Full-scale accelerated carbonation of waste incinerator bottom ash under continuous-feed conditions.

Bottom ash (BA) is the dominant residue derived from the incineration of municipal solid waste or refuse-derived fuel (RDF). Costs for the disposal of the material chiefly depend on the leachability of salts and trace metals which may be cut by ageing the BA for several months to promote carbonation via uptake of carbon dioxide (CO2). Enhanced exposure to CO2 sources has been referred to as accelerated carbonation. Here we report on the successful implementation of the accelerated carbonation of BA in a continuously fed full-scale rotating drum reactor. The reactor was operated with the fine fraction (< 20 mm) of BA from an RDF incinerator and the exhaust of a combined heat and power unit was used as the reactant gas. The system was tested in 15 experiments and the process efficiency was addressed by maximizing the reactor loading and minimizing the BA residence time. Results confirmed that the reactor loading depended on the rotation-normalized mass flow rate of BA where the slope and intercept of the characteristic varied with the design of the reactor discharge and the use of mixing tools. According to leaching test results, BA residence times as low as 60 min were sufficient to render the carbonated BA a non-hazardous waste and convert it to a material suited for geotechnical applications. This outperforms previous laboratory findings and opens new perspectives for implementing the accelerated carbonation at incinerator sites.

Towards optimized drum composting: evaluation of the radial mixing performance of a model substrate on the laboratory scale.

The rotating drum composter (RDC) is one of the most widespread reactor systems for biowaste treatment, worldwide. Nevertheless, knowledge on optimum operating conditions including, e.g. fill level, turning frequency, and mixing tool configuration is sparse. This study investigated the effect of static mixing tools (SMTs) on mixing in a rotating drum at high fill levels (60-80%). The methodological approach encompassed mixing experiments in a laboratory RDC using soaked wheat grains as a model material. The temporal course of material blending was quantified in terms of the entropy of mixing using digital image analysis. Experiments without SMTs showed the evolution of unmixed cores. With a single SMT, mixing was superior even at fill levels >70% while peripheral unmixed zones persisted when overly long SMTs were used. The results of this study may help to derive optimal process conditions for RDCs operated at high fill levels.

Continuous-feed carbonation of waste incinerator bottom ash in a rotating drum reactor.

Carbonation is a key process in the aging of waste incinerator bottom ash (BA). The reaction with CO2 decreases the BA alkalinity and lowers the leachability of amphoteric trace metals. Passive ageing over several months is usually performed in intermittently mixed BA heaps. Here we aimed at accelerating the process in a rotating drum reactor continuously fed with the BA and the reactant gas (10 vol-% CO2, volumetric flow rate 60 L/min). In one test, the gas was heated and humidified. Since carbonation depends on the specific CO2-supply, experiments were conducted at varied BA residence time (60, 80, and 100 min). Residence time was calculated by mass balancing and confirmed by the breakthrough time of two tracers. Leachates and solid phase properties of the treated BA served to evaluate the carbonation performance. The residence time of BA could be adequately controlled by the reactor loading and feed rate. A residence time of 80 min was sufficient to reduce the BA leachability such as to comply with the German regulatory standards for non-hazardous waste, whereas the untreated BA was hazardous waste. Decreased alkalinity was indicated by lower leachate pH and Ca(OH)2 contents of the BA as compared to the input. Leachate concentrations of amphoteric trace metals (Pb, Zn, Cu) decreased by at least one order of magnitude while oxyanions became slightly more mobile upon carbonation. In view of relatively short residence times and stable process performance, the rotating drum reactor seems promising for a full-scale implementation of BA carbonation.

A fast and simple method to monitor carbonation of MSWI bottom ash under static and dynamic conditions.

Accelerated carbonation may be employed to improve the leaching behaviour and the geotechnical properties of MSWI bottom ash (BA). Here we report on a novel method to monitor and evaluate the progress of carbonation in both static and dynamic reactor systems. The method is based on following the pressure drop in the gas phase induced by the CO2-uptake of BA and was benchmarked against carbonate contents as measured by thermogravimetry. Laboratory results demonstrated the serviceability and reproducibility of the method. Complementary logging of relative humidity and temperature showed constant moisture conditions and self-heating induced by the exothermal carbonation reaction, respectively. Under dynamic conditions BA carbonation was higher than in the static reactor. Consistently, the self-heating was more pronounced. After a reaction time of 120 min the pressure records indicated a CO2-uptake of 1.5 g CO2/100 g BA (static tests) and of 2.6 g CO2/100 g BA (dynamic tests). The proposed method is suited to study carbonation processes at minimum analytical expense and integrates over the small-scale heterogeneity of BA.