Optimising industrial cogeneration for reliable energy supply

Paul Boughton

Niclas Krantz looks at the engineering of industrial cogeneration plants in order to manage energy supply even during in-house or grid disturbances. Proper requirement management and dynamic analysis will enable superior results.

Industrial cogeneration power plants are valuable sources of energy to process industries like pulp and paper, steel, petrochemical and cement.Many plants use waste products for fuelling the boilers, which reduces the energy cost drastically.

More important is that these plants can be used to increase the reliability of energy supply. If the cogeneration plant is optimized so that it can keep up steam and electricity supply even during and after disturbances in house or in the electric grid, it can strongly increase the competitiveness in terms of improved availability, product quality, and productivity, especially in developing markets where the power grids suffer from frequent outages and poor power quality.[Page Break]

Grid back-up

In Sweden, the availability of power of the public power system is very high, and still the cogeneration plants serve as an important back-up in the event of thunder lightning strikes and other disturbances in the grid.

Cogeneration plants are generally far more complex than conventional thermal power plants. Multiple boilers, several steam branches, steam accumulator, steam reductions, batch steam consumption, starting and stopping of electric equipment and weak grid connection are some reasons for the complexity.

If the best possible performance is required, it also requires state of the art engineering skills and methods. Solvina has developed superior methods for Engineering and Engineering Management to guarantee high quality results. We will describe the use of systematic Requirement Management and advanced Dynamic Simulator Based Engineering in a Cogeneration project (Fig. 1).

The whole idea is to work with clear goals (requirements), to include the whole plant dynamic behaviour in analyses, verify and validate project results and deliveries continuously, and not close a project until all requirements have been met.[Page Break]

Requirement management

In complex industrial plants, many stakeholders require different things of the plant. The owners want high efficiency and long plant life, the regulators require fulfilment of environmental law, the maintenance department requires easy maintenance, the plant itself requires certain quality of delivery, certain operating conditions that must be handled etc. The list is long, and there is a broad mix of very high-level requirements as well as detail requirements that have to be considered and mixed together.

In order to design, tune and operate a plant in the most optimal way, a lot can be won if an efficient requirement management (RM) system is in place. The RM system is meant to be an important part of the quality plan and aims at giving the following positive aspects.

- Requirement breakdown from high level to detail requirements (traceability of detail requirements to their origin; creates a good overview).

- Clarify responsibilities for different project activities, and plant components.

- Detect faults and deficiencies early in the project

- Enables planned V&V (Validation and Verification) requirement fulfilment.[Page Break]

Cogeneration project

The benefit of using dynamic simulator technology is that it can be used through the whole project, from the early design phases to commissioning and training.

Extreme operating conditions and faults can be studied carefully, and tuning can be made in early stages to meet the performance requirements. Expensive reengineering during startup can be avoided, and the simulator can preferably be used for operator training.

Consequently, the start-up can be made faster, which saves a lot of money due to improved productivity (Fig. 2).

We will describe a recent Cogeneration project where the methods above have resulted in superior plant performance.

One of the major pulp and paper manufacturers in Sweden has recently modernised its mill. A new recovery boiler and a new turbine were purchased.

The existing boilers operate at a lower pressure than the new boiler, so a turbine with two steam inlets and four steam outlets was chosen, one for each internal steam branches with consumers (Fig 3). Solvina got the task to develop the whole steam system control, and hence integrate the new equipments into the existing plant. This was a huge task which required advanced engineering tools.[Page Break]

Operating Requirements

In the early stages of the project, it was agreed with the customer about the operating requirements for the cogeneration power plant. The plant should maintain stability in all normal operating conditions, which were defined and agreed upon. In addition, different disturbances, either internal or external, should cause a minimum of plant trips and outages, in order to achieve best possible productivity.

In a design process it is important to define the different operating conditions and events thoroughly in order to get the right design criteria.

Some of the operating conditions and events that were important for the control design were:

1. Batch steam consumption.

2. Boiler trip.

3. Trip of steam consumer.

4. Loss of main grid and transfer to island operation.

5. Load changes in island operation.

We can observe two different aspects of these requirements. The requirements on the steam system control include events and behavior in the electric grid, which means that analysis must be performed in several technical disciplines. A holistic approach is required. The other aspect is that these requirements are defined on plant level and have to be transferred into detail requirements for control systems, turbine, boiler, generator, etc. One example of dynamic requirement is island operation (Isolated grid). In this case it is not enough to just require island operation capability, but the quantitative aspects must be defined, such as frequency drop after a load change, time constants etc. [Page Break]

Engineering

A simulator model was developed of the whole steam system and parts of the power system. For a given operating condition, analysis could then be made on the whole system, the boilers, steam system, turbine frequency/power control and generator voltage control. Hence, the controls were designed and tuned even for the most complex cases.

It was checked that the plant could handle all the operational conditions set up in the requirements. This was an important stage of verification. Commissioning could then be carried out smoothly without any major changes of settings.[Page Break]

Importance of practical validation

The commissioning is important for the final Validation and Verification of the performance requirements. In order to verify the requirements, real tests were conducted as far as possible. One of the most difficult cases to verify, Island operation, was tested using Solvinas unique HardWare In the Loop (HWIL) simulator based test device, SolvSim Power Station.

The test equipment is connected to the governor and simulates an island network, so that the governor acts as in real island mode while the machine still runs synchronized to the main grid. The plant and the simulator are running together in real-time in closed loop operation.[Page Break]

Results

The results of these efforts were stable and reliable operation with very few outages and large amounts of energy savings. In addition, the plant start-up became very short since the control was already optimised in the simulator. Furthermore the simulator was used for operator training so that the plant staff knew what to expect when the plant was started.

These successful results were reached very much thanks to careful requirement management, and multi discipline engineering using dynamic simulators.l

Enter √ www.engineerlive.com/asia

Niclas Krantz is Managing Director is with Solvina International, Västra Frölunda, Sweden. www.solvina.se