Maximising Electrical Efficiency at Waste to Energy Plants

Thomas Norman takes a look at some of the technologies under development aimed at boosting the electrical efficiency of waste to energy facilities.

Recycling Waste to Energy
31 January 2011

The practice of incinerating waste from businesses and households began approximately 100 years ago. Since then, the overall purpose of waste incineration 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.

In Denmark waste is not only an environmental matter, but a means of ensuring a reliable energy supply in the future, as Danish incineration plants produce both district heating and electricity from waste.

In a few years the plants will be replaced by more effective and dependable plants, increasing the production of clean electricity for the benefit of people and the environment worldwide.

Tomorrow's efficient incineration plants will produce even more electricity. But how does this work on a purely technical level?

More efficient plants

Waste to Energy facilities provide plant owners with high revenues from energy production, and are no longer used solely for waste disposal. At the same time, they must operate like efficient power plants with the lowest possible environmental impact.

Today there is a concerted focus on optimising the 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 rather than 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?

When developing a new, efficient plant, there is a need to be aware of the importance of not accounting solely for electrical efficiency. It is the plant's overall efficiency and emissions output that determine its quality.

It is not difficult to achieve a high level of electrical efficiency if the turbine's condenser is cooled, for example with sea water.
This, however, is not a sustainable solution, as it would result in cooling, and thereby losing 40% to 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 incineration 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, and is 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 electrical efficiency of 27%.

If, for example, sea water from the Limfjord in Denmark was used at this plant to cool the turbine's condenser, electrical 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?

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 incineration 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 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 electrical 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 incineration 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 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.

Incineration technology of the future

In the immediate future, requirements for incineration 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 two-step waste incineration will constitute the solutions that lift steam data to an even higher level.

Two-step waste incineration is based on dividing the incineration 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 incineration in the second phase, which takes place further down on the combustion grate.

The flue gas from the pure incineration is led up through an extra superheater, where the steam temperature can be raised even more.

Thomas Norman is Technology Development Department Head at Babcock & Wilcox Vølund