Sewage Sludge Self-Heating and Spontaneous Combustion. Field, Laboratory and Numerical Studies

Abstract

Self-heating in compost piles results in unsuitable doors, smoke production and fires, having strong negative environmental impacts. Field, laboratory and numerical studies were accomplished trying to reproduce and understand the conditions where selfheating and combustion may take place. Inside a compost pile, built from solid obtained after municipal waste water treatment, oxygen, methane, carbon monoxide and carbon dioxide concentrations and temperature change with time and in depth. Electrical conductivity and pH showed only sligth changes. In field piles temperature increased with time until get a maximum about 90°C. While no spontaneous combustion was observed after 6 months in field experiments, in laboratory studies carried out in a closed bottom cylinder, self-ignition was observed and a maximun temperature of about 400 °C was reached.Numerical simulations describe the field piles internal heat generation, concluding that a minimum of 1.8 m height is required to reach a self-ignition condition.

Review on Self-Heating of Biomass Materials: Understanding and Description

“…Both the biological activity of microorganisms and chemical oxidation of cellulosic materials were considered and the diffusion of oxygen in the pile was also coupled. The model was applied to predict self-heating in compost piles of sewage sludge and the critical conditions and pile temperature and configuration for spontaneous ignition. , The model was extended by Zambra et al to include the heat effect of water vaporization and consider the effects of moisture on both the biological activity and chemical oxidation and by Luangwilai and co-workers to incorporate the effects of convective heat and oxygen transport, , the effects of the moisture on the biological activity and chemical oxidation, and the water evaporation and condensation in the pile. , The developed model was recently applied to describe the self-heating process in stockpiles of wood bark and evaluate the impact of humidity change on self-heating and spontaneous ignition …”

“…The state-of-the-art numerical models ,, theoretically describe the transport processes involved in the self-heating process and are therefore able to fully cover the process factors including pile size and configuration, ambient conditions, , and air flow inside and outside the pile . However, to cover the reaction and material related factors requires characterizing the properties and especially the kinetics of different biomass materials. , In particular, the submodels for the heat sources, that is, microbial activity and chemical oxidation, are still not so mechanistically based.…”

Sewage Sludge Disposal and Applications: Self-heating and Spontaneous Combustion of Compost Piles - Trace Metals Leaching in Volcanic Soils After Sewage Sludge Disposal

“…The iterative procedure ended at each time step when the maximum difference between iteration for φ =T,C ox satisfied, at each control volume, the convergence criteria (Escudey et al, 2008). Data obtained from numerical calculations were plotted considering daily output at 12:00 a.m.…”

“…These assumptions are incorporated in the second term on the right-hand side of the Equation (2). Details in the formulation of the term representing the heat generated by the biomass have been given by Chen and Mitchell (Chen & Mitchell, 1996) and some parameter values used in the mathematical model may be obtained from the literature (Sidhu et al, 2007;Escudey et al, 2008;Moraga et al, 2009). …”

Experimental study of torrefied wood fixed bed: Thermal analysis and source term identification

“…Self-heating is defined as a spontaneous phenomenon deriving from an unintended accumulation of heat within a reactive material. Several carbonaceous materials are affected by this situation involving mainly coal [9][10][11], lignocellulosic biomass [12,13] (hazelnut shells, olive pomace, wood [14][15][16][17]) and wastes (compost [18], sewage sludge [19]). Self-heating may occur as a consequence of chemical, physical or microbiological exothermic processes at low temperature.…”