Using onsite wood wastes and sludge Energy for Industrys 12MW Heat Plant at WPI Residues to Revenues

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Using on-site wood wastes and sludge Energy for
Industrys 12MW Heat Plant at WPIResidues to
Revenues 2005Mike Suggate
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Contents

• Project background
• Technologies adopted, and why
• Fifteen months on - some of the lessons

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WPIs project drivers

• High and uncertain energy costs
• Reliance on LPG
• Obsolete and unreliable bark burner
• Disposal of 20,000t pa pulp sludge
• Need to reduce costs

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The commercial relationship

• WPI/EFI energy partnership
• Heat supply
• 12 MW thermal fluid heater
• Commissioned June 2004
• Long-term electricity supply contract
• Energy efficiency initiatives
• Contractually committing EFI to deliver services
  via a BOOT contract

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Thermal energy

• Previously
• Pre-heaters - 3 MW
• Bark burner - 4 MW
• LPG - 6 MW
• Total - 15 MW
• Project objective To minimize LPG use, utilize
  waste
• Pre-heaters - 3 MW
• Heat plant - 810 MW
• Balance LPG

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Available fuels

• Woodwaste
• Bark from site de-barker
• Imported sawdust, other wood wastes
• Chipped forest residue
• Stockpiled waste
• Pulp sludge
• Mix of biological and mechanical

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Pulp sludges

• Expensive to landfill, new resource consent
  difficult
• Screw presses achieve 60 - 75 moisture content
• Additional drying stage required
• At 50 moisture content CV 10 GJ/tonne
• Clinkering was major risk
• Alkali metals (sodium) from bleaching process

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Heat plant selection criteria

• Fuel is the main driver
• Other considerations
• Reliability
• 15-year commitment, performance guarantees
• Life-cycle costs
• Re-use of existing equipment
• Primary heat plant options considered
• Fluidised bed
• Grate

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Fluidised bed combustion

• Ideal for wet fuels (65 max)
• Thermal mass of sand bed gives stable combustion
• Generally less risk of clinkering
• More complex and expensive
• Operating costs higher sand and fluidising
  energy

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Grate combustion

• Available in smaller boiler sizes
• Vibrating pin hole or moving grates
• Fuel burns on grate, in suspension
• Lower capital operating costs
• More sensitive to changes in fuel quality
• Biomass moisture limit around 60

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The solution adopted

• Easteel heater with...
• Babcock and Wilcox vibrating grate technology
• Detroit stoker
• Intech thermal fluid coil
• Flue gas recirculation
• Low gas velocities
• Waste heat recovery for sludge drying
• Disc type sludge dryer
• Upgrades to WPIs sludge treatment plant
• Substantial upgrades to existing fuel supply
  systems

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The Heat Plant
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The process
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Wood screening hogging
Steam to sludge dryer
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Existing bark silo
Sludge surge bin and conveyor
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Heat plant
Sludge dryer
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Thermal fluid to flash dryers
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Why a vibrating grate?

• Track record with NZ pinus radiata, bark and
  sawdust
• US reference site burning pulp sludges
• Clinkering risk manageable
• Acceptable performance on wet fuels,
• With pre-heated under-grate air
• Fluidised bed technology was discounted
• Tests on WPI fuels indicated high propensity to
  clinkering
• Complex and more expensive

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Fuel challenges

• Very wet
• High levels of fines
• Variability
• Alkali metals
• Rock contamination
• Blending issues
• Abrasion

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Effect of high moisture

• Leads to low efficiency, unstable combustion
• Plant more expensive
• Excess moisture must be heated and vaporised
• EFI guarantees
• MCR at 62 moisture
• Can burn up to 65
• May require supplementary fuels
• Coal, oil

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Fuel sizing

• Size is important
• Stable combustion requires good ash bed on grate
• Fines
• Burn above grate in suspension
• Carry over and block fly-ash systems
• Mixing/blending is a key
• Wet and fine fuel is the killer

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Waste sludge to fuel

• The practice of landfilling is ending
• Solution - convert sludge to fuel
• Optimising WPI treatment plant performance
• Doubling dry solids density
• Installing disc dryer to dry sludge to gt 50
  solids
• Specific design of combustion systems

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Fuel handling

• Upgrades included new hogger, screens, conveying
  systems
• Inherited silo
• Screws and conveyors
• Very high erosion rates from pumice, rock
• Trialling different materials for flights on
  screws
• Continuing excessive wear

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Ash-handling fuel contamination

• Pumice abrasion was anticipated
• Rocks were not and remain a major issue
• Re-engineering/replacement
• Grate ash conveyor
• Fly-ash rotary valves
• Ongoing grate/refractory damage
• Treatment at source/prevention is work in
  progress

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Combustion air

• Problems were
• Performance spec not met
• Combustion unstable
• Fine ash/fuel blown through heater
• Redesign for
• Less but hotter primary air under grate
• More secondary air and turbulence
• Results
• Transformed plant performance
• On fine, wet fuels

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Plant performance

• Capable of greater than 12 MW
• MCR on 62 moisture fuel
• Average load higher than anticipated
• Stable, essentially unattended, operation
• Efficiency tested at 55, on 61 wet fuel
• Guarantee is 48, on 62 wet fuel
• Availability guarantees
• First year gt90
• Subsequent years gt95

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Ongoing projects

• Supplementary coal systems
• Fuel - blending and quality improvement
• Rock removal, or control
• Abrasion-proofing high-wear plant items
• Reinstatement of steam pre-heaters
• Plant optimisation ongoing

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How do WPI view the project?

• Reduction in LPG consumption by gt3 million litres
  pa
• All sludge burnt - avoided land filling
• Technical issues, but these resolved
• New plant without capital outlay
• Performance guarantees achieved
• Benefits of EFI and Easteel expertise realised
• Outsourcing of project risks has paid off

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In conclusion

• A very difficult technical application
• The plant had a range of teething problems
• Now achieving project goals, guaranteed
  performance
• Main residual issue is fuel-caused abrasion
• Good initial design eliminated potential
  problems
• Performance on wet/fine fuel
• Slagging, clinkering
• Drying and burning sludge
• Key to the projects success a close working
  partnership between Energy for Industry, WPI and
  Easteel

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Contact us P 04 381 1330 W www.energyforindustr
y.co.nz