The optimization of flames in combustion processes in power plants is an important part of maximizing power plant efficiency. By using emission spectroscopy, digital image processing and intelligent software tools, pollutant emissions can be minimized and combustion safety ensured.
An important problem inherent in the thermal extraction of energy is the associated generation of waste products, in particular the emission of pollutant gases and dust. Every operator aims to maximise the effectiveness of the combustion process, but will often need to minimise emissions, too. The possibilities for a simultaneous fulfilment of these conditions are highly limited. Boundary conditions such as pollutant emission, fuel, combustion safety and the mechanical limitations of the power station components, strongly reduce the flexibility of the operator.
Low operational flexibility is highly unsatisfactory in view of the high level of competition that now exists in many energy markets, and there is an obvious need for the use of intelligent systems to help operators meet their objectives. Intelligent systems are able to optimise combustion in power plants at all load conditions, helping operators to balance revenue needs with environmental objectives. Germany-based Orfeus Combustion Engineering has designed such a system, which the company believes can achieve this aim.
In modern power stations, a large potential exists with regard to the optimization of the flames and combustion process. A great deal of information can be obtained through the analysis of individual flames. This can be done using modern sensor techniques where the analysis is converted into the form of an on-line control. The Orfeus system’s strengths are the conversion of the acquired information and the provision of ‘ready’ regulated quantities for the respective burner at every load condition and every fuel quality.
The Orfeus system
Orfeus is more than a sensoric system. It determines the flame status with special sensors and processes this information as well as essential data from the process control engineering. For example, the analysis of coal mill data, performance of the accompanying burners and the connected flue gas conditioning are absolutely necessary to optimize a coal-fired power plant system. With Orfeus, it is possible to optimize the combustion process with the help of an adaptive multi-variable controller about the individual flames integrally.
Orfeus is an optimizing combustion control system at individual burner level with simultaneous adherence to plant safety. It combines emissions spectroscopy, modern video technology and knowledge-based systems in order to diagnose combustion and gasification processes and to minimize pollutants in thermal power plants, gas turbine systems, gasification plants, and so on.
In addition to monitoring temperature and fluctuations in the burner, the Orfeus system makes it possible to detect a diverse array of thermally excited particles and metal ions online. It is distinguished by the following features:
- Measurements are made without coming into direct contact with the medium inside the combustion chamber.
- Spectroscopy with combined fluctuation measurement is used to help describe the internal condition of the flames and the overall combustion process, and enables local and chronological determinations to be made of radiation emitters and ions in different wavelength ranges. The chronological and local fluctuations in flame intensity in the burner field is drawn upon as a measure of turbulence. This correlates to the turbulence of the flow field and to the particle load of the flame.
- The video system depicts the external condition of the flame and combustion process, and is employed to gain information related to the geometric and chronological flame characteristics according to different time scales.
The information gained from these processes is used to develop an optimal control strategy for the combustion chamber with the help of neural nets, fuzzy logic and classic model generation. The Orfeus system calculates the best possible regulated output for a chronologically and locally optimized process based on three primary characteristic areas: the temperature field; reaction field; and turbulence field. It also makes use of the methods for optimizing non-linear systems with respect to several target quantities (adaptive status-oriented control). The following quantities serve as regulated outputs for control and optimization of the individual burners or overall combustion: fuel feed, fuel quantity, primary air quantity, primary and secondary air distribution or burn-out air, graduated air quantity, degree of swirling, grid speed in incinerators, and so on.
Hierarchy and structure
The internal structure of the Orfeus system is hierarchically designed, based on the Orfeus and boiler sensoric systems. The intermediate level is data compaction, which is topped by data management. On top of this there is a stage with ‘decision agents’. All these are managed by the overall optimization target.
This pyramid structure is interconnected to function as a multi-variable controller, consisting of four main separate working blocks.
The attribute generator: This determines the flame attributes through the Orfeus sensor system and in addition, the boiler and plant characteristics through the local process control system. The Orfeus attributes from video, fluctuation, and spectroscopy are fed into the adaptive multi-variable controller with their average value, and as time-related and local changes.
The adaptive self-learning multi-variable controller: This is the heart of the system and is supplied with data from the attribute generator and the optimizer and works interactively with the manipulated variables.
The optimizer: This is the section in which the optimizing targets and limiting values of the plant are parametrised. The working principle of the optimizer is that a target direction is defined, e.g. improvement of the efficiency with simultaneous minimization of the NOx value and of total air ratio and without exceeding a plant limit value in respect of CO. In parallel to this, other mandatory plant-specific values are to be met.
The manipulated variables for targeted on-line optimization: These are grouped into the systems “Firing”, “Mill/Burner (Level)” and “Individual burner systems”.
Given that fuel quality and burner load often change, automatic burner control allows operators to achieve, according to their objectives, optimized combustion with regard to emissions, production of pollutant particles and plant efficiency.
Orfeus acquires information on the flame radiation characteristics that were previously calculated with chemical models. Other advantages to be gained with the system include:
- Increased boiler efficiency
- Optimization of the fuel-air ratio for individual burners
- Optimization of the fuel-air ratio in the overall combustion process
- Reduction of the excess air
- Control over extinguishing of the flame on an individual burner
- Reduction of the amount of coal in the ash
- Reduction of particles deposited inside the boiler combustion chamber
- Decrease in NOx emissions through primary measures on the combustion side
- Improvement of the ignition stability via online control of the ignition point on an individual burner
- Decrease in the consumption of expensive support fuels
- Improvement of the liquid-gas ratio in the flue gas conditioning aggregates
- Early recognition and correction of wear and coal mill operation; correction of negative effects on the combustion process
- Determination of several measurands (data validation)
- Cessation of discontinuous check of the fuel distribution.
The first complete Orfeus system installed in a power plant unit is at Tiefstack Unit 2.1 owned by the Hamburgische Electricitätswerke AG utility. It comprises six Orfeus lances, each fitted with a video camera and four glass fibres, and with the corresponding number of spectrometers.
Orfeus Combustion Engineering has examined the performance of its system at Tiefstack and has found the following results:
- With the Orfeus system the fuel-air ratio increases and the total air volume decreases. Both are taken to be positive effects with regard to the operational performance of Orfeus.
- With the Orfeus system the CO concentration is substantially lower than without it.
- The optimization strategy was implemented correctly and effectively.
- Boiler operation using Orfeus proves to be clearly more positive regarding residual oxygen at the combustion chamber walls, helping to prevent corrosion.