pid strategy for primary/secondary control of wood boiler


Thread Starter



You will quickly realize I am not from a engineering background, but have a process that I am looking for advice. A little background, I am a homeowner with a 2000 gal. firetube batch fired wood gas firing boiler,it has a refractory secondary burn chamber with a type k thermocouple that burns the wood gas at observed temps of 1800-2100degf. The flue gas travels through hx tubes radiating the heat into the 2000 gal of surrounding water, the flue gas leaves the appliance at 240-300degf. The boiler is draft induced, approx 350cfm and has a high fire output about 450kbtu/hr. Typically a 2. 5 hr burn will put about 1 million btu into storage that travels through pex pipe to a flatplate hx to integrate to the house distribution system.

This boiler was designed with non adjustable premetered primary/secondary combustion air system, as you could imagine the with a batch burn the o2% was low for the first 30 minutes, moderated to and through a acceptable % for the next hr and was way over aired for the remainder of the burn. So I installed a insitu flue gas analyser that outputs a 4-20mA signal based on a o2% range of 20. 9-0%. I fabricated independent p/s air inlets into the appropriate locations in the firebox and ducted to 2 modulating dampers. The o2%-mA signal is input into a Watlow eze-zone pid controller with independent outputs for each damper. In theory with no fire the primary damper is open and the secondary damper is closed, as the fire intensity increases the primary damper starts to close, limiting the amount of wood gas production and the secondary damper starts to open, controlling the % of o2 in the flue gas. As the fire dies down the dampers eventually return to their starting positions.

The controller is setup that output 1, heating or reverse acting controls the primary air damper, and output 3,cooling or direct acting controls the secondary air damper. I am told this will control above and below setpoint. The damper movement is regulated by the heating and cooling proportional bands a shared integral [time] and dead band%. Through a lot of trial and probably more error the settings that seem to work best are htg prop 28, cooling prop 22, integral 35sec. and deadband -12. 5%. Starting with a pv of 20. 9%o2 and driving to a setpoint of 6, it will dive to 3 and slowly rise to sp. then it will vacillate 6. 1-6. 2-6. 3 and correct to sp and vacillate about 3 tenths either way until burn reaches a point beyond setpoint control towards end of fire. I feel this is a vast improvement over the stock setup, but i thought a "properly tuned controller" might yield better results. I have heard of analysing and configuring tools to write a algorithm based on specific feedback, or is a different method or controller better?

I am sorry for the length of this and thanks in advance.