Biomass Combustion

With increased emphasis on renewable, low CO2 emitting power generation, biomass combustion has become has become a growing segment of fuels market. Biomass combustion presents unique challenges for boilers, especially in plants where coal combustion systems are modified to utilize biomass such as waste wood or bark.

Biomass is often of varying composition, moisture, and heat content, making combustion control difficult. The usual result is operation with increased excess air levels for stable combustion. Add to that the generally lower Btu levels in biomass (relative to coal), and primary air fans are often required to operate continously at maximum capacity, while steam output is derated.

Combined OFA, FGR, and SNCR on Wood Burning Stokers

In 2008, Steag Energy Services (formerly ESA) designed and installed combustion modifications and NOx control systems on two stoker boilers at Xcel's Bayfront station in Ashland, WI. The twin stoker units are capable of firing wood or coal. Installation of Steag Energy Services's (formerly ESA) technologies was part of Xcel Energy's strategy to convert the entire station exclusively to biomass combustion.

The combustion modifications included an overfire air system to reduce NOx emissions, a flue-gas recirculation system to stabilize conditions at the stoker grate, and sweep air modifications to improve fuel delivery. A multi-zone urea SNCR system with auto-retract injectors was also installed on each unit to further reduce NOx. Steag Energy Services (formerly ESA) used CFD modeling in the design of the OFA, FGR and SNCR systems.

The extreme winter temperatures of northern Wisconsin also presented significant challenges in the design of a urea SNCR system for year-round operation. A urea storage facility was installed that included a 10,000 gallon heated urea storage tank, as well as an indoor unloading area for the delivery truck. A heat-traced urea transfer and circulation system tied the urea storage facility to the SNCR process skid.

The limited space in the boiler house presented a considerable challenge to the installation of the OFA and FGR fans and related duct work and dampers. FGR fans were installed on the rooftop; OFA fans under the boilers. Steag Energy Services (formerly ESA) provided startup commissioning, tuning, optimization testing, and operator training for each new system component to insure their smooth integration into full-time operation.

Combustion Modifications for Flyash Control on Bark Burning Stokers

In 2006, Steag Energy Services (formerly ESA) designed combustion modifications on a bark burning stoker boiler at Boise Cascade's mill in DeRidder, Louisiana. The stoker unit had been upgraded to increase the steam production capacity, but was facing issues achieving the expected load due to increased high carbon flyash carryover. Steag Energy Services (formerly ESA) was selected to design combustion modifications to improve the boiler combustion to allow increased bark firing.

The combustion modifications included redesign and replacement of the overfire air system to complete the bark combustion. A flue-gas recirculation system was incoporated to stabilize conditions at the stoker grate. Modifications to the sweep air system were recommended to improve fuel delivery.

Analysis of Combined Biomass, Coal, and Natural Gas Combustion for Process Kilns

In 2007-09, Steag Energy Services (formerly ESA) conducted computer simulations of the combustion of biomass, coal, and natural gas in a rotary kiln for US Steel. Flame shape and heat delivery to processed iron ore was studied under various co-firing combinations of the three fuels.