PLANT

Full steam ahead

Lumber mill saves with biomass boiler.


Twin Rivers Paper eliminated the use of bunker oil and recovered its $17.5 million investment in less than four years.

Twin Rivers Paper Co.’s lumber division in Plaster Rock, NB, has tallied significant savings by replacing two oil-fired boilers with a new high-efficiency unit and Dutch oven biomass boilers that run on mill waste, mostly composed of bark.

The division employs 200 people and specializes in supplying softwood dimension lumber and studs for residential and commercial markets. The company operates year-round, except for a one-week annual maintenance shutdown; however, the boiler and kilns run 24/7 all year.

In 2008, the company approved the purchase of a $10.5 million biomass boiler with an additional $4 million for two new kilns and sawmill line upgrades, because the existing biomass boilers were undersized, at the end of their life and unlikely to meet new licensing requirements. Oil-fired boilers were needed to supply additional steam, which required more than 5.5 million litres of bunker C oil each year. Oil costs were reaching $3 million annually.

Maintenance superintendent Earle Fawcett said the 350-green-tonne capacity fuel bin feeds bark into the combustor with a step-grate design, whereby the biomass is conditioned or dried and then gasified and leftover carbon burned.

“This design allows for maximum efficiency, because it limits the air intake at the critical burning stage and adds air later to reduce emissions,” said Fawcett.

The gasification system eliminates the need for an electrostatic precipitator to control emissions, which means additional savings and the production of only 1.5% ash from combustion.

The flue gases heat water and generate steam, while an economizer preheats the boiler feed water. A recovery system channels heat normally rejected from the boiler grate cooling system to in-floor tubes in the fuel bins that dry and preheat the biomass, further increasing overall boiler efficiency.

The door is open for future power generation of five to seven megawatts because the new boiler is designed for expansion from the current 600 pounds of pressure, and a superheater could be added inside to accommodate a steam turbine.

“Less than four years later, the project has already reached its payback period goal, and the biomass boiler is operating smoothly; we have barely used the back-up oil boiler since 2008,” says Fawcett.

The kilns were extremely inefficient, but he said the company worked with a manufacturer who integrated a recovery system that takes heat from kiln exhaust with an exchanger to treat incoming air, reducing the amount of steam required and lowering related biomass requirements of the boiler.

A weight scale system helps to drive optimal kiln performance, and variable frequency drive fan motors are used in many parts of the drying process. With the new kilns, the mill now heat-treats all of its lumber, saving the company additional money.

Source: Natural Resources Canada

This article appears in the July/August 2013 edition of PLANT.