sludge2energy: A way to energy-autarkic operation of sewage treatment plants
Preface
When we speak about the future of sewage sludge disposal today, regional and supraregional differences are evident. Everyone seems to talk about agricultural or thermal utilisation and phosphorus recovery but everyone also seems to interpret these topics most differently.
For many states the recovery of materials contained within sewage sludge still plays an important role. This applies to both landscaping and sludge spreading on agricultural land. The fertilization effect of sewage sludge and especially its phosphorus content is normally sufficient to cover the nutrients demand of typical agricultural land.
On the other hand, there are a lot of countries where the agricultural application of sewage sludge is met with much scepticism due to its potential heavy metal pollution and content of organic pollutants, such as PFT. In these countries there has been a clear trend towards concepts for thermal sewage sludge utilisation for some years already, partly combined with the approach to recover the phosphorus contained within sewage sludge.
Our search for energetically optimal utilisation finally resulted in our sludge2energy project which is presently in the implementation stage on WWTP Straubing, Bavaria as a pilot project of HUBER SE and under the EU Life06 project.
- Thermal sewage sludge utilisation on WWTP Straubing
- The treatment steps of the sludge2energy system
1 sludge2energy process
1.1 General description
Basically, the sludge2energy is the decentralised combination of sewage sludge drying followed by mono-combustion and power generation by means of a micro gas turbine. The main system components are a belt dryer and a grate stoker furnace for dried sludge.
A HUBER Belt Dryer BTplus with a process temperature of about 120°C is used as drying plant.
The industrial combustion methods used for sewage sludge incineration, such as story furnaces or fluidised beds, are for cost reasons inapplicable on the target scale. HUBER SE developed therefore in cooperation with a plant manufacturer a small grate stoker furnace with 1 MW thermal output that meets the requirements of the German standards 17. BlmSchV.
Presently, phosphorus recovery from the generated residual ash is in the planning stage as last consequential step of sewage sludge utilisation.&nbs
1.2 Sewage sludge drying
1.2.1 The aspects of sewage sludge drying
There are a number of reasons that speak for additional drying of sewage sludge after its mechanical dewatering. These are some of the main arguments:
- Reduced sewage sludge volumes
- Easier to store and transport product
- Easier to convey and dose product
- Microbiological stabilisation and hygienic safety
- Increased thermal value
Especially the latter is of importance for later thermal treatment. Frequently, the dry substance content achieved through mechanical dewatering is insufficient for autarkic combustion, or prior additional drying is required for technical reasons.
1.2.2 HUBER Belt Dryer BTplus
- Sketch of the HUBER Belt Dryer BTplus
- Detail view of the HUBER Belt Dryer BTplus
In the HUBER Belt Dryer BTplus convective sewage sludge drying takes place at medium temperature.The sewage sludge is dried by the process air streaming through the installed two-belt dryer and the air is enriched with water.
After intermediate storage the sludge is fed into the continuously operating dryer by a conveying unit. A main aspect of the operation principle of the HUBER belt dryer is the feed system that places the sewage sludge onto the first belt. The well-proven pelletizer distributes the dewatered sewage sludge over the full active width of the upper belt.
The layer of sewage sludge on the upper belt has a well-permeable structure that consists of defined individual sludge lines. The upper belt transports the sewage sludge into the area with air flow. The process air is heated by means of a heat exchanger. While the air streams through the belts covered with sewage sludge, the air is being cooled and loaded with sewage sludge water.
The exhaust air is directed through another heat exchanger. Heat is extracted from the air flow and re-introduced into the supply air flow via the respective heat exchanger. This type of heat recovery reduces the thermal energy demand enormously. The condensate produced by air cooling is discharged.
The exhaust air is further cooled by a washer. Fine dust particles, odours and other components are removed before the exhaust air is biologically clarified in another treatment stage. The exhaust air treatment complies with German TA standards (Technical Instructions for Air Pollution Prevention).
1.3 Sewage sludge combustion
1.3.1 Grate stoker furnace
- Grate stoker furnace
- Sewage sludge combustion of the Zauner company, source: Zauner
The thermal utilisation of the dried sludge takes place in an oven with grate furnace that provides a high flexibility for the fuels used and the benefit of easy and reliable operation.
For optimised combustion the grate zones are equipped with individual controls and air supply. Grate de-ashing is performed automatically. Flue gas recirculation represents the first stage of exhaust gas treatment.
1.3.2 Exhaust gas cleaning
The reduction of nitric oxides is accomplished by means of well-proven firing measures. The combustion temperature of at least 850 °C provides favourable preconditions for this type of denitrogenisation. Also other applicable burning technology requirements have of course been taken into account, for example the German 17. BlmSchV [5] concerning temperature control.
Acid noxious gases contained within the exhaust gas, such as SO2 and HCI, are removed in a dry-sorptive process. A cyclone separator is installed prior to the pulse-series filter to pre-filter the fly ash.
1.3.3 Micro gas turbine
The used Turbec micro gas turbine system is a modular high-efficiency system for the generation of power and heat. The system can be applied in all processes where hot exhaust gases are directly used for drying, cooling,
2 Energy balance
2.1 Mass flow evaluation
The plant on the WWTP Straubing designed for 200,000 PE presently treats about 35,000 m3 wastewater per day. After anaerobic sludge treatment and dewatering by means of centrifuges this is an annual volume of almost 9,000 t sludge dewatered to on average 28-29 % DS.
The drying plant has been dimensioned on the basis of the following paramete
Type of sludge: | Municipal, dewatered, digested sewage sludge |
Sludge volume dewatered to 29% DS: | approx. 9,000 t/a |
Dry substance: | approx. 2,600 t/a |
Operating time: | approx. 7,500 h/a |
Sludge throughput: | approx. 1,200 kg/h |
Initial solids content after dryer: | min. 65 % |
Water evaporation capacity: | approx. 665 kg/h |
Heat transfer medium: | warm water mit 140°C |
Temperature difference: | approx. 10 – 20 K between flow and return |
The combustion plant design is based on the following mass balances and technical data:
Sludge volume dried to 65% DS: | approx. 525 kg/h |
Combustion capacity: | max. 1 MW thermal |
Ash output: | max. 250 kg/h |
Flue gas volume: | approx. 3,400 Nm³/h |
Micro gas turbine capacity: | approx. 80 kW electric |
Burn-out zone: | min. 850°C for 2 s |
Hot air heat exchange: | 520 kW thermal |
Water WT: | 350 (865) kW thermal |
2.2 Energy balance of the sludge2energy process
The thermal energy content of the dried sludge is a substantial value for the creation of an energy balance.
On the basis of the data specified under 2.1, the amount of available energy is 1,020 kWh resulting from 65% DS concentration, assumed ODS of 50% and about 7,000 kJ/kg thermal value of the sludge. Calculating with the according boiler efficiency, about 800 kWh thermal energy can be generated. After deduction of further thermal losses in the micro gas turbine about 700 kWh thermal energy effectively remain for the drying process.
With a thermal energy factor of about 0.85 kWh/kg WVD the resulting energy consumption of the dryer is 565 kWh when drying 1,200 kg/h to 65 % DS. This means there even is a surplus of energy in view of the a.m. 700 kWh.
The electric energy factor of the dryer can be reduced to approx. 0.06 kWh/kg WVD when a high process temperature of about 120 °C and final drying degree of only 65 % DS are selected. Assuming this electrical energy factor, the resulting electric energy consumption is approx. 40 kWh, which is an annual electric energy consumption of 300 MWh/a.
So, another 40 kWh are available to operate the sewage sludge combustion plant. This is enough to cover the calculated present power consumption for combustion.
3 Outlook
After some months of operation it will certainly be able to present the actual energy balance. From today’s perspective we can say that energy-autarkic sewage sludge combustion can be achieved at Straubing with the combination of a belt dryer, combustion and micro gas turbine. A substantial approach to saving energy is the energetic optimisation of the belt dryer and best possible utilisation of the combustion exhaust heat.
Meanwhile, the operators even have gone a step further and are thinking about possibilities how to recover the phosphorus contained within the sewage sludge ash from mono-combustion. The necessary space is available on site so that there is a good chance that the thermal sewage sludge disposal line on WWTP Straubing will soon be completed with another innovative treatment stage.
