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Combustion Processes in the Forest Products Industry
An informal presentationby Kenneth M. Nichols, Weyerhaeuser Company
Presented to Students and Faculty of theUniversity of Washington
at the ME 523 Seminar series – Energy and EnvironmentOctober 20, 2004
Principle areas of energy use within Weyerhaeuser Company
• Pulp and paper manufacture
• Wood products manufacture
– Kraft pulp– Mechanical pulp– Containerboard– Printing papers– Other
– Lumber– Plywood– OSB (Oriented Strand
Board)– LSL (Laminated Strand
Lumber– Composites (particleboard,
fiberboard)– Other
Pulp and Paper
Weyerhaeuser energy data 2003(acquired from http://www.weyerhaeuser.com/environment/sustainability)
Pulp and Paper manufacture
Biomass Fuel (67%)• Recovery Boilers (spent pulping
liquor)
• Power Boilers (wood-waste, sludge, rejects)
Fossil Fuel (25%)• Power Boilers
• Lime Kilns
• NCG Incinerators
Purchased Electricity (7%)
yr
yr
Production Rates
Raw Materials-Wood Fiber• Whole timber or
chips are delivered to the mill
• The logs are cut and debarked
• Debarked logs are chipped in preparation for pulping
Kraft (or Chemical) PulpingIn chemical pulping, the objective is to separate the wood fibers by dissolving the lignin with chemicals and heat.
)
Chemical Pulping• In the digester, wood
chips are mixed with chemicals and cooked with steam to separate the fibers in the wood.
• The material holding the fibers together, lignin, is dissolved in this process. (Most wood contains 18-30% lignin)
Chemical Pulping
Pulping processes are chosen for yield and pulp strength.
– Increased lignin removal reduces yield
– Overly aggressive chemistry degrades the cellulose and reduces pulp quality
PULPING PROCESS
Chipper
Washers
Digester
Precipitators
Pulp
LimeLimestone
Evaporators
Smelt
Clarifier
ChemicalMixing Tank
Black Liquor Tank
Recovery Furnace Green Liquor Tank
TreesSlakerCausticizersWhite
LiquorTank
Typical Recovery Boiler
Liquor Composition:
Carbon 37.2%Na 18.2%Sulfur 3.7%hydrogen 3.5%Oxygen 35.1%Inerts 0.3%Potassium 1.4%Chlorine 0.6%
The Papermaking ProcessChemical Recovery Boiler
• Concentrated black liquor (70% solids) is sprayed into the furnace
• Droplets are formed, which dry, swell, pyrolyze, and burn
Black liquor combustion model
Typical Fiber Sourcing
Recycled Virgin Fiber
Other1%1%
40%40%
59%59%
PULPING PROCESS
Chipper
Washers
Digester
Precipitators
Pulp
LimeLimestone
Evaporators
Smelt
Clarifier
ChemicalMixing Tank
Black Liquor Tank
Recovery Furnace Green Liquor Tank
TreesSlakerCausticizersWhite
LiquorTank
Function of Lime Kiln
• The causticizing process that produces white liquor from green liquor consumes lime (CaO) and produces lime mud (CaCO3) as a by-product.
• The function of the lime kiln is to convert CaCO3 back to CaO for reuse in the causticizing process.
CaCO3 ↔ CaO + CO2
Flame Chains Feed Spirals
Solids Flow
Gas Flow
Dam or Nose Ring
Lime Kiln Zones
PreheatCalcination Drying
Distance Along Kiln (ft)
Tem
pera
ture
(°F)
2500
3000
0 50 100 150 200 250 3000
500
1000
1500
2000
GasSolids
The zones in the kiln are easy to distinguish using the bulk solids temperature profile. The drying zone is the region where the bulk solids temperature is constant, at a value of about 175°F. The bulk solids temperature in the calcining zone is constant at 1600°F, though the surface temperature of the solids can be as much as 500°F higher. The peak flame temperature is near 2800°F, with an exit gas temperature around 400°F.
Bulk Solids
Solids Surface
Flame Chains Feed Spirals
Solids Flow
Gas Flow
Dam or Nose Ring
Wood Products
Weyerhaeuser energy data 2003(acquired from http://www.weyerhaeuser.com/environment/sustainability)
Wood products manufacture
Biomass Fuel (53%)• Boilers (bark, sawdust, sanderdust,
board trim, rejects)
• Burners
Fossil Fuel (23%)• Boilers
• BurnersPurchased Electricity (17%)
Purchased Steam (7%)
Comparison with other WeyCosites
Approximate illustration of current fuel distribution
Desired fuel distribution
Example of tramp air source