Wood Boiler Firing

[Sam654]

My background is with operation and maintenance of utility systems in the chemmical processing industry, primarily oil and Nat Gas fired with some waste fuels, mostly positive pressure boilers. I've recently changed jobs acting as a contractor managing the operation of a wood fired boiler system (among other systems) that utilizes 200 psi steam directly injected into a hot water system (cascade heaters) that provides domestic heating and hot water to a large installation. The boiler has a wet scrubber system whose performace parameters are monitored by the state EPD. The boiler was restarted ~ 2 years ago after several years of shutdown for retubing and scrubber installation. I have two natural gas boilers that are rarely run due to fuel cost (wood chips are free).

Over the years as new buildings have been built, they were designed with stand alone systems with each building. As a result, the steam demand of the boiler has significantly reduced. The boiler is rated for 100K #/hr and is currently running (barely) at ~10k (15% O2, 1200+ ppm CO). I'm being pressured (but resisting) to decrease air for "more efficient operation". I drop air and combustibles shoot up. That also doesn't help the BOD loading to the WWTP from the scrubber. I have several concerns with operations at this low load.
1. Boiler operations - The facility owners (local govt) are extremely cost conscious. They want to decrease the pressure on the boiler to <15 psi, essentially making it a large hot water heater. To the boiler designers out there, is this a concern with natural circulation? I'm concerned already with circulation at these low loads. Will lowering pressure help or hinder circulation?
2. Sootblowing - With oil fired boilers, I typically had operators raise the firing rate to around 50% to ensure soot was carried through the boiler. I cannot raise load this high on this boiler. Is this boiler being set up for a potential explosion? Again, I'm not as familiar with wood firing vs. fossil fuels.
3. Boiler performance - Prior to my arrival, they had an inverter installed on the ID fan which helped with scrubber performance however, focus is more on diff. press. across the scubber than maintaining a negative pressure on the furnace. Again, not the best way to operate in my opinion.

There is consideration of smaller boilers but, they would be ~2 years away if started now. In my opinion, the system needs a complete redesign or we may end up on the evening news. Comments on my concerns? Agree or disagree? Can anyone provide me with more ammunition for my arguement? As a note, my predecessors were less than successful.

 

[macmet]

Regarding the boiler design, can you tell us what the configuration of the boiler is? A-type?

In terms of sootblowing, does the boiler have any sootblowers installed on it now? If not, you may be able to add a steam or compressed air system for soot blowing. It will depend on what type of boiler you have.

In my days of working at a biomass energy company, draft on the furnace was one of the most critical controls on the system. Personally I would be concerned if you were no longer controlling to maintain that. But maybe there are designs out there where it is not a factor.

 

[Sam654]

It's a stoker and we have steam soot blowers.

 

[GHartmann]

I would defintely be concerned about lowering the pressure to 15 psig for a 200 psig boiler and the effect on the natural circulation of the boiler. What can happen is that the lower pressure results in "steam blanketing" of the circulation coils due the lower boiling temperature / pressure ; possibly leading to overheating of your tubes. If possible I would try to contact the boiler manufacturer.

Good luck.

 

[macmet]

By stoker, I believe you are referring to the combustion chamber. What is the actually boiler type?

Is it a field erected boiler or packaged boiler?

 

[rmw]

Explosion probably isn't a worry, but having enough steam pressure to do a proper cleaning job is.

Do you have an auxiliaries you could put on steam? What about driving the ID fan with steam from the boiler?

Your situation isn't pretty.

rmw

PS: I iterate once again the question about what type boiler it is. Who is the manufacturer?

 

[Sam654]

It's a Riley Stoker, field erected. Keeler makes a similar unit. The furnace side wall tubes are feed from a side drum that runs perpendicular to the generation section.

I'll contact B&W. I thought I would pick the minds here before making the call.

rmw
You make a VERY good point concerning the soot blowing pressure. My concern was not only cleaning the tubes but, getting the soot out of the boiler with the low velocities at low fire.

 

[davefitz]

Low load burners- you can permanently de-rate the unit if you replace the air register and burner with a a lower heat input burner , preferrably a california style low nox unit. Lower the fan speed as well, and the air flow measuring device should be replaced with one that is accurate at the new steady load- the original flow element will not be accurte below 25% MCR flow, regardless of the accurace of the DP xmitter.

if combustion gas flow unbalance is causing issues, you can superimpose gas recirculation GR over the total air flow . This would also help with sootblowing issues you discussed. If the unit does not have an air heater or an effective economizer, you can add a small air heater or economizer in the air flow duct that feeds the GR fan. To ensure good GR fan reliability , it is essential to ensure the air to that fan is as cool as feasible.

Lowering the drum pressure to 15 psig is strongly discouraged- the velocity of the steam in the evaporator tubes and the steam risers will be too high, and will tend to cause a "negative thermal hydraulic sensitivity " characteristic to occur- the tube that absorbs the most heat will draw the least amount of water flow . Normally , lowering the drum pressure linear with load helps circulation, but if the drum pressure is dropped far below that linear relationship then the opposite circulation change occurs. Also, a very low drum pressure will prvent the drum driers and chevrons from properly separating steam from water and you will end up with excessive water carryover and steam purity issues .

If 10% MCR is the new steady load, then a min drum pressure of 25 psig should be the objective ( based on 200 psig original) , and if transient load increases are expected, then raise drum pressure during those load spikes. This might require that you add a steam outlet pressure reducing valve. Also, the pressure drop from the BFP thru the drum feed control valve might be excessive, so that valve may need new trim or you should lower the feed pump motor speed to 50% original speed .

I you do fire nat gas at only 10% MCR heat intput, other issues appear, such as corrosion at the stack and exit flue, and if an air heater is provided, then a condensed water leg may form in the air heater and lead to gas flow instability- the old rule of thumb that minimum permitted air flow on a boiler is 25% MCR was due to a boiler explosion in 1962 (?) at a PSE+G boiler that had a horizontal shaft air heater and that had plugged with condensed water during low load operation at 10% air flow. Also, a conventional flowmeter will not provide adequate accuracy below 25% MCR flow- there may yet be NFPA rules on that subject. The combustion gas flowarate can be artificailly increased to 25% MCR by adding a 10-15% GR flow.