This works because of Germany's high electric rates, $0.30/kWh in 2014. The waste to energy plants get about 500kWh/ton of garbage, or $150 of electrical energy. They can pay $50/ton and make a profit work out.
I'm curious, what makes the German electrical rate so high? Is it taxes guiding the market toward renewables and ecologically friendly sources? Perhaps a regulatory induced shortfall of production? Maybe a happy oligopoly?