Modernising Mehrum: 40 green MW from a coal plant

12 July 2004

With tightening emissions limits and the looming reality of carbon trading, middle-aged coal fired plants in Europe might be considered to face a difficult future. One notable exception is the imported-hard-coal fuelled Mehrum plant, near Hanover.

Although Mehrum is over 25 years old, this station can now look ahead with a reasonable degree of confidence, even optimism, following extensive upgrades carried out over the last couple of years aimed at increasing competitiveness and cutting carbon dioxide emissions.

The plant, owned by E.On together with the nearby municipalities of Hanover and Braunschweig (through Enercity and BS Energy respectively), has been the beneficiary of E.On's programme to systematically modernise its fleet of large coal fired plants, with other recent projects including major upgrades at Scholven and Wilhelmshaven, for example.

Mehrum, equipped with a once-through Benson boiler supplied by Steinmueller, was built in response to the 1973 oil crisis and designed for mid merit order, load following, operation, frequently starting up and shutting down. It has undergone nearly 4000 start-ups to date and has averaged no more than about 4100 full-load operating hours per year. However, in recent times, with liberalisation of the German market resulting in closure of several large plants with high fixed costs, and rising electricity prices, much higher capacity factors are expected in the future, strengthening the economic case for the upgrades. The load factor rose to 70% in 2003, and a similar figure is projected for this year.

A concerted programme to improve the economics of Mehrum was launched in 2000. Modifications were made to permit burning of supplementary fuels such as petcoke and sewage sludge (up to about 12 % of total fuel supply), with installation (in 2002) of a system to feed such sludge to each of the plant's six existing coal mills. And in the summer outage of 2003 (28 May to 16 July), following two years lead time of design and planning, the following modifications were carried to increase efficiency, add "green MW", and reduce CO2 emissions:

{blob}Complete upgrade (by Siemens) of the steam turbine.

{blob}Replacement (by Balcke Durr) of the cooling tower internals, with substitution of plastic for asbestos-containing materials, resulting in bigger heat exchange surfaces and more efficient heat exchange.

{blob}Installation (by Babcock Borsig Service) of a system to recover heat from the flue gas for combustion air preheating.

The overall effect of these upgrades has increased output by some 40.5 MW - about 8 MW more than originally anticipated - taking the plant's installed capacity from 712 MWe to over 752 MWe. Since 1998, the number of employees at the plant has gone from around 230 to 130, so in terms of MW per people employed productivity has nearly doubled. Production costs in 2003 were 2.74 euro cents per kWh.

This rise in installed capacity has been achieved entirely through increased efficiency, entailing no additional fuel consumption. Plant efficiency has gone from 38.45% to 40.4%, with a saving in CO2 emissions of 193 000 t per year (at a cost of around 5.6 euro/t).

Of the added MW, it is estimated that the cooling tower upgrade has contributed around 4 MW and the flue gas heat recovery some 6.5 MW. But by far the biggest contributor, accounting for around 30 of the new found green MW, is the turbine upgrade, which underpins the whole strategy of increasing Mehrum's profitability and maximising the asset's productive life.

Downloading a new steam turbine

The steam turbine upgrade has amounted to a complete rebuild with - to use the fashionable term - "download" of the latest generation design tools, materials and technologies into the 25 year old plant. The result is essentially a state of the art steam turbine, with an overall efficiency 3.8 percentage points higher than the original, 70s vintage, equipment.

The Mehrum steam turbine consists of one barrel-type high pressure (HP) turbine, one intermediate pressure (IP) turbine and two low pressure (LP) turbines. The upgrade consisted of replacing the rotors, the blading and the inner casings in all sections, and optimisation of the entire steam path.

In the case of the HP turbine, a 16th row of blading was added to reduce the thermodynamic loading on each stage and increase efficiency, necessitating a new shaft with dimensions different from those of the original HP turbine shaft.

The new HP blades, both stationary and moving, are of Siemens' 3DS type, with a variable degree of reaction. The 3DS concept uses three-dimensional steam flow analysis. An iterative process optimises the profile and airfoil design as well as the degree of reaction and contour of the flow path for each stage. 3DS blades are also twisted in the circumferential direction, helping to reduce secondary losses at the blade root and tip.

The efficiency of the new HP turbine is 93.6%, believed to be a world record for a retrofitted HP machine.

The IP turbine upgrade consisted of installing a new rotor, a new inner casing, 3DS high efficiency blading and optimised inlet and exhaust flow guides.

The LP turbine upgrade was designed to further optimise steam flow through use of high efficiency blading profiles, minimised clearance losses and reduced exhaust losses through larger exhaust areas, increased from 6.3 m2 to 8 m2.

The L-0 (??) rotating blades are free-standing, made of ??????????, with side entry root, while all the other blade rows are of interlocked integral shrouded design.

The last stage stationary blades are hollow, with either suction slots or a heating system (which in Mehrum case??) to avoid the formation of large water drops, thus reducing water droplet erosion.

The diagram below shows the efficiency gains achieved for each stage of the turbine.

The project has also included replacement of the turbine instrumentation & control and stress monitoring system, contributing to greater operational flexibility (faster start ups, higher output ramp rates, lower minimum load and improved part load efficiency).

Flue gas heat recovery

While the revamped steam turbine was the key to the Mehrum upgrade, the additional 30MWt (6.5MWe) contributed by the flue gas heat recovery system installed during the same outage is also significant, and indeed is one of six emission reduction projects carried out within the scope of the Hesse Tender. The Hesse Tender is a joint initiative of the Hesse Environment Ministry, financial institutions (including Dresdner Bank) and industry to "test the project-based mechanisms of the Kyoto Protocol." The six projects will result in a reduction in CO2 emissions of 1.3 million tones between 2005 and 2009, and will be monitored continuously during this period.

The Mehrum plant has two flue gas lines, each with a Ljungstroem air preheater and its own wet flue gas desulphurisation unit.

In line 2 a gas/gas heat exchanger is used to cool the flue gas before entering the FGD and then reheat it on leaving the FGD. Downstream of the FGD units, the flue gas in line 1 and the reheated flue gas from line 2 are mixed to reach the temperature required by regulations for discharge into the stack (73 deg C).

In line 1 the flue gases leaving the air preheater, at 150 deg C, were fed directly to the wet FGD where the temperature was decreased by quenching. This is where the opportunity for heat recovery derives from. Calculations suggested that the flue gas could be cooled to 85 deg C before entering the FGD unit, yielding a potential of 30 MWt of heat to be recovered via a flue gas cooler.

But how best to use this heat? In the original configuration the air entering the Ljungstroem air preheater was itself preheated using a steam/air heat exchanger fed with steam extracted from the IP steam turbine. This was originally intended for only occasional use, eg in cold conditions and start ups, but is in fact operating almost continuously because of the sometimes problematic nature of the imported coal employed at Mehrum.

The idea of the flue gas heat recovery system installed at Mehrum is to use the heat recovered from the flue gas to provide this air pre-preheating, instead of IP steam extraction, with consequent efficiency gains.

The system uses the Powerise heat recovery system developed by Babcock Borsig Service. This employs corrosion resistant heat exchangers made of plastic tubes in a U configuration inserted in the flue gas flow, with water in the U-tubes used as the heat transfer medium.

Cost effective MW

The significant efficiency gains achieved overall, from the combined effect of turbine modernisation, flue gas heat recovery and cooling tower upgrade are shown in the graph below, while the costs of these and other major projects carried out at Mehrum in 2002 and 2003 are shown in the table.

Included in the table is a new fly ash silo being built to accommodate the expected rise in load factor, to deal with the higher ash content of the coal that Mehrum imports and to help the plant better manage the seasonal nature of the market for flyash, which is used in construction markets that tend to peak in the summer when power generation is lower.

Adding together the costs associated with the increased efficiency, the per-kW cost of the new capacity is around 700 euro, making this a cost effective way of adding new generation, and that is without allowing for benefits that might accrue from carbon trading in the future.

The details as to how carbon trading will pan out for Mehrum are not yet clear, but the position should be clarified as we approach the autumn when the National Allocation Plan numbers will be finalised. As a result of the upgrades the plant has reduced its CO2 emissions by around 1-2%, but CO2 emissions certificates will be based on outputs over the period 2000-2002 when the plant's load factors were low, around 55%. (?? but plant would benefit if basis is per MWh)

However, what we can say is that with demand for its product rising and its costs competitive, to say nothing of the planned nuclear phase-out, the commercial prospects for Mehrum look good, and show what can be achieved by a far from young coal plant in the uncertain era of emissions trading

Features of the L-0 rotating blade: full 3D design; freestanding; thin trailing edge; supersonic airfoil; flame hardened leading edges

Assembly of a blade ring for the new LP turbine

The barrel type HP turbine during assembly at Mulheim

Installation of the IP rotor

Installation of the IP inner casing

Installation of the LP inner casing upper half

Efficiency gains with the new steam turbine

Flue gas line 1, before and after modification. In the modified line heated water from the flue gas cooler is used to preheat air before it enters the Ljungstrom air preheater, instead of IP steam extracted from the turbine, as used in the original configuration

Side view showing layout of modified line 1

Efficiency gains following 2003 summer outage, compared with 2000 figures.



Installation of the IP rotor Installation of the IP rotor
Mehrum Mehrum
The barrel type HP turbine during assembly at Mulheim The barrel type HP turbine during assembly at Mulheim
Assembly of a blade ring for the new LP turbine Assembly of a blade ring for the new LP turbine
Flue gas line 1, before modification Flue gas line 1, before modification
Cylindrical airfoil Cylindrical airfoil
Efficiency gains with the new steam turbine Efficiency gains with the new steam turbine
Flue gas line 1, after modification Flue gas line 1, after modification
Side view showing simplified layout of modified line 1 Side view showing simplified layout of modified line 1
Twisted airfoil (since late 70s) Twisted airfoil (since late 70s)
3DS (since late 90s) 3DS (since late 90s)
Installation of the IP inner casing Installation of the IP inner casing
Efficiency measurements following 2003 summer outage, compared with 2000 figures Efficiency measurements following 2003 summer outage, compared with 2000 figures

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