Abstract
This article explores the utilisation of power plant simulator & designer software (PPSD) in modelling power islands, with a specific focus on biomass-based power plants. The article showcases the modelling procedure with respect to full plant simulation through the use of a case study. The model was validated against measured operational data and provides an accurate model for the power station. The thermal model was used in conjunction with a data analysis study to establish one of the root causes of reduced export of electrical power from woody biomass-fired power island over recent years.
Introduction
The growing importance of sustainable and renewable energy sources has led to the development and operation of biomass power plants. The use of forestry remnants and rejected lumber from the paper and pulp industries serves as a promising biomass fuel for power generation. This is the case for the independent power island which utilises these sources of biomass to export approximately 25MWe of electricity to the national grid in accordance with a power purchase agreement (PPA). It is important to consider the entire system from the pre-boiler to condenser when modelling such power islands. Since changes in the thermal layout, geometry and steam supply can affect areas such as the boiler efficiency, thermal efficiency and operating costs, which go unnoticed when various components are considered in isolation.
Accurate modelling and simulation of such power plants are crucial for optimising their design, operation and performance. The use of PPSD provides a powerful tool that facilitates this process in a fast and sufficiently accurate manner.
A power island firing woody biomass was selected as the primary case study since John Thompson currently operates and maintains the plant on behalf of the owner, hence operational data was readily available for model tuning. The developed model was used to illustrate the potential such models can provide to the operator of the plant when troubleshooting performance issues on site and provide a possible resource that can be utilised in boiler operator training programmes.
Simply put, PPSD is a standard calculation program that captures the heat, mass and energy transfer in various heat exchanging devices. The following sections highlight the major components utilised in the modelling of the power island for both the flue gas scheme and water/steam side scheme.
Flue gas scheme
The flue gas scheme of the simulation primarily captures the mass, energy and heat transfer to/from any heat exchanger in the flue gas flow path. In addition to the flue gas flow path, the combustion air flow path is also incorporated into this scheme. Figure 1 highlights the flue gas path, the air path, and the furnace for the power island.
One of the critical aspects of modelling biomass power plants is simulating the heat transfer process within the furnace. For utility scale combustion, the Blokh or Gurwich method is often employed due to its effectiveness in capturing the complex heat transfer mechanisms involved to provide an estimate on the heat absorbed by the furnace walls, the exit flue gas temperature and radiative heat transfer component exiting the furnace in a computationally efficient manner. The flue gas path captures the flue gas interactions (i.e. heat and mass transfer) between the various heat exchangers, from the furnace through to the superheater and heat recovery towers, eventually ending off at a node that represents the flue gas conditions before the stack. It is helpful if the flue gas scheme is set up to correspond to the general arrangement of the boiler, making the thermal model a lot easier to convey to boiler operators and other engineers.
Water/steam side scheme
As the name suggests, the water/steam side scheme captures the corresponding heat exchanger that incorporates water or steam as its working fluid. In addition to the water side heat, mass and energy considerations, the furnace walls and subsequent circulation system as well as the balance of plant (BOP) components are captured in this scheme. Figure 2 provides an overview of a power island’s water/steam scheme highlighting the main boiler water/steam flow path and the BOP components.
PPSD offers a comprehensive library of pre-defined BOP components, allowing users to assemble a complete power plant model. They are configured, interconnected and simulated to represent the behaviour of the power island. As with the flue gas path, the water/steam path should also correspond to the general arrangement of the boiler. Since it is important to ensure the correct water/steam flow direction, input temperatures and qualities are captured in each heat exchanger, as this will affect the mechanism of heat and energy mass transfer in the system.
Thermal model development and validation
As with all numerical modelling approaches, validation is paramount for establishing a sufficiently accurate and representative model. The thermal model development and validation was performed as follows:
1. The model was developed using available drawings and operating manuals capturing the geometric layout and configuration of the power island, from the boiler to the BOP setup.
2. Validation was performed using available plant performance data. From this data the various fouling factors and operational assumptions were tuned to minimise the errors between the operational data and the model predictions at 100% MCR.
3. Using the tuned model, further low load cases were conducted using the thermal model and compared to measured data to showcase the model’s suitability for various load cases.
The thermal model was shown to provide sufficiently accurate predictions for load range from 40% to 110% MCR, with errors ranging from 0.5 to 6% for the various parameters of interest, which included the flue gas path temperatures, the heat exchanger steam loads and the water/steam temperatures.
Modelling results
One of the primary issues that the operators at the power island in question have experienced since the performance tests were conducted is that the exported electrical power delivered has decreased in the past year. In conjunction with the development of the thermal model, a detailed data analysis of the available operational data was conducted to investigate and try to pinpoint the cause of the problem through the comparison of year-on-year data. Having access to the operational data allowed for further tuning of the thermal model.
One of the key parameters of interest was the exhaust steam pressure or turbine condenser vacuum pressure. The data analysis showed a vast operational difference in comparison to previous years. The developed thermal model proved useful in determining the effects the turbine condenser vacuum pressure has on the amount of power that can be generated. Figure 3 illustrates this effect, showing that for an increase in the exhaust pressure the generated power will decrease.
These findings tied up well with the data analysis study. The findings were subsequently presented to the operational staff of the power island. Further discussions confirmed that they have had issues with air ingress into the steam ejection system, resulting in a higher operational condenser pressure.
Conclusion
Modelling power islands, especially those using biomass as a fuel source, is a complex but essential task for design, optimisation and performance assessment. PPSD provides a powerful platform to achieve this by incorporating advanced modelling techniques like the Blokh or Gurwich method for furnace heat transfer and captures of the BOP components have on the entire system. The entire system, from the pre-boiler plant to the condenser, can be modelled.
The power island PPSD model was validated against both performance tests and current operational data. Further use of the developed thermal model highlighted the effects the turbine condenser vacuum pressure has on the generated power. These findings correlated well as to what was being seen by the operational staff and from the data analytical study.
The accuracy, computational efficiency and simple user interface make PPSD a valuable tool for modelling power islands, allowing for the investigation into the interactions between the boiler and BOP equipment.
By Brad Rawlins
Design Engineer – Industrial Watertube Boiler Business Unit
John Thompson