Energy Efficiency of Rotary Kilns
Following my article temperatures in rotary kilns now I will have a look to the energy efficiency.
Heat requirements of indirect gas heated rotary kilns
Indirect heated rotary kilns still need a temperature difference from heating gas to kiln-shell to educt. The higher the temperature difference the better and faster the heat transfer into the educt.
But only the energy difference from G1 to G4 can be used. The higher the required product temperature, the higher is the off-gas temperature G4.
But the heating gas temperature G1 is limited. With low budget heating muffle systems often below 900°C. Higher industrial standard is appr. 1.250°C and maximum standard is normally 1.600°C.
So for processing tires at 500°C (temperature P1) and a heating gas temperature of 950°C you have an energy efficiency of about 33%. Means you need 3 times more energy consumption than really need for pyrolysis process. And you still producing waste heat for ???
If you want to activate biomass you have to follow the grey lines. With a lower heating gas temperatures it is impossible to do a correct activation (in an indirect heated rotary kiln) ...
In small-scale plants a counter flow around the kiln (or screw-feeder) will be used to supply at the end of the kiln the higher temperatures and at the educt input the lower gas temperatures to get a better efficiency. But the total heat transfer is limited due to the heat capacity of the gas flow and its flow-speed.
In case of electrical heated rotary kilns there is no gas flow and no significant heat losses, so you can calculate in every temperature range a energy efficiency of > 95%.
The electrical heated MIDI-system is available for low-temperature thermolysis of plastic and tires but also for #activation of #biomasses.
The most heard argument "electric power is to expensive" is an old statement. Of course, until last year the electric energy produced by coal power plants was more expensive than direct fired kiln systems. But when you put into the calculation the CO2-taxes and the heat-efficiency then the game changes and the green electric powered system becomes interesting.
Requirements of direct heated rotary kilns
With the high temperature kilns of #schmidtclemens you can realise now kiln shell temperatures up to 1.250°C (see article )
This allows a new view to calcination processes in direct heated rotary kilns, too. The new #suc kilns you can realize calcination processes up to 1.200 °C.
A standard MIDI-system has an installed electrical heating of 480 kW. The table shows roughly a comparisation to direct heated rotary kilns with counter gas flow.
For the same energy input with a direct heated rotary kiln you need in the example 14.205 Bm3/h. For the possibility of heat transfer from gas to educt you have to lower the gas flow and this will increase the kiln size... and CAPEX ... and in future due to CO2-tax the OPEX, too.
If you operate with product temperatures below 1.200°C let us check the possibilities!