Combustion of a substitution fuel made of cardboard and polyethylene: influence of the mix characteristics--experimental approach

摘要

This article presents an experimental study of the combustion of substitution fuels elaborated from compressed mixes of cardboard and polyethylene (PE). These components are representative of two classical classes of waste materials: materials derived from wood and plastics. The combustion of these fuels has been experimentally characterized in terms of combustion rate, and quantity of PolyAromatic Hydrocarbons (PAH) pollutants emitted. The temperature levels reached within the fuel sample are also reported and discussed. A parametric study has been performed with three characteristics of the fuel as the parameters: (i) the size of the elements before mixing; (ii) the proportion of PE in the mix; (iii) and the apparent density of the 'bricks' that were prepared. Experiments were conducted using a standard calorimeter cone. This device leads to a quasi-1D situation, and good repeatability has been observed. A special sample holder and a PAH sampling system were adapted to the system. The samples were irradiated with a flux of 50 kW m~(-2). No air was blown through the samples, and the ash layer formed at the surface was not removed. It was observed that combustion occurs with two different stages. During the first stage, the fuel is devolatilized, and a flame is formed at the surface. It was observed that the duration of this period was proportional to the fuel density. The mass loss rate (kg s~(-1) m~(-2)) appeared not to depend upon the brick characteristics. In second stage, the fuel is oxidized. The mass loss rate is again very similar from one brick to another. It is approximately 10 times smaller than during the devolatilization stage. An examination of the temperature levels at three locations inside the bricks indicates that there is not a thin combustion front propagating through the sample. As a consequence of this, despite the large quantity of energy released by the combustion, the temperature reached remains between 700 and 900 deg C, which is very close to the surface steady state temperature resulting from the surface irradiation. PAHs are formed during the flame period. The PAH specified here are those formed inside the flame at the brick surface. In the case of an industrial application, it must be emphasized that these PAHs are likely to react downstream depending on the furnace conditions. The density of the fuel and the size of the elements have no impact on these emissions. Our results show that this is the percentage of PE that controls the emissions. We showed that the introduction of more than 30 percent of PE (expressed in micrograms per gram of PE) leads to very high PAH emissions. Moreover, for mass fractions of PE larger than 30 percent, heavy PAH, which are more toxic than light PAH, are formed in majority. In conclusion, if PE mass fractions lower than 30 percent are used, such substitution fuels allows one to recover the available energy of these materials, while solving the environmental and technical problems usually encountered when burning these materials individually.