María Francisca Gómez-Rico

Interrelation between the kinetic constant and the reaction order in pyrolysis

Abstract A numerical method for calculating the kinetic constants of the thermal decomposition of heterogeneous solids is proposed, considering the interdependence between the pre-exponential factor and the reaction order. This method is applied to the pyrolysis of poly-(ethylene terephtalate) with satisfactory results. The kinetic constants calculated with this method during the optimization goes preferably to parameters with correct physical meaning, and therefore, a lower number of iterations in the optimization calculation system is needed. The interdependence of the pre-exponential factor and the reaction order restricts the possible solutions of the calculations, and produces more valid solutions in the calculation procedure.

PCDD/F and dioxin-like PCB concentrations during municipal solid waste biomethanation and subsequent composting

PCDD/F and dioxin-like PCB concentrations were compared in different samples of a municipal solid waste (MSW) treatment plant: the initial MSW fraction that enters the biomethanation from the digester, the semi-solid digestate obtained after biomethanation of MSW, and the solids after composting the digestate since the final product is destined for land application and special attention must be paid to these compounds for environmental considerations. The initial MSW sample showed low concentrations of PCDD/Fs, although in the biomethanation output sample the concentration was more than ten times higher. The difference was even more significant for PCBs. In compost samples concentrations for both PCDD/Fs and PCBs were in the same range as in biomethanation or lower. Nevertheless, concentrations found for all samples were low and these treatments do not pose a major problem for the environment in the working conditions used.

Volatile organic compounds released from thermal drying of sewage sludge.

Thermal drying of sewage sludge is a treatment to obtain a material that can be used as co-fuel for some chemical industries, but some volatile organic compounds (VOCs) can be emitted and consequently must be considered. In this work the emissions of these VOCs were studied. The sewage sludge produced by a domestic water treatment plant was dried in a simple laboratory scale distillation/evaporation apparatus at different temperatures (80, 90, 100 and 120 oC). The sludge was heated and a flow of air released water and volatile compounds. The water was condensed and separated, and the VOCs were analysed in the resultant gas fraction. More than forty compounds were identified and the total yield ranged from 545 to 591 mg kg dry weight, without significant differences for the temperatures used. The compounds with the highest yields were dimethyl disulfide, toluene, dimethyl trisulfide and 1,2,4trimethylbenzene, with great differences in yields from the rest of the compounds found. Between the compounds identified, several types were found: aliphatic hydrocarbons, aromatic hydrocarbons, halogenated or not, and compounds with sulphur. Since most of the identified VOCs are carcinogenic, toxic or harmful by inhalation, there is a need to control the emitted gases from thermal drying.

Thermal degradation of organic pollutants in sewage sludge

Sewage sludge from a domestic water treatment plant was heated in an air atmosphere at different temperatures (105-500 oC) for 1 hour in order to study the degradation of some organic pollutants. The compounds studied were: linear alkylbenzene sulphonates (LAS) with an alkylic chain of 10-13 atoms of carbon, and nonylphenolic compounds, including nonylphenol (NP) and nonylphenol ethoxylates with 1 and 2 ethoxy groups (NP1EO+NP2EO). These compounds were studied because they frequently exceed the limits proposed for sludge application to land in Europe. The lowest temperature did not show variations with respect to the initial sludge, but at 250 oC the concentrations of LAS and NP were reduced by around 30% and NP1EO+NP2EO disappeared. Logically, at 500 oC all compounds studied were totally removed. The study was completed with a thermogravimetric analysis of LAS, NP and NP1EO+NP2EO separately carried out at different times of treatment and different temperatures. The results showed that an increase of the treatment time at moderate temperatures (<250 oC) reduced the concentrations of NP and NP1EO+NP2EO, which can be interesting since probably the beneficial properties of the sludge for the soil are practically not changed.