We opted to use a grate furnace at our waste-to-energy plant. This tried and tested technology has been continuously further developed and improved over the last few years. Waste-to-energy plants equipped with this technology have proven to be extremely reliable.
From receiving waste to recovering energy: this is how our waste-to-energy plant works:
The facility was designed so that waste can be delivered by both rail and road. We can handle a number of different container systems: when waste is delivered by rail, the container is loaded onto a truck by gantry crane, weighed and then transported to the unloading area. Deliveries by road are first weighed using the weighing system at the entrance to our plant and then a visual check is carried out to ensure the waste corresponds with the EWC code (european waste catalogue) listed in the declaration form. Once this has been completed, the trucks are driven to the unloading area which is reached via a ramp. Here, they are reversed towards the tipping bay where their waste can be unloaded into the bunker.
Two waste cranes (overhead grab cranes) keep the area behind the tipping area free of waste. Moreover they are used to mix and stockpile the waste as well as to load the feed chutes to the incineration lines. The waste bunker is made of reinforced concrete and also includes an integrated bunker for the bottom ash.
The bottom of the feeder is equipped with a charging ram which measures out the waste and then pushes it onto the combustion grate. During the incineration process, the hydraulically driven reciprocating grate transports the waste towards the end of the grate. The waste is incinerated at high temperatures in accordance with the "17. BImSchV" (17th Ordinance of the Federal Emissions Control Act). The combustion air needed for this process is supplied as primary air from below the grate and as secondary air in the combustion chamber for the post-combustion stage. The gases generated by this process are used to produce steam. The non-combustible contents of the waste are discharged as bottom ash. The first two sections of the grate are cooled down with water. This helps to considerably reduce wear and tear and enables the grate cooling system and supply of combustion air to be separated from one another.
The steam production process starts in the combustion chamber where evaporator tubes have been installed as gas-tight membrane walls. Suitable refractory materials are used to protect the walls from the effects of the flames. The primary air is extracted from the waste storage bunker and fed - in a controlled fashion - into the five air zones below the grate. The secondary air system ensures that the temperature remains at the same level throughout the combustion chamber and controls the distribution of O2. The secondary air is extracted from the boiler house via an air preheater (secondary APH) and then injected into the combustion chamber from two sides. The swirling air flow created by this process helps to further intensify the way the combustion gases are mixed. By varying the volumes of primary and secondary air, it is possible to adjust the distribution of the air to suit the properties of the fuel. Any remaining bottom ash drops through a chute at the end of the grate into an ash extractor filled with water. Chutes are also used to convey the small amount of fine-grained material to the ash extractor which, in turn, transports the ash into the bottom ash bunker. A bottom ash crane loads the ash onto trucks so it can be sent for further recycling.
The SNCR (selective non-catalytic reduction) process is used to reduce NOX by injecting ammonia in an aqueous solution into the 1st boiler pass. In order to be able to adapt to different operating conditions, this solution can be injected at several different points so that it can be placed in the area with the best possible reaction temperature, i.e. between 850°C and 950°C. The reaction between the NOX and NH3 leads to the creation of the harmless substances nitrogen and water.
The energy in the hot flue gas is used to produce direct steam in the steam generator (40 bar / 400°C). The steam generators have been designed as vertical four-pass boilers. The sheathing walls act as evaporator walls and are constructed in a tube-fin-tube design. They create the boiler's natural circulation system together with the boiler drum, the vertical downpipes, the connecting and overflow pipes and the evaporators. The 2nd pass is designed as a radiating pass with a platen tube evaporator. The protective evaporators, tube bundle and contact heating surfaces of the superheater are located in the 3rd pass. The boiler's 4th pass contains the economiser (feed-water preheater). The feed-water enters the boiler via the economiser and flows into the boiler drum, from where it is fed into the circulation system. The steam generated in the evaporators also flows into the boiler drum where it is separated from the water and then superheated in the superheater stages. Injection cooling systems have been installed between the superheater stages to control the temperature of the steam.
Click here to see the inside of the boiler combustion chambers.
Further pollutants are removed once the nitrogen oxides (NOX) have been broken down. To be able to do this, the flue gases are fed into the flue gas cleaning system once they leave the steam generator.
The semi-dry flue gas treatment system has two main components: the reverse flow scrubber and the fabric filter. Once it leaves the boiler, the flue gas is fed into the reverse flow scrubber. To begin with, an ideal state is created for the pollutants to be removed from the gas by injecting water into the system at high pressure. The gas is then mixed intensively with specific amounts of calcium hydroxide and activated carbon to bind the acidic and metal components in the flue gases and, where applicable, the dioxins and furans so they can be extracted by the fabric filter. The majority of the materials picked up by the filter are fed back into the reverse flow scrubber so that any remaining reactive calcium hydroxide and activated carbon can be reused. This helps to reduce the need for new supplies of calcium hydroxide and activated carbon and keeps volumes of residue to a minimum. Supplies of calcium hydroxide and activated carbon are stored in separate silos.
An induced draught fan is located beyond the fabric filter. The induced draught maintains the negative pressure in the combustion chamber and flue gas treatment system and transports the flue gas to the chimney. The emissions in the chimney are continuously measured to monitor levels of CO, total carbon, particulates, NOX, Hg, HCI and SO2. In addition, oxygen and moisture levels are measured as well as the volume, temperature and pressure of the flue gas. An emissions calculator logs and classifies the various measurements and then evaluates them in accordance with the "17. BImSchV" (17th Ordinance of the Federal Emissions Control Act). The waste residue from the semi-dry treatment process is stored in a silo together with the boiler ash and then sent by silo truck for further recycling.
The direct steam from the steam generator's superheaters is fed into the turbine / generator to produce electricity. The electricity is fed into the grid and used to cover the plant's own requirements.
Steam is extracted from a number of points in the turbine. Medium pressure (MP) steam (25-27 bar) is used to supply the soda works with process steam, to remove soot from the boilers as well as to preheat the combustion air when heating values are low. Low pressure (LP) steam (6 bar) is used for preheating the feed-water and for the SNCR process.