April 1, 2010

OPG Charts Move from Coal to Biomass

Les Marshall, Chris Fralick, and Daryl Gaudry, Ontario Power Generation

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:

• Modifications that promote the expeditious removal of wood from the mill and reduce the volume of recirculating product in the pulverizer.

• Further study into the minimum safe velocity required for effective pneumatic transportation.

• Modification or complete replacement of the burners.

Corrosion. Corrosion — via KCl formation — is a known concern for pure wood combustion. Much of the current research on this topic involves the lack of sufficient sulfur to promote formation of the less-aggressive K ₂SO₄. It may prove to be sufficient to merely "dope" the pellets with a sulfur-bearing compound.

Boiler Performance. Changes to the superheater/reheater tube banks may be necessary to improve steam temperature performance (possibly including a materials upgrade to address corrosion). The air heater mass imbalance will need to be addressed, but it may not be feasible to simply convert the existing PA heater into a second SAH.

Emissions Controls. The SO₂ and heavy metal emissions related to wood firing are naturally very low. The NO_x performance observed during the Atikokan trials is also very encouraging, possibly eliminating the need for additional NO_x controls. The OPG experience at Atikokan regarding wood ash collection in a cold side ESP has been good.

Looking Forward

OPG’s biomass test program at the Nanticoke and Atikokan Generating Stations is part of the overall development of biomass as a fuel to replace coal in some of its coal-fired generating units.

To further develop the business case for the biomass option, OPG’s biomass program is focused on the following:

• Determination of unit conversion modifications (including all safety measures) and unloading and storage facilities required for commercial scale operation.

• Assessment of different biomass fuels (energy crops, agricultural by-products, wood), including analysis of balance-of-plant combustion-related issues through pilot scale experiments.

• Fuel supply chain analysis, including biomass availability (both agricultural and wood-based) and transportation logistics.

• Analysis of the complete economic model associated with the development of the biomass option (including capital costs and revenue structure).

• Complete greenhouse gas life cycle analyses of biomass (both agricultural and wood-based) compared to coal.

• Stakeholder involvement through continuing to work closely with the various government sectors (Ministry of Energy and Infrastructure; Ministry of Northern Development, Mines, and Forestry; Ministry of Agriculture, Food, and Rural Affairs; and Ministry of Environment) and an extensive stakeholder network.

The development of a biomass industry in Ontario represents an exciting opportunity for OPG’s coal fleet. The biomass program has the potential to contribute to the expansion of Ontario’s renewable energy portfolio by contributing dispatchable, renewable biomass energy.

—Les Marshall (les.marshall@opg.com) is the senior technical officer at Ontario Power Generation (OPG) in Nanticoke, Ontario, Canada. Daryl Gaudry (daryl.gaudry@opg.com) is the production supervisor of operations at OPG’s Atikokan Generating Station in Atikokan, Ontario. Chris Fralick (chris.fralick@opg.com) was formerly the manager of chemical and environmental services at the Nanticoke Generating Station and is currently production manager at OPG’s Thunder Bay Generating Station.