Many integrated pulp and paper mills built in the twentieth century are still in operation and are largely self-sufficient — some are even capable of generating 75 percent of their own power by burning waste wood. There’s a lot to learn from them about biomass fuel handling.
However, most air handling systems on both boilers and kilns operate inefficiently. They are either the original systems, or old installations that have been modified over the years to address evolving emissions requirements, inadvertently creating system inefficiencies.
In many older pulp and paper mills, old ductwork, expansion joints, and air heaters have developed air in-leakage that has significant adverse system effects on boiler and kiln operations. Historically, typical boiler and kiln system upgrades that have been implemented to address these issues include:
- Thermally efficient refractories
- Longitudinal lifter sections and dams
- Additional chain sections
- Improved kiln seals
- Planetary coolers
- New air heaters or economizers
- New or modified scrubbers and/or venturi sections
- New or modified precipitators
What all of these changes have in common is that they increase system pressure drop. This additional drop changes the point on the fan curve at which the induced draft (ID) fan operates. As a result of these system upgrades/modifications, then, the fan ends up operating at a very inefficient point on the fan curve, which causes excessive energy consumption, which equates to significant increases in plant energy costs.
Additionally, many boiler and kiln ID, forced draft (FD), and overfire air (OFA) fans were oversized by the original boiler and kiln OEMs. Often, these original oversized fans are not modified to meet the new system pressure drop and flu gas flow requirements that results when new equipment is installed to improve boiler and kiln operation.
Many plant owners have not addressed these issues because they are either not aware of their significance or haven’t been presented with a good range of solutions. Typically, firms that are called in to analyze pulp and paper air handling systems will recommend a complete ID fan replacement, which is often cost prohibitive.
An alternative, cost-effective approach is to retrofit an air handling system, working with as much existing equipment as possible. Relative to a complete overhaul, this strategy requires minor capital investment and can be carried out in a comparatively short period, minimizing plant downtime.
There are three areas ripe for air system retrofit:
- FD fans
- ID fans
- Multiclone dust collectors
Fans can be modified to run more efficiently in a variety of ways, including tipping/de-tipping the fan blades and changing the fan blade design. In many cases a more efficient fan impeller can be retrofitted into the existing fan housing while re-using the existing foundations, bearings, motor, and coupling.
Multiclone dust collectors can often be replaced and/or retrofitted to a larger tube design that reduces system pressure drop and improves collection efficiency. This approach not only saves ID fan horsepower consumption but reduces wear and maintenance costs for downstream equipment, including the ID fan, dampers, ductwork, venturi scrubber, and precipitator.
Recently, ProcessBarron employed these techniques at a paper mill in the Southeastern United States, saving the plant 1,000 hp a year, or $500,000 annually. ROI was achieved within two years.
Read our case study to find out more about this example of cost-effective pulp and paper mill air handling optimization. Or, give us a call at 1-888-663-2028.