Cofiring of Coal and Biomass Under Oxycombustion Conditions:

NOx Formation and PM Characterization

Ben KumferScott Skeen

Richard Axelbaum

Laboratory for Advanced Combustion & Energy ResearchDept. of Energy, Environmental & Chemical Engineering,

Washington University in St. Louis, MO

Oxy-Fuel with Carbon Capture

Cofiring Biomass:

•Potentially Carbon negative means of power generation

•Utilizes renewable fuels

biomass

CO2

[CO2] > 95% (dry)

air separation

unit

purification

compression

coal

FGR

geo -sequestration

O2

N2air

• High CO2 concentration in flue gas

• Parasitic load for air-separation

• Large reduction in NOx can result through RFG

• Potential for improved boiler efficiency

Conventional vs. Oxy-Combustion

Conventional Coal/Air Combustion

air

coal+airPrimary

Oxy-Coal Combustion

air

Secondary

Secondary

O2+RFG

fuel+RFG+O2

Secondary

Secondary O2+RFG

Primary

Questions:

Is there an optimum set of PO and SO compositions for NOx?

What is the influence of oxy-combustion on ash properties?

What are the consequences of cofiring biomass under air-fired and oxy-combustion conditions?

30 kW, Horizontally-Fired Test System

4

Fuels:•Coal: Powder River Basin, HV = 30 MJ/kg•Waste Sawdust, HV = 22 MJ/kg

Instruments (Thermo Inc):• 100:1 Dilution probe system for gas sampling• CEM w/ chemiluminescence NOx analyzer• Dekati ELPI particle analyzer

ELPI

diffusion dryer

Fuel Properties

5

coal

10 µm

10 µm

sawdust

Air-Fired Results: Variable Primary Stoichiometry

stO

POO

stair

POairPO m

m

m

m

,

,

,

,

2

2

coal+PO

SO

SO

Constant PO Flow Rate Constant Thermal Input

axial tangential (full swirl)

SO flow:

Effects of Variable Secondary Swirl

coal+PO

SO

SO

Air-fired vs. Oxy-combustionRun conditions: synthetic oxidizer: 30%(v) O2, 70% CO2, in PO and SO 3%(v) O2 in exhaust 30 kW

= 5.2 m3/hr

SO_tangential / SO_total

Variable [O2] in PO and SO

30 kW 3%(v) O2 in exhaust

constant total O2:CO2

coal+PO

SO

SO

axial SO

tangential SO

tangential SO

Sawdust Cofiring

sawdust sieved 20 mesh

sawdust sieved 50 mesh

Summary: NOx

Nitric oxide emissions are strongly dependent on flame attachment

Flow rate reductions required under oxy-fuel conditions can dramatically influence flame hydrodynamics and thus NOx

An optimum PO & SO composition was observed

Fuel particle size is an important consideration for NOx when cofiring with biomass.

PM Size Distribution & Morphology

5.0 µm

100% coaldpaero = 5.18 µm

1.0 µm

cofired, (50% wt. sawdust)dpaero = 330 nm

5.0 µm

cofired, (50% wt. sawdust)dpaero = 5.18 µm

Elemental Analysis: Oxycoal

Elemental Analysis: Cofired

Effect of Temperature

• Oxidizer is a mixture of O2 and CO2

• Formation of submicron PM increases with O2 concentration.

• This effect is due to increased combustion temperature.

Acknowledgments

Consortium for Clean Coal Utilization at Washington University http://www.c3u.wustl.edu/

DOE UCR Program

Ameren UE

Extras

16

Base/Acid ratio, an indication of slagging propensity, is higher in submicron PM.

B/A = (Fe2O3+CaO+MgO+Na2O+K2O)/(Al2O3+SiO2+TiO2)

Effect of Temperature

Critical PO Stoichiometry

• No SO Swirl

• 40 vol.% O2 in PO

• 28 vol.% O2 in SO

• Vary thermal input (i.e. vary λPO)

coal+PO

SO

SO