The plant consists of two lines with a total annual waste throughput of 200,000 tonnes and a thermal capacity of 47MW per line. Hitachi Zosen Inova (HZI) delivered the complete combustion system including the boiler, as well as the flue gas treatment system. HZI’s task was to build a highly efficient EfW plant using the latest technology available. The plant achieves high overall energy efficiency by exporting process steam to a nearby paper factory, using heat for district heating, and having little flue gas at low temperature at the stack.
The Renergia energy-from-waste (EfW) plant is located close to Lucerne, Switzerland, and started operation with the first waste fire in January 2015. The plant consists of two lines with a total annual waste throughput of 200,000 tonnes and a thermal capacity of 47MW per line. Hitachi Zosen Inova (HZI) delivered the complete combustion system including the boiler, as well as the flue gas treatment system. HZI’s task was to build a highly efficient EfW plant using the latest technology available. The plant achieves high overall energy efficiency by exporting process steam to a nearby paper factory, using heat for district heating, and having little flue gas at low temperature at the stack.
The Renergia plant is designed for a mixture of municipal and industrial solid waste with relatively high net calorific value in the range of nine to 16MJ/kg.
It is the first plant equipped with HZI’s new continuously moving INOVA grate. The forward acting grate is inclined at 10° and consists of two lanes and four zones, of which the first two zones are water-cooled. The first pass of the boiler is fully clad down to the grate, without any refractories. The three radiation passes are each cleaned with one shock pulse generator.
The primary air is preheated, and the secondary air is injected at two stages together with recirculated flue gas (RFG). The RFG increases plant efficiency, and is extracted directly after the electrostatic precipitator to keep the volumetric flow through the flue gas treatment system as small as possible. The flue gas treatment system consists of sodium bicarbonate injection with fabric filter, a DeNOx catalyst, a second fabric filter with calcium hydroxide, and lignite coke injection. This special two-stage design with two fabric filters was chosen to cope with the very low emission limits requested by the operator to reach the same emission limits as with the wet scrubber installed in the old plant in Lucerne, which was in use until 2015. These very low emissions limits could be achieved with the flue gas treatment system built.
Low excess air combustion
The plant was originally designed for waste combustion with variable O2 content at the boiler exit in the range of 3–7 vol% (wet).
Today the plant is operated with very little excess air. In fact, the combustion process now runs at O2 concentrations of 2–3 vol% at the boiler exit. This corresponds to a total air lambda of less than 1.20. The benefits of having such little excess air are twofold.
First, the energy efficiency of the plant is increased by reducing the energy consumption of the fans and lowering thermal flue gas losses. Second, less ammonia is consumed for reduction of NOx in the DeNOx catalyst. The amount of uncontrolled NOx formed in the combustion process is markedly reduced when little O2 is present in the combustion process.
The Renergia plant now achieves very low levels of uncontrolled NOx, in the range of 150 mg/Nm3 (@ 11% O2) or even lower.
For operation at such low O2 levels without significant CO emissions, a very stable combustion process is required. This starts with the proper design of the combustion chamber and the primary and secondary air systems, and ends with an appropriate bunker management set-up in combination with an excellent combustion control system.
Combustion control system
Renergia was the first plant equipped with the new HZI combustion control system (CCS).
The CCS system, comprising standard PID control logic, is further supported by additional sensors and self-learning logic in the so-called CCS+. CCS+ is capable of detecting the waste layer height on each grate element, the current ram feeder efficiency including waste slip, the net calorific value (NCV) of the currently burnt waste, and the position and curvature of the fire end line. With this information available, CCS+ adjusts all actuators according to the requirements of the changing combustion conditions (NCV, waste density, etc).
The new CCS/CCS+ in combination with good bunker management and a relatively high net calorific value results in excellent live steam stability, with hardly any drops in live steam production. In fact, the current live steam flow standard deviation is lower than 1%; in other words, the plant runs within a margin of 3% more than 98% of the time.
Thanks to the CCS+ system it has been possible to eliminate most of the manual interventions typically required for the stable combustion of municipal and industrial waste. Together with a fully automated crane system, it enables the plant to be run with only two shift operators responsible for both lines, including the crane. During a shift, fewer than two manual changes of a single parameter are needed. Another outstanding feature of HZI’s new CCS is the very short commission time, as it relies on pre-set parameters and self-learning logic.
In combination with the fully clad boiler and the absence of thermal energy storage in the refractories, the CCS also enables rapid load changes of more than 0.5% per minute. This is very important if a plant is providing electricity as an operating reserve.
In fact, it takes less than two hours from first waste ignition until operation at full load, without having burners installed.
In response to the current waste market situation, Renergia recently called for a study to increase waste throughput. So the plant already runs at 7% overload without any apparent problems. The plan is to further increase the load to more than 113% in the near future.
The feasibility of running the plant at 115% has already been demonstrated for short periods.
The ease with which the load can be increased can be attributed to the relatively large margins built into the design, and the fact that the plant operates with low excess air. On top of this, the new HZI combustion control system in combination with the CCS+ system greatly facilitates an increase in load, as this set-up assures a very stable combustion process with very little fluctuation.