Comparative technical and economic analysis of innovative methods for waste heat recovery from flue gases for boiler type BKZ 220-100

. Two alternative schemes for waste heat recovery from flue gases of boiler type BKZ 220-100 in the Stepnogorsk TPP (Kazakhstan) are presented. The technical solutions are innovative because they create conditions for deep heat recovery even when using battery emulsifiers to purify the gas flow. A characteristic feature of the schemes is the purification of a small part (10-15%) of the stream by means of a bag filter and the mixing of the stream with the main gas flow consisting of moist gases after a battery emulsifier. An analysis and assessment of the technical and economic feasibility of the implementation of the two alternatives has been carried out.


Introduction
Waste heat recovery from flue gases of industrial and steam boilers is always a topical issue for industry and energy [10,14]. Each decrease in the flue gas temperature increases the rate of fuel use, reduces heat losses with the exhaust gases q2 and directly affects the efficiency of the boilers [3,5,9,10,19]. In addition to the direct benefits of fuel economy through heat recovery both the amount of harmful components and CO2 emissions are reduced.
However, it should be noted that despite the undeniable advantages of waste heat recovery methods, there are sometimes insurmountable obstacles to the implementation of such projects. For example, the sulphur content of the fuel is essential [4] to the process -it determines the permissible limit temperature to which the gases can be cooled without causing condensation and low-temperature corrosion on the heating surfaces of the disposal facilities [3]. Sometimes there are energy schemes where it is difficult at first glance to assess the technical and economic feasibility of implementing a waste heat recovery system, especially when more than one technical solution is offered [5]. In the present work, a technical and economic analysis of two proposals for the utilization of waste heat from the exhaust gases of the steam generator BKZ 220-100F [7] in Stepnogorskaya TPP -Kazakhstan [4] is carried out.

Problem description
At the Stepnogorskaya CHP, three steam generators of the BKZ 220-100F type are in operation. No. 5, 6 and 7), burning Ekibastuz coal [4,7]. The average temperature of flue gases for the boiler No. 5 under consideration is very high, 183 °C, which leads to significant heat losses with the flue gases of up to 9  9.5%. The average efficiency of boiler No. 5 does not exceed 84%, which gives grounds to look for opportunities to reduce the temperature of flue gases and thus increase boiler efficiency. It is also important to note that the existing emulsifier second generation [8] cleans the flue gases to a satisfactory degree and there is no reason at this stage to recommend a transition to another ash collection system. A significant disadvantage of wet ash collectors is their high-energy consumption for the circulation of flushing water. Due to the fact that humid gases after emulsifiers have a high relative humidity (in practice, 100%) heating of these is required before entering the pipe to prevent secondary condensation and subsequent sulfuric corrosion in the stack [3,6]. A disadvantage of the presented scheme is the non-utilisation of the heat of the exhaust gases. With a high temperature of 183 °C, the gases after passing through an air preheater second stage enter a battery emulsifier which purifies the solid particles they contain. Thus, the potential from 183 to 125 °C is lost due to the wet method of gas purification used. Moreover, given that the gases purified in the emulsifier are wet (in practice their relative humidity is 100%), they should be heated additionally from 50 to 75 °C before entering the stack by mixing the flow of wet gases with part of the hot air after the air preheater 1 stage (air temperature is 380 °C). The diagram shows that the incoming air from the atmosphere is not heated by a steam heater, but this is done by mixing the outside air with part of the heated air after an air heater of the 1st stage (air temperature is 380 °C).
The preliminary assessment shows a high degree of heat loss, both with the heating air after the air heater 1st stage and with the unused potential of the flue gases before they enter the emulsifier.

The essence of the innovative method for waste heat recovery
There are two schemes for waste heat recovery from the exhaust gases, which create conditions for the elimination of the heating of the exhaust gases after the battery emulsifier [1], without using preheated air from a second stage air heater. These schemes will be analysed in detail to assess their technical and economic feasibility.

Description of the proposed method for heat waste recovery (variant 1)
The proposed method to improve energy efficiency provides for a more complete use of thermal energy of flue gases and the elimination of the need for additional heating in front of the stack. This can be achieved by directing a portion of the gas stream (11.5%) into a bag filter, keeping the same temperature, and then mixing this portion with the main stream (88,5% from the flue gas flow) in the existing mixing chamber to achieve the desired overall temperature.
The main flow of flue gases (88.5%) after the air heater at stage 1 enters an additional air heater-utilizer, with a heat output of 3019 kW. In it the temperature of the flue gases is reduced from 183 to 150 °C, while the captured heat is used to heat the outside air from 5 to 67 °C, which should enter the stage 1 air heater. The produced heat power is able to completely eliminate the old inefficient scheme of heating the air before the air heater. However, it should be noted that the additional air heater-utilizer should be made of thermosyphons to ensure a corrosion-free regime of its heating surfaces in the winter season [6]. After the air heater-utilizer the gases enter the existing battery emulsifier for wet purification of the gases. At the outlet of the emulsifier, the temperature of the gases is reduced to 55 °C at high relative humidity (100%). Therefore, before entering the stack, the gases are mixed in a mixing chamber, with the hot stream of purified gases coming from the bag filter. The temperature of the mixture (75 °C) is normatively determined by the condition to avoid secondary condensation of the gases in the stack.
The aerodynamic resistance of the bag filter, which will be connected in parallel with the emulsifier, does not differ from the resistance of the battery emulsifier, which will allow for the use of the existing exhauster without any changes.
Thus, several positive effects are simultaneously achieved: • Energy consumption for heating flue gases with hot air is eliminated; • Part of the heat energy of flue gases is recovered; • Significant cleaning of gases from ash is carried out; • Emulsifiers are unloaded, the volume of flush water directed to the ash dump is reduced, which reduces the load on the emulsifier and flush water pumps and theoretically makes it possible to reduce their power consumption (for example, by installing a frequency control when it is economically justified by the current prices and tariffs); • If there is market demand and economically viable transportation opportunities, baghouse ash can be sold for use in construction, agriculture or other industries.

Description of the proposed method for heat waste recovery (variant 2)
The second option uses two energy-saving units: a water economizer and an additional air heater.
Some of the hot gases (11.5%) after the 1st stage economizer at a temperature of 289 °C are fed to an additional economizer with a heat output of 1307 kW, in which the gases are cooled down to about 180 °C and then enter a bag filter for gas purification. The economizer is supplied with network water with an initial temperature of 55 °С and a flow rate of 35 m 3 / h and is heated to 87 °С and is again fed to the district heating network. In order to comply with the permissible operating temperatures (t<200°С) of the gases before they enter the bag filter, the water flow in the economizer acts as a regulating factor in the various operating modes. The purified gases after the bag filter at a temperature of 178 °C enter a mixing chamber, where they are mixed with the wet and cooled to 55 °C gases after the battery emulsifier. The mixing temperature should be in the range of 72-75 °C to avoid secondary condensation in the chimney.
Due to the reduction of the gas flow (up to 88.5%) before the air heater 1st stage, the velocity of the gases in the tubular heat exchanger decreases and as a consequence the gas temperature increases from 183 to 190.5 °С. Under these conditions, the main gas flow will enter an additional thermosyphon-type air heater with a heat output of 4355 kW, where the gases will be cooled down to 150 °C before entering the battery emulsifier for purification. In this case, the air heater will heat the outside air from 5 to 87 °C and will replace the existing inefficient method for heating the outside air with hot air after a second stage air heater.
In Fig. 3 is a diagram illustrating the operation of the method for utilization of waste heat from gases.

Analysis of results
Numerical calculations have been performed with specialized software for calculation of energy steam generators in accordance with the widely used Normative method for calculation of boilers [18]. Table 1 presents data on the technical parameters for baseline, variant 1 and variant 2 of waste heat recovery. The technical parameters are determined under real conditions, as the values are accepted as average for all boilers in 2020.
Full thermal calculations of the waste heat recovery units [11,12] (air preheater in combination with a bag filter and economizer) have been made, and the production, installation and commissioning costs have been estimated at European prices. The data from the calculations are presented in Table 2. The ecological payments that can be avoided as a result of the realised savings from coal are also estimated. One of the most important criteria for assessing the feasibility of an investment related to energy efficiency is the minimum investment per unit of energy saved (EUR / MWh) min. This criterion is characterized by a high degree of objectivity, especially in countries where the price of fuels is many times lower than those on the global market. According to the data in Table 1, it can be seen that Option 1 is superior in this criterion.
The analysis shows that the most economically viable is variant 1 in which the investment for 1 MWh of saved energy is 9.71 EUR. This assessment is complex and includes a complex dependence on several criteria: fuel price, operating time of the steam generator, exhaust gas temperature, average boiler load, etc.

Financial analysis
The results from the performed technical economic analysis are used to prioritize the two options proposed in Table 3. This set of energy efficiency measures represents a CAPEX module, which can be used successfully by the company's management in making investment decisions [13,15,16].
In Table 2  The economic life has been set at 10 years for both options; Real interest rate -set at 10% based on the conditions of bank financing in Kazakhstan; the inflation coefficient for 2020 according to data from the State Statistics Service of Kazakhstan is 6.9% [14]; The calculations have been obtained using the 'ENSI economy' v6 software product, with the results presented in Table 3.
The proposed prioritization scheme is strictly informative offering decision-makers a possibility to compare and select the most attractive option.

Environmental and other project benefits/impacts
The main environmental effect of installing waste heat recovery units (economizer with back filter and airheater) are the reduced CO2 and NOx emissions [17]. The different options with included economizer, air heater and back filter are estimated to reduce coal consumption by 6981 to 10662 tons/year, CO2 emissions from 10113 to 15445 t CO2/year and NOx emissions from 44.9 to 68.5 t NOx/year, depending on the selected technology and equipment.
The project's environmental impacts for three investment options are summarized in Table 4.

Conclusion
1. The proposed innovative waste heat recovery methods, comprising a combination of an additional air preheater and a bag filter or alternatively an additional economizer, air heater and a bag filter, is a topical option for steam generators and boilers using "wet methods" for flue gas purification (scrubbers, emulsifiers of the first and the second generation). Both options presented create excellent conditions for deep utilization of waste heat from the flue gases. 2. The conducted technical and economic analysis shows the expediency of the implementation of both options. There is a slight advantage of the second option in terms of simple payback period, NPV and NPVQ, but the difference is insignificant.