While there are currently around 11,000 plant operators across Europe participating in the EU ETS on a mandatory basis, there are around 1,900 operators in Germany.
Whether a plant is subject to emissions trading ultimately depends on certain threshold values. All combustion plants with a rated thermal input of at least 20 MW are obliged to participate in the EU ETS. Installations in energy-intensive industries such as steelworks, cement plants, refineries must exceed an industry-specific production output in order to be subject to the emissions trading obligation.  

Since 2013, the chemical and aluminium industries have also been largely covered by the EU ETS, as since then, in addition to CO2, the much more environmentally harmful greenhouse gases nitrous oxide (N2O) and perfluorinated hydrocarbons (PFC) are also subject to emissions trading.
Air traffic has also been integrated into emissions trading since 2012. All aircraft operators that operate flights within an EU-ETS member state or between EU-ETS member states are currently affected.

There is no reason to participate in EU emissions trading voluntarily. Those who do not meet the statutory thresholds are not allowed to participate.