Biomass Conversion & Cofiring Solutions
Modern combustion processes are required to be fuel-flexible and accommodate variable fuel diets to ensure cost effective power generation. Substituting carbon neutral fuels, such as biomass for coal, reduce carbon dioxide (CO2) emissions, a significant greenhouse gas. However, there are many complexities to biomass combustion, as gas phase temperature distribution changes and ash-melting temperatures may decline, potentially leading to slagging, fouling, corrosion issues and loss of MWt output. Mobotec is a world leader in full biomass conversion as well as cofiring – our experience and expertise has been critical to the success of many projects around the world.
Core Competences in Biomass Conversion and Co-firing
Mobotec prioritises several commercial objectives: combustion efficiency and minimal thermal derate, ensuring regulatory emissions compliance, and providing expertise on fuel quality and its impact on combustion efficiency and balance of plant.
Mobotec’s core competences in biomass co-firing and conversion include:
- Fuel handling evaluation
- Biomass burner analysis and replacement
- CFD in-furnace combustion modelling
- NOx abatement by ROFA and/or Rotamix
- Furnace exit gas temperature evaluation and impact on steam side thermal efficiency analysis
- Slagging and fouling evaluation
- Superheater/reheater corrosion evaluation and control strategy
- PM control evaluation
The ability to convert a boiler to partial or full biomass firing capacity is heavily dependent on the ability to predict how the furnace combustion system will operate with the alternative fuel. The core of Mobotec’s capabilities focuses on a facility-wide evaluation intended to assess the suitability of the boiler and associated equipment for a conversion of this type, and to identify what modifications might be required. Our core competence in fundamental combustion system analyses allows us to resolve many combustion dynamic challenges, and our engineering expertise can be called upon to conduct a system wide feasibility assessment of the balance of plant. By determining an optimised firing configuration, identifying fuel size distribution, predicting effects of heat transfer changes, and establishing potential for slagging, fouling and corrosion, Mobotec evaluates and manages the most important aspects of a conversion.
Our experience with numerous biomass conversion projects allows Mobotec to assist clients in selecting which boilers are best suited for biomass conversion with the least amount of equipment modification. Mobotec has many successful biomass conversion projects in operation, ranging from biomass cofiring with coal to 100% biomass conversions of pulverised coal and grate boilers.
Burner Retrofit or Replacement
When converting to biomass, existing coal burners need to be evaluated, and retrofits or replacements will be eventually needed to meet biomass burning configuration requirements. Biomass flames are substantially different from coal flames, and can become detached when directly switching from coal to biomass, as indicated by CFD simulations below.
Above: CFD modelling of coal-to-wood conversion for a 500 MW wall-fired furnace. Modelling shows significant combustion differences between fuels and burner settings.
In partnering with burner experts, Mobotec has the full capability to evaluate coal burners to meet modifications for biomass cofiring or conversion needs. The following aspects are evaluated:
- Biomass transport pipe air flow and nozzle size
- Biomass burner tip design
- Elimination of existing primary air
- Secondary and tertiary air flow and nozzle resize
- Secondary and tertiary air nozzle spin van design
And the evaluation will focus on the following issues:
- Burner tip erosion
- Biomass particle ignition control and flame stability
- Near-burner flame temperature control and NOx control
Combustion Optimisation and NOx Abatement
The Mobotec Rotating Opposed Fire Air (ROFA) System has proven to be essential for any conversion effort of pulverised coal (PC)/Stoker coal-fired boilers to biomass firing. The ROFA System relaxes some of the fuel requirements with its ability to control the combustion process to match biomass combustion dynamics to the limitations imposed by existing furnace design.
The ROFA System is a state-of-the-art and comprehensive combustion improvement solution that injects air through an array of specially designed nozzles, breaking up stratified gas flow and providing excellent mixing and heat distribution in the boiler. The ROFA system increases the volume of the unit available for combustion, which results in superior mixing of the volatile species released from the biomass and better control of boiler furnace temperature distribution. By controlling oxygen distribution and residence time, the Mobotec system provides more complete combustion of biomass char particles and lowers NOx emissions.
Depending on the application, the ROFA system will:
- Minimise disruption to combustion dynamics resulting from fuel variability
- Accommodate and optimise low NOx burner dynamics
- Vastly improve burner zone flame stability
- Significantly reduce unburned carbon in ash
- Accommodate flexible fuel injection locations in the furnace to control steam temperature
- Minimise unit thermal de-rates as well as reduce CO2, NOx, and other flue gas emissions