Oxygen Enhanced Biomass Combustion

Paper from the AFRC 2014 conference titled Oxygen Enhanced Biomass Combustion by Y. Xue.

The use of biomass as an energy source is getting more attention not only as a renewable energy source but also for reducing the CO2 foot print of electrical energy production. One of the primary problems with utilizing biomass, in particular low grade biomass, in industrial processes is flame stabilization and burnout of the fuel. Biomass particles cannot be economically ground to the fine particle size typical of coal combustion, creating problems for conventional burners, such as biomass flame instability and incomplete combustion. Air Liquide/BYU is developing an oxy-enhanced biomass combustion technology that can improve biomass combustion performance by introducing oxygen to the burner without substantial geometric changes. The impact of oxygen injection in a swirl-stabilized burner on flame ignition and particle burnout was investigated by measuring LOI, flame imaging, flame intensity, and flame temperature. The results showed that a small amount of oxygen enrichment (less than 25% overall oxygen enrichment) can stabilize the biomass flame and produce ash closer to complete combustion. NO emission were also monitored and oxygen injection can be used to improve flame stabilization and burnout without increasing NO formation.

PUBLIC Oxygen Enhanced Biomass Combustion Yuan Xue, Hwanho Kim, and Kenneth Kaiser Air Liquide, Delaware Research and Technology Center, Newark, DE 19702 Steven Owen and Dale Tree Mechanical Engineering, Brigham Young University Provo, Utah 84602 2 2014 Research & Development PUBLIC World leader in gases, technologies and services for Industry and Health Introduction An inexpensive way to reduce net CO2 emissions is to displace the combustion of fossil fuels with biomass Technical challenges with cofiring Pulverizing to a size similar to coal is expensive Large particle size affects flame ignition and burnout Studies have shown oxygen enrichment can increase burnout and mass loss rates [Luo 2009, Fang 2006, Daood 2011] A possible drawback: increased probability of NO formation 2 3 2014 Research & Development PUBLIC World leader in gases, technologies and services for Industry and Health Objective Local oxygen enrichment to promote biomass flame characteristics Promote flame ignition and stabilization Increase particle burnout without significantly increasing NOx emissions Potential for increasing firing rate of biomass fired boilers/furnaces with oxygen boosting (gas volume impact) by improving burnout and keeping NO emission level 3 4 2014 Research & Development PUBLIC World leader in gases, technologies and services for Industry and Health Combustion Facility at BYU (Provo, Utah) 4 Pilot scale test facility: 150 kWth down-fired Burner Flow Reactor (BFR) Limited combustion space Bulk Bag Unloader Twin Screw Feeder BFR with view ports 5 2014 Research & Development PUBLIC World leader in gases, technologies and services for Industry and Health Burner Description Pipe-in-pipe geometry with three channels Flexible swirl control Movable block swirl generator S = Gθ / RGz where Gθ is axial flux of angular momentum, Gz is axial flux of axial momentum, R is burner radius Zero swirl Mediumswirl Maximum swirl Movable block Flow Geometry 2 1 2 h R R S B R                2 1 1 cos( ) 1 tan( ) tan 2 cos( ) 1 tan( ) tan 2 2 sin( )                                          m m m z               (1) 6 2014 Research & Development PUBLIC World leader in gases, technologies and services for Industry and Health Wood Pellets Analysis 6 Proximate analysis (wt%) Wood Pellet Bituminous Coal Ultimate analysis (wt%) Wood Pellet Bituminous Coal Moisture 4.91 3.34 Carbon 49.85 73.81 Volatilematter 78.67 34.64 Hydrogen 5.42 4.12 Fixed carbon 15.93 55.97 Oxygen 38.9 10.63 Ash 0.47 6.05 Nitrogen 0.06 1.15 Sulfur 0.37 0.9 Heating value (Btu/lb) 7,942 13,353 7 2014 Research & Development PUBLIC World leader in gases, technologies and services for Industry and Health Wood Pellet Particle Distribution Wood pellet particle size much larger than pulverized coal Impact the flame ignition and stabilization Impact the burnout of the flame • Reduced energy efficiency • Potential accumulation on down stream equipment 0 10 20 30 40 50 60 70 80 90 100 0.1 1 10 100 1000 Cumulative Volume Percent Diameter (microns) Particle Size Distribution (PSD) Coal PDS Wood Pellet 8 2014 Research & Development PUBLIC World leader in gases, technologies and services for Industry and Health Experimental Setup 8 mf mf cf W W W LOI  (%)  Ash was collected by a cyclone, which catches particles 2 μm and above Loss on ignition measured by test method ASTM D7348 9 2014 Research & Development PUBLIC World leader in gases, technologies and services for Industry and Health Experiment Air combustion case as baseline Strategic oxygen local enrichment (less than 3%) to maximize the benefits Injection locations Oxygen velocities and momentum Constant equivalence ratio and flow field 10 2014 Research & Development PUBLIC World leader in gases, technologies and services for Industry and Health Results - Oxygen Enrichment with Constant Equivalence Ratio Local oxygen injection promotes early flame ignition. The flame luminosity (temperature) increased in the near burner zone For burnout and NO emission: NO level has moderate increases with oxygen enrichment, the impacts is negligible when the swirl number is below 0.8 LOI data shows oxy enrichment has a minimal impact on biomass burnout 11 2014 Research & Development PUBLIC World leader in gases, technologies and services for Industry and Health Results - Oxygen Enrichment with Constant Flow Field Local oxygen injection promotes early flame ignition. The flame luminosity (temperature) increased in the near burner zone Local oxygen enrichment significantly improvedwood pellet flame: NO emission increased slightly when the oxygen concentration is less than 23%, but increased more when the oxygen concentration is higher than 23% LOI results showed the burnout improved with local oxygen enrichment 12 2014 Research & Development PUBLIC World leader in gases, technologies and services for Industry and Health Conclusion and Next Step Biomass flame was promoted by local strategic oxygen injection with only 3% oxygen overall enrichment Flame ignition was promoted Flame luminosity (temperature) was increased in the near burner zone Particle burnout was improved by local oxygen enrichment Promote combustion efficiency of biomass fuels Reduce impact of the unburned particle accumulation on the downstream equipment NO level can be controlled Increased for certain conditions (mostly less than 20%) Depending on operating condition and oxygen enrichment level Next step Improve burnout performance while minimizing NOx impact at constant equivalence ratio by adapting O2 injection technology 13 2014 Research & Development World leader in gases, technologies and services for Industry and Health PUBLIC THANK YOU FOR YOUR ATTENTION Follow @AirLiquideGroup www.airliquide.com