A Model of the self-heating mechanismsobserved in stockpiles of biosolids

Rachael AganettiAymeric Lamorlette

Graham ThorpeDominique Morvan

Victoria University, Melbourne, AustraliaAix-Marseille University, France

• Background – Biosolids and the Western Treatment Plant• The Problem – Spontaneous Combustion of Biosolids• The Physics of Self-Heating• Development of diffusive-advective Models• Parametric study on the permeability• Strategies to avoid auto-ignition

OUTLINE

• Located in the West of Melbourne• 10,500ha treatment plant• Half of Melbourne’s sewage treated at

the WTP• 40b Litres of recycled water produced

per year• 30,000 tonnes of Biosolids processed

per year

THE WESTERN TREATMENT PLANT (WTP)

Biosolids Management is regulated by the Victorian Environmental Protection Agency (EPA 2004)

For the highest treatment grade (T1)• Dewater sludge to >10%

w/w solids• Long term storage >3 years• Ensure no recontamination• Does not generate offensive • odours

BIOSOLIDS MANAGEMENT AT THE WTP

SmokeSmouldering

Fire

Elevated temperature during stockpilingduring storage

Loss of production and material

Occupational health and safety hazard

SPONTANEOUS COMBUSTION

Heat Generation• Biological activity• Oxidative reaction• Phase change of moisture Heat Loss• Heat diffusion• Heat advection in the fluid

phase

THE PHYSICS OF SELF-HEATING

P O R O U S M E D I U M

Heat Generation• Biological activity• Oxidative reaction• Phase change of moisture Heat Loss• Heat diffusion• Heat advection in the fluid

phase

THE PHYSICS OF SELF-HEATING

H E A T G E N E R A T I O N

Heat Generation• Biological activity• Oxidative reaction• Phase change of moisture Heat Loss• Heat diffusion• Heat advection in the fluid

phase

THE PHYSICS OF SELF-HEATING

D I F F U S I V E C O O L I N G

Heat Generation• Biological activity• Oxidative reaction• Phase change of moisture Heat Loss• Heat diffusion• Heat advection in the fluid

phase

THE PHYSICS OF SELF-HEATING

A D V E C T I V E C O O L I N G

Heat Generation• Biological activity• Oxidative reaction• Phase change of moisture Heat Loss• Heat diffusion• Heat advection in the fluid

phase

THE PHYSICS OF SELF-HEATING

T H E R M A L R U N A W A Y

EQUATIONS• Momentum:

• Average Energy:

• Oxygen Concentration:

BOUNDARY CONDITIONS• Solid Inlet/Outlet heat flux: Mixed condition• Fluid Inlet temperature: Dirichlet condition• Fluid Outlet temperature: Neumann condition• Solid/Fluid temperature ground condition: • Oxygen inlet concentration: Dirichlet condition• Oxygen outlet concentration: Neumann condition

Boundary condition for the average energy equation?

EQUATIONS AND BOUNDARY CONDITIONS

oxOeffOOO SCDCut

C −∇=∇+∂

∂222

2 2εε

( ) ( ) oxcbiobeffairpeffp SQSQTkTuCt

TC ++∇=∇+

∂∂ 2ρερ

Thqs ∆=ϕ

0=qfϕambientTT =

0=mϕambientOO CC

22=

Adiabatic

( )

−−=

RT

ECAS c

Occox exp12

ερ

( )

−+

−

−=

RT

EA

RT

EA

S cbbio2

2

11

exp1

exp

1 ερρ

TTT sf ==

ρ∂u∂ t

=−μκ uϵ−∇ p−ρ gβΔT

NUMERICAL SIMULATIONS : typical pile at WTP

H

L2θ

Fixed dimensions• H=5m• L=20m• Ө=18° Tested permeabilities● Κ=[10-11;10-7]m2

Critical permeability?

RESULTS : Temperature and velocity fields

K=10-11 m2

RESULTS : Temperature and velocity fields

K=10-7 m2

RESULTS : Critical permeability

CONCLUSION : strategies to avoid auto-ignition

• For WTP typical stockpiles, diffusive cooling is not sufficient to avoid ignition

• Buoyancy driven flow allows to avoid ignition for permeabilities above a critical value (depending on the pile geometry)

• Biosolid particle size managment theoretically allow to increase pile permeabilities (Moment theory adapted to the Kozeny-Karman law) to reach the critical value

• Forced convection : effect of external wind on the pile internal flow ?