR code to do these calculations. You can get equilibrium products from GasEq. Zero out the NO moles in the PFR calculation. Run at a constant temperature. You want to choose the time so that you can clearly see the action in the curve. Problem 3: For the conditions of Problem 2 at an equivelence ratio of 1, compare (on the same plot) the NO versus time curve to the NO versus time curve computed using Turns Eq. (5.7). Problem 4: A Utah coal sample has the following ultimate analysis (wt%) C=73.2, H=3.1, N=0.9, S=0.9, O=3.8, Ash=18.1. If all the coal nitrogen goes to NO, estimate the NO product concentration (ppmv). To estimate this, compute the product composition for CO2, H2O, and N2, ignoring N, S, and O in the coal (that is, get CxHyNz and burn it, but you can ignore the Nz to get products; or you can do the full CxHyNzSpOq but the answers differ by only 5% and we are estimating). How does this number roughly compare to the equilibrium NO for the stoichiometric methane/air flame? ...
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- Winter '12