Lessons learned from the deployment of flexible steam turbines for utilising steam generated from sewage sludge incineration

by Cornelia Liebmann

The disposal of sewage sludge has been a widely discussed topic for decades. Spreading sewage sludge on fields add the valuable fertiliser phosphorus to the soil, but it also releases the pollutants which are contained in the sewage sludge.

On 03/10/2017, the amended Sewage Sludge Ordinance (AbfKlärV) came into force in Germany, increasing the relevance of the discussion. According to the current Sewage Sludge Ordinance, sewage treatment plants with more than 50,000 or 100,000 population equivalents may only recycle sewage sludge on a soil-related basis until 2029 or 2032 respectively. Once this period has expired, sewage sludge with a minimum of 20 g phosphorus/kg must undergo phosphorus recovery (according to Article 5 Section 3(1) No. 1 AbfKlärV).

An alternative to pure recovery is thermal pre-treatment of the dried sludge, e.g. in sewage sludge mono-incineration plants, with subsequent phosphorus recovery.

In addition to examining the investment costs for the incineration plant, it thus makes sense to optimise the process technology of the individual mechanical components in terms of electricity generation, depending on the size of the sewage sludge incineration plant.

If steam is generated during sewage sludge incineration, then it must be ensured that this steam is also utilised for drying the sewage sludge by using combined heat and power (CHP) and also by increasing the efficiency of the overall process.

Electricity generation from sewage sludge with optimised drying and incineration

Sludge treatment is the term utilised to designate all processes thath improve the usability or the transportability and storability of sludge. Sludge treatment processes include digestion, thickening, hygienisation, stabilisation, dewatering, drying and incineration, including subsequent conversion of the waste heat into electricity [1] [2]. The treatment processes mentioned are thus not pure alternatives to each other. On the contrary, combining them optimises sewage sludge utilisation.

Drying processes when using steam and the influence on electricity generation

Using dried sewage sludge as opposed to wet sludge, which comes directly from the sewage treatment process, provides several benefits:

• Reduced amount of sewage sludge

• Better storage and transportability

• Better conveyance and metering capabilities

• Inhibition of microbiological processes and establishment of hygienic

safety

• Good miscibility

• Increased calorific value (firing is nevertheless often necessary)

Drying processes are essentially divided into direct and indirect processes. [3]

Tab. 1: Thermal media utilised, temperature and pressure ranges as well as associated drying units [4].

Optimisation of the drying parameters as regards power-heat coupling and increasing the electricity yield

Heat-controlled operation

As far as the use of steam turbines for CHPP is concerned i.e. where the steam from the incineration is converted into electricity and subsequently used for drying, the most effective way to increase electricity generation is to design the dryer in such a way that the steam can be used at the lowest possible pressure and temperature. Since the composition of the sewage sludge can vary greatly, it also helps to design the steam turbine for a variable exhaust pressure. This means that fluctuations in drying can also be compensated for by the drying temperature.

If the dryer is also operated at partial load, then it is important to ensure that the steam pressure provided downstream of the turbine can be reduced so that the dryer or its heat transfer surface can be fully utilised.

The electricity yield of the steam turbine can be increased by operating the dryer with a lower steam pressure.

Overall, the heat-controlled operation is limited by the fact that only the amount of sewage sludge which is required as the necessary amount of steam for the dryer can be incinerated, or the excess steam generated must be cooled accordingly via emergency coolers. If the plant environment allows, then the excess steam can also be decoupled and used for heating purposes for other consumers. Usually, however, sewage sludge incineration plants are not located near users of heating energy, making the use of emergency coolers the most common type of system.

The challenge in operation is the frequent start-stop cycles of the steam turbine, which result from the strongly varying supply and quality of the sewage sludge on the one hand and the varying steam demand of the dryer on the other.

Based on the modular design of its KK&K MONO turbines, Howden Turbo has developed the highly standardised BASE type series (Fig. 1), which has been optimised to meet these requirements. The robust action profiles are ideal for saturated steam applications. Thanks to its overhung design, the turbine can be started quickly and is equipped with an oil-free control system. This enables the turbine generator to react immediately to fluctuating steam quantities and to synchronise quickly. [5]

The BASE type series is staggered into four performance levels, striking a cost-optimised balance between optimum dimensioning and a high degree of standardisation with corresponding repeat parts and standardised project documentation.

Fig. 1: Turbogenerator KK&K® BASE [6]

Electricity-controlled operation

Another way to use steam after incineration and supplying it to the drying process is to convert it into electricity via an extraction-condensation turbine.

Only just over a third of the steam generated is needed for drying sludge. It thus makes sense to convert the remaining steam into electricity in a second stage.

The drying plant has the lowest investment costs when using 10 bar(a) steam. On the other hand, the highest electricity yield is achieved when the dryer is operated with the lowest possible steam pressure. In principle, dryers can also be operated with 3.5 bar(a) steam but they are correspondingly larger and more expensive.

As with heat-controlled operation, the steam turbine can also be designed with a controlled sliding extraction pressure for electricity-controlled operation. This enables the drying temperature and quantity to be controlled based on the composition of the sewage sludge and the amount of heat required for drying. Moreover, the electricity yield can be optimised by this mode of operation.

Example of incineration at the Ulm sewage treatment plant

In the high-pressure steam section, the steam from the two 40-bar boilers is expanded to a pressure of 3 bar(a) to 5 bar(a), depending on the steam requirement for drying. After the steam has been extracted for the dryer, then the steam that is not required is expanded to 0.08 bar(a) in the second steam section. The condensation stage was not designed for the maximum steam flow, but rather for about 60% of the live steam quantity. It was assumed here that steam is always needed for drying sewage sludge. This design also optimises the electricity yield, as the steam turbine's condensing machine thus operates more at full load and hence at maximum efficiency. This adapted design has been validated in operation over the last ten years.

A KK&K TWIN (Figure 2) was thus selected for this application. In this compact design, two steam parts are mounted on a common, integrated gearbox, which drives a generator. The installation space required is thus only slightly larger than for a single-stage turbine. The control valve of the condensing steam section ensures a constant extraction steam pressure when the live steam and/or extraction quantity fluctuates, or, in the case of fluctuating steam demand for drying, as described above, it regulates the extraction steam pressure and quantity as required.

A water-cooled vacuum condensation plant with a Taprogge plant is used for condensation. In this case, the capacitor is a shell and tube heat exchanger, although plate exchanger devices are also occasionally used.

Evacuation by means of steam jet ejectors or water ring pumps is necessary in order to operate the apparatus under vacuum conditions. Steam jet ejectors are normally used for these small systems. The turbine exhaust pressure is determined exclusively by the temperature level of the cooling water. Treated process water is used and the capacitor is continuously cleaned by the Taprogge plant in the Ulm sewage treatment plant.

The steam turbine plant has been operating without any malfunctions for ten years.

Fig. 2: Turbogenerator KK&K^® TWIN [6]

Lessons learned from operation

The project planning of steam turbines for sewage sludge incineration plants is made more difficult in Europe by strict regulations on feeding the generated electricity into the grid. In Germany in particular, the Directive for Connection and Parallel Operation of Generating Plants on the Medium-Voltage Grid requires our machinery to be certified in accordance with VDE4110. The grid operator requires a plant certificate from the operator, which serves as the basis for the grid operator's approval for feeding the electricity into the grid. Since turbogenerator plants have to undergo to the individual testing procedure, this must be created every time for each plant.

The regulatory requirements and the expected costs prevent the planned conversion of electricity and/or retrofitting of small decentralised CHP plants.

Personnel should be provided with sufficient training to ensure optimal operation of the plants. In addition, the various components of the overall plant cannot always be optimally coordinated during the commissioning phase. This must be implemented later during operation of the plant. In this case, it would be important to implement the planning specifications for optimisation by the plant personnel accordingly.

Summary and outlook

To sum up, the requirements for combined heat and power generation in sewage sludge incineration plants, with subsequent conversion to electricity, depend on the individual circumstances. Depending on the mode of operation and the size of the plant, it must be decided which optimisations make the most sense. In this context, it must also be considered whether it involves a small plant with a strong focus on investment costs or a larger plant with a focus on the amount of electricity generated. Based on the modular design concept of the compact industrial steam turbines, it is easy to offer the right turbine for different requirements and many other possible applications.

Sources

[1] Gujer, W.: Siedlungswasserwirtschaft. [Urban water management] Berlin Heidelberg: Springer-Verlag, 2007.

[2] Brandt, S.: Nutzung von Klärschlamm als Rohstoffquelle – Aktueller Stand in Deutschland und in der Europäischen Union sowie Perspektiven für die Zukunft. [Use of sewage sludge as a source of raw materials - Current status in Germany and the European Union and perspectives for the future] Master thesis, University of Rostock, https://docplayer.org/4022212-Nutzung-von-klaerschlamm-als-rohstoffquelle. html (Version: 02/07/2019).

[3] Roskosch, S.; Heiecke P.: Klärschlammentsorgung in der Bundesrepublik Deutschland. [Sewage sludge disposal in the Federal Republic of Germany] at- ric Heidecke, 2018.

[4] DWA 379 Deutsche Vereinigung für Wasserwirtschaft Abwasser und Abfall e. V.: Leaflet  :  DWA-M 379 Klärschlammtrocknung. [Sewage sludge drying] Hennef, 2004.

[5] Heimann, C.: Howden Turbo GmbH: Howden KK&K® Dampfturbinen für die Abwärmenutzung. [Howden KK&K® steam turbines for waste heat recovery] 2018.

[6] Howden Turbo GmbH: Webinar – Steam Turbines overview, https://www.howden.com/en-gb/products-and-services/steam-turbine (Version: 02/07/2019).