Power: Business And Technology For The Global Generation Industry

In general, the flame conditions on the burners firing wood were observed to be bright but somewhat detached from the burner nozzle. With the mill stabilized, the operations staff began to make stepwise adjustments to the airflow and spin vane settings on these burners.

The final objective considered during this test involved NOx emissions. Previous cofiring trials in the U.S. and Europe tend to show a modest but repeatable reduction in NOx emissions when cofiring with wood. Furthermore, the level of the NOx reduction tends to increase with an increase in the wood energy input.

However, the observations with wood cofiring at the Atikokan GS are that for biomass input levels in the 20% range, NOx emissions are mostly unchanged when compared with the baseline lignite performance, with one exception. Corrected NOx emissions did drop approximately 10% with a 10% reduction in the primary airflow to the dedicated wood pulverizer in conjunction with operational adjustments to the associated burner row. A single burner row represents about 20% of the furnace energy input.

Atikokan’s Attempts at 100%
Wood Firing

In July 2008, a series of tests were conducted at the Atikokan GS with the ultimate objective of assessing the unit’s potential to operate on 100% wood pellet fuel. A number of significant observations were made during this program and are discussed in the following sections.

Startup with Wood. During the previous single mill trials, boiler load was always carried with the remaining four mills operating with coal. The July 2008 test program included trials with all five mills firing wood pellets, making the transition from coal to wood firing rather challenging. The test team determined that starting the unit on wood fuel (following initial firing on natural gas) would solve these logistical difficulties and provide important information as well. The mill coordination curves were modified for future tests to allow for better control during both start-up and stable operation.

Air Heater Mass/Energy Balance. The use of cold PA on a single mill has a noticeable impact on the heat transfer performance of the primary air heater (PAH). With all five mills operating on essentially cold PA, system performance was seen to degrade dramatically. Operating at unit MCR on pure wood, the PAH gas outlet temperature was seen to increase by more than 40C, to approximately 200C, and then stabilize at this level following adjustment of the gas-balancing damper. The additional gas flow to the secondary air heater (SAH) caused the outlet gas tem-

perature to increase to almost the same level. Long-term operation with such elevated temperatures might create issues for induced fan shaft growth and the integrity of the stack liner and associated components.

Wood Pulverizer Trip and Restart. Another key issue when considering a complete fuel conversion of this type regards the implications of a pulverizer trip on wood fuel. Mill #3 was tripped from very high load ( 8. 6 kg/s) and subsequently restarted with a deep bed of wood dust around the grinding elements. Aside from a brief spike in the mill motor current (this also occurs during a restart with lignite) the restart proceeded smoothly and the mill returned to normal operation on wood without any further issues.

Steam Temperatures. The previous trials at relatively modest levels (<20%) of wood input did not result in significant impact on the boiler’s thermal performance. However, at wood energy input of 67% and 100%, both the main steam and hot reheat steam temperatures were observed to be well below their design values on coal. This is the expected trend for the fuels involved, but the magnitude of the temperature depression was rather surprising.

Primary Air System Limitations. The capacity of the existing PA system was found to be marginal at full unit load on 100% wood pellet firing. The cold primary airflow available to each mill was approximately 18 kg/s—some 10% below the target value.

Pulverizer Throats. As part of an existing maintenance upgrade program, Atikokan GS has been replacing the original equipment manufacturer static throats in the pulverizers with a third-party rotating throat design. In general terms, those mills with rotating throats operate with about half the pulverizer differential of a static throat. The rotating throat also appears to stabilize faster, and there is a small benefit in a reduced time for the clearing cycle.

NOx Emissions. As noted previously, the various cofiring trials at Atikokan yielded fairly flat results with respect to NOx reductions. NOx was shown to be sensitive to the high transport velocities used in the test program, but the final results were generally similar to those for the base lignite coal. However, operation with higher levels of cofiring and with 100% wood firing resulted in a definite change in NOx performance—from 0.79 kg/MWh to 0.53 kg/MWh. The baseline NOx rate for Atikokan firing lignite at MCR is 1. 50 kg/MWh. The value of 0.53 kg/MWh is equivalent to other OPG units with selective catalytic reduction technology installed.

Heat Rate. Low final steam temperatures and elevated flue gas exit temperatures both have an obvious negative impact on the heat

rate of the unit. The expected degradation in heat rate is on the order of ~4%.

Challenges Related to Complete
Conversion from Coal to Wood

The Atikokan biomass test program has included operational trials with up to 633 MJ/s of wood input. This is a remarkable achievement for an unmodified pulverized coal ( PC)-fired boiler, but a number of issues will require attention and investment to enable safe, commercial operation. Any fuel conversion might require physical equipment modifications in addition to changes in operation. The complete conversion from coal to wood for a utility PC-fired boiler is expected to result in a number of unique challenges, discussed below.

Safe Material Handling. Later in 2008 the Atikokan plant experienced a dust explosion while bunkering wood pellets in preparation for further tests. Review of this incident has lead OPG to conclude that major modifications to coal-handling systems must be made in order to ensure the safe handling of biomass fuels. Specific findings include these:

■ Receiving systems should include the capability to screen deliveries of pellets to remove dust and fines that might have been generated during transportation.

■ Fire and explosion detection and suppression systems are needed.

■ A thorough review of the design and limitations of existing bunkers, conveyors, and transfer points is necessary.

In addition to the dusting risks associated with use of coal conveyor systems, other techniques used for the bulk solids handling of a fuel like wood pellets are quite different from those traditionally used by utility sites with coal. Possible considerations include the following:

■ The need to protect the pellets from the elements during transportation to the site and during long-term storage.

■ Including provisions for significant covered storage in the site design.

Firing Systems. In addition to the mandatory retrofit of additional safety systems, several other areas of operation would require further attention to fully employ the dedicated milling concept on a commercial basis: