CO2 reduction in rotary kiln processes

Due to the efforts to reduce CO2 #emissions in general, I have been asked several times in recent weeks what is feasible for rotary kilns .

Classically, rotary kilns in the waste pyrolysis sector are operated with the flue gas from #pyrolysis gas combustion. In my article on energy efficiency , I described how high the heat loss or how low the heat efficiency is, especially at higher process temperatures, if one wants to stay at normal combustion chamber temperatures.

In the past, electric power was often a no-go for industrial rotary kilns because it was significantly more expensive than natural gas. Now, however, times are changing. Electricity from our own PV or wind power plants is much cheaper to obtain, while the price of natural gas is very volatile.

Many rotary kilns are used in particle or pigment production. Here during #calcination, higher process temperatures are required than in classical pyrolysis (500°C). Therefore, directly heated rotary kilns have often been used here, often with a process temperature above 700°C.

Direct heating with electricity is normally not possible due to contamination and the risk of short circuits. Indirect heating with gas is not economical with a simple burner solution at the higher temperatures due to the low thermal efficiency of the rotary drum. Complex recuperation solutions are necessary or much simpler: switch to electrical heating.

In the example calculation (pyrolysis at 500°C) with the MASTER module, we have an electrical heating capacity of 2.2 MW, almost 100% of which is available as process heat, since there is no air exchange in the heating muffle. In comparison, the gas heating with the thermal efficiency of over 30% still seems acceptable.

Indirectly heated rotary kilns must have a wall temperature that is higher than the process temperature so that heat transfer can take place. Therefore it has to be checked if this is a problem for the process (e.g. ash softening). With indirect flue gas heating at 900°C process temperature, much more expensive materials are needed for the combustion chamber insulation alone, and the heat efficiency is not very good either. In this respect, electric heating seems a cost-effective standard solution.

Especially from the point of view of rising CO2 taxes, heating with fossil raw materials is coming under the scrutiny of economic efficiency.

When looking at the process, another optimization opportunity arises:

• In a direct gas-fired process, large volumes of air are moved and the entrained particles must be filtered at great expense. CO2 capture #CCS is relatively expensive due to the quantities of gas involved. For example, in the first stage of calcining CaCO3, only CO2 is released. The gas quantities are relatively small and should actually be very pure, making further processing cost effective.
• In other calcination processes, for example, only water is separated. Here, too, the indirectly heated process has decisive advantages in terms of equipment costs and investment.

If you want to have the calculation with your individual values please contact me. Any other questions are also welcome :-)

By the way: Electrically heated pyrolysis rotary kilns can be used as boosters for older coal-fired power plants . With the MASTER, up to 15 MW of gas energy can be released at 2 MWel. With this pyrolysis gas, older coal-fired power plants can be fired and then also comply with the latest emission standards.

And if they need raw materials, we should talk about the MIDI and thermolysis ...