A rapidly changing waste market and strict environmental regulations make it necessary for waste to energy operators to run their thermal treatment plants in an increasingly flexible manner in terms of waste throughput, energy efficiency and load profile. Legal requirements for the emissions must nonetheless be kept while the economic returns must be maximised.
A good example of a facility that has recently taken steps to satisfy these fundamentally contradictory requirements is the KVA Turgi combined heat and power (CHP) waste to energy facility in Switzerland. The plant feeds 16 MW of heat to the local district heating network and delivers up to 9 MW of electricity to the power grid. The ratio between the amount of heat and electricity generated at the plant varies, with district heating always taking priority.
Keen to investigate new ways to improve plant profitability and benefit from the higher prices of the electricity spot market, KVA Turgi volunteered to act as a pilot for new software being developed. The aim of the project was to quantify the potential revenues of selling surplus power on the spot market.
The Turgi facility comprises two combustion lines with grates and two steam turbines. Although the plant already demonstrated a very high level of modernisation, with high energy efficiency, the operator was interested in innovative solutions to further increase the plant profitability and operational flexibility. To fulfil these requirements, an advanced control system was needed.
Grate Combustion Optimisation
Together with Austrian process control specialist, VOIGT+WIPP Engineers GmbH, power and automation technology provider ABB installed and commissioned a multi-objective optimisation control system - the WACS Grate Combustion Optimisation (GCO) module - as an add-on to the existing Advanced Combustion Control (ACC) system.
 |
Flexible: the system enabled plant operators to select between multiple operational modes |
The system is intended to optimise the incineration of waste on moving grates by processing plant data such as process measurements, and providing a set of optimal setpoints for the plant actuators.
This is in order to reach given targets such steam ?ow rate and oxygen concentration in the ?ue gases, etc., while minimising plant costs and greenhouse gas emissions. At the heart of the system is a multi-input, multi-output dynamic process model, identified via step tests and historical data, which allows a fast response of the controlled variables.
The final solution relies on state of the art technologies, like Model Predictive Control (MPC), automatic fire-on-grate-position estimation, as well as adaptive setpoints and automatic one-button target optimisation (Operation Mode Selector). These features make it possible to achieve maximal fuel conversion efficiency and lowest flue gas losses, notwithstanding a high variability of the waste quality.
The installation of the WACS GCO system enabled operators at Turgi to select between multiple plant operational modes. In addition to the standard mode, 'maximal steam stability' with +/-3% maximal deviation from target, the WACS GCO can easily be configured to follow preselected targets, such as 'maximal waste throughput', 'minimal CO emissions', and 'maximal thermal efficiency'. Moreover the plant manager can define additional plant-specific optimisation targets.
Three grate sections with adjustable hydraulic movements, a hydraulic waste feeder system, four primary air plenums and two secondary air nozzle rows are available to control automatically the current operational mode to its targets. The system has been operating at the Turgi facility since the end of 2010.
District Heat Forecasting
In addition to the grate optimisation technology, the Turgi plant has also benefitted from the installation of the district heating forecasting solution developed by ABB. Known as WACS District Heating Forecasting (WACS DHF), the system enables waste to energy plants to position themselves as independent electricity providers and sell surplus electric power on the spot market at potentially higher prices.
Generating a very precise forecast of the electricity to be sold on the spot market is not a trivial task. It is in fact not enough to only consider the production capacity of the boiler and derive the resulting electricity that could potentially be produced. The amount of thermal energy absorbed by the district heating system needs to be estimated as well. This is largely because electricity that can be sold on the spot market is that which is produced with the thermal energy, left after the district heating demand has been fulfilled.
KVA Turgi's operators not only had the goal of increasing the already high degree of efficiency and availability at the plant, but also to increase its profitability. As a pilot project, the WACS DHF was installed with the aim of quantifying the potential revenues on the electricity spot market.
In order to realistically judge the quality of the forecast, which is of the greatest importance, the pilot installation comprises a complete forecast system, with WACS DHF running online with real time data. The additional modules "WACS Electricity Forecasting" and "WACS Energy Spot Market" allow the plant manager to act as an IPP by directly accessing the stock market.
Since being installed the system has proven itself capable of predicting electricity production at the Turgi facility. The only difference between the pilot installation and a fully operational system is that the WACS electricity forecasting module is restricted to calculations of the size of the revenues earned and penalties incurred.
No electricity sales were concluded as KVA Turgi is currently bound by a long-term contract with a local utility. However, the forecasts were so accurate that considerable revenues would have been earned under the conditions prevailing at the time.
Silvia Bardi is product manager for waste to energy at ABB.
web: www.abb.com/powergeneration
Waste Management World Issue Archives
