More electricity from waste
Tomorrow's efficient waste-to-energy plants will produce even more electricity. But how does this work on a purely technical level?
In Denmark waste is not only an environmental matter, but a means of ensuring a reliable energy supply in the future, as Danish waste-to-energy plants produce both district heating and electricity from waste. In a few years the plants will be able to be replaced by more effective and dependable plants, which will increase the production of climate-friendly electricity for the benefit of people and the environment worldwide.
People's views on waste are constantly changing, as is the technology used to manage waste. The practice of combusting waste from companies and households began approximately 100 years ago. Since then, the overall purpose of waste combustion has changed from disposing of or reducing the volume of waste, to seeing it as a potential resource to be used as efficiently as possible.
More efficient waste-to-energy plants
The plants are therefore no longer used solely for waste disposal, as they now provide plant owners with high revenues from energy production. At the same time, the waste-to-energy plants must operate like efficient power plants that impact the environment as little as possible.
Today there is a concerted focus on optimising the waste-to-energy plants' production of electricity in order to achieve more efficient energy production.
Widespread interest in increasing electricity production in Denmark and the rest of Scandinavia should be seen in the light of price movements in electricity and district heating.
As it stands, it is more advantageous to produce electricity instead of district heating. Outside of Scandinavia, district heating is not particularly widespread, which is why there is even greater focus on producing electricity. But how can the desire for electricity efficiency be met?
Babcock & Wilcox Vølund, which is among the world's leaders in knowledge of waste-based energy production, technology and equipment used in waste and biomass combustion, will soon be ready with new power plants capable of producing much more electricity. When we develop a new, efficient plant, we are aware of the importance of not accounting solely for electricity efficiency. It is the waste-to-energy plant's overall efficiency and emissions output that determine its quality.
It is not difficult to achieve a high level of electricity efficiency if the turbine's condenser is cooled with e.g. sea water. This, however, is not a sustainable solution, as it would result in cooling, and thereby losing 40-70% of the waste's total energy content. But what, then, is a technologically sound solution?
Reno Nord as a role model
Reno Nord line 4 in Aalborg, Denmark, is a state of the art waste-to-energy plant that produces both electricity and district heating. The plant was built using today's best solutions with respect to the combustion grate, operation control, materials and boiler design. The plant is thus able to operate stably and safely with relatively high steam data of 425°C and 50 bar. One reason for this high level of efficiency is that the cycle has been optimised through preheating the condensate by flue gas cooling after the boiler.
Reno Nord uses 97% of the waste's energy, with an electricity efficiency of 27%. If, for example, sea water from the Limfjord in Denmark was used at this plant to cool the turbine's condenser, electricity efficiency would increase to approximately 35%, but total energy use would drop to 35%, as opposed to today's level of 97%. On the whole, the current solution is much more efficient.
How do we obtain more electricity?
Over the last five years, Babcock & Wilcox Vølund's engineers have focused on optimising electricity production, and our experience has shown that there are five factors that have crucial influence on a plant's ability to produce electricity:
• Steam temperature
• Steam pressure
• Temperature of the cooling agent used on the turbine's condenser
• An optimised cycle
• Stable and robust operation.
The steam temperature is primarily limited by corrosion on the superheaters due to the very aggressive flue gas. High steam temperature can, in part, be obtained by improving the corrosion resistance of the materials used in the superheaters, and, in part, through stable and robust operation control of the waste combustion and the full cycle, so that the flue gas and steam temperatures are stabilised. Doing so prevents great temperature fluctuations, which can accelerate the corrosion process.
High pressure can be obtained by using Inconel coating in the evaporator part of the boiler. The higher the pressure, the more Inconel. As such, the surfaces of the boiler's evaporator part are protected from increasing corrosion resulting from the increasing vaporisation temperature, which, in turn, results from increasing pressure.
When a waste-to-energy plant produces district heating, low temperatures may not be used in the turbine's condenser. On the other hand, it is possible to use two-step condensation, i.e. one step at high temperature and one step at low temperature. This results in partial condensation at low temperatures, thereby improving electricity efficiency.
The entire cycle can be optimised by preheating the condensate before the feed water tank. By using e.g. flue gas cooling, grate cooling and steam extraction from the turbine's low-pressure end, half and full percentage points can be added to the efficiency rating.
It can be difficult to create uniform operating conditions, as the waste's composition can vary significantly. However, stable and robust control ensures that the combustion process is optimal, and that the entire process runs without significant fluctuations.
As such, all settings can be put closer to their upper limits without resulting in e.g. problems with corrosion. At the same time, the turbine's efficiency is better assessed over a longer period of time because the plant runs stably without significant upward or downward adjustments.
Waste-to-energy technology of the future
In the immediate future, requirements for waste-to-energy plants' production of electricity will grow significantly, pushing steam data up to the ranges of 440–480ºC and 70–160 bar. Today there are plants that experiment with such steam data, and the future will show what provides the best balance between electricity production and operating costs.
In the long term, concepts such as WasteBoost™ and two-step waste combustion will constitute the solutions that lift steam data to an even higher level. The two concepts are under development and are part of Babcock & Wilcox Vølund's R&D programme. WasteBoost™ is based on utilising a pure gas derived from gasification as fuel in an external superheater.
Two-step waste combustion is based on dividing the combustion process into two phases by building a partition wall inside the furnace. In the first phase, chlorine and other aggressive substances are extracted from the waste to provide a purer combustion in the second phase, which takes place further down on the combustion grate. The flue gas from the pure combustion is led up through an extra superheater, where the steam temperature can be raised even more.
Thomas Norman is Head of Technology Development Department at Babcock & Wilcox Vølund
Sectional drawing of Reno Nord's waste-to-energy plant in Aalborg, Denmark. The plant is highly efficient and utilises 97% of the waste's energy.
Even more electricity will come from waste combustion in the future.
