Process improvement at WtE Bern with Shock Pulse Generators

With Shock Pulse Generators, the cleaning is carried out at lower power per cleaning process, but much more frequently. This avoids the “sawtooth profiles” in the flue gas temperatures known in many plants, the ash is discharged more evenly and the alternating stress on the units is reduced. The comparison of three operating periods before and after the change of the empty pass cleaning system shows that the maximal as well as the average flue gas temperatures were significantly reduced. The reduction of the flue gas temperature at the inlet to the horizontal pass is an important prerequisite for being able to operate the plant permanently at a capacity of 110%. At the same time, the energy requirement for boiler cleaning was reduced.

The work was carried out by Martin GmbH and Explosion Power GmbH in cooperation with ewb. The PI process data platform is used to evaluate the operating data.

The energy center belongs to “Energie Wasser Bern” (ewb), which operates the following energy plants at the Forsthaus site:

A waste-to-energy plant (German: KVA, (59 MW thermal capacity), a wood-fired CHP plant (32 MW), a gas-and-steam combined-cycle power plant CCGT (131 MW) and two peak-load boilers (2×24 MW). The waste-to-energy plant (KVA) was commissioned in 2012, as part of the Bern Forsthaus energy center. This waste-to-energy plant replaces the former waste-to-energy plant of the city of Bern which was located at Warmbächliweg.

The thermal part of the new single-line WtE Bern with a design capacity of 110,000 t/a consists of a grate incinerator with a reciprocating grate and a 4-pass boiler with 3 vertical empty passes and one horizontal pass (steam output nominally 73 t/h at 40 bar(a) and 400°C). It is followed by an electrostatic precipitator, raw gas catalyst and external economizer. A wet scrubber and fabric filter are used for flue gas cleaning. The produced steam is expanded in an extraction-condensing turbine.

The plant was originally equipped with a ShowerCleaning System (SCS) to clean the empty passes of the boiler. The cleaning effect results on one hand from the impulse when the water droplets hit the surface (like water spray of a high-pressure cleaner) and on the other hand from the sudden evaporation of the water that has penetrated the ash deposits and the resulting strong increase in volume by its evaporation. The water is introduced via a hose equipped with a spray head, which is inserted into the boiler through a nozzle in the boiler ceiling for the respective cleaning process. ShowerCleaning is usually used in such a way that the cleaning process is only triggered when the deposit layer reaches a considerable thickness, since spraying bare walls can lead to severe wear of the walls.

For the afore mentioned reasons, a ShowerCleaning system is used to clean at intervals of several days to once per week. This in comparison to most other boiler cleaning systems, which are usually used several times a day. A resulting major disadvantage of ShowerCleaning Systems is the characteristic “sawtooth profile” of flue gas temperatures in downstream boiler sections and other plant components. A one-month profile of flue gas temperature upstream of the final superheater with application of ShowerCleaning is shown below.

In the period shown, the flue gas temperature upstream of the final superheater reaches peak values up to 700°C before it is reduced by approximately 60°C by the next cleaning process. Flue gas temperature above 650°C have a negative effect on the corrosion rate and on the service life of the superheater and are worsening the cleanability of the deposits in the horizontal pass.

Since more frequent cleaning processes with ShowerCleaning are undesirable (a lot of water is required per cleaning process, high material stress, higher flue gas energy losses), ewb decided in 2018 to replace the ShowerCleaning with Shock Pulse Generators, as part of a planned capacity increase.

With Shock Pulse Generators, the boiler is cleaned by an impulse of pressure waves triggered by the sudden combustion of small quantities of a mixture of a combustible gas (natural gas or methane) and an oxidation medium. In contrast to manual methods applying the same principle of action (e.g. blast cleaning with inflatable balloons, detonating cords or cartridges), the Shock Pulse of SPGs is triggered automatically whereby an instant incineration of the mixture takes place in a stable, pressure-resistant device outside of the boiler. The generated pressure wave is directed via a valve and a discharge nozzle into the boiler, where it vibrates the flue gas, the boiler tubes and walls in such a way that the deposits are cleaned off. Due to the spherical spread of the shock pulse, the cleaning is effective in all directions and reaches even the shaded areas of tube bundles. This enables gentle cleaning of boiler heating surfaces in all temperature ranges from the furnace to the economizer, without increased mechanical stress on the heating surfaces.

Three Shock Pulse Generators are used for cleaning the empty passes in the WtE Bern; one EG10XL in the second pass and two opposite EG10L in the third pass. Two units are used in the third pass because a center wall divides the boiler into two segments, each about 4 m wide. The first pass does not need to be cleaned separately, among others because the SPG installed in the second pass is still sufficiently effective there. The frequency of the Shock Pulses (SP) is adjusted as required. At average, one SP per hour per SPG is applied. Pneumatically operated single impact cylinders are used to clean the bundles in the horizontal pass.

Since May 2018, the empty passes have been cleaned exclusively with Shock Pulse Generators. At that time, the target value for the live steam output was still maintained at the original value of 73 t/h, the flue gas temperature upstream of the final superheater SH3 shows considerably lower fluctuations. The average temperature was 600 °C (blue line), which means a reduction of 40 °C compared to operation with ShowerCleaning. At 615°C, the maximum value was even 75°C lower than operating without SPGs. As a result, the injection water quantity is also much more constant.

The aim of ewb was to permanently increase the steam output from the original 73 t/h to 80 t/h. In addition to optimizing the utilization of energy, this results in an increase in waste throughput from 110,000 t/a to around 140,000 t/a. The following graph shows that the nominal output could be increased significantly by 10% to 80 t/h from mid-2019 without undesirably high flue gas temperatures occurring upstream of the final superheater.

After the increase in performance in mid-2019, the maximum target value (set by the operator) for the live steam output is 80 t/h, the average flue gas temperature upstream of the final superheater SH3 (blue line) was 617°C, i.e., despite a 10% higher load it was still 25°C lower than with ShowerCleaning. At 632°C, the maximum value was still well below the critical limit of 650°C.

Ewb is very satisfied with the current situation. The SPGs are in continuous use since they were commissioned and are also maintained by ewb staff.

In 2020, Explosion Power GmbH introduced the SPGr series, which differs in such a way, that the shock wave in the Shock Pulse Generator is generated by incinerating small amounts of natural gas (or methane) with air instead of oxygen. The main advantages of the SPGr series include the significantly longer maintenance interval and that no technical oxygen must be provided. Since the launch of the SPGr series, over 130 units have been sold, with over 50 already in service.

The two types of the SPGr series, SPGr10 and SPGr16 cover the entire performance range of the EG10 series and the TwinL, respectively extend to even higher performance.

From February 2023, the EG10XL in the 2nd pass will be replaced by an SPGr10 for an initial period of 2 years in order to benefit from longer maintenance intervals and lower maintenance costs.

Acknowledgment