In the search for ways to improve the efficiency of enterprises in the energy sector, as well as other industrial facilities that use equipment that burns fossil fuels (steam, hot water boilers, process furnaces, etc.), the issue of using the potential of flue gases is not raised in the very first place.
Meanwhile, relying on the existing calculation standards developed decades ago and the established standards for selecting key performance indicators of such equipment, operating organizations lose money by releasing them literally into the pipe, simultaneously worsening the environmental situation on a global scale.
If, like the commandβIf you think itβs wrong to miss the opportunity to take care of the environment and the health of your cityβs residents with benefits for the companyβs budget, read the article on how to turn flue gases into an energy resource.
Studying standards
The key parameter that determines the efficiency of the boiler unit is the flue gas temperature. The heat lost with exhaust gases makes up a significant part of all heat losses (along with heat losses from chemical and mechanical underburning of fuel, losses with physical heat from slag, and also heat leaks into the environment due to external cooling). These losses have a decisive influence on the efficiency of the boiler, reducing its efficiency. Thus, we understand that the lower the flue gas temperature, the higher the efficiency of the boiler.
The optimal flue gas temperature for different types of fuel and operating parameters of the boiler is determined on the basis of technical and economic calculations at the earliest stage of its creation. At the same time, the most useful use of exhaust gas heat is traditionally achieved by increasing the size of convective heating surfaces, as well as developing tail surfaces - water economizers, regenerative air heaters.
But even despite the introduction of technologies and equipment for the most complete heat recovery, the flue gas temperature, according to the current regulatory documentation, should be in the range:
- 120-180 Β°C for solid fuel boilers (depending on fuel moisture content and boiler operating parameters),
- 120-160 Β°Π‘ for oil-fired boilers (depending on the sulfur content in it),
- 120-130 Β°C for natural gas boilers.
The specified values ββare determined taking into account environmental safety factors, but primarily, based on the requirements for the performance and durability of the equipment.
So, the minimum threshold is set in such a way as to eliminate the risk of condensate in the convective part of the boiler and further along the path (in the flues and chimney). However, to prevent corrosion, it is not at all necessary to sacrifice the heat that is released into the atmosphere instead of doing useful work.
Corrosion. We exclude risks
We do not argue that corrosion is an unpleasant phenomenon that can jeopardize the safe operation of a boiler plant and significantly reduce its assigned service life.
When flue gases are cooled to the dew point temperature and below, water vapor condenses, together with which NOx, SOx compounds pass into a liquid state, which, reacting with water, form acids that have a destructive effect on the internal surfaces of the boiler. Depending on the type of fuel burned, the temperature of the acid dew point can be different, as well as the composition of the acids that precipitate as condensate. The result, however, is the same - corrosion.
The flue gases of natural gas boilers mainly consist of the following combustion products: water vapor (H2O), carbon dioxide (CO2), carbon monoxide (CO) and unburned combustible hydrocarbons CnHm (the last two appear during incomplete combustion of the fuel, when the mode combustion is not debugged).
Since atmospheric air contains a large amount of nitrogen, among other things, nitrogen oxides NO and NO2, collectively referred to as NOx, appear in the combustion products, which adversely affect the environment and human health. Combining with water, nitrogen oxides form corrosive nitric acid.
When fuel oil and coal are burned, sulfur oxides, called SOx, appear in the combustion products. Their negative impact on the environment has also been widely studied and is not questioned. The acidic condensate formed when interacting with water causes sulfurous corrosion of heating surfaces.
Traditionally, the flue gas temperature, as shown above, is chosen in such a way as to protect the equipment from acid precipitation on the heating surfaces of the boiler. Moreover, the temperature of the gases must ensure the condensation of NOx and SOx outside the gas path in order to protect not only the boiler itself, but also the flues with a chimney from corrosion processes. Of course, there are certain norms that limit the permissible concentrations of emissions of nitrogen oxides and sulfur, but this does not in any way negate the fact that these combustion products accumulate in the Earth's atmosphere and fall out in the form of acid precipitation on its surface.
Sulfur contained in fuel oil and coal, as well as the entrainment of unburned particles of solid fuel (including ash) impose additional conditions for flue gas cleaning. The use of gas cleaning systems significantly increases the cost and complicates the process of flue gas heat recovery, making such activities unattractive from an economic point of view, and often practically unpaid.
In some cases, local authorities set a minimum flue gas temperature at the mouth of the stack to ensure adequate dispersion of flue gases and the absence of a flue. In addition, some enterprises may voluntarily adopt such practices to improve their image, as the general public often interprets the presence of a visible smoke plume as a sign of environmental pollution, while the absence of a smoke plume can be seen as a sign of clean production.
All this leads to the fact that under certain weather conditions, enterprises can specially heat flue gases before they are released into the atmosphere. Although, understanding the composition of the flue gases of a natural gas boiler (it is analyzed in detail above), it becomes obvious that the white "smoke" that comes out of the pipe (with the correct combustion mode setting) is for the most part water vapor formed in as a result of the combustion reaction of natural gas in the boiler furnace.
The fight against corrosion requires the use of materials that are resistant to its negative effects (such materials exist and can be used in plants using gas, oil products and even waste as fuel), as well as organizing the collection, processing of acid condensate and its disposal.
Technology
The introduction of a set of measures to reduce the flue gas temperature behind the boiler at an existing enterprise ensures an increase in the efficiency of the entire installation, which includes the boiler unit, using, first of all, the boiler itself (the heat generated in it).
The concept of such solutions, in essence, boils down to one thing: a heat exchanger is installed in the section of the gas duct up to the chimney, which perceives the heat of the flue gases with a cooling medium (for example, water). This water can be either directly the final heat carrier that needs to be heated, or an intermediate agent that transfers heat through additional heat exchange equipment to another circuit.
The schematic diagram is shown in the figure:
The collection of the resulting condensate takes place directly in the volume of the new heat exchanger, which is made of corrosion-resistant materials. This is due to the fact that the dew point temperature threshold for moisture contained in the volume of flue gases is overcome precisely inside the heat exchanger. Thus, not only the physical heat of the flue gases, but also the latent heat of condensation of the water vapor contained in them is usefully used. The apparatus itself must be calculated in such a way that its design does not provide excessive aerodynamic resistance and, as a result, deterioration in the operating conditions of the boiler unit.
The design of the heat exchanger can be either a conventional recuperative heat exchanger, where heat is transferred from gases to liquids through a separating wall, or a contact heat exchanger, in which flue gases directly come into contact with water, which is sprayed by nozzles in their flow.
For a recuperative heat exchanger, the solution of the issue of acid condensate is reduced to the organization of its collection and neutralization. In the case of a contact heat exchanger, a slightly different approach is used, somewhat similar to the periodic purging of the circulating water supply system: as the acidity of the circulating liquid increases, a certain amount of it is taken into the storage tank, where it is treated with reagents, followed by the disposal of water into the drainage sewer, or directing it into the technological cycle.
Individual applications of flue gas energy can be limited due to the difference between the temperature of the gases and the demand for a certain temperature at the inlet of the energy-consuming process. However, even for such seemingly impasse situations, an approach has been developed that relies on qualitatively new technologies and equipment.
In order to increase the efficiency of the flue gas heat recovery process, innovative solutions based on heat pumps are increasingly being used as a key element of the system in world practice. In certain industrial sectors (e.g. bioenergy) such solutions are used in the majority of commissioned boilers. In this case, additional savings of primary energy resources are achieved through the use of not traditional vapor-compression electric machines, but more reliable and technological absorption lithium bromide heat pumps (ABTN), which require heat rather than electricity (often this can be unused waste heat). , which is present in abundance in almost any enterprise). Such heat from a third-party heating source activates the internal cycle of the ABTN, which allows you to convert the available temperature potential of the flue gases and transfer it to more heated environments.
Experience the Power of Effective Results
Cooling of the flue gases of the boiler using such solutions can be quite deep - up to 30 and even 20 Β° C from the initial 120-130 Β° C. The resulting heat is quite enough to heat water for the needs of chemical water treatment, make-up, hot water supply and even heating systems.
Fuel economy in this case can reach 5Γ·10%, and increase in the efficiency of the boiler unit - 2Γ·3%.
Thus, the introduction of the described technology allows us to solve several problems at once. This:
- the most complete and useful use of the heat of flue gases (as well as the latent heat of condensation of water vapor),
- reduction of NOx and SOx emissions into the atmosphere,
- obtaining an additional resource - purified water (which can be useful at any enterprise, for example, as a feed for a heating system and other water circuits),
- elimination of the smoke plume (it becomes barely visible or disappears altogether).
Practice shows that the expediency of applying such solutions primarily depends on:
- the possibility of useful utilization of the available flue gas heat,
- the duration of the use of the received thermal energy in a year,
- the cost of energy resources at the enterprise,
- the existence of an excess of the maximum permissible concentration of emissions for NOx and SOx (as well as the severity of local environmental legislation),
- method of neutralizing condensate and options for its further use.
Source: habr.com