NorskeCanada Mill Uses Coal as Auxiliary Fuel To Improve Wood Waste Boiler Performance

British Columbia mill "earns environmental respectability while being politically incorrect" with good communication to community, provincial, and national leaders

NorskeCanada's Elk Falls mill is an integrated printing papers, kraft, and containerboard operation located in Campbell River, B.C., on the East Coast of Vancouver Island. Production facilities include a 1,100-mtpd kraft mill, a 1,600-mtpd thermomechanical pulp mill, three paper machines, a market pulp drier, and a containerboard machine.

The steam plant consists of two fossil fuel auxiliary boilers and a large hog fuel (wood waste) fired traveling grate boiler constructed in 1980. In 2001, the Canadian industry's first wet electrostatic precipitator was installed on this boiler to reduce emissions and deal with public complaints. In the spring of 2002, a three-month trial was run where coal was cofired with hog fuel in the large boiler.

POWER BOILER DESCRIPTION. The No 5 power boiler (Figure 1), a large hog fuel boiler, burns wood waste along with a small amount of natural gas or Bunker C fuel oil. This Babcock & Wilcox boiler produces 50.5 kg/sec (400,000 lb/hr) of 4,140 KPa (600 psig) steam. The furnace is water cooled with a membrane wall furnace enclosure. The furnace is rectangular in cross section, approximately 13.9-m wide (45-ft., 6-in.) by 6.1-m deep (20 ft.). The floor of the furnace is comprised of a Detroit Stoker rotograte.

Approximately 3 m (10 ft.) above the grate surface, the front and rear wall tubes are bent inward at an approximately 45-degree angle forming a throat. The intention of these arches is to create a lower combustion zone for hog fuel in the furnace. The front and rear wall tubes then bencl back out at a 60-degree angle and return to the original wall location.

High-pressure overfire air nozzles are installed at the arches to provide increased turbulence in the combustion space immediately above the grate. There are oil and natural gas burners located on both sidewalls, just above the wall arches. Full steam load may be generated utilizing natural gas or oil alone.

As a result of the boiler's unique design, optimal hog burning is difficult to achieve. Unstable furnace air conditions are prevalent, as the lower hog combustion zone below the boiler arches competes for oxygen with the fossil fuel combustion zone above the arches.

A recent retrofit to this boiler was a wet electrostatic precipitator. Built in 2001, this precipitator allows Elk Falls to achieve significantly lower paniculate emissions levels while burning salt-laden coastal hog fuel.

BOILER OPERATION. During operation, fuel is blown onto the grate through windswept spouts. Additional combustion air is fed through the grate as underfire air and above the fire as overfire air. Auxiliary fuel-either Bunker C oil or natural gas-is fired into the boiler through four guns located above the combustion arches (Figure 2).

Firing the auxiliary fuel at this location carries the steam load when high moisture limits the amount of heat available from hog fuel. Unfortunately, the auxiliary fuel is too far from the grate to offer appreciable combustion assistance to the waste wood fire.

Aside from changing fuels, the operator's only other option is to adjust airflow. Increased underfire air carries the small wood particles up into the furnace cavity and into the auxiliary fuel flame. Here, the wood particles must compete for oxygen, and since the particles are less volatile, the lack of sufficient oxygen extinguishes the wood flame.

The result is incomplete combustion and excessive carryover of partially burned fly ash. Also, the hog fuel piles up on the grate and goes through a cycle of smoldering and flaring that causes excessive blow back into the boiler building, creating a workplace environment problem.

If the auxiliary fuel could be used to help dry out the wet hog fuel rather than just supporting the steam load, a number of advantages may be possible. The fire on the grate will be hotter, so the fuel will dry out more quickly and be less sensitive to large swings in moisture.

If better control of the fire could be achieved, the mill could expect to use less combustion air and much less underfire air. The wood fuel would be expected to burn more completely with a resultant decrease in unburned combustion products and reduced fly ash carryover (Figure 3). It seems illogical to use volatile natural gas or oil auxiliary in a boiler designed to burn solid fuel.

In the Campbell River area, there is a mine producing low-sulfur coal located about 20 minutes away from Elk Falls. Given the location and price of this option, coal was selected for an auxiliary fuel trial over other available choices.

INITIAL PUBLIC CAMPAIGN. The public s visualization of coal is a "turn-of-the-century" practice with boilers belching black smoke and covering the countryside with soot and coal dust. It doesn't matter how much or how the coal is burned-either as a primary fuel or as an auxiliary-the mental image is negative. The mill understood this proposal to burn coal in the Elk Falls boiler would be viewed as being very politically incorrect, so it was important that local sentiment be assessed before proceeding.