Industrial steam boiler with capacity control

A natural-gas-fired water-tube boiler raising 50 t/h of steam at 42 bar and 400 C, with its three circuits and its capacity control on one flowsheet. Fuel: methane at 3 bar to the windbox. Combustion air / flue gas: an FD fan and a regenerative air preheater feed the burner at 15% excess air; the flame (adiabatic, 2252 K) then gives up its heat to the radiant evaporator, the superheater, the economizer and the air preheater in turn, and leaves the stack at 377 K with 3.0% O2 dry. Steam: deaerated feedwater is pumped to 48 bar, warmed in the economizer, boiled in the evaporator and separated in the drum; saturated steam is superheated to 672.2 K and 2% is blown down. Capacity control: a steam-flow controller (FIC, integral action) sets the firing rate to hold the steam demand, a ratio station keeps combustion air at 15% excess, and the drum-level balance makes feedwater equal steam plus blowdown. At design it fires 49.92 mol/s of fuel (40.0 MW LHV); the evaporator takes 25.3 MW, the economizer 7.7 MW and the superheater 5.7 MW, for a direct efficiency of 96.7% LHV / 87.1% HHV. The heat-transfer banks are rated by UA, so a change of load redistributes the heat instead of reproducing it: run the sensitivity (70-100% load) and the superheat falls from 673 K to 605 K at 70%. Property method: pr-steam, Peng-Robinson for the gas with IAPWS-95 steam tables for pure water; plain Peng-Robinson puts 18% too little heat in the superheater. Bounds: no radiation or casing loss (a real boiler loses 1-2%, so efficiency reads high, and more so at part load); no dissociation in the flame; the radiant section is a fixed-UA exchanger, which overstates its absorption at deep turndown; no attemperator; and level and ratio control are the steady-state balances, not dynamic loops.