A substantial part of Europe’s sludge and slurry mountain can be converted into gas for electricity production, if a new invention from Finnish company Outotec turns out to be successful. Together with Stuttgart University the company has invented a new drying-and-gasification process that should make this kind of energy production affordable. According to Ludwig Hermann of Outotec, the process has an additional advantage in that it leaves a nutrient-rich residue that can be used as fertiliser, preventing the loss of valuable nutrients from agricultural land.
Researchers around Europe are looking for ways to turn sewage sludge and farmyard slurries into energy. Such energy conversion has great potential. The European market produces a total of 140 million tons of slurry per year measured in dry substance, which is concentrated in the livestock-intensive regions of north-west Europe. One ton of dry slurry produces about 12,000-15,000 MJ (3.3-4.2 MWh) of gas (a mixture of H2, CH4, CO, CO2, tars and minor impurities). This is equivalent to about 40 bcm (billion cubic metres) of natural gas with heating value of 38 MJ/m³. A country like Germany uses some 70-80 bcm of natural gas per year, so this is a substantial amount.
The problem is how to do this cost-effectively. To date, the biggest barrier has been the moisture content of the sludge. At typically 90 per cent it cannot be combusted in traditional incineration plants. It is also difficult and costly to transport this wet waste.
Common solution
Thus, before sewage sludge and slurry can become a reliable fuel source for power generation, efficient moisture extraction is essential. Researchers from the Finnish engineering and material sciences company Outotec say they have come up with a solution. In an interview with Energy Post, Ludwig Hermann, global commercial product manager at Outotec in Frankfurt, explains the background.
With stricter enforcement of existing directives on the use and recovery of nitrates expected, it will become almost impossible to incinerate this form of biowaste in power plant
“Our researchers in Frankfurt were investigating the potential of wet biowaste, while our colleagues in Skellefteå in Sweden were working on a dryer concept for wood-based feedstocks. When the two teams compared projects, it became clear that there was mutual benefit to joining efforts and finding a common solution. “
The two research teams from Outotec soon realised that drying was in fact the first stage of a two-phase process, says Hermann. “The Frankfurt team had been looking at the potential of biowaste and generating energy from chemical fuels for some time. Working together with our colleagues in Sweden, we developed a proposition that, in addition to producing dried mass for combustion in mixed or specialist generation plant, could apply gasification techniques to produce more universally applicable synthetic fuel gas.”
From this proposition, the “Demonstration of Efficient Biomass Use for the Generation of Green Energy and Recovery of Nutrients” project, or DeBugger, was born.
Clear logic
The first element in the project is the design of a closed-loop steam dryer, developed from the original idea of Outotec’s Swedish researchers. The second element is a gasification plant. The plant features a dual-circulating fluidised bed gasifier, which is used for the thermal treatment of the dried substrate to produce synthetic fuel gas.
“There is a clear logic to developing these solutions in tandem,” explains Hermann. “The gasification process requires a dry feedstock, so the drying process is essential. However, a percentage of the gas produced by the gasifier can be used to power the dryer. Once in production, it’s a very efficient energy-conversion unit.”
Whereas a typical drying plant would require 800 kWh to evaporate a ton of water, initial tests conducted by Hermann and his team showed that the DeBugger dryer would use less than half that amount. “Compared to the process for evaporating water from wet biomass or the effluents of traditional anaerobic digestion plants, this is a very carbon- and cost-effective system,” says Hermann. “What’s more, by producing gas fuel rather than solid fuel, it can provide the feedstock for a far greater number of non-specialist power generation plants. It has a far more universal application than standard combustion processes.”
Secondary market
But the Debugger concept not only offers a way of converting sludge and slurry into energy. It could also help solve another problem which has to do with fertilisation of agricultural land.
Currently, if sludge and slurry are removed and incinerated, whether in mixed combustion plants, cement kilns or municipal incineration facilities, this process dilutes nutrients such as phosphates to the point that they are beyond recovery. Agriculture thus loses valuable sources of soil enrichment.
Alternatively, spreading excessive amounts of manure on fields creates exactly the opposite problem: over-fertilisation of soils and eutrophication (over-nutrition) of inland and coastal waters. This produces algal blooms that damage delicate ecosystems, while diverting valuable nutrients away from crop-growing.
This focus on the circular economy couldn’t have come at a more favourable time for us
With stricter enforcement of existing directives on the use and recovery of nitrates expected – Germany and Switzerland are even preparing legislation mandating phosphorous recovery – it will become almost impossible to incinerate this form of biowaste in power plants. According to Hermann, the DeBugger gasification process offers a solution to the problem of nutrient recovery as it leaves a concentrated, nutrient-rich solid residue, enabling compliance with more stringent regulation.
Hermann sees this as a secondary market and an important element in the commercialisation of their prototypes. “Certainly synthetic gas fuel is the primary output, but the beauty of this closed loop system is that by recycling plant nutrients for controlled fertilisation in agriculture it has a much wider environmental application,” he says. “It expands the commercial possibilities for municipal utilities and others who wish to adopt it.”
Circular economy
In fact, Hermann sees their project as a pioneer for the recently launched EU Circular Economy, for which the European Commission will present a new strategy later this year “There is a growing sense within Europe that future economic growth will involve, to some extent, more products being manufactured from secondary raw materials. Indeed, waste can be considered a valuable resource. This requires business models that retain physical goods for longer and keep them in efficient productive use for longer. This focus on the circular economy couldn’t have come at a more favourable time for us.”
The DeBugger project has established a clear path to commercialisation, says Hermann. It is supported by KIC InnoEnergy, the organisation founded by the European Institute of Innovation and Technology (EIT), to support the development and commercialisation of sustainable energy solutions in Europe. The financial support from KIC InnoEnergy has enabled the Swedish team to commission the first prototype of the dryer and conduct a series of comprehensive tests into its functionality, the feeding system, the steam parameters, and the reliability of the system in a commercial setting. The first pilot, with a capacity of several hundred kilograms per hour, was unveiled at a waste-water treatment plant in Skellefteå, Sweden earlier in the summer.
KIC InnoEnergy also introduced the project team to researchers at the University of Stuttgart, one of its academic partners, and one of the leading research institutions for the type of double-fluid gasification process used in the DeBugger gasifier. Researchers at Stuttgart and Outotec are now looking at the impact of different feedstocks, such as sludge, manure, or chicken litter, how they behave in the system, and the quality of the gas they produce.
“We know that sewage sludge and manure can produce fuel gas that has similar qualities as gas produced from forest residues; and we know that we can achieve that at higher gasification temperatures. What we need to investigate is what happens at lower temperatures,” explains Hermann. “We can gasify at 650 degrees rather than 850, for example, but we want to understand more about what that does to the quality of the end product.”
Research is also continuing into improving the quality of solid residue for use in controlled fertilisation. Hermann points out that the double-fluid test bed system means that the separation of contaminants from solid residues is a possibility, which the team believe has positive implications for the usability of the end-product, as well as the synthetic gas produced.
DeBugger is targeting large farmers, cooperatives, municipal companies and service companies as possible clients, says Hermann. The company is already in discussion with a number of municipal utilities and private companies, mainly in Germany and the Netherlands, willing to act as a test bed. “We’re in the stage between R&D and full implementation,” explains Hermann. “But we believe the proposition is a strong one. We are on course to have proofs of concept up and running by the second half of this year. Our goal is avoid the need for subsidies and achieve full commercialisation before the end of 2016.”
Helalley Abdelhady Hady Helalley says
Excellent and practicable technique , I hope to replicate at my authorized area .
Thanks