Combustion of large solid fuels in cement rotary kilns

The cement industry has a significant interest in replacing fossil fuels with alternative fuels in order to minimize production costs and reduce CO2 emissions. These new alternative fuels are in particular solid fuels such as refuse derived fuel (RDF), tire-derived fuel (TDF), meat and bone meal (MBM), waste wood, sewage sludge, paper and plastics.

The alternative fuel share of the total energy varies significantly from region to region, but the general trend is towards increased alternative fuel utilization. Solid alternative fuels typically have physical and chemical properties that differ from traditional solid fossil fuels. This creates a need for new combustion equipment or modification of existing kiln systems, because alternative fuels may influence process stability and product quality.

Process stability is mainly influenced by exposing the raw material bed in the rotary kiln to reducing conditions, which increases the tendency for deposit formations in the rotary kiln material inlet end, kiln riser duct and lower cyclone stages. Clinker quality may also be affected by minor compounds from the fuel ashes or from unburned carbon leaving the rotary kiln with the clinker.

This thesis provides an insight into the utilization of solid alternative fuels in the material inlet end of rotary kilns. This position is interesting because it allows utilization of large fuel particles, thereby eliminating the need for an expensive shredding of the fuels. The challenge, however, is that the solid fuels will be mixed into the cement raw materials, which is likely to affect process stability and clinker quality, as described above.

The mixing of fuels and raw materials was studied experimentally in a pilot-scale rotary drum and was found to be a fast process, reaching steady state within few drum revolutions. Thus, heat transfer by conduction from the cement raw materials to the fuel particles is a major heat transfer mechanism rather than convection or radiation from the freeboard gas above the material bed.

Consequently, the temperature of the cement raw materials becomes a factor of great importance for heating the fuel particles. Combustion of different alternative fuels has been investigated experimentally in a pilot-scale rotary furnace under conditions similar to those in the material inlet end of cement rotary kilns.

The main focus was on tire rubber and pine wood which are relevant fuels in this context. Heating, drying and devolatilization of alternative fuels are fast processes that primarily depend on heat transfer and fuel particle size. Devolatilization of a large wood or tire particle with a thickness of 20 mm at 900°C is for example around 2 minutes.

By contrast, char oxidation is a slow process which may greatly reduce the amounts of solid fuels to be utilized in the material inlet end of rotary kilns due to the limited residence time. Several parameters control the rate of char oxidation: a) bulk oxygen concentration, b) mass transfer rate of oxygen to char particles, c) conversion pathway, d) bed material fill degree and e) char particle size and shape.

Parameters such as temperature and rotational speed only have a minor influence on char oxidation subject to the conditions in cement rotary kilns. Models for devolatilization and char combustion of tire rubber and pine wood have been developed and compared with experimental results. The models may be modified to qualitatively predict conversion times in industrial rotary kilns.

This will, however, require further model development and preferably validation against full-scale data. Sulfur release from cement raw materials during alternative fuel combustion have been investigated both experimentally and with thermodynamical equilibrium calculations. Known effects of temperature and gas atmosphere on the decomposition of sulfates in the raw materials were confirmed.

In addition, new knowledge was obtained regarding the effects of alternative fuel types and fuel particle sizes on sulfur release: Particularly tire rubber led to a high sulfur release. For all tested alternative fuels, the sulfur release from raw materials was observed to primarily take place during fuel devolatilization where the rapid formation of reducing agents such as CO led to high sulfur release from the raw materials.

It was found that the overall sulfur release could be reduced by using larger fuel particles due to a slower devolatilization rate and a reduced tendency for local reducing conditions.