For the Steam Chief or Operations Manager that is responsible for a biomass fired furnace winter can be a time of dread. Every day there seems to be different reasons, excuse, incident or issue that has caused the facility not to make budget.
It seems like there is no end and no way around it. Winter production just sucks.
Too often this is a case of complacency. That’s the best we can do…its winter.
Why is winter so difficult?
This can be a case of historically created issues. Those little changes that have developed and built over the years of operations. More often its a misguided combustion philosophy. Typically implemented with the best of intentions but not taking the full operating spectrum into consideration.
How do we make this better? What can we do differently?
First, how much is it curtailed and why?
If the limit is mechanical, such as the ability to keep the infeed system running due to freezing or fuel flow then yes that is an issue. If you can’t get the fuel into the furnace, then production will be impacted.
If the limit is the ID Fan due to moisture content obviously due to the amount of snow and ice in your fuel… then you also have a problem.
I will get back to these two issues a little further down in this post so don’t assume that if this is your problem then ALL is lost.
HOWEVER, what we have found to be a major issue in many locations is treating the fuel as if it were the same year-round.
So many facilities run on a fuel feed rate to air ratio that is fixed. That’s great for a gas fired burner but not even close for biomass.
Your biomass fuel changes continuously. Moisture content, BTU value, contamination, size distribution etc. and that is before you take the variation between bone dry and frozen lumps of ice. Your process control system must be as dynamic and responsive as your fuel. All of those moving parts must work in harmony and the tune you are trying to harmonize on is changing with your fuel.
If you have dry sawdust it burns almost instantaneously, if that same sawdust is in a wet or a frozen lump then its not going to burn the same. You can’t supply the same air flows and expect the same results. If your control cannot distinguish between variations in the fuel then you are not going to get the performance you require.
Often response control is disguised under limited adjustments excess O2 or limits based on perceived moisture content or fuel quality, but these are generalizations and typically not specific enough.
Unfortunately, too many believe that you just have to live with this. The operator will adjust the fuel quality and moisture content the best that they can and that is all that can be done.
You fuel is continuously changing. The operator can’t keep up on the best of days. Realistically they have too many other things to take care of, they are going to set parameters where it gives them the least trouble and go about their day.
This isn’t due to being uncaring or unmotivated. This is due to self preservation. Everyone wants to do a good job.
Depending on the weather conditions and the fuel sources an operator may experience 5 or 6 major changes in a 12-hour shift. These changes are typically not measured. Where they become apparent is when the output starts to drop or an imbalance on the grate or the bed shows up. With a grate system the operator goes out and looks at the grate and sees that it is heavy, light or out of balance from side to side and makes an adjustment to the feeders. This imbalance has probably built up over the past two hours and may not even exist anymore at the back of the furnace. The operator can’t see past the wall of flame that exists to know the current state. What he or she sees at the front of the furnace is history and probably not representative of the current conditions.
The probability of over or under correction is great and then the chasing starts.
Every unit and every fuel source is different. From Fluidized beds to mechanical grate systems, feeders, air distributors and nozzles, every aspect has its impact and each of these impacts are continuously changing dependant upon the current fuel condition.
There are many moving parts and every one of them must be operating in conjunction with each other to get the greatest output of your system.
Again, using a grate system as an example there may be segments of the grate that can be moved at different speeds to each other. The undergrate air distribution may be one large plenum covering the full grate vs multiple zones with individual air flow control.
The general premise is to mix the fuel and air in the appropriate areas of the furnace. Sounds simple. On any grate system the air is going to take the path of least resistance. If a hole is created in the bed due to an interruption in the fuel flow then the air is going to pass through the section in the grate that has the least fuel. The air goes where you need it the least. If not addressed a short term fuel flow issue can have an impact over a much greater duration.
Depending upon the mechanical configuration this can be a challenge, however if you respond to the parameters in the proper manner you can achieve greater efficiencies and a more responsive operation.
We achieve this by applying our supervisory control to your existing system. By monitoring the response and current state of all control parameters we can adapt and adjust for the changes in fuel and some mechanical issues to achieve maximum performance.
The steam produced by the fuel feed rate, the ID Fan loading, the overfire to undergrate air ratio can be used to determine the fuel quality and moisture content and adjust the parameters of the furnace to suit.
In simplified terms low moisture content fuel is going to use less undergrate air and more over fire air than a wet or frozen fuel. If a wet or frozen fuel is present, then putting the undergrate air to the back of the furnace to dry the fuel has a major impact on the sustained output. In the case of a fluidized bed the loading of the bed with more fuel and giving the fuel the retention time within the bed to dry achieves a similar action. Other parameters need to be considered in both scenarios, this is a simplified general action.
If the fuel is so wet that the ID Fan is running at its limit, then to keep the operation within safe operating conditions this limit can be monitored and used to adjust feed rates and air flows to sustain a safe maximum operation. Conditions can also be adjusted to create that maximum heat release based upon the limit.
If the fuel flow is impaired, then by modifying the parameters at the fuel entry area of the furnace the bed can be somewhat maintained and a reduced impact experienced. The recognition and response to issues can greatly reduce their overall impact.
These are a brief glimpse into the variations in response that is required to achieve sustained performance and efficiency from your furnace.
The return is that you get a greater output from your winter operations and possibly year round.
This is not a simple one size fits all implementation. It takes time, it takes a coordinated effort from all parties and often takes some coordination of expectations. Every situation is different and needs to be addressed as such.
The outcome is that you lead your team into a more effective operation that is easier to manage and much less stressful for everyone.