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Gasifying sludge and slurry: Europe’s circular economy in action

Dr Dominic Buchholz

Research from Outotec and Stuttgart University, supported by KIC InnoEnergy, shows that green energy and nutrient recovery provide powerful incentives to explore and commercialise innovative solutions for processing and converting Europe’s sludge and slurry mountain, as Dr Dominic Buchholz explains.

The potential of sewage sludge and farmyard slurries for energy conversion has long been recognised, if not yet fully realised. To date, the biggest barrier has been the moisture content. At typically 90 per cent it precludes combustion in traditional incineration plants and presents the logistical problem of transporting wet waste. The costs of both have, in large part, reduced the commercial viability of such waste for all but the most specialist areas.

Nonetheless, there is a palpable sense of lost potential that colours discussions regarding biomass conversion in general and farmyard waste in particular. The European market alone produces a total of 140 million tonnes of slurry measured in dry substance, which is concentrated in the livestock-intensive regions of north-west Europe.

For sewage sludge and slurry to be considered a reliable fuel source for power generation, efficient moisture extraction is essential. However, the same principles apply to forestry and timber waste and the manufacture of wood pellets for fuel - as separate teams at Outotec, the Finnish engineering and material sciences company, discovered.

The company’s researchers at its Frankfurt site were considering the potential of wet biowaste, while 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.

However, the two research teams from Outotec soon realised that drying was in fact the first stage of a two-phase process. Ludwig Hermann, commercial product manager for metals, energy and water at Outotec in Frankfurt explains: “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.”

Debugger project

From this proposition, the Demonstration of Efficient Biomass Use for the Generation of Green Energy and Recovery of Nutrients project, or DeBugger, was born.

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 tonne 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.”

Interestingly, the key drivers towards commercial adoption of the DeBugger dryer and gasifier are likely to be found in more stringent directives on land use, as well as stricter enforcement of existing directives on the use and recovery of nitrates, and forthcoming legislation in Germany and Switzerland mandating phosphorous recovery.

Current means of disposing of sludge and slurry present two distinct problems. The first is that incineration, whether in mixed combustion plants, cement kilns or municipal incineration facilities, dilutes nutrients such as phosphates to the point that they are beyond recovery. Agriculture thus loses valuable sources of soil enrichment.

The alternative solution of 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, which produces algal blooms that damage delicate ecosystems, while diverting valuable nutrients away from crop-growing.

With greater controls in place regarding phosphate recovery, it is almost impossible to incinerate this form of biowaste in power plants converted for co-incineration of biomass. However, the DeBugger gasification process leaves a concentrated, nutrient-rich solid residue, it overcomes the problems of nutrient recovery and enables compliance with more stringent regulation.

Commercial possibilities

Hermann and his team see 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.”

In fact, Hermann sees their project as a pioneer for the recently launched EU Circular Economy. “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. It’s also going to raise the profile of solutions like ours, and the role they can play in delivering environment, economic and even social benefits. 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. Initial concepts and early stage research evolved into a project that is now 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.

“DeBugger’s sustainability credentials have always been strong,” says Dr. Christian Müller, CEO of KIC InnoEnergy in Germany. “Firstly, despite the underlying complexity of the technology, the project actually offers a relatively simple proposition for addressing two complementary but separate environmental challenges. Secondly, it is perfectly aligned with the need for a more diversification and sustainability in power generation for Europe. And thirdly, it demonstrates the often-overlooked role that energy from chemical fuels can play in any future energy mix.”

Müller also believes that DeBugger’s strength comes from its commercial prospects. “We invest in projects we believe will come to fruition and make a difference,” he says. “One of the stand-out features of the DeBugger project is that it doesn’t just serve a niche market: this is a technology with broad application that serves two distinct industry sectors, boosts the return on investments made in mixed combustion plants, and enables biomass use in unconverted gas-fired plant. However you look at it, there will be demand for their product.”

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 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 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.

The company is already in discussion with a number of municipal utilities and private companies willing to act as a test bed. “We’re in the interesting stage between R&D and full implementation,” explains Hermann. But we believe the proposition is a strong one. We believe the benefits are clear. And we are on course to have proofs of concept up and running by the second half of this year. Our goal remains full commercialisation before the end of 2016.”


Dr Dominic Buchholz is Technology Officer for Energy from Chemical Fuels at KIC InnoEnergy



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