The importance of coal for grid reliability

Rick Perry, Secretary of Energy, commissioned a report to study electric grid reliability.  The report was favorable and well received by the coal and nuclear industries while being criticized by environmental groups and the solar industry according to the Washington Post.  

While this study will no doubt be a lightning rod in the current political climate, it is also worth noting that Germany, often regarded as the leader in clean and renewable energy, is currently drawing 40-50% of its power demand from coal fired power plants.  Furthermore, a near blackout in January indicates that Germany's ability to wean itself off of coal may be more difficult than anticipated.  You may read more about Germany's grid reliability discussion here.

CO2 Capture Possibilities and Breakthroughs

One of the greatest challenges politically and technically for CO2 capture, utilization, and storage (CCUS) is modeling the subterranean reservoir to ensure that CO2 is safely contained in the appropriate rock formation.  Until very recently, most computational models relied on Darcy's law to explain the movement of fluids through porous rock structures.  However, a recent discovery at Imperial College in London has shown that Darcy's Law does not adequately explain underlying physics of fluid movement in these applications.  Researchers at Imperial College are working to improve models that will aid in more accurate predictive modeling of CO2 injection and migration.  This will hopefully reduce costs and risks associated with CO2 storage and increase acceptance of the technology.

While most of the large coal fired power plants in the western US have been studied for potential applications of CCUS including the Navajo Generating Station (NGS), no plants have moved forward with the technology.  NGS is located in the 4 corners area of the US and features geologic structures which contain natural CO2 reservoirs.  The most notable of these reservoirs is the McElmo Dome in southwester Colorado.  This naturally occurring CO2 deposit is located approximately 250 miles from NGS and is the origin for the 500 mile long Cortez Pipeline which feeds CO2 to enhanced oil recovery (EOR) activities in the West Texas oil fields.  

Clearly the geology of the region is amenable to CO2 storage, and infrastructure already is in place to transport CO2 500 miles for EOR, so economical CO2 capture is needed to retain coal assets in the region while meeting CO2 emissions reduction targets.  To support the initial deployment of such technologies, Senator Heitkamp of North Dakota, among others, has introduced legislation that would increase the tax credit for CO2 captured and stored or used for EOR under IRS section 45Q.

Will this tax incentive combined with advancing modeling tools, existing infrastructure, and amenable geology be enough to bring CCUS to the western US and give plants like NGS and others a long term future?

Slagging and Fouling Causes

Slagging and fouling is a core portion of our business at Efficient Fuel Additives.  Often times boiler fouling and slagging can be managed, or partially mitigated by combustion tuning.  Power Magazine featured an article which describes many of the combustion tuning modalities that can be investigated and utilized to reduce slagging and fouling.

The article discusses appropriate furnace exit gas temperatures in relation to ash fusion temperature as well as channelling of flue gas through the superheater section.  While these are obviously critical operational parameters, they cannot always be controlled to optimum specifications.

For example, boilers burning fuels that are not boiler designed specified fuels may not be able to achieve the appropriate differences between furnace exit gas temperature and ash fusion temperature while still maintaining proper heat balance.  Many boilers may be looking for alternative fuel sources for a variety of reasons such as lower cost, production or renewable tax credits, lower emissions, depletion of existing mines, etc. that force operators to consume alternative fuels.  When this occurs, slagging and fouling can be significant as a result of constraining factors within the boiler interacting with different ash properties.  

Finally, constraints such as NOX emissions limitations may force combustion modifications such as staged combustion leading to expanded reducing conditions throughout the furnace.

This is why Efficient Fuel Additives works not only to optimize combustion to reduce slagging while maintaining emissions compliance, but we also examine the ash properties to tailor a specific fuel additive program to meet each individual boiler's needs in a cost effective fashion.

Tangential Fired Combustion - A Navajo Generating Station Asset

The Navajo Generating Station is configured with tangential fired boilers.  In tangential fired combustion, often abbreviated T-fired or corner fired, the boiler consists of jets of air and fuel typically at multiple levels in the furnace.  Tangential fired boilers differ significantly from wall fired boilers in that β€œthe furnace is the burner.”  Wall fired units have arrangements of multiple burners where each burner controls the mixing of the air and fuel and therefore combustion and flame characteristics of each coal jet.  In wall fired units, primary air is used to convey the coal through, most commonly, the center of each jet with secondary and tertiary air providing the necessary air for oxidation of the fuel.

Tangential fired boilers facilitate combustion through a large fireball formed by jets of coal and air blown into the furnace at the corners of the boiler box.  This creates significant turbulence and mixing of air and fuel.  The advantage of such designs is that it allows for control of coal feed at each level, and can bias air flow at each corner.  This creates opportunities for fine tuning the combustion within the furnace to better control emissions such as NOX, as well as maintain proper heat balance and facilitate complete char burnout and such work has a long history of success from the Department of Energy and Southern Company.

For these reasons, tangential fired boilers have been able to operate with extremely low NOX emissions levels such as 0.12-0.15 lbs/MMBtu while still achieving acceptable LOI in the ash of less than 1% for Powder River Basin (PRB) coals.  For many plants this can be the difference between having to install expensive selective catalytic reduction (SCR) to manage NOX emissions levels, or being able to achieve compliance solely through the use of combustion controls and boiler tuning.  While these results represent some of the best in the industry, this is not achievable for all units.  Various factors such as CO emissions limits constraints, furnace/boiler design, fuel type, and operator engagement and skill can all create limits on NOX reduction potential for a given unit.

Furthermore, because these units have the ability to adjust the height of the fireball in the boiler by adjusting corner jet tilts as well as by taking offline lower level jets while maintaining higher level jets, these units can do a very good job of maintaining heat balance within the boiler.  This is critical for efficient operation at a variety of firing rates and during load following operation where load must be adjusted up or down based on renewable generation output from wind and solar.  Increasingly, coal plants are being required to load follow rather than provide baseload power for which they were designed.  Having the ability to control combustion precisely and under a wide operating envelope positions these plants well to adjust to the new role of coal fired power generating stations within the US if they are equipped with knowledgable engineers, skilled operators, advanced combustion controls, and the drive to compete in an ever changing energy landscape.

While this video features an oil fired unit, the tangential fired concept is nicely illustrated with a boiler camera.

An End to Coal Gasification w/CO2 Capture at Kemper County

Utility Dive is reporting that Kemper Generating Station which is owned by Mississippi Power, a Southern Company subsidiary, will be run solely on natural gas and that lignite coal, which was mined in close proximity to feed Kemper's gasifiers will no longer be considered as a fuel for the plant.  The plant, which has been known in the CO2 capture community for its cost overruns and delays will utilize natural gas to feed its combustion turbine and heat recovery steam generator (HRSG) that comprises the combined cycle portion of the power plant.  The coal gasifiers paired with Selexol CO2 capture system will no longer be utilized.

Unfortunately this will undoubtably be cheered by those opposing coal utilization with CO2 capture, however, it is also an opportunity to revisit and add focus to other CO2 capture technologies such as post combustion CO2 capture and oxy-fuel combustion.  While these technologies do not offer the thermodynamic benefits of a combined cycle generation arrangement, the technologies are based on much more wide spread coal combustion technologies as well as slag control and boiler fouling control and mitigation.  

Navajo Generating Station Case Study

The Navajo Generating Station has been in the headlines recently as it's fate is uncertain when the utility owners voted earlier in the year to shut the plant down earlier than expected as they no longer wish to own/operate the plant.  However, a new lease extension for the plant has been signed that will keep the plant operating for two more years.  If the plant is to continue operations beyond 2019, it must prove it can be economically competitive in the region so that a new buyer will have incentive to purchase the plant and continue long term operations.

Peabody has said that they will provide a reduced coal price to new ownership, which should help the economics of the plant, possibly attracting new ownership.  

Over the next couple of weeks, we'll take a look at the Navajo Generating Station and explore why it has some technical merits which make it a compelling generating asset including:

  • Boiler design
  • Heat rate
  • Combustion flexibility for NOx control
  • CO2 capture possibilities