Researchers of the Institute of Chemical Engineering Fundamentals and Plant Engineering at the Technical University of Graz in Austria have been using FLUENT to optimize the design of an industrial biomass furnace. Biomass fuels, such as wood, are a popular alternative in countries that are rich in this resource. Use of biomass fuels in place of fossil fuels, such as coal and oil, leads to reductions in CO2 emissions, so the design and optimization of biomass furnaces have received increased attention during the past few years.

The Graz researchers optimized the geometry and investigated the influence of air staging and flue-gas recirculation on the combustion process of a recently designed low-NOx bio-mass grate furnace with a nominal boiler capacity of 550 kW (thermal). They used simple empirical models for estimating mass and heat flux boundary conditions at the biomass fuel bed on the grate surface. The biomass furnace under investigation has a primary combustion zone above the fuel bed that extends to an array of secondary air nozzles. The flue-gas has a residence time of 0.6 - 0.8 seconds in this zone, where the lean combustion conditions give rise to NOx reduction. The secondary combustion zone immediately after the nozzle array runs as an air-rich burnout zone. Flue-gas recirculation is used in the furnace for temperature control. Depending on the operating conditions, it can be fed into the secondary combustion zone and/or into the primary combustion section below the grate.

Several furnace designs and operating conditions were tested. The temperature and flue-gas composition were tracked through both the primary and secondary combustion zones to assess the performance of each furnace design. In addition, the arrangement of nozzles was varied to optimize the highly turbulent mixing of secondary air (and/or recirculated flue-gas) with the combustible gases. The primary air ratio in the furnace was varied, depending on the grate system and biomass fuel used, which can vary from bark to wood chips. The Graz engineers also investigated varying the total flue-gas-recirculation ratio, selective injection of recirculated flue gases, and the variation of the amount of excess air.

Biomass fuel bed

It was concluded that, despite some simplifications in the CFD physical model inputs, heat production and environmental performance of the furnace were successfully optimized with the CFD simulations that were performed. Based on the simulations, a first prototype of the biomass furnace has been built, and is now undergoing testing.

The authors of this R&D work were selected as winners of the Poster Award for the topic Biomass Production and Utilization R&D - Combustion at the First World Conference and Exhibition on Biomass for Energy and Industry, June 2000, Seville, Spain.

Contours of CO concentration in the flue gas on selected planes before and after the secondary air/fuel nozzles

References:

1. R. Scharler, I. Obernberger: Numerical Modeling of Biomass Grate Furnaces, Proceedings of the 5th International Conference on Industrial Furnaces and Boilers, April 2000, Porto, INFUB (Ed.), Rio Tinto, Portugal.
2. R. Scharler, I. Obernberger, G. Längle, J. Heinzle: CFD Analysis of Air Staging and Flue Gas Recirculation in Biomass Grate Furnaces, Proceedings of the 1st World Conference and Exhibition on Biomass for Energy and Industry, June 2000, Seville, Spain.

For more information, contact:
Robert Scharler,
Institute of Chemical Engineering Fundamentals and Plant Engineering, Graz,
Technical University of Graz,
A - 8010 Graz, Inffeldgasse 25,
Austria,
Tel.: +43 (0)316 481300 33,
Fax: +43 (0)316 481300-4,
E-mail: scharler@glvt.tu-graz.ac.at
Web: http://vt.tu-graz.ac.at/bios

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