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ScienceDirect Energy Procedia 72 (2015) 119 – 126

International Scientific Conference “Environmental and Climate Technologies – CONECT 2014”

Efficiency diagram for district heating system with gas condensing unit Girts Vigants*, Gundars Galindoms, Ivars Veidenbergs, Edgars Vigants, Dagnija Blumberga Riga Technical University, Institute of Energy Systems and Environment, Azenes iela 12/1, Riga, LV 1048, Latvia

Abstract The integration of a flue gas condensing unit in a district heating (DH) system source increases the efficiency of the DH system due to the partial recovery of flue gas heat. A multi-regression equation obtained linking the gas condensing unit efficiency indicator with several statistically significant independent variable parameters based on the statistical analysis of the DH system data in Ludza, Latvia, is discussed in this paper. Parameters related to the interaction between the gas condensing unit and the DH system, boiler capacity and the influence of the DH network return temperature on the efficiency of the gas condensing unit is discussed in more detail. To assess the efficiency of the gas condensing unit and to show the distribution of the boiler house heat load between the boiler and the gas condensing unit during the heating season, a diagram is proposed. As a result of the installation of a gas condensing unit after the boiler, the primary energy savings during the heating season is 11.8 %. © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment. Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment Keywords: Gas condensing unit; Heat recovery; Heat load; Boiler house; Boiler efficiency

1. Introduction With the increased use of woodchips as a fuel for heat sources in central heating systems, efficient use and optimal operation of the boiler house becomes a matter of discussion and research. If the moisture content of the woodchips is high, they are burned with low efficiency. In practice, the moisture content may be 45 %, or in winter

* Corresponding author. Tel.: +371 28316444 E-mail address: [email protected]

1876-6102 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment doi:10.1016/j.egypro.2015.06.017

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it may reach 55í60 %. Fuel moisture and water, which is formed by the burning of the hydrogen in the fuel, produce water vapor which is then discharged from the boiler together with the flue gas, thereby increasing heat loss in the exiting waste gases to 20 % of the combustion heat released [1]. The vapor content of the exhaust gas may reach 10í20 % of the volume of the flue gas [2]. In this case, boiler efficiency can be improved 10í15 % by equipping the boiler with a flue gas condensing unit installed behind the boiler [3, 4]. A flue gas condensing unit can recover some of the latent heat by condensing the water vapor; it can also recover some physical (sensible) heat by flue gas cooling, thus recovering up to 50 % of the heat lost from the exiting flue gas [1]. ~2500 kJ of latent heat are lost with every kilogram of steam emitted into the atmosphere. The higher the moisture content of the fuel, the more heat can be recovered by the condensation process. Studies [5, 6] have shown that up to 30í35 % of the boiler heat produced can be recovered by using deep flue gas cooling. Vapor condensation in flue gases is a complex process involving heat and mass transfer between the wet gases with a high content of non-condensable gases and water droplets in the case of a direct contact heat exchanger [7]. Unlike indirect contact gas condensing units, droplets do not have a distinct heat and mass transfer surface. The heat transfer between the gas and the surface on one side and the surface of the water on the other side produces thermal resistance in an indirect contact gas condensing unit and causes the temperature difference. As a result of a direct contact gas condensing unit, water can be heated to a temperature that is about 10–15 °C higher [1]. This paper discusses the operation during the 2012/2013 heating season of the direct contact gas condensing unit installed after the woodchip boiler at the Ludza heating system boiler house. The condensing unit is made in Latvia. The results of the study have been used to make a diagram that assesses the distribution of the boiler load between the boiler and the gas condensing unit and fuel savings during the heating season. Boiler house capacity is selected according to consumer load. The amount of fuel the boiler house will consume per year is calculated according to the load. In the case of a boiler house with a flue gas condensing unit, it is possible to use a smaller amount of fuel due to the heat recovered by the flue gas condensing unit. 2. Methodology The object of the research is the municipal boiler house in the town of Ludza, Latvia, which is fitted with an 8 MW woodchip boiler equipped with a direct contact gas condensing unit and also a diesel-fired peak load boiler. The boiler plant’s operating experience has shown that, in the case of very low outdoor temperatures, the woodchip boiler capacity can be increased to 10 MW. The gas condensing unit schematics, description and the connection to the DH system are discussed in detail in a separate publication [8]. The research was done by analyzing the performance data from the Ludza heating system during the 2012/2013 heating season using correlation and regression statistical analysis techniques. STATGRAPHICS Plus software was used for the statistical processing of the data. The test data set contains 474 operation modes and 17 parameters. Data was recorded every three hours per day during the heating season. Data analysis of the industrial experiment is performed to establish a statistically significant set of operating parameters, which significantly affects the heat recovery efficiency of the gas condensing unit, as well as to obtain a multiple regression equation of binding parameters and to assess its adequacy. To evaluate the performance of the gas condensing unit, an efficiency indicator of the gas condensing unit capacity is used, which is defined as:

Ec

N co ˜ 100 / N b ,%,

(1)

where Nco – gas condensing unit capacity, MW; Nb – boiler capacity, MW. The gas condensing unit efficiency indicator shows the part of the boiler capacity that can be recovered by the deep cooling of flue gas.

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As a result of the statistical analysis of the data, an equation is obtained linking the gas condensing unit efficiency indicator with seven independent variables:

Ec

7.84433 + 0.0793491 ˜ G t - 1.82416 ˜ K sh + 0.394416 ˜ K sv -

(2)

1.96617 ˜ N b - 0.0255361 ˜ t g2 + 0.966587 ˜ t k2 - 1.07522 ˜ t r .

An analysis of variance shows that all seven independent variables are statistically significant at the 95 % confidence level. Equation (2) explains 94.86 % of the variability in Ec. Independent variables of the equation: Nb– boiler capacity, MW; Gt– flow of water in DH system circuit, m3/h; tg2– flue gas temperature after flue gas condensing unit, °C; tk2– water temperature after gas condensing unit and before heating system heat exchanger, °C; tr– water return temperature of heating system before heat exchanger, °C; Ksh– spray water ratio in horizontal part of gas condensing unit, kg/kg.d.g; Ksv– spray water ratio in vertical part of gas condensing unit, kg/kg d.g; kg d.g.- kg dry gas. The range of the variables changes during the experiment shown in Table 1. Table 1. Range of variable changes Parameter Unit Range

Gt

Ksh

Ksv

Nb

tg2

tk2

tr

m3/h

kg/kg.d.g.

kg/kg.d.g.

MW

°C

°C

°C

107í216

0.034í1.005

3.19–22.04

2.8–9.4

50í70

51í64

43.3í58.3

The following equation is used to determine the spray water ratio: ‫ܭ‬ௌ ൌ

ீ೎ ήఘೢ ήଵ଴଴଴ ௅೏೒

ǡ

(3)

where Gc– amount of spray water delivered through spray nozzles in horizontal and vertical parts of gas condensing unit, m3/h; Ldg– amount of dry flue gas, kg/h; ȡw– density of water, kg/m3. The spray coefficients Ksv and Ksh are used depending on whether the water is sprayed into the gas condensing unit’s horizontal evaporative part or vertical condensation part. 3. Analysis of the results The independent variable parameters from equation (2) can be divided into two groups: x related to the gas condensing unit: Ksh, Ksv, Tg2, Tk2; x related to the interaction between the gas condensing unit and the district heating system: Gt, Nb, tr. One of the essential parameters of the heating system network interaction is the return temperature of the heating system network [1]. To initiate the condensation of water vapor, the flue gas needs to be cooled to the dew point

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Condenser efficiency indicator Ec, %

temperature. A further temperature drop will determine the amount of condensation of the vapor and the total latent heat recovery. This will determine the effectiveness of the gas condensing unit. The adiabatic dew point temperature of the flue gas is close to 65 °C. This is influenced by the moisture content of the flue gas [9] and the oxygen content of the exhaust gas [10]. In order to achieve condensation, the temperature of the spray water in the vertical part of the gas condensing unit must be lower than the dew point temperature. The spray water is cooled in the heating system heat exchanger by the heating system network return water. The heating system water thus acts as a cooler, and the efficiency of the cooler is determined by the return water temperature. Changes in the gas condensing unit efficiency indicator depending on the network return temperature are shown in Figure 1. 30 25 20 15 10

y = -0,7022x + 49,698 R² = 0,5111

5 0 40

45

50 55 DH return temperature tr, °C

60

Fig. 1. Changes in gas condensing unit efficiency indicator depending on the heating system network return temperature

Condenser efficiency indicator Ec, %

It can be seen that by lowering the return water temperature by 1 °C, the value of the gas condensing unit efficiency indicator increases by 0.7 %. The heating system network return temperature is a variable value and depends on the outdoor temperature, system heat load, control mode (qualitative or qualitatively-quantitative) and performance of consumer heating units. Influence from all of the above-mentioned indicators describes the distribution of the data shown in Figure 1. In order to cover the variable heat load during the heating season, it is necessary to vary the boiler capacity. Measured boiler capacity and the corresponding changes in indicator values can be observed in Figure 2. 30 25 20 15 10 5

y = 0,1849x2 - 4,2999x + 33,214 R² = 0,5791

0 2

3

4

5 6 7 Boiler capacity Nb, MW

8

9

10

Fig. 2. Changes in gas condensing unit efficiency indicator depending on the capacity of the boiler

A data analysis of the DH system indicates that an increase in boiler capacity also increases the capacity of the gas condensing unit. However, in cases of higher boiler capacities, the ratio between the gas condensing unit and

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boiler capacities is smaller than in the case of lower boiler capacities. The value of the gas condensing unit efficiency indicator decreases non-linearly when boiler capacity increases and data scattering is observed. The changes in the average efficiency indicator are described by the second degree equation

‫ܧ‬௖  ൌ ͲǤͳͺͶ ή ܾܰ ʹ Ȃ ͶǤʹͻͻ ή ܾܰ  ൅ ͵͵Ǥʹͳ

(4)

70

28

60

24

50

20

40

16

30

12

20

8

10

4

0

0 -30

-20

-10 Ambient temperature, °C

0

Boiler capacity Nb, MW

DH return temperature, °C

The DH system operating modes depend on the outdoor temperature, and the mode determines the necessary boiler heat capacity to cover the required consumer load and also the DH network parameters of water flow, outgoing temperature and return temperature. Outdoor temperature is an important but not the only factor that determines the boiler capacity and network return temperature. Changes in boiler capacity and return temperature observed during the heating season depending on the outdoor temperature are shown in Figure 3.

Temperature Capacity

10

Fig. 3. Changes in boiler capacity and network water return temperature depending on the outdoor temperature

It can be seen that the changes of two parameters have the same focus - by increasing boiler capacity, the network water temperature increases. The reduction of the gas condensing unit efficiency indicator by increasing boiler capacity (see Figure 2) can be explained due to the higher return temperatures in the DH network. To achieve a higher capacity of the gas condensing unit during very low outdoor temperatures, the DH network return temperature must be lowered. This can be achieved by increasing the energy efficiency of the consumer buildings, by reducing heat loss in DH networks or by using the heat pumps in the gas condensing unit [4] or the DH network circuits [11, 12]. In order to assess the operation of both pieces of equipment to cover the required boiler house load, the graph seen in Figure 4 was created.

Condenser efficiency indicator Ec, %

30

y = 16.612x - 80.882 R² = 0.8594

25

y = 12.24x - 84.339 R² = 0.8475

y = 29.274x - 86.392 R² = 0.8808

Reg. 3 MW

20 y = 9.7046x - 80.778 R² = 0.7962

15

Exp. 3 MW Reg. 5 MW Exp. 5 MW Reg. 7 MW

10

Exp. 7 MW Reg. B,5 MW

5

Exp. 8,5 MW

0 0

2

4 6 8 Boiler house capacity NBH, MW

10

Fig. 4. Boiler and gas condensing unit impact on boiler capacity provision

12

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Four different values for changes in boiler house capacity are taken into account: 3 MW, 5 MW, 7 MW and 8.5 MW. The corresponding graph was obtained from experimental data by choosing the mentioned change values for boiler house capacity with the range of ±0.1 MW. For example, the capacity value of 3 MW describes data in which boiler capacity values are within the range of 2.9 MW í 3.1 MW. Capacity of the gas condensing unit and energy efficiency values corresponding with each characteristic boiler capacity form experimental points through which the graph is drawn. If the gas condensing unit is turned off, its efficiency is 0 and the boiler house capacity is determined by the boiler capacity. It can be seen that the graph has lines with different slopes that form a beam of rays. For each experimental point, the efficiency indicator value was calculated using a multi-regression equation (2). As can be seen, the match of the points is good and the regression equation can be used to create a graph. The diagram in Figure 5 was created to show the interdependency of boiler capacity, boiler house capacity and the changes in the gas condensing unit energy efficiency indicators. This diagram is formed by a beam of rays in which each line corresponds to a boiler capacity ranging from 3 MW to 10 MW in increments of 1 MW. An average gas condensing unit energy efficiency indicator value is created using equation (4). The indicator changes depending on the boiler capacity are characterized by the curve shown in the diagram. Points 1 and 2 are shown to explain the use of the diagram. If the boiler house capacity at Point 1 is 6.8 MW, then line 1-2 intersects the average gas condensing unit efficiency curve at Point 2, where the indicator value is 12.5 %. A line representing a boiler capacity of 6 MW goes through Point 2. This means that the boiler house capacity is covered by a boiler with a capacity of 6 MW and by a gas condensing unit with a capacity of 0.8 MW. Condenser efficiency indicator Ec, %

25 22.5 20 17.5 15

2

12.5 10 7.5 5 2.5 1

0 2

3

4

5 6 7 8 9 10 Boiler house capacity NBH, MW

11

12

13

Fig. 5. Distribution of boiler house capacity between the boiler and the gas condensing unit

If the boiler house capacity and gas condensing unit energy efficiency indicator values are known, boiler capacity can be determined by the equation: ܰ௕  ൌ  ܰ஻ு௕ Ȁሺͳ ൅ ‫ܧ‬௖ ሻ.

(5)

Boiler house capacity is determined by the consumer heat load converted to the boiler house outlet. In such a case, NBH = Qth. The load changes during the heating season are characterized by the load duration graph, which can be seen in Figure 6 (upper curve).

Girts Vigants et al. / Energy Procedia 72 (2015) 119 – 126 12,00

HeatloadQth,MW

10,00 8,00 6,00

Heatload Boilercapacity

4,00

Condenser capacity

2,00 0,00 0

1000

2000

3000

4000

5000

Loaddurationʏ,h

Fig. 6. Distribution of boiler house load in the case of an installed gas condensing unit

By using the diagram shown in Figure 5, it can determined what part of the load is covered by the boiler and gas condensing unit. The changes in boiler and gas condensing unit loads during the heating season are shown in Figure 6. By knowing the boiler house and gas condensing unit capacities and their duration, it is possible to calculate the summary energy produced by the boiler and gas condensing unit together during the heating season. The results are shown in Figure 7. 30000

Producedenergy,MWh

25000 20000 15000 10000 5000 0 Total

Boiler

Condenser

Fig.7. Energy produced during the heating season: total, by the boiler and by the gas condensing unit

An analysis of the results shows that it is possible to save 11.8 % of energy during the heating season if a gas condensing unit is installed. 4. Conclusions As a result of a statistical analysis of the DH system data, a multi-regression equation is obtained linking the gas condensing unit efficiency indicator with several statistically significant independent variable parameters. The independent variable parameters can be divided into two groups: those related to the gas condensing unit, and those related to the interaction between the gas condensing unit and the DH system. The adiabatic dew point temperature of the flue gas is close to 65 °C. In order to achieve condensation, the temperature of the spray water in the vertical part of the gas condensing unit must be lower than the dew point

125

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temperature. The heating system water thus acts as a cooler, and the efficiency of the cooler is determined by the return water temperature. An analysis shows that by lowering the return water temperature by 10C the value of the gas condensing unit efficiency indicator increases by 0.7 %. A data analysis of the DH system indicates that an increase in boiler capacity also increases the capacity of the gas condensing unit. The value of the gas condensing unit efficiency indicator decreases non-linearly from 22 % to 8 % if the boiler capacity is 3 MW and 10 MW, respectively. An equation is obtained to calculate the average efficiency indicator. A diagram is offered showing the interdependency of boiler capacity, boiler house capacity and the changes in the gas condensing unit energy efficiency indicators. The diagram is verified in an analysis of the operations of the municipal DH system in the town of Ludza, Latvia. It is determined that during the heating season 88.2 % of the total energy produced is produced by the boiler and 11.8 % is produced by the gas condensing unit. This shows that the primary energy savings in the case of gas condensing unit usage is 11.8 %.

Acknowledgements The work has been supported by the National Research Program “Energy efficient and low-carbon solutions for a secure, sustainable and climate variability reducing energy supply (LATENERGI)”.

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