The use of sewage sludge as an alternative fuel in cement kilns will be banned in Switzerland from 2026 and in Germany from 2029 due to upcoming phosphorous recovery legislation. With these countries acting as bellwethers for European waste handling trends, has the time come for the widespread gasification of sewage sludge?
There are several ways to use biomass, including waste biomass like sewage sludge, as a source of energy in cement production. The most obvious is full combustion in an atmosphere of excess oxygen. This is a popular method in Germany, where 46% of sewage sludge was co-incinerated in 2020. However, there will be a ban on the co-incineration of sewage sludge in Germany in 2029 and Switzerland will outlaw the practice in 2026. Considering that these countries are often the first movers in waste management trends, there could be very large quantities of sewage sludge that can no longer be disposed of in this way in the medium term.
The best developed technological solution to handle this material is mono-incineration, but it is also thermally inefficient. This is because sewage sludge must be as much as 55% moisture to keep incineration temperatures below 900°C. Otherwise, the ash liquifies, harming the process. Heating the moisture in the sewage sludge reduces the amount of energy that can be effectively recovered for other uses, for example energy generation.
An alternative to full combustion of sewage sludge is gasification of sewage sludge in a reduced atmosphere where there is not enough O2 to oxidise all of the organic matter. This produces synthesis gas (syngas), which contains combustible hydrogen (H2) and carbon monoxide (CO), as well as water (H2O), carbon dioxide (CO2) and nitrogen (N2). The syngas is then used as a conventional gaseous fuel.
In practical terms, we must dry the sewage sludge to less than 10% moisture, for example by using waste heat from energy-intensive industrial processes like cement production. It will then be possible to gasify the sewage sludge at 800 - 900°C to generate syngas. This allows the true calorific value of dried sewage sludge to be released and utilised.
Fluidised bed reactor
Kopf SynGas, part of the family-owned Sülze Group, has made industrial gasifiers for more than 20 years. Its fluidised bed reactors deliver hot air from below into a bed of sewage sludge, causing it to act like a liquid. The temperature within the reactor is constant at 850°C. Syngas rises out of the top of the reactor before heading to cooling and dust removal steps. Kopf SynGas has developed a proprietary syngas cooler and has partnerships with ceramic filter manufacturers such that it can produce dust-free syngas at 400°C for industrial use.
Typically, the syngas produced using Kopf SynGas' process comprises 10 - 15% hydrogen, 12 - 23% CO and more than 50% nitrogen (Table 1). This is simply due to the composition of the air used in the gasification process. The company is currently researching processes that involve oxygen-enriched atmospheres, in a drive to increase the calorific value of the resulting syngas.
| Component / Property | Value |
| H2 (%) | 10 - 15 |
| H2O (%) | 5 - 12 |
| CO (%) | 12 - 23 |
| CO2 (%) | 8.5 - 12 |
| CH4 (%) | 3 - 5 |
| N2 (%) | Balance |
| Tars (Gaseous) (mg/Nm3) | 4000 |
| Heating Value (MJ/Nm3) | 3 - 5 |
Table 1: Composition of syngas manufactured using Sülze Kopf SynGas' fluidised bed reactor.
Ideal for cement plants
Cement and lime plants are well-placed to make use of syngas as a fuel. Many are used to handling gaseous fuels and the presence of waste heat sources enables effective drying of the sewage sludge prior to on-site gasification. There are possibilities to distribute syngas to calciners, as well as main burners in some cases.
In real world installations, Kopf SynGas would supply one or multiple parallel running reactors, occupying approximately 20m x 20m. They would be able to supply syngas at a temperature of at least 350°C after transport in a pipeline over distances of up to 150m.
Comparison with natural gas
Kopf SynGas carried out CFD modelling to assess how much syngas could substitute for primary fuels in cement production. This modelled natural gas and syngas with identical exhaust temperature at the burner outlet and identical thermal energy. While the flame shape and size are similar, the syngas flame is actually hotter. However, it is slower due to the lower calorific value. Despite this, syngas is still suitable as an alternative to natural gas.
It is also worthwhile to compare syngas to brown coal with the same boundary conditions. We need a higher mass flow of syngas due to its lower heating value. However, at the same time there needs to be less air, as the syngas is already partially oxidised. In the end, the overall mass flows are very similar. There do, of course, need to be changes to the feeding system, but overall gas volumes and exhaust systems are unaffected.
Operational experience
Kopf SynGas has developed a unique patented two-stage gasification reactor, which is located in Koblenz, Germany. The two steps are to reduce the development of tars. The 1.2MW facility is exactly as could be built at a cement plant and is entirely automated, keeping the temperature static in the reactor over periods of many weeks. The plant can be shutdown within 30 seconds, if required.
Gasification is more feasible than mono-incineration, due to the greater efficiency that the process offers. There are a number of synergies between existing equipment in cement and lime plants, with a real-world-ready industrial process from Sülze Kopf SynGas ready to go.
