Condensing economizers for large scale steam boilers

. An energy analysis and assessment of the feasibility of the development and implementation of a condensing economizer (CE) for steam generator TGM-96A has been made. The condensing economizer is characterized with a very high heating capacity of 21 to 23.5 Gcal/h and will cool the fuel gases from 123°C down to 47°C. The utilized heat will be used to heat three water loops: two loops with District Heating Network water (DHN) and one with raw water from the Dnieper River. The expected improvement in the efficiency of the steam generator is 10.55%. Complete thermal calculations of the condensing economizer have been carried out along with a definition by flow and temperature of the heat carriers. Based on the calculations of the heat and mass transfer processes, the thermal capacity of the economizer under the conditions of partial condensation of water vapor from the flue gases was estimated. The analysis presents an objective assessment of the project’s investments, as well as other financial indicators and environmental benefits that give the investor the opportunity to put the project into operation.


Introduction
The efficient use of fuels in large energy boilers is predominantly related to the utilization of wasted heat from the exhaust gases. One of the ways to increase the energy efficiency is to use state-of-the-art condensing economizers to utilize the heat from the exhaust gases of the steam generators [1,2,3]. Common practice shows that at this stage deeper heat utilization from boilers burning natural gases, methane, and propane butane is more promising [2,3]. The efficiency of the boilers burning natural gas is in the range of 82-94% [4], estimated by the lower heating value (LHV). At temperatures between 110-220°C for the exhaust gases, the heat losses are estimated at 6-12% [1,4]. In recent years, the socalled condensing boilers, in which the gas is cooled at temperatures below the dew point (58-60°C), have gained popularity [1,2,9]. In these boilers both the physical heat of the gases and the latent heat of condensation are utilized, which allows the fuel efficiency to be significantly increased by up to 15%. For example, in [5] it is shown that when the gas is cooled down to 0°C, the amount of heat during condensation is 11.9% relative to the LHV of the natural gas. However, due to the absence of a source with such a low temperature, the gases are cooled to 20-40°C, which to some extent reduces the effect of using condensation heat.
A number of studies [6] have found that the condensation of water vapors from flue gases dramatically increases the cost of each cooled degree. The most important factor in this cost increase is the initial moisture in the flue gases; the higher the moisture content, the higher the cost. To increase the efficiency of the conventional boilers by 1%, a reduction of gas temperature by 15-20°C is needed. The same share of efficiency with condensing boilers can be achieved with cooling the gases by 2-3°C only. The aforementioned studies [2,6] prove that when the exhaust gas is cooled from 150°C to 40°C the fuel economy reaches 10-12%. Moreover, the heat loss of the exhaust gases, related to the upper LHV, is 2-3%, with efficiency reaching 96-97%. If the heat balance of the steam generator is made by the lower combustion heat, the efficiency reaches 105-107% [1, 2, 3, 10].

Subject of this energy analysis
CHP-5 is the largest thermal power station in Ukraine with an installed thermal capacity of 1874 Gcal/h and installed electric power of 700 MW. The plant has four units, two power units of 100 MW and two of 250 MW. The plant has two vertical water-tube steam generators, model TGM-96A; two co-current vortex steam boilers, model TGMP-314А; three water heating water-tube boilers, model PTVM-180; and two hot water boilers, model KVGM-180. The main fuel for the steam generators is natural gas, and fuel oil, M100, is used as a standby fuel.
CHP-5 provides domestic hot water to the 850,000 inhabitants of Kyiv, as well as heat energy to 3 million square meters of industrial buildings.
The heat energy is supplied through 6 main pipelines with diameters from 900 mm to 1200 mm.
For both thermal and electricity production 800,000 m 3 of natural gas, with a calorific value of 34,330 kJ/Nm 3 is consumed annually.
The construction of CHP-5 started in 1960. The first and second power blocks were put into operation in 1971-72 with the third and fourth in 1974-1976.
The subject of this energy analysis are the two TGM-96A steam generators. The gross thermal efficiency of the steam generators, according to their load, is within  = 92% to 94% at exhaust gases temperatures of 123°С to 125°С.
The project's goal is to assess the opportunity for condensing the flue gases below the dew point cooling temperature, using a Condensing Economizer (CE) thereby increasing the efficiency of the steam generators by 8% to 10% -and also reducing natural gas consumption and CO 2 and NO x emissions while saving significant quantities of heat in CHP-5's regeneration cycle and the heat distribution network.

Analysis of the Parameters for Determining the Degree of Flue Gas Cooling
Today, indirect surface-type heat exchangers, using water as their coolant, are the most common. Their critical advantage is the provision of optimal price, performance and technological solution.
The key factor, that influences the residual heat of the flue gases utilization in this type of regenerative heat exchanger is the quantity and temperature of the cooling fluid (usually water) [7]. The secondary factors affecting the performance and design characteristics include: type of heat exchanger; pitch and arrangement of the heat exchanger's tubes; velocities of the gases and the water; pressure drop in water and gas flow; composition of the exhaust gases (including excess air ratio and air humidity); and heat exchange surface area/type [ 8,9,10].

Boilers' Capacity and Quantity of Exhaust Gases
Based on the data provided by KTE:  Boiler #1 operates about 2,648 hours per year at an average capacity of 206 Gcal/h (240 MW);  Boiler #2 operates about 5,204 hours at an average capacity of 230 Gcal/h (268 MW). Both boilers do not operate simultaneously with the CE -it operates with either boiler and will be switched from boiler to boiler as appropriate.
During three of the winter months, the average capacity of one boiler is about 259 Gcal/h, which is when the conditions for condensing mode are the best. Therefore, when determining the amount of exhaust gases required for calculating the size of the CE it is assumed that the maximum boiler capacity is 260 Gcal/h (302 MW).
Taking into consideration the quantity and the composition of the fuel and the combustion products, and the excess air ratio (which must not exceed 1.4), the amount of exhaust gas is assumed to be 411,000 Nm 3 /h. The analysis uses this method of estimating exhaust gas volume instead of calculating the theoretical amount of gas at full boiler capacity.

Exhaust Gas Temperature
The temperature of the exhaust gases at different time periods and loads ranges is between 110°C and 130°C. This analysis assumes an average exhaust gas temperature of 123°C.

Quantity and Temperature of Water Flows for Gas Cooling
A CE has one to several water inlets and outlets, with water flow rates and temperatures sufficient to cool the exhaust gases below the dew point of the water vapor in the flue gases at the respective partial pressure.
At CHP-5 it is possible to use three types of water streams: district heating net (DHN) water; additional water to refill the district heating system; or fresh water from the Dnieper river for the CE's technological needs.

District Heating Network (DHN) Water
Using water from the DHN's return water is a significant advantage for the CE as there is sufficient water at a temperature close to the condensation temperature of the water vapor in the flue gases.
It is possible to build the CE with two or three independent water loops and outlets in the exhaust flue gases, using DHN water with a flow rate of 2000 m 3 /h to 3000 m 3 /h. The circulation pump with a required flow rate and head will be used to transport the water. Another possibility is to use the head of the network connected pumps, installed before the boiler system, for running the water through the economizer. (The different strategies will be considered during the CE's design process.) The DHN return water temperature ranges between 52°C and 55°C in the summer months -June, July and August. During the winter season the water temperature is between 46°C and 50°C. Experience shows that when water temperature rises from 53°C to 55°C the outlet gas temperature is usually higher than 56°C to 58°C (when sufficient cooling water flow rate is present). At this temperature, the CE partially condenses the exhaust gases. Therefore, during the 4 summer months, the inlet water does not ensure a stable condensing process in the CE. During the winter period, the network water in the CE can utilize a power of 10 to 15 Gcal/h, and in summer 8 to 10 Gcal/h. The distance between the proposed CE site and the heating network is about 250m.

Water Added to Compensate the Water Losses from the District Heating
System (Makeup Water) The CE could use the district heating system's makeup water. The chemically treated and purified makeup water, with a flow rate of 400500 m 3 /h and a temperature of about 30°C, could be used by the CE for providing year-round condensation of water vapor in the exhaust gases at almost all loads of the heating networks and the boilers. If this water source is used, it is recommended that the makeup water is heated by 10°C before entering the deaerator and added back to the heating network.
Adding water with such properties is an advantage ensuring condensation at the CE and saving about 8 Gcal/h to 10 Gcal/h of thermal energy output.
Circulation would be provided by a pump with suitable head and flow rate. The distance from the CE's proposed site to the makeup water source is about 80m to 100m.

Fresh Water from the Dnieper River for Technological Needs of the Plant
The Dnieper River's waters could also serve as the CE's cooling water. Water circulation is supported by a pump with suitable pressure and flow rates. It is very likely that the river water would need to be treated. The distance to the water source (i.e., the Dnieper River) is about 80 -100 m.  Table 1 presents the average annual flow and temperature data for the different possible water sources.

Condensation Conditions
The easiest and most straightforward way to significantly reduce CHP-5's fuel use is deep flue gas cooling to and below, the dew point temperature of water vapor in the flue gases and further use the latent heat of condensation.
The maximum amount of heat utilized from the flue gases is represented by the amount of heat released under full condensation (at a temperature below the dew point t=0 o C and moisture content of d=0 g/kg dry gas) related to the lower heat value (LHV) of the fuel. The utilized latent heat is 11.9% more, related to the LHV -depending on the amount of air in the flue gases and its moisture content, the partial pressure of the gas mixture, the temperature of the gases, the characteristics of the cooling surface, etc. -this percentage may vary. There is no precise theoretical relationship used for determining the percentage of heat recovery in industrial-scale condensation in pipe economizers due to the significant physical, construction/design and other factors that affect the process.
However, a number of studies have shown that the condensation of water vapor in flue gases significantly depends on the excess air ratio, which changes the condensation temperature [8]. High excess air ratios may compromise the economizer's performance by limiting condensation and collecting a reduced share of the latent heat. By increasing the excess air ratio (α), flue gas condensation is hindered, in turn requiring deeper cooling of the flue gases. Therefore, an excess air ratio of α < 1.4 before and after the economizer is recommended, to allow the process of water vapor condensation in the flue gases to start at a temperature higher than 53.5°C (Figure 1). Reducing and maintaining the excess air ratio (α) at the lowest possible level is important for operating the CE in an efficient manner.
In conclusion, there are three potential water sources with the needed quantities and at the required temperatures able to cool and condense the flue gases from the boilers. The CE's maximum output is expected to be in the range of 21 to 23.5 Gcal/h (24.5 to 27.3 MW), which represents an increase in efficiency from 9% to 10.5%. Generally, it is possible to achieve an additional increase of about 1% to 2%, but this requires complex engineering and equipment, which are not cost effective.   2 shows the general appearance of the CE installed at Kyiv's CHP-6 after the ALSTOM water-heating boiler with a capacity of 180 Gcal/h. The thermal capacity of the CE is 15.6 Gcal/h, saving 8% to 10% of the hot water boiler's fuel consumption. This analysis assumes the following will take place at CHP-5: the CE will operate with an average real load on the boiler (see Table 2) of 222 Gcal/h; the boilers will operate with an 8.6% increase in their efficiency (which is both theoretically, reasonably and practically possible), the CE's average annual power value is 21 Gcal/h, and the CE has a stable and reliable operation and maintains its design parameters.
The average annual performance indicators of the CE, which can be switched between the two boilers, are presented in Table 2.
A schematic of CHP-5's condensing economizer is shown in Figure 3. The figure shows the three water inlets and outlets and their temperatures as well as the temperatures of the incoming and outgoing flue gases used in the calculations.
Thermal calculations of the condensing economizer are performed with specialized software. Input data accepted for the analysis of the condensing economizer are presented in table 3.
The parameters that most strongly affect the degree of utilization of the residual heat of the flue gases in the concerned recuperative heat exchanger types are the flow rate and cooling fluid temperature.
The next other significant factors influencing the efficiency and design features are the type, step and pipes arrangement, optimal velocities of gases and water, acceptable resistance losses on the water and gas path, composition of the exhaust gases, incl. excess air coefficient, air humidity, and total heat exchange surface.
Input data analysis shows that when using the condensing economizers (CE) a couple of closed water loops with flow rates and temperature sufficient to cool the gases below flue gas dew point temperature at the respective partial pressure.
Three types of water flows are possible at CHP-5: network water supplied by the district heating network; additional water to fill the district heating system; raw water from the Dnieper River for the power plant's technological needs The temperature of the returning network water is in the range from 52 to 55 о С during the summer period (June, July and August). During the winter season the temperature is between 46 -50 о С.
The performed analysis and thermal calculations show that both in a winter and summer mode when the inlet water temperature in CE is in the range of 46 -56 о С and the optimal flow rate of the used network water is lower than gases will always be greater than the flue gas dew point temperature ( i.e. this loop will operate as an ordinary economizer without condensation of water from the flue gases. In this case the flue gases outlet 75 о С.
In terms of the above, the partial condensation of water vapor in the flue gases will take place in the second or third contour of the at the same temperature and flow, condensation of water vapor in the flue gases will not occur again, but such a process will instead surface.
Therefore a third loop with make-up water at a flow rate of 32 о С, is considered, while the gases will be cooled to about expected to reach more than 60% (it is well known that practical results and empirical relations for degree of condensation are not available not only for different construct but also for different condensation terms).
The results of reaching high levels of condensation can be loop with filtered water for technological needs temperature lower than 25 о С. Average steam boiler output, N Flue gas temperature after steam boiler, t Excess air ratio in the burning chamber, Excess air ratio after boiler, Boiler efficiency (Gross) Total operating hours for both boilers Fig. 3. The schematic of the condensing economizer proposed for implementation at CHP lower than 1000 m 3 /h, the temperature of the exhaust lue gas dew point temperature (which is 56-58 о С), i.e. this loop will operate as an ordinary economizer without condensation of water vapor the flue gases outlet temperature will also drop to about the partial condensation of water vapor in the flue gases will take CE. If main water runs in the second loop again , condensation of water vapor in the flue gases will not instead be initiated only on a part of the pipe wall up water at a flow rate of 420 m 3 /h and temperature of is considered, while the gases will be cooled to about 45 о С, and condensation is expected to reach more than 60% (it is well known that practical results and empirical relations for degree of condensation are not available not only for different constructions of condensation can be achieved by using a third loop with filtered water for technological needs from the river at up to 620 m 3 /h and a annual data for one boiler incorporated with condensing economizer The schematic of the condensing economizer proposed for implementation at CHP-5 Table 3 presents the input data (composition and calorific value of the fuel, steam generator parameters, etc.) needed for the CE calculations. Table 4 summarizes the calculation parameters used for the analysis of the CE. It is well known that carbon steel corrodes rapidly when exposed to hot condensing flue gases. When there is a presence of higher concentration of acidic components in the combustion gases, corrosion is accelerated.
Flue gas dew point corrosion occurs when these aggressive acids condense on carbon and stainless steels in the convection sections, flue ducts, and stacks [11]. The amount of contaminants in the fuel is directly correlated with the concentration of acid droplets, and therefore with the degree of corrosion itself. To mitigate these corrosive processes in the proposed CE more resistant materials in the construction of flues are installed.The flue gases will be discharged into the atmosphere via a chimney made of polymer composite materials with a high corrosion resistance and operational lifespan up to 50 years. This will avoid secondary condensation of water vapor in the chimney without applying the very common methodology of mixing condensed flue gases with hot gases before the chimney.
In order to avoid low temperature corrosion over the heating surfaces, ribbed pipes with extruded aluminum ribs are recommended for the CE. The calculations are made on the basis of pipes D=32 mm and wall thickness of 2.3 mm with extruded aluminum ribs 320 pcs/m and a rib height of 15 mm. For the primary loop, where condensation of water vapor does not occur, carbon steel pipes can be used -boiler type st20 /GOST 9567-75; ГОСТ8733; DIN 2391; EN 10305-1/.
For the second and third loop surfaces where the process of water vapor condensation takes place and carbon and nitrogen oxides from the exhaust gases are dissolved in the condensate, the pН of the condensate is lower than 7. This suggests the use of alloy steel pipes with universal application type 304, 304L, 304H, which boast better corrosion resistance to nitric acid with concentrations below 65%, as well as most other organic and inorganic acids.
It is mandatory that stainless-steel pipes resistant to carbonic acid and nitric acid in all are used in all water contours. It is recommended that the CE is designed as a vertical structure to permit a downward flow of the gases. Additionally, when the CE's condensate is to be used for technical needs or the district heating network considerations should be made to ensure that it is collected and neutralized (to a pH>7)..

Project benefits
The project's benefits are estimated based on calculations of the CE's energy use and efficiency:

Heat energy & natural gas savings
After installing the CE, CHP-5 would use the waste heat of the boiler's exhaust gases. The boiler's exhaust gas temperature will be reduced from 123°C to 47°C. DHN water (or heated feeding or raw water) will be running and heating in the CE's heat pipe bundles. Assuming an average boiler power of 222 Gcal/h, an average power of the CE about 21 Gcal/h and 7,800 operational hours per year, the CE will produce 163,800 Gcal/year. Considering the annual heat losses in the pipes to the CE of 477 Gcal/year, the estimated heat savings are 163,323 Gcal/year. 23,500,000 Nm 3 /year of natural gas would be needed to produce such an amount of thermal energy.

Natural gas cost savings
 Natural gas prices in Ukraine are regularly changing and the price change can be significant. Also, there are 2 different natural gas prices, depending on the type of energy (heat or electricity) produced by natural gas: "heat natural gas price" and "electrical natural gas price". KTE staff recommended that this SPFS use the "heat natural gas price", because the CE produces heat that otherwise (when the CE does not work) is produced by a hot-water boiler producing only heat. The data for "heat natural gas price" for 2018, was provided by KTE, and is set in calculation model. This price is: 6,763.51 UAH/1000 Nm 3 (229.51 EUR/1000 Nm 3 ).  Extended forecasts for the changes in price of natural gas should be used in a more detailed feasibility study.  For a natural gas price of 229.51 EUR/1000 Nm 3 ,the annual natural gas cost savings alone would amount to 5,393,454 EUR. This results in a 2.05 years estimated payback period, which is highly promising. With the installation of the CE at CHP-5, annual CO 2 emissions will drop by 44,791 t/year as a result of the 23,500,000 Nm 3 of natural gas saved each year. However, 8,502 MWh/year, of additional electricity will be used to power the water pumps, which will increase CO 2 emission by 8,723 t/year.

Conclusion
The energy analysis and assessment of the feasibility of the development and implementation of a condensing economizer (CE) for steam generator TGM-96A showed that:  There is considerable potential for the recovery of waste heat from the boiler and boiler efficiency can be increased by 10.55% using a condensing economizer;  The CE's maximum output is expected to be in the range of 21 to 23.5 Gcal/h (24.5 to 27.3 MW);  The energy saving measure would result in the following: o Electricity consumption of circulation pumps will increase the overall CE electricity consumption: The pumps' annual electricity consumption is 8,502 MWh