Environmentally Oriented
Modernization of Power Boilers

To cite this book, please use the following reference:

Pronobis M.
Environmentally Oriented Modernization of Power Boilers.
Elsevier, 2020.
http://modernpowerboilers.org

or this BibTeX entry:

@book{pronobis2020modernpowerboilers,
  title = {Environmentally Oriented Modernization of Power Boilers},
  author = {Marek Pronobis},
  publisher = {Elsevier},
  year = {2020},
  note = {\url{http://modernpowerboilers.org/}}
}

1. Introduction We are witnessing ground-breaking changes in the global energy sector. The construction of new large units is becoming more and more risky as there is no guarantee of a return on expensive investments in a realistic period of operation. This situation is a consequence of the reduction of CO₂ emissions and other environmental requirements. In this context, the modernization of existing units is of great importance because, at relatively low capital costs, it is possible to continue their operation. Drastic ecological requirements and strict competition in the energy market pose serious challenges both for companies producing utility boilers, as well as power plants and heat and power plants that use them. These challenges have a direct impact on the design and operation of boilers. Properly carried out modernization can extend the boiler’s lifetime and enable compliance with environmental protection regulations that are constantly being tightened up.
2. Boiler Efficiency and Thermal Losses The basic way to decrease the impact of a boiler on the environment is to reduce fuel consumption and pollutant emissions. The emissions are identified by means of direct measurements, while fuel consumption depends on the thermal efficiency of the boiler. It is determined in a formalized manner based on relevant standards. The differences between these methods can be significant. Hence, the comparison of results from before and after modernization must be made using the same method. This book is based on the standard BS EN 12952-15, which is in force in the European Union. Chapter 2 presents basic information on thermal balancing of boilers based on this standard.
3. Modernisation to Reduce the Flue Gas Loss The flue gas or stack loss can be decreased by reducing the two parameters of influence: the temperature of the flue gas t_GO and the air excess number λ_GO. The lowering of the t_GO can be obtained through the following modernization activities:
   • reconstruction of convection surfaces (superheaters and economizers),
   • reconstruction of air heaters,
   • application (or modification of the existing one) of a cleaning system of the heating surfaces,
   • and installation of additional heat exchangers as waste heat recovery systems.
   The main limitation of lowering the t_GO is to prevent condensation of dew from the flue gas on the boiler elements. However, the t_GO should be selected taking into account the economics of boiler operation. The algorithm for selecting the optimal flue gas outlet temperature has been presented. Reduction of excessively high λ_GO can be obtained by reducing the ingress of “false air” as well as by modernization of combustion system to allow operation with lowered air number.
   1. Lowering of flue gas temperature
   2. Selection of the minimum flue gas temperature at the boiler outlet
   3. Optimisation of flue gas outlet temperature
   4. Lowering the air excess number in the boiler
4. Reduction of Nitrogen Oxide Emissions Nitrogen oxides produced in combustion processes of boiler furnaces are a dangerous environmental pollutant. The amount of nitrogen oxides emitted by the boiler depends mainly on the fuel characteristics, the furnace type determining the temperature level in the combustion region, and the oxygen concentration in the initial section of the flame. A reduction in NOx concentration in flue gas is realized by means of primary methods, based on the modification of the furnace processes, and secondary methods, based on flue gas treatment using appropriate reacting substances. Practically all primary methods make use of the following two basic processes: staging of air and staging of air and fuel, sometimes supported by recirculation of flue gases to the combustion area. Secondary methods are realized on the basis of two fundamental processes: SCR - Selective Catalytic Reduction and SNCR - Selective Non-Catalytic Reduction. There are other technologies also, but their dissemination is limited.
   1. Formation of nitrogen oxides
   2. Impact of operating conditions of the furnace on emissions of nitrogen oxides
   3. Methods of reduction of nitrogen oxide emissions in PF boilers
   4. Secondary methods of NOx reduction
   5. NOx reduction methods without the use of ammonia or urea
   6. Combined methods of NOx control
   7. The future of NOx emission reduction methods
5. Modernisation of Fuel Grinding Systems Although the mill systems are only auxiliary elements in pulverized fuel boilers, their impact on the correct operation of the boiler is considerable. Through modernization, it is possible to achieve improvement in the operation of the mill system to obtain an optimal particle size distribution PSD. Many modernizations are connected with the need to meet NOx emission limits. Another criterion, particularly important due to the increased share of renewable energy sources in power systems, is the need to increase the flexibility of the power plant operation. Important issues are preventing the deposition of pulverized fuel in pipelines and to reduce the wear of the grinding system components. Coal mills can also be used for reducing harmful emissions from the boiler. Appropriate modernization can not only decrease the sulfur content in the pulverized fuel, and thus the SO₂ emissions, but also the emissions of arsenic and mercury. An important objective of modernization of the grinding systems is to equalize fuel and air streams to individual burners, which is needed to ensure the required NOx emissions and combustibles content in ash.
   1. Quality of pulverised coal
   2. Coal mills
   3. Modernisations of coal mills arising from low-NOx combustion
   4. Modernisation to improve the operating conditions of pulverisers in dynamic states
   5. Modernisation of pulverisers to reduce harmful emissions
6. Replacing Coal with Other Fuels Modernization of boilers for the combustion of fuels other than anticipated during design may be necessary for a number of reasons. The following cases are possible:
   • modernization caused by switching to another grade within the same type of fuel, e.g., transition from anthracite to coal with a high content of volatiles,
   • modernization caused by switching to another type of fuel, e.g., transition from coal to gaseous or liquid fuel,
   • modernization enabling combustion of more than one fuel by introducing new fuel while maintaining the possibility of burning the current fuel.
   In particular, problems related to the replacement of coal with natural gas (including coal-mine methane), blast furnace gas or low-quality syngas as well as heating oil were analyzed. The transition from hard coal to lignite or coal-water slurry in the PF boiler was also discussed.
   1. Introduction
   2. Replacement of coal with natural gas
   3. Replacement of coal with blast furnace gas and low quality syngas
   4. Replacement of coal with fuel oil
   5. Replacement of hard coal with lignite
   6. Modernisation for the combustion of various fuels in the same boiler
7. Adaptation of Boilers for Biomass Burning The combustion of various forms of biomass, including the utilization of biomass-containing waste, may be the cheapest option for reducing greenhouse gas emissions. Materials of this kind are more often burned together with coal in steam generators. There are also modernizations of power boilers, as a result of which coal is completely replaced with biomass. Quite often, to facilitate biomass burning, PF boilers are converted into grate or fluidized bed boilers.
   Circulating fluidized bed/bubbling fluidized bed boilers or stoker-fired boilers that have been designed for coal combustion can be quite easily adapted for biomass combustion. This adaptation is generally limited to the addition of an appropriate fuel supply system. Therefore, the considerations in this chapter are mainly limited to the description of modernizations aimed at adapting PF boilers to biomass combustion or its co-firing with coal. This chapter also deals with the modernization of power boilers for the co-incineration of waste and waste derived fuels.
   1. Types of biomass used in the power industry
   2. Adaptation of PF boilers for biomass burning
   3. Complete replacement of coal with biomass
8. Harmful Phenomena in Modernised Boilers Modernization of boilers to reduce the harmful impact of the energy sector on the environment may result in various unfavorable phenomena, such as different types of corrosion, erosion, and deposition of various substances in the flue gas path. They cause serious operational problems, which in extreme cases may make modernization difficult or even impossible. The chapter presents high and low temperature corrosion phenomena of boiler components on the flue gas side, as well as threats to boiler reliability resulting from harmful phenomena on the water side. In addition, problems related to erosion by fly ash, as well as fouling and slagging of heating surfaces are presented. An important problem of ammonium bisulfate deposition in boilers equipped with SNCR and SCR systems has also been described.
   1. HT corrosion on the flue gas side
   2. LT corrosion on the flue gas side
   3. Fly-ash erosion
   4. Fouling
   5. Slagging
   6. Condensation of sulphates
9. Conversion of an Existing Boiler to a Condensing Boiler The need to reduce CO₂ emissions is one of the biggest challenges facing today’s power systems. Increasing the boiler efficiency directly reduces the emission of not only greenhouse gases, but also other harmful substances. The most effective method to increase efficiency is to recover latent flue gas heat. In this case, modernization consists in converting the existing boiler into a condensing boiler. The greatest increase in efficiency is achieved by the use of this technology in boilers fired with fuels with a high content of hydrogen or water. It should be taken into account that the actual increase in boiler efficiency is greater than the ratio of gross calorific value to net calorific value due to the significant lowering of the flue gas outlet temperature and the recovery of its physical enthalpy. Flue gas condensation systems are designed in two ways: as a single-stage recuperative heat exchanger and two-stage systems with the first stage of the cooling recuperator and the second stage in the form of scrubber condensing water vapor and cleaning the flue gas. Both of these solutions are presented as examples of industrial applications.
   1. Condensing technology
   2. Industrial applications
10. Increasing Flexibility of Boiler Operation In recent years in Europe, the priority has been the production of electricity from renewable sources (RES), which work in a stochastic manner. In such a situation, fossil-fuel power plants are required to balance power grids by compensating for the variable electricity supply from RES.
    To determine the flexibility of operation, the following parameters of the boiler are taken into account: maximum and minimum possible load, resilience to frequent start-ups, and large and rapid load changes. Also, the boiler should meet the needs of the power unit, e.g., maintaining high cycle efficiency in a wide range of loads. For these purposes, extensive modernizations of boilers may be necessary. An example may be the introduction of an indirect combustion system replacing or supplementing the direct connection between burners and coal pulverizers.
    In the last section, an innovative cogeneration cycle for hot water boilers is presented. Depending on temporary needs, this system can work either as a source of only hot water or as a cogeneration unit.
    1. Adaptation of the boiler to work with a load higher than nominal
    2. Lowering the minimum boiler load
    3. Frequent start-ups and large and rapid load changes
    4. Increasing flexibility of boiler pressure parts
11. Interactions Between Emission Reduction Systems Obtaining ever-lower emissions of harmful substances requires not only devices with very high efficiency, but also ensuring optimal operating conditions for each of the systems in their simultaneous operation. Various emission reduction technologies interact and their interaction can be beneficial as well as adverse for the operation of the power unit. This chapter describes interactions between devices that reduce particulate matter emissions, as well as sulfur and nitrogen oxides emissions.
    1. Introduction
    2. Interactions between NOx reduction systems and dust removal systems
    3. Interactions between SOx reduction systems and dust removal systems
    4. Influence of flue gas dedusting and NOx reduction systems on wet FGD