Unsteady 2d coupled heat and mass transfer in porous media with biological

International Journal of Heat and Mass Transfer 52 (2009) 5841–5848

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International Journal of Heat and Mass Transfer
journal homepage: www.elsevier.com/locate/ijhmt

Unsteady 2D coupled heat and mass transfer in porous media with biological and chemical heat generations
N.O. Moraga a,*, F. Corvalán b, M. Escudey c, A. Arias c, C.E. Zambra a
aDepartamento de Ingeniería Mecánica, Universidad de Santiago de Chile, Alameda, 3363 Santiago, Chile Departamento de Ingeniería Geográfica, Universidad de Santiago de Chile, Alameda, 3363 Santiago, Chile c Departamento de Química, Universidad de Santiago de Chile, Alameda, 3363 Santiago, Chile
b

a r t i c l e

i n f o

a b s t r a c t
The coupled two-dimensional heat and oxygendiffusion in a compost pile of sewage sludge obtained from domestic wastewater treatment was studied. The unsteady, coupled, non-linear mathematical model, solved through the finite volume method, includes the volumetric heat generation caused by the action of aerobic bacteria and by the oxidation of cellulose in a porous media. The numerical simulation allows the prediction for the pile shape and sizeeffects on the heat generated and on oxygen consumption. Temperature and oxygen concentration transient distributions within compost piles are presented for different geometries. Heat (temperature) and mass transfer (oxygen) results indicate that the pile height has an important effect on the heating process and that keeping the compost pile height lower than 1.7 m the self-combustion of sewagesludge can be avoided. Ó 2009 Elsevier Ltd. All rights reserved.

Article history: Received 10 February 2009 Received in revised form 31 July 2009 Accepted 31 July 2009 Available online 18 September 2009 Keywords: Auto-ignition Self-heating Compost pile Finite volume simulation Heat and mass transfer diffusion

1. Introduction Sewage sludge is the inevitable end product of municipal wastewatertreatment processes worldwide. For temporary disposal, the sewage sludge can be accumulated in compost piles. Municipal solid waste landfills often develop scenarios of self-heating causing negative environmental impacts by odors, gas generations and smoke production. Auto-ignition and resulting fires at landfill has been a non-desirable outcome in compost piles worldwide [1,2]. The conditionsleading to spontaneous combustion in selfheating systems were revised for Buggeln and Rynk [3]. This sludge has particular biochemistry characteristics (organic carbon and micro-organisms) that combined with the compost process originate a very complicated system to analyze. Chemical reactors have been proposed to study energy generation by carbonaceous materials [4]. Numerous efforts have been madefor describing non-equilibrium systems using different strategies. One of the methods used in three studies is the statistical thermodynamics [5–7]. The thermodynamic coupling is not considered in this study. The use of a mathematical model coupling heat and mass transfer along the use of finite volume simulations is the way used in this work to investigate the diffusion in the compost pile and theauto-ignition processes. Finite differences, finite volumes and finite elements methods have been used along with
* Corresponding author. Tel.: +56 2 718 31 10; fax: +56 2 681 24 31. E-mail address: nelson.moraga@usach.cl (N.O. Moraga). 0017-9310/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2009.07.027

different mathematical model in theprediction of the fluid dynamics and convective heat transfer in enclosures with porous media [8–21]. Exothermic chemical reactions in porous media has been investigated by using numerical methods [22,23]. Effect of concentrations and temperature on the coupled heat and mass transport in liquid mixtures was studied for Demirel and Sandler [24]. The self-combustion phenomena originated by internal heat...