The EU energy strategy for 2020 and 2050 sets specific targets for the transition of the current European energy system and energy market. The aim of the strategy is to encourage a low-carbon energy system with decreased greenhouse gas (GHG) emissions (by 50% compared with 1990 levels until 2050), increased energy efficiency, and a larger share of renewable energy sources (RES).1 All these developments set new challenges in the conventional thermal power sector. Under these new market conditions, modern, highly efficient natural gas combined-cycle (NGCC) power plants cannot be competitive in several countries and lose market share. Hard coal and lignite power plants are often requested by grid operators to stay in operation as the backbone of the electricity generation system and to increase their operational flexibility, in order to cover the increasing fluctuations of the residual load due to the intermittent RES.2
Most efforts to improve flexibility in existing hard coal and lignite plants begin with measures taken to improve the flexibility of firing systems. Indirect firing systems may play a key role through utilization of pulverized coal dust or pre-dried lignite dust that can be stored in intermediate silos. In addition, the development of new ignition systems without expensive auxiliary fuels enables successful ignition and stable combustion conditions using only electricity. This reduces start-up costs and increases flexibility. This article discusses new developments in firing system technologies. Additional information can be also found in the literature.3–6
FLEXIBILITY REQUIREMENTS AND CHALLENGES
Increasing flexibility in coal power plants is not a straight- forward task, because several operating parameters must be optimized under a high number of constraints. In general terms, the key targets toward increasing flexible plant operation are:
- reduction of minimum load
- increase of ramp up/down rates
- reduction of start-up cost and start-up time
- increase of maximum load period
In parallel, the above-mentioned targets must be achieved under the following conditions:
- lowest investment and operating costs
- highest plant efficiency rate and lowest CO2 emissions, and
- by always keeping within flue gas emission limits
FIGURE 1. Overview and comparison of flexibility measures and impact on the operating mode
A graphical representation of these parameters is depicted in Figure 1. Several of these targets are not fully complementary to each other. Hence, new design principles need to consider a broad range of plant operating modes, so that plant operating parameters can be adjusted and optimized based on system operators’ and market demands.
An overview of the current state−of−the−art technical parameters related to flexible operation of coal plants is provided in Table 1 for (1) older plants commissioned in the 1990s, (2) newer plants commissioned aGer 2000 representing the state of the art, and (3) future plants following highly flexible design characteristics.
OVERVIEW OF FLEXIBILITY INCREASE MEASURES
Mitsubishi Hitachi Power Systems Europe (MHPSE) has pre− sented a comprehensive overview of possible technical measures for retrofit and flexibility increase in existing boilers in several papers.5−7 A short list of the key measures is provided in Table 2 with an acceding order from the “simpler” or “limited” measures to the more “advanced” or “extensive” measures. Similarly, the measures presented on the top of each class are the most “limited” ones within this class. The classification provides only initial guidance and may differ between cases. Furthermore, additional checks on low−load operation are required before undertaking any retrofit measure. The checks have to be carried out within the framework of a comprehen− sive study−and−measurement campaign and include checking:
- current instrumentation and control system installed in each plant and the upgrade possibilities
- the boiler’s static and dynamic stability with different load changes and the planning of retrofit measures
- all other main plant components apart from the boiler (steam turbine, condenser) as well as the balance of the plant (fans, pumps)
- flue gas emissions performance in low−load and dynamic operation (NOx, SO2, CO, particulates)
FLEXIBILITY INCREASE MEASURES (SELECTED EXAMPLES): INDIRECT FIRING
A key bottleneck to increasing the flexibility in existing hard coal and lignite boilers is the firing systems. A possible ret− rofit through installation of additional indirect firing systems can contribute to overcoming limitations and extending the operating range of existing boilers. Indirect firing systems can include an additional pulverized fuel storage (Figure 2). During normal boiler operation the pulverized fuel produced can be partly stored in an additional coal dust silo. The dried fuel dust can be used (1) as supporting fuel for combustion stabilization in low−load operation, (2) as supporting fuel in case of very low−quality fuels, and (3) as a start−up fuel alternative to oil or natural gas during start−ups and shut−downs.
TABLE 1. State-of-the-art and future targets in operating parameters related to plant flexibility
FIGURE 2. Indirect firing system
In indirect firing systems the fuel dust is directly injected into the boiler via a special burner. For these applications MHPSE developed the DST−burner (Figure 3), suitable for indirect fir− ing of different pre−dried fuels. Due to the high turn−down ratio, the DST−burner may be used in a broad load range dur− ing start−up and shut−down, leading to savings in conventional start−up fuels of up to 95%. Furthermore, in lignite power plants the potential integration of an external pre−drying sys− tem may be used for the production of pre−dried lignite, which can be utilized as start−up and supporting fuel in existing and future lignite power plants (Figure 4).
DEVELOPMENT ACTIVITIES: ELECTRIC IGNITION SYSTEMS
To reduce the consumption of costly auxiliary fuels such as oil and natural gas, MHPSE is evaluating the possibility for ignition of solid fuels by electric start-up technologies. Two technologies are currently in development: the electrically heated burner nozzle and the plasma ignition system.
FIGURE 3. DST-Brenner® burner for dried fuel dust (1-core air, 2-fuel, 3-secondary air, 4-tertiary air, 5-fuel nozzle, 6-swirler)
TABLE 2. Possible measures to increase flexibility in existing power plants and expected impact
FIGURE 4. Lignite pre-drying system can aid increase in flexibility of current and new power plants.
The electrically heated burner nozzle is designed for start-up of further burner levels when increasing the boiler load; the plasma ignition system is designed for cold, warm, and hot start-up. The concept is to induce ignition of pulverized fuels through the radiation heat from and through contact with the burner nozzle, which is electrically heated (Figure 5). The proof of concept was successfully demonstrated in 2013 with industrial-scale experiments. The first prototype, modified DS® burners with electrically heated nozzles, has been installed in a 300-MWe CHP plant providing electricity and heat to the city of Hannover and nearby industries (Gemeinschaftskraftwerk Hannover).
FIGURE 5. (a) Bituminous coal ignition with electrically heated burner nozzle: proof of concept; (b) installation of DS® burner with electrically heated nozzle in PP Hannover
FIGURE 6. 70-kW plasma flame incorporated in a 30-MW DS®-burner during the cold commissioning tests
Ignition using a plasma flame (Figure 6) is possible given that plasma is a highly reactive blend of electrons, radicals, atoms, and molecules. Development aims to optimize the plasma flame in low NOx swirled burners for safe ignition of a wide range of fuels while minimizing the necessary plasma power. The implementation of such electric ignition systems aims to reduce supporting fuels and maintenance costs of the complex infrastructure and/or storage of heavy fuel oil, light fuel oil, and gas start-up systems, which require regular safety inspections.11
CONCLUSIONS
This article summarizes recent developments and state-of-the- art technology using firing systems to increase flexible plant operation on hard coal and lignite boilers. Depending on coal quality and market conditions, today’s boilers and combustion systems can be optimized for maximum flexibility with reasonable capital investment. If necessary, coal-fired power plants can be designed for fast-load ramps as well as minimum load operation at 15–20% or lower independent of fuel type. For this application, indirect firing systems are already considered as state-of-the-art technology. Electrical ignition concepts are also currently under development and in a prototype stage. Additionally, the article provides a list of measures toward plant flexibility and provides a ranking of these measures from the simpler concepts to the concepts with the higher complexity. All flexibility options have to be evaluated case by case and take into account the particular technical and economic boundary conditions of each considered case.
Source: Michalis Agraniotis, Malgorzata Stein Brzozowska, Christian Bergins, Torsten Buddenberg, Emmanouil Kakaras - Innovation & New Products Department, Mitsubishi Hitachi Power Systems Europe
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