Reduction of Nitrogen Oxide Emissions from Wood Chip Grate Furnaces

• Research unit: EU RFP
• Project number: 95.0859-1

Key words (English)

Wood combustion; NOx reduction; air staging; fuel staging; flue gas recirculation

Alternative project number (English)

EU project number: JOR3-CT96-0059

Research programs (English)

EU-programme: 4. Frame Research Programme - 5.1 Nonnuclear energies

Short description (English)

See abstract

Partners and International Organizations (English)

Tiba-Müller AG, Joanneum Research (A), Chalmers University (S), TU Wien (A),VTT Energy (SF), Abo Akademi (FIN), Kvaerner (S), Universität Zaragoza (E)

Abstract (English)

Air staging and fuel staging have been applied as primary measures for NOx reduction in automatic wood furnaces. Air staging was investigated in a prototype low NOx furnace which was additionally equipped with measures to decrease the temperature in the reduction zone and with a process control device to adjust the primary excess air. To investigate the potential of fuel staging, a laboratory furnace with two independent under stokers in series was designed and realised. For high nitrogen content in the fuel and thus for high NOx emis-sions, a higher reduction rate was achieved than for low nitrogen content. The main process parameters in the reduction chamber are the excess air ratio, the temperature, the mixing quality and the residence time.
With air staging at optimum conditions, app. 50% reduction can be achieved for native wood and app. 75% for chipboard. Optimum conditions are: a slightly under stoichiometric excess air ratio in the reduction zone and a reduction temperature of app. 1'100 0C - 1'200 0C with a residence time of app. 0,5 5. The average reduction rate in practice can be reduced signi-ficantly due to non ideal conditions and/or varying operating parameters. To enable an opti-mum temperature for different fuel humidities, measures are needed to influence the temperature. Flue gas re-circulation and partial heat extraction in the primary chamber have been applied to lower the temperature. The amount of re-circulated flue gas can be adapted in function of the temperature to control the temperature in the reduction zone. Since the highest NOx reduction is achieved at the lowest possible overall excess air, CO/Lambda control with setpoint optimization was applied to ensure optimum conditions tor NOx reduction and to increase combustion efficiency.
Fuel staging shows an even higher potential of NOx reduction than air staging. Furthermore it seems to be less sensitive to the excess air ratio in the reduction zone and it reaches a significant NOx reduction at temperatures app. 200 0C lower than air staging. Since app. 25% to 35% of the fuel has to be fed as secondary fuel, two feeding systems are needed.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 95.0859-1