Centrifugal Induced Draft Fans for Power Generation

Power plants using coal or cofiring with biomass should replace their equipment with newer, higher-efficiency industrial fans.
By Doug Jones | February 25, 2020

As new sources of renewable energy have emerged, coal-fired power generation has decreased. However, coal plants—including plants cofiring coal with biomass fuels—have prevailed as a major source of energy worldwide. According to the U.S. Energy Information Administration, despite ongoing retirements of coal-fired electric generation units, coal still represents approximately a 32 percent share of the total U.S. electricity generation, surpassed only by natural gas. In addition, according to the International Energy Agency, parts of Asia have seen the use of coal-fired power generation increase in recent years. 

While investment in new coal plants has been limited, existing facilities have faced an increased need to modernize their outdated equipment and retrofit their applications with robust, high-performance, and high-efficiency industrial fans in order to remain viable in an increasingly competitive market.  In addition, coal and biomass cofired plants require ongoing maintenance and upgrades for existing fans to keep them running smoothly.

A heavy-lifter in the fuel-firing process, induced draft (ID) fans are a common area where solid fuel power plants are replacing existing solutions with new, high-efficiency equipment. The following information explains how ID fans impact the fuel-firing process and describes key considerations for choosing new fans, or upgrading existing equipment to increase efficiency while withstanding conditions—ultimately saving long-term operating and maintenance costs.

Induced Draft (ID) Fans in the Fuel-Firing Process
Coal-fired plants generate power by burning coal—either alone or in combination with a biomass fuel like wood or wood waste—in a boiler to produce steam. The steam produced flows into a turbine, which spins a generator to create electricity. The steam is then cooled, condensed back into water, and returned to the boiler to start the process over.

Fans that are used to evacuate a space or create negative air pressure in a system are referred to as induced draft fans. In these applications, ID fans are positioned downstream of the boiler to draw gases and fly ash out of the boilers and through a dust collection system. The airstream is then directed up a stack downstream of the fan.  

Below are five major factors to consider when specifying or upgrading ID fans for coal/biomass-firing applications. Note that this is not an exhaustive list and does not replace partnering with a trusted fan manufacturer that can help match the right technology to your application.

 Fan Performance Requirements
The demands for fan performance are high in cofiring applications. At the upper extreme, fans must be able to support high volumetric flow in excess of 1 million actual cubic feet per minute (ACFM) and generate upward of 35 inches H2O (8,710 Pascal) in fan total pressure. Note that the fan creates negative inlet pressure to induce a draft out of the boiler, and positive pressure at the outlet forces the airstream out through the ductwork and stack. To achieve such rigorous performance specifications, these ID fans are enormous and often run on 10,000-plus horsepower motors.    

 Energy Efficiency and Operating Costs
Energy efficiency is a major factor in equipment selection for power generation applications because an inefficient fan detracts from the power that is being generated. A centrifugal fan constructed with airfoil blades—blades that are shaped like an airplane wing—provides the highest efficiency, consumes minimal power, and reduces operating costs. Airfoil fans are the most common for this application due to their efficiency; however, a backward curved fan may be used occasionally, depending on the application requirements.

Harsh Environmental Conditions 
Fly ash and hot flue gases create a hot and dirty environment for air moving equipment—and the ID fan is right in the middle of this process. To withstand conditions, ID fans should be constructed with wear-resistant materials like heavy-duty carbon steel, with surfaces most susceptible to wear covered with liners or overlay material. Common liner materials include A514 for the wheel liners, tungsten carbide over the blade nose, and A36 for the fan housing liners. 

ID fans must also be able to withstand the high temperatures of the combustion process—from normal operating temperatures to, potentially, short bursts of very high heat in the case of a boiler malfunction.  For example, normal operating temperature might require the fan to withstand 300-400 degrees Fahrenheit (150-205 Celsius) for extended periods of time. A design temperature of least 400-plus is used for stress analysis with ample safety factor; and an excursion temperature specification of 750-plus ensures that the major fan components can survive an unexpected burst of high heat, although the fan should be shut down in the event of a malfunction.
 
Particulate Buildup and Wear
Another factor to consider when specifying fans is the impact of particulate buildup on the fan, which can reduce performance, efficiency and reliability. It is important to note that the buildup of fly ash on the fan is not uniformly distributed and can fall off the blades in uneven patterns, resulting in rotor imbalance and subsequent increased vibration that can cause unexpected downtime. A reputable fan manufacturer will select the appropriate blade geometry to limit particulate buildup, and recommend preventative maintenance procedures to extend the life of the fan.   

Bearings and Instrumentation 
Due to the size of this fan type, hydrodynamic bearings are used almost exclusively. It is not uncommon to see journal diameters up to 14 inches. Circulating oil via a lubrication skid is required to properly lubricate and cool the bearings. In addition, any fan deployment should include bearing instrumentation to help monitor the health of the rotor and bearings.

For example, dual prox probes are placed 90 degrees apart, in an X and Y configuration, and measure relative location of the shaft within the bearing journal. Velometers are used to measure vibration of bearing housing (typically in X, Y and Z directions). Dual thermocouples provide redundancy and, if mounted on opposite sides of the bearing thrust surfaces, an indication of the direction fan is thrusting. The fan vendor should dictate acceptable limits of vibration and temperature for safe operation.

Conclusion
When specifying or upgrading ID fans, it is important to carefully consider the application requirements and constraints. Selecting equipment that can withstand the precise demands of the application will keep operations running smoothly, ensure efficiency and reduce costly unexpected downtime. Partnering with a knowledgeable and experienced industrial fan manufacturer can help you customize the right solution for your application.
 

Author: Doug Jones
Staff Engineer, New York Blower Company
www.nyb.com
djones@nyb.com