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Flue gas purification and heat recovery: A biomass fired boiler supplied with an open absorption system
Applied Energy 96 (2012) 444–450
Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Flue gas purification and heat recovery: A biomass fired boiler supplied with an open absorption system Lars Westerlund a,⇑, Roger Hermansson a, Jonathan Fagerström b a b
Division of Energy Engineering, Lulea University of Technology, S-971 87 Lulea, Sweden Energy Technology and Thermal Process Chemistry, Umeå University, SE-901 87 Umeå, Sweden
a r t i c l e
i n f o
Article history: Received 30 August 2011 Received in revised form 14 December 2011 Accepted 29 February 2012 Available online 27 March 2012 Keywords: Open absorption system Particle reduction Heat recovery
a b s t r a c t A new technique for energy recovery combined with particle separation from flue gas has been tested in this project. A conventional small boiler for biofuel produces besides heat also particles to the environment through the flue gas. Decreasing the impact on the environment is desirable. Increased efficiency can be obtained if the temperature and water content of the flue gas can be further reduced. Installing an open absorption system in the heat production unit fulfils both these demands. An experimental unit has been built and tested in the last 2 years. The results show a reduction of particles in the flue gas by 33–44% compared to the ordinary system. At the same time the heat production from the unit increased by 40% when fired with wet biofuels. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Small biofuel boilers (100 kW) have a high efficiency today but still the amount of particles in the flue gas is too high [1,2]. The purification methods available on the market are too expensive for this size of boiler [3]. The open absorption system can be beneficially used in such heat production units. At the same time as the amount of particles in the flue gas is reduced the main part of the remaining heat in the flue gas after the convection part is recovered. The energy supplied to the system is mainly heat that is produced by the boiler, and only a small amount of electricity is needed for circulation pumps. Sorption technology has advantages compared to conventional heat recovery systems since latent heat in the water vapor can be utilized better [4]. The dew point limitation, a major obstacle to ordinary heat recovery appliances, does not apply to sorption systems. Ref. [4] also concludes that sorption systems have better economic and environmental benefits than condensing boiler system. To our knowledge the open absorption system has not earlier been used for heat recovery and particle reduction in flue gas from biomass boilers. It is assumed that the technique used will give the same particle removal efficiency as a wet scrubber but a more efficient heat recovery. The open absorption system consists mainly of three components: the absorber, the generator and the condenser (see Fig. 1). The working medium (water) is produced by an external system (primarily from biofuel). The flue gas is brought into contact with ⇑ Corresponding author. Tel.: +46 920 491000; fax: +46 920 491047. E-mail address: [email protected] (L. Westerlund). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.02.085
the absorption solution in the absorber. Water vapor is absorbed by the solution and the flue gas is dried, cooled and scrubbed of particles. The diluted solution is pumped to the generator where the absorbed water is separated from the solution by primary heat supply. The concentrated solution is transported back to the absorber in a closed loop. The water vapor is condensed in the condenser giving off primarily the latent heat. The condensed and chilled water is separated from the system after the condenser.
2. Test facility At Luleå University of Technology comprehensive research work concerning the open absorption system has been performed for a long time [5–7]. Development plants for different applications have been constructed, mainly for drying purposes [8,9]. In the actual case the conditions for the absorption solution were more severe, since particles and chemical reactions could ruin the stability of the solution. The facility was connected to an existing boiler and with limited space for the equipment. A long distance between the absorber and generator caused large heat losses during the experiments. 2.1. System description The open absorption system was constructed with stainless steel and integrated with the boiler as shown in Fig. 2 where the absorber and generator are clearly seen. The third main part, the condenser, is the heat exchanger (HEX 4). The cross-current absorber consists of a channel, filled with packings, to create good
L. Westerlund et al. / Applied Energy 96 (2012) 444–450
445
Nomenclature Roman letters Q_ heat transfer rate (kW) V_ volume flow (m3/s) T temperature (°C) TT temperature transmitter and also temperature value (°C) _ m mass flow (kg/s) Cp specific heat capacity (J/kg K) h flue gas enthalpy (kJ/kg dry mass flow flue gas) q density (kg/m3) Subscripts absorber 1 energy from the flue gas absorber 2 energy to liquid flows ref reference value (0 °C) conv heat transfer rate from conventional boiler (kW)
contact between the absorption solution and the flue gas. The flow direction for the solution versus the gas gives the absorber its name; the solution flows vertically while the gas flows horizontally. The generator consists of a number of vertical tubes where the hot flue gas flows inside. The tubes are surrounded by absorption solution. Heat transferred from the flue gas is used for evaporation of absorbed water. A part of the hot gas from the boiler is used as heat input for the generator and the total gas flow passes the convection part of the boiler before entering the absorber. The by-pass over the absorber is closed during a normal run. The number of particles in the flue gas is reduced during contact with the absorption solution in the absorber. At the same time the flue gas is cooled and dried. After the absorber the flue gas flows to the chimney. The particles collected by the solution are separated by a small continuous flow through the filter. The heat from the flue gas increases the temperature of the solution when it flows vertically through the absorber. In HEX 1 and 2 this heat is transferred to the district heating system. The steam produced in the generator is condensed and chilled in HEX 4 giving off heat to the district heating system and the condensate is finally drained from the system. The district heating system is lastly heated through the convection zone in the boiler. Diluted absorption solution from the absorber to the generator is preheated in HEX 3. Concentrated solution to the absorber is cooled when giving off the heat in this heat exchanger.
boiler heat transfer rate from boiler (kW) generator heat transfer rate to generator (kW) extra heat transfer rate according to increased area (kW) conc concentration of absorption solution (mass fraction) frac abs part of total flue gas flow which flows through absorber (mass fraction) Abbreviations HEX heat exchanger TT temperature transmitter and also temperature value (°C) LT level transmitter FM flow meter CU control unit CV control valve P pump
2.2. Instrumentation For an overall view of the system conditions, the temperatures (TTs) and liquid flows (FMs) were measured at the positions shown in Fig. 3. Psychrometer (dry and wet bulb temperature) was used to establish the flue gas conditions after the absorber. Measurements were taken at 10 s intervals. To control the system, the temperature in the generator should be kept at a constant value. This is done by controlling the flow of solution to the generator and the amount of hot gas to the apparatus. If the temperature in the generator is above the set point value, the diluted solution flow to the generator increases by increasing the opening of the control valve (CV1). If it is fully open and the temperature is still too high, the damper (CV2) decreases the flow of hot gas to the generator. The liquid level in the generator should be constant and is controlled with the level transmitter (LT1) and control valve (CV5). 3. Method 3.1. Heat and mass balances A heat balance of the absorber consists of heat given off by the flue gas ðQ_ absorber 1 Þ and heat taken up by the district heating system and solution flows ðQ_ absorber 2 Þ according to Eq. (1). The mass flow, temperature and heat capacity of the absorption solution change and have to be included in a total balance.
_ dry flue gas hafter absorber hbefore absorber Q_ absorber 1 ¼ m Q_ absorber 2 ¼ qwater V_ water C p;water ðTT3 TT1Þ _ solution to abs C p;solution to abs ðTT12 T ref Þ þm _ solution from abs C p;solution from abs TT8 T ref m
Fig. 1. The open absorption system.
ð1Þ
The flue gas enthalpy (h) was calculated with normal psychrometric correlations. The heat capacity value for the dry flue gas was determined by adding each spices specific heat capacity value multiplied by their mass fraction. To estimate the conditions of the flue gas before the absorber the psychrometer could not be used because of temperatures above 100 °C. The water content was determined through software Fluegas knowing the water content in the fuel and the content of oxygen in the flue gas. The total mass flow of dry flue gas from the boiler, was resolved with Fluegas and through manual measurements of the total volume of gas flow. The condensate volume flow was measured with flow meter (FM 2 in
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Fig. 2. The open absorption system integrated with the boiler.
Fig. 3. Control equipment and measurement points in the test facility.
Fig. 2). The volume flow of concentrated absorption solution to the absorber was determined by registration of the opening time for valve CV5 in Fig. 3. During one opening period the volume of 1.55 l flows to the absorber. The mass flow of diluted solution from the absorber could then be calculated by adding the mass flow of condensate to the concentrated solution mass flow. Heat balances for HEX 3 and 4 were also performed. On the primary side of HEX 3 the latent heat for water vapor was included in the balance.
The total heat transfer rate was calculated with the mass flow in the district heating system and the temperature difference between TT1 and TT5. A mass balance for absorbed water from the flue gas _ water absorbed Þ and condensate ðm _ condensate Þ was performed according ðm to the following equation.
_ dry flue gas xbefore absorber xafter absorber _ water absorbed ¼ m m _ condensate ¼ q V_ water m water
ð2Þ
L. Westerlund et al. / Applied Energy 96 (2012) 444–450
The absorbed water from the flue gas and condensate flow out of the system should be equal when running the plant in constant conditions. The humidity ratio and other variables were determined according to earlier descriptions. 3.2. Heat recovery To compare the technique with a conventional boiler system and a water scrubber the following concepts and calculations were performed. Conventional heat transfer rate: Heat transfer rate from the boiler without the open absorption system was calculated according to Eq. (3). The large area in the generator caused a decreased flue gas temperature which is not the case using only the convection part in the boiler. It had therefore to be withdrawn to get the total heat transfer rate.
Q_ conv ¼ Q_ boiler þ Q_ generator Q_ extra
ð3Þ
Q_ boiler ¼ qwater V_ water C p;water ðTT5 TT4Þ _ conc solution C p;conc solution ðTT11 TT10Þ Q_ generator ¼ m _ condensate ðr water þ C p;steam ðTT13 TT11Þ þm Q_ extra
ð4Þ
þ C p;water ðTT11 TT10ÞÞ _ ¼ mdry flue gas hordinary hwith open abs system
In Eq. (4) the generator heat power includes heating of the absorption solution, heating of water (liquid) and superheating of steam. In Q_ extra using enthalpy values include the energy content in the moisture. Moisture content of the fuel: Water content in the fuel (mass part). The district heating system temperature to heating unit: Measured before HEX 1 i.e., TT1. Theoretical possible improvement: Calculated as the energy possible to retrieve from the flue gas if the energy content after the boiler is decreased to the level in the flue gas leaving the absorber. This value is divided by the conventional heat transfer rate.
_ dry flue gas hafter the boiler hafter the absorber =Q_ conv Q_ th: improv ¼ m
ð5Þ
Measured improvement: Total measured heat transfer rate in the district heating system, divided by the conventional heat transfer rate.
Q_ meas improv ¼
Q_ HEX 1 þ Q_ HEX 2 þ Q_ HEX 4 þ Q_ boiler _ frac abs m
!, Q_ conv
ð6Þ
Improvement rectified by heat losses: Total measured heat transfer rate increased by theoretically calculated heat losses, divided by the conventional heat transfer rate. Improvement with a water scrubber: A theoretical calculation of an ideal scrubber without heat losses. The apparatus works with a temperature equal to the district heating system temperature to the heating unit (TT1) and a flue gas temperature after the scrubber 5 °C above this temperature. Reheating of the flue gas after the scrubber is not included in the calculations.
_ dry flue gas hafter the boiler hafter the scrubber =Q_ conv Q_ water scrubber ¼ m
was studied using a micro manometer and pitot tube in different points on each cross-sectional area. The lengths of straight lines before and after each cross-sectional area were fulfilled. The results showed that only 64% of the total volume flow passed through the absorber depending on a high pressure drop over the absorber. The flow resistance is proportional to the square of the volume flow in each circuit (absorber/damper-CV3), and the quota for these flow resistances could then be determined. Even if the temperature changes, this quota is almost constant as long as the flue gas can be treated as an ideal gas. The conditions for the flue gas before and after the absorber are known. With the ideal gas law it is possible to calculate the specific volumes before/after the absorber. The part of mass flow of dry flue gas passing through the absorber can then be determined in each experiment. 3.4. Particle sampling and analysis Particle collection was performed with a 13-stages low pressure impactor from Dekati (DLPI). The particles were separated according to different aerodynamic diameters in the total range 0.03– 10 lm. Two impactors were used at the same time, before and after the absorber. The gas flow to the impactors was taken isokinetically. By comparing these results the purification of the flue gas could be established. Analysis of the particles’ chemical composition was performed with a scanning electron microscope with an attached energy dispersive X-ray detector (SEM-EDS). 4. Results Analysis of the experimental values was essentially based on heat and mass balances. Heat losses from the total system to the room were calculated at 11.4 kW. This value was used in the evaluation of all experiments. An increased electrical input using the open absorption system consists of four liquid pumps and an increased pressure rise for the flue gas fan. In the experimental unit the increased electrical input was estimated at 2.2 kW. The distance between different parts of the system and small volume flows makes instantaneous comparison of measured values difficult. The part of mass flow of dry flue gas that passed through the absorber was 65–66% of the total mass flow of dry flue gas from the boiler for all experiments. The flue gas temperature after the boiler was for all experiments in the range of 150–180 °C. 4.1. Heat transfer rates Heat transfer rates to the district heating system are presented in Fig. 4. Each heat exchanger (HEX 1, 2 and 4) and boiler is shown,
ð7Þ
3.3. Gas flow through the by-pass A leakage is normal for a damper (CV3) in a flue gas system. To determine the magnitude of this leakage measurements were performed with closed damper. The air temperature was 20 °C during the measurements and several different volume flows were investigated. The total gas flow from the boiler and after the absorber
447
Fig. 4. Heat transfer rates to the district heating system.
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L. Westerlund et al. / Applied Energy 96 (2012) 444–450 Table 2 Comparison of heat improvement between installation of an open absorption system and installation of a water scrubber.
Fig. 5. Temperature levels for district heating system and flue gas leaving the system.
Moisture content of the fuel (%)
Improvement rectified by heat losses (%)
Improvement with a water scrubber (%)
50.9 51.7 52.1 52.0 48.0 28.0
44 46 45 41 47 25
23 27 25 19 29 16
is only 1–2 °C higher than water temperature even though there are two drops in temperature, flue gas to absorption solution and absorption solution to water in the district heating system. The heat exchanger (HEX 1) has a large heat transfer area. Not only the temperature but also the relative humidity of the flue gas leaving the absorber is of interest for the heat transfer rate. If the relative humidity of the gas leaving the absorber increases, the heat transfer rate will decrease. The appearance in Fig. 5 around 16 h was caused by external interference. 4.4. Mass balance
Fig. 6. Absorbed water from flue gas and condensate mass flow.
and the total heat transfer rate (total) and heat transfer rate from the absorber (abs) are included. The heat transfer rate from the absorber includes HEX 1 and 2. 4.2. The absorption solution The relative humidity of the flue gas leaving the absorber is 38– 45% RH. This value is somewhat higher than the design data due to the high absorption rate and absorption of CO2 in the solution. The concentration of CO2 is stabilized at a low level and separated from the absorption solution during heat supply in the generator. After the experiments the capability of the absorption solution from the generator was compared with new solution that had never been in contact with flue gas. The results showed only small differences in absorption capability. 4.3. Heat transfer between flue gas and district heating system Fig. 5 shows the temperature of the district heating system (TT1) before heat exchanger (HEX 1) and the dry temperature of the flue gas leaving the absorber (TT25). The flue gas temperature
Comparison of calculated absorbed amount of water from the flue gas in the absorber with measured condensate mass flow after HEX 4 is illustrated in Fig. 6. Average values for these variables correspond well. The time difference is large between absorbed water and condensate from the generator, since large volumes of absorption solution are used in the absorber and generator and only a small amount of water is absorbed. Instantaneous values can therefore not be used. 4.5. Heat recovery improvement Results from six different experiments are shown in Tables 1 and 2. Used concepts in the table are explained in Section 3.2. The increased electrical input is not included in the comparison. Tables 1 and 2 show that increased moisture content of the fuel results in improved heat recovery. A lower temperature in the district heating system has the same influence. The heat losses in the experimental set-up were large. The last column in Table 1 constitutes the most accurate comparison for the open absorption system with a conventional boiler. These values are only for an ideal unit, since some heat losses will always arise. With decreasing water content in the fuel the improvement decreases of course. The open absorption system is superior to a water scrubber since the flue gas is dried using the absorption system. The energy in the moisture is taken care of which increases the improvement. Including reheating of the flue gas after the scrubber increases the difference between the systems. Increased return temperature of the district heating system gives a reduction of the heat recovery that is less significant for the open absorption system compared to the water scrubber.
Table 1 Increase of the heat transfer rate from the system during different moisture contents in the fuel and different temperatures in the district heating system to the heating unit. Conventional heat transfer rate (kW)
Moisture content of the fuel (%)
The district heating system temperature to heating unit (°C)
Theoretically possible improvement (%)
Measured improvement (%)
Improvement rectified by heat losses (%)
65 70 61 70 42 80
50.9 51.7 52.1 52.0 48.0 28.0
46.4 41.0 44.4 52.3 35.1 38.3
50 46 45 41 55 26
23 29 25 24 19 10
44 46 45 41 47 25
L. Westerlund et al. / Applied Energy 96 (2012) 444–450
449
Fig. 7. Size distribution of particle mass, the upper curve for each fuel shows results before the absorber and the lower curve value after the absorber.
4.6. Reduction of particles in the flue gas Particles in the flue gas are collected by the absorption solution during contact in the absorber. The results indicate that the mass from particles in the flue gas before the absorber is dominated by particles in the interval 0.1–0.3 lm, see Fig. 7, a normal size when biofuel is used [1,10]. According to [11] the particle removing efficiency for the wet scrubber is very low in this range. The small size of the particles makes them harmful for humans and should be reduced as much as possible. The percentile reduction that takes place in the absorber is evenly spread, no particle size is over-represented. The removal efficiency is in the same range as reported by Gröhn et al. [10].
Table 3 Particle measurement results in flue gas, fuel: wood chips.
Run 1 Run 2 Run 3
Before absorber at 10% O2 (mg/Nm3)
After absorber at 10% O2 (mg/Nm3)
Reduction of particles (%)
18 23 18
11 16 12
35 31 34
Table 4 Particle measurement results in flue gas, fuel: wood waste.
Run 1 Run 2
Before absorber at 10% O2 (mg/Nm3)
After absorber at 10% O2 (mg/Nm3)
Reduction of particles (%)
51 39
29 21
42 46
Table 5 Particle measurement results in flue gas, fuel: wood and red canary grass.
Run 1 Run 2
Before absorber at 10% O2 (mg/Nm3)
After absorber at 10% O2 (mg/Nm3)
Reduction of particles (%)
14 13
9 8
36 40
Table 6 Particle measurement results in flue gas, fuel: wood waste and red canary grass.
Run 1 Run 2
Before absorber at 10% O2 (mg/Nm3)
After absorber at 10% O2 (mg/Nm3)
Reduction of particles (%)
40 26
25 14
39 47
For each fuel two sample periods were taken, the results are presented in Tables 3–6. The chemical composition of the particles does not change during the flow through the absorber. Measured components were Na, Si, P, S, Cl, K and Zn. Inspection of the filter house and used filter was performed after the last experiment. The amount of particles was not possible to establish, but a high accumulation could be observed. The color of the absorption solution in the absorber and in the filter house differs strongly, a good function of the filter can therefore be assumed. 5. Conclusions The purification of the flue gas worked satisfactorily, 33–44% of the particles in the flue gas were separated. The boiler used, produces relatively small amounts of particles compared to other incinerating plants. A positive result is that the separation of small harmful particles is efficient. The particles can simply be removed from the system with a filter. No observable damage of the absorption solution was possible to establish during the experiments. The heat recovery from the flue gas increased the heat production from the plant by about 40% when wet fuels were used. The increased electrical input is estimated at 2.2 kW and not included in the comparison. A commercial unit will use significantly less electricity. Acknowledgments This work was carried out thanks to funding from the Swedish National Energy Administration (STEM), Swebo Bioenergy and the County Administrative Board of Norrbotten. The particle measurements were performed by the Department of Applied Physics and Electronics at Umeå University. References [1] Messerer A, Schmatloch V, Pöschl U, Niessner R. Combined particle emission reduction and heat recovery from combustion exhaust – a novel approach for small wood-fired appliances. Biomass Bioenergy 2007;31:512–21. [2] Ghafghazi S, Sowlati T, Sokhansanj S, Bi X, Melin S. Particulate matter emissions from combustion of wood in district heating applications. Renew Sust Energy Rev 2011;15:3019–28. [3] Ronnback M, Jones F. Avskiljning av stoft med rökgaskondensering anpassade till biobränsleeldning <10 MW. SP Rapport 2010:81; 2010 [Swedish only]. [4] Riffat SB, Zhao X, Doherty PS. Application of sorption heat recovery systems in heating appliances – feasibility study. Appl Therm Eng 2006;26:46–55. [5] Westerlund L, Dahl J. Open absorption system: experimental study in a laboratory pilot plant. Appl Energy 1991;38:215–29. [6] Westerlund L, Dahl J. Absorbers in the open absorption system. Appl Energy 1994;48:33–49.
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[7] Westerlund L, Dahl J, Hermansson R. Heat and mass transfer simulations of the absorption process in a packed bed absorber. Appl Therm Eng 1998;18:1295–308. [8] Westerlund L, Dahl J. Use of an open absorption heat-pump for energy conservation in a public swimming-pool. Appl Energy 1994;49:275–300. [9] Johansson L, Westerlund L. An open absorption installed at a sawmill. Description of pilot plant used for timber and bio-fuel drying. Energy 2000;25:1067–79.
[10] Gröhn A, Suonmaa V, Auvinen A, Lehtinen K, Jokiniemi J. Reduction of fine particle emissions from wood combustion with optimized condensing heat exchangers. Environ Sci Technol 2009;43(16):6269–74. [11] Kim HT, Jung CH, Oh SN, Lee KW. Particle removal efficiency of gravitational wet scrubber considering diffusion, interception and impaction. Environ Eng Sci 2001;18(2):125–36.
Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Flue gas purification and heat recovery: A biomass fired boiler supplied with an open absorption system Lars Westerlund a,⇑, Roger Hermansson a, Jonathan Fagerström b a b
Division of Energy Engineering, Lulea University of Technology, S-971 87 Lulea, Sweden Energy Technology and Thermal Process Chemistry, Umeå University, SE-901 87 Umeå, Sweden
a r t i c l e
i n f o
Article history: Received 30 August 2011 Received in revised form 14 December 2011 Accepted 29 February 2012 Available online 27 March 2012 Keywords: Open absorption system Particle reduction Heat recovery
a b s t r a c t A new technique for energy recovery combined with particle separation from flue gas has been tested in this project. A conventional small boiler for biofuel produces besides heat also particles to the environment through the flue gas. Decreasing the impact on the environment is desirable. Increased efficiency can be obtained if the temperature and water content of the flue gas can be further reduced. Installing an open absorption system in the heat production unit fulfils both these demands. An experimental unit has been built and tested in the last 2 years. The results show a reduction of particles in the flue gas by 33–44% compared to the ordinary system. At the same time the heat production from the unit increased by 40% when fired with wet biofuels. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Small biofuel boilers (100 kW) have a high efficiency today but still the amount of particles in the flue gas is too high [1,2]. The purification methods available on the market are too expensive for this size of boiler [3]. The open absorption system can be beneficially used in such heat production units. At the same time as the amount of particles in the flue gas is reduced the main part of the remaining heat in the flue gas after the convection part is recovered. The energy supplied to the system is mainly heat that is produced by the boiler, and only a small amount of electricity is needed for circulation pumps. Sorption technology has advantages compared to conventional heat recovery systems since latent heat in the water vapor can be utilized better [4]. The dew point limitation, a major obstacle to ordinary heat recovery appliances, does not apply to sorption systems. Ref. [4] also concludes that sorption systems have better economic and environmental benefits than condensing boiler system. To our knowledge the open absorption system has not earlier been used for heat recovery and particle reduction in flue gas from biomass boilers. It is assumed that the technique used will give the same particle removal efficiency as a wet scrubber but a more efficient heat recovery. The open absorption system consists mainly of three components: the absorber, the generator and the condenser (see Fig. 1). The working medium (water) is produced by an external system (primarily from biofuel). The flue gas is brought into contact with ⇑ Corresponding author. Tel.: +46 920 491000; fax: +46 920 491047. E-mail address: [email protected] (L. Westerlund). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.02.085
the absorption solution in the absorber. Water vapor is absorbed by the solution and the flue gas is dried, cooled and scrubbed of particles. The diluted solution is pumped to the generator where the absorbed water is separated from the solution by primary heat supply. The concentrated solution is transported back to the absorber in a closed loop. The water vapor is condensed in the condenser giving off primarily the latent heat. The condensed and chilled water is separated from the system after the condenser.
2. Test facility At Luleå University of Technology comprehensive research work concerning the open absorption system has been performed for a long time [5–7]. Development plants for different applications have been constructed, mainly for drying purposes [8,9]. In the actual case the conditions for the absorption solution were more severe, since particles and chemical reactions could ruin the stability of the solution. The facility was connected to an existing boiler and with limited space for the equipment. A long distance between the absorber and generator caused large heat losses during the experiments. 2.1. System description The open absorption system was constructed with stainless steel and integrated with the boiler as shown in Fig. 2 where the absorber and generator are clearly seen. The third main part, the condenser, is the heat exchanger (HEX 4). The cross-current absorber consists of a channel, filled with packings, to create good
L. Westerlund et al. / Applied Energy 96 (2012) 444–450
445
Nomenclature Roman letters Q_ heat transfer rate (kW) V_ volume flow (m3/s) T temperature (°C) TT temperature transmitter and also temperature value (°C) _ m mass flow (kg/s) Cp specific heat capacity (J/kg K) h flue gas enthalpy (kJ/kg dry mass flow flue gas) q density (kg/m3) Subscripts absorber 1 energy from the flue gas absorber 2 energy to liquid flows ref reference value (0 °C) conv heat transfer rate from conventional boiler (kW)
contact between the absorption solution and the flue gas. The flow direction for the solution versus the gas gives the absorber its name; the solution flows vertically while the gas flows horizontally. The generator consists of a number of vertical tubes where the hot flue gas flows inside. The tubes are surrounded by absorption solution. Heat transferred from the flue gas is used for evaporation of absorbed water. A part of the hot gas from the boiler is used as heat input for the generator and the total gas flow passes the convection part of the boiler before entering the absorber. The by-pass over the absorber is closed during a normal run. The number of particles in the flue gas is reduced during contact with the absorption solution in the absorber. At the same time the flue gas is cooled and dried. After the absorber the flue gas flows to the chimney. The particles collected by the solution are separated by a small continuous flow through the filter. The heat from the flue gas increases the temperature of the solution when it flows vertically through the absorber. In HEX 1 and 2 this heat is transferred to the district heating system. The steam produced in the generator is condensed and chilled in HEX 4 giving off heat to the district heating system and the condensate is finally drained from the system. The district heating system is lastly heated through the convection zone in the boiler. Diluted absorption solution from the absorber to the generator is preheated in HEX 3. Concentrated solution to the absorber is cooled when giving off the heat in this heat exchanger.
boiler heat transfer rate from boiler (kW) generator heat transfer rate to generator (kW) extra heat transfer rate according to increased area (kW) conc concentration of absorption solution (mass fraction) frac abs part of total flue gas flow which flows through absorber (mass fraction) Abbreviations HEX heat exchanger TT temperature transmitter and also temperature value (°C) LT level transmitter FM flow meter CU control unit CV control valve P pump
2.2. Instrumentation For an overall view of the system conditions, the temperatures (TTs) and liquid flows (FMs) were measured at the positions shown in Fig. 3. Psychrometer (dry and wet bulb temperature) was used to establish the flue gas conditions after the absorber. Measurements were taken at 10 s intervals. To control the system, the temperature in the generator should be kept at a constant value. This is done by controlling the flow of solution to the generator and the amount of hot gas to the apparatus. If the temperature in the generator is above the set point value, the diluted solution flow to the generator increases by increasing the opening of the control valve (CV1). If it is fully open and the temperature is still too high, the damper (CV2) decreases the flow of hot gas to the generator. The liquid level in the generator should be constant and is controlled with the level transmitter (LT1) and control valve (CV5). 3. Method 3.1. Heat and mass balances A heat balance of the absorber consists of heat given off by the flue gas ðQ_ absorber 1 Þ and heat taken up by the district heating system and solution flows ðQ_ absorber 2 Þ according to Eq. (1). The mass flow, temperature and heat capacity of the absorption solution change and have to be included in a total balance.
_ dry flue gas hafter absorber hbefore absorber Q_ absorber 1 ¼ m Q_ absorber 2 ¼ qwater V_ water C p;water ðTT3 TT1Þ _ solution to abs C p;solution to abs ðTT12 T ref Þ þm _ solution from abs C p;solution from abs TT8 T ref m
Fig. 1. The open absorption system.
ð1Þ
The flue gas enthalpy (h) was calculated with normal psychrometric correlations. The heat capacity value for the dry flue gas was determined by adding each spices specific heat capacity value multiplied by their mass fraction. To estimate the conditions of the flue gas before the absorber the psychrometer could not be used because of temperatures above 100 °C. The water content was determined through software Fluegas knowing the water content in the fuel and the content of oxygen in the flue gas. The total mass flow of dry flue gas from the boiler, was resolved with Fluegas and through manual measurements of the total volume of gas flow. The condensate volume flow was measured with flow meter (FM 2 in
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Fig. 2. The open absorption system integrated with the boiler.
Fig. 3. Control equipment and measurement points in the test facility.
Fig. 2). The volume flow of concentrated absorption solution to the absorber was determined by registration of the opening time for valve CV5 in Fig. 3. During one opening period the volume of 1.55 l flows to the absorber. The mass flow of diluted solution from the absorber could then be calculated by adding the mass flow of condensate to the concentrated solution mass flow. Heat balances for HEX 3 and 4 were also performed. On the primary side of HEX 3 the latent heat for water vapor was included in the balance.
The total heat transfer rate was calculated with the mass flow in the district heating system and the temperature difference between TT1 and TT5. A mass balance for absorbed water from the flue gas _ water absorbed Þ and condensate ðm _ condensate Þ was performed according ðm to the following equation.
_ dry flue gas xbefore absorber xafter absorber _ water absorbed ¼ m m _ condensate ¼ q V_ water m water
ð2Þ
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The absorbed water from the flue gas and condensate flow out of the system should be equal when running the plant in constant conditions. The humidity ratio and other variables were determined according to earlier descriptions. 3.2. Heat recovery To compare the technique with a conventional boiler system and a water scrubber the following concepts and calculations were performed. Conventional heat transfer rate: Heat transfer rate from the boiler without the open absorption system was calculated according to Eq. (3). The large area in the generator caused a decreased flue gas temperature which is not the case using only the convection part in the boiler. It had therefore to be withdrawn to get the total heat transfer rate.
Q_ conv ¼ Q_ boiler þ Q_ generator Q_ extra
ð3Þ
Q_ boiler ¼ qwater V_ water C p;water ðTT5 TT4Þ _ conc solution C p;conc solution ðTT11 TT10Þ Q_ generator ¼ m _ condensate ðr water þ C p;steam ðTT13 TT11Þ þm Q_ extra
ð4Þ
þ C p;water ðTT11 TT10ÞÞ _ ¼ mdry flue gas hordinary hwith open abs system
In Eq. (4) the generator heat power includes heating of the absorption solution, heating of water (liquid) and superheating of steam. In Q_ extra using enthalpy values include the energy content in the moisture. Moisture content of the fuel: Water content in the fuel (mass part). The district heating system temperature to heating unit: Measured before HEX 1 i.e., TT1. Theoretical possible improvement: Calculated as the energy possible to retrieve from the flue gas if the energy content after the boiler is decreased to the level in the flue gas leaving the absorber. This value is divided by the conventional heat transfer rate.
_ dry flue gas hafter the boiler hafter the absorber =Q_ conv Q_ th: improv ¼ m
ð5Þ
Measured improvement: Total measured heat transfer rate in the district heating system, divided by the conventional heat transfer rate.
Q_ meas improv ¼
Q_ HEX 1 þ Q_ HEX 2 þ Q_ HEX 4 þ Q_ boiler _ frac abs m
!, Q_ conv
ð6Þ
Improvement rectified by heat losses: Total measured heat transfer rate increased by theoretically calculated heat losses, divided by the conventional heat transfer rate. Improvement with a water scrubber: A theoretical calculation of an ideal scrubber without heat losses. The apparatus works with a temperature equal to the district heating system temperature to the heating unit (TT1) and a flue gas temperature after the scrubber 5 °C above this temperature. Reheating of the flue gas after the scrubber is not included in the calculations.
_ dry flue gas hafter the boiler hafter the scrubber =Q_ conv Q_ water scrubber ¼ m
was studied using a micro manometer and pitot tube in different points on each cross-sectional area. The lengths of straight lines before and after each cross-sectional area were fulfilled. The results showed that only 64% of the total volume flow passed through the absorber depending on a high pressure drop over the absorber. The flow resistance is proportional to the square of the volume flow in each circuit (absorber/damper-CV3), and the quota for these flow resistances could then be determined. Even if the temperature changes, this quota is almost constant as long as the flue gas can be treated as an ideal gas. The conditions for the flue gas before and after the absorber are known. With the ideal gas law it is possible to calculate the specific volumes before/after the absorber. The part of mass flow of dry flue gas passing through the absorber can then be determined in each experiment. 3.4. Particle sampling and analysis Particle collection was performed with a 13-stages low pressure impactor from Dekati (DLPI). The particles were separated according to different aerodynamic diameters in the total range 0.03– 10 lm. Two impactors were used at the same time, before and after the absorber. The gas flow to the impactors was taken isokinetically. By comparing these results the purification of the flue gas could be established. Analysis of the particles’ chemical composition was performed with a scanning electron microscope with an attached energy dispersive X-ray detector (SEM-EDS). 4. Results Analysis of the experimental values was essentially based on heat and mass balances. Heat losses from the total system to the room were calculated at 11.4 kW. This value was used in the evaluation of all experiments. An increased electrical input using the open absorption system consists of four liquid pumps and an increased pressure rise for the flue gas fan. In the experimental unit the increased electrical input was estimated at 2.2 kW. The distance between different parts of the system and small volume flows makes instantaneous comparison of measured values difficult. The part of mass flow of dry flue gas that passed through the absorber was 65–66% of the total mass flow of dry flue gas from the boiler for all experiments. The flue gas temperature after the boiler was for all experiments in the range of 150–180 °C. 4.1. Heat transfer rates Heat transfer rates to the district heating system are presented in Fig. 4. Each heat exchanger (HEX 1, 2 and 4) and boiler is shown,
ð7Þ
3.3. Gas flow through the by-pass A leakage is normal for a damper (CV3) in a flue gas system. To determine the magnitude of this leakage measurements were performed with closed damper. The air temperature was 20 °C during the measurements and several different volume flows were investigated. The total gas flow from the boiler and after the absorber
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Fig. 4. Heat transfer rates to the district heating system.
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L. Westerlund et al. / Applied Energy 96 (2012) 444–450 Table 2 Comparison of heat improvement between installation of an open absorption system and installation of a water scrubber.
Fig. 5. Temperature levels for district heating system and flue gas leaving the system.
Moisture content of the fuel (%)
Improvement rectified by heat losses (%)
Improvement with a water scrubber (%)
50.9 51.7 52.1 52.0 48.0 28.0
44 46 45 41 47 25
23 27 25 19 29 16
is only 1–2 °C higher than water temperature even though there are two drops in temperature, flue gas to absorption solution and absorption solution to water in the district heating system. The heat exchanger (HEX 1) has a large heat transfer area. Not only the temperature but also the relative humidity of the flue gas leaving the absorber is of interest for the heat transfer rate. If the relative humidity of the gas leaving the absorber increases, the heat transfer rate will decrease. The appearance in Fig. 5 around 16 h was caused by external interference. 4.4. Mass balance
Fig. 6. Absorbed water from flue gas and condensate mass flow.
and the total heat transfer rate (total) and heat transfer rate from the absorber (abs) are included. The heat transfer rate from the absorber includes HEX 1 and 2. 4.2. The absorption solution The relative humidity of the flue gas leaving the absorber is 38– 45% RH. This value is somewhat higher than the design data due to the high absorption rate and absorption of CO2 in the solution. The concentration of CO2 is stabilized at a low level and separated from the absorption solution during heat supply in the generator. After the experiments the capability of the absorption solution from the generator was compared with new solution that had never been in contact with flue gas. The results showed only small differences in absorption capability. 4.3. Heat transfer between flue gas and district heating system Fig. 5 shows the temperature of the district heating system (TT1) before heat exchanger (HEX 1) and the dry temperature of the flue gas leaving the absorber (TT25). The flue gas temperature
Comparison of calculated absorbed amount of water from the flue gas in the absorber with measured condensate mass flow after HEX 4 is illustrated in Fig. 6. Average values for these variables correspond well. The time difference is large between absorbed water and condensate from the generator, since large volumes of absorption solution are used in the absorber and generator and only a small amount of water is absorbed. Instantaneous values can therefore not be used. 4.5. Heat recovery improvement Results from six different experiments are shown in Tables 1 and 2. Used concepts in the table are explained in Section 3.2. The increased electrical input is not included in the comparison. Tables 1 and 2 show that increased moisture content of the fuel results in improved heat recovery. A lower temperature in the district heating system has the same influence. The heat losses in the experimental set-up were large. The last column in Table 1 constitutes the most accurate comparison for the open absorption system with a conventional boiler. These values are only for an ideal unit, since some heat losses will always arise. With decreasing water content in the fuel the improvement decreases of course. The open absorption system is superior to a water scrubber since the flue gas is dried using the absorption system. The energy in the moisture is taken care of which increases the improvement. Including reheating of the flue gas after the scrubber increases the difference between the systems. Increased return temperature of the district heating system gives a reduction of the heat recovery that is less significant for the open absorption system compared to the water scrubber.
Table 1 Increase of the heat transfer rate from the system during different moisture contents in the fuel and different temperatures in the district heating system to the heating unit. Conventional heat transfer rate (kW)
Moisture content of the fuel (%)
The district heating system temperature to heating unit (°C)
Theoretically possible improvement (%)
Measured improvement (%)
Improvement rectified by heat losses (%)
65 70 61 70 42 80
50.9 51.7 52.1 52.0 48.0 28.0
46.4 41.0 44.4 52.3 35.1 38.3
50 46 45 41 55 26
23 29 25 24 19 10
44 46 45 41 47 25
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Fig. 7. Size distribution of particle mass, the upper curve for each fuel shows results before the absorber and the lower curve value after the absorber.
4.6. Reduction of particles in the flue gas Particles in the flue gas are collected by the absorption solution during contact in the absorber. The results indicate that the mass from particles in the flue gas before the absorber is dominated by particles in the interval 0.1–0.3 lm, see Fig. 7, a normal size when biofuel is used [1,10]. According to [11] the particle removing efficiency for the wet scrubber is very low in this range. The small size of the particles makes them harmful for humans and should be reduced as much as possible. The percentile reduction that takes place in the absorber is evenly spread, no particle size is over-represented. The removal efficiency is in the same range as reported by Gröhn et al. [10].
Table 3 Particle measurement results in flue gas, fuel: wood chips.
Run 1 Run 2 Run 3
Before absorber at 10% O2 (mg/Nm3)
After absorber at 10% O2 (mg/Nm3)
Reduction of particles (%)
18 23 18
11 16 12
35 31 34
Table 4 Particle measurement results in flue gas, fuel: wood waste.
Run 1 Run 2
Before absorber at 10% O2 (mg/Nm3)
After absorber at 10% O2 (mg/Nm3)
Reduction of particles (%)
51 39
29 21
42 46
Table 5 Particle measurement results in flue gas, fuel: wood and red canary grass.
Run 1 Run 2
Before absorber at 10% O2 (mg/Nm3)
After absorber at 10% O2 (mg/Nm3)
Reduction of particles (%)
14 13
9 8
36 40
Table 6 Particle measurement results in flue gas, fuel: wood waste and red canary grass.
Run 1 Run 2
Before absorber at 10% O2 (mg/Nm3)
After absorber at 10% O2 (mg/Nm3)
Reduction of particles (%)
40 26
25 14
39 47
For each fuel two sample periods were taken, the results are presented in Tables 3–6. The chemical composition of the particles does not change during the flow through the absorber. Measured components were Na, Si, P, S, Cl, K and Zn. Inspection of the filter house and used filter was performed after the last experiment. The amount of particles was not possible to establish, but a high accumulation could be observed. The color of the absorption solution in the absorber and in the filter house differs strongly, a good function of the filter can therefore be assumed. 5. Conclusions The purification of the flue gas worked satisfactorily, 33–44% of the particles in the flue gas were separated. The boiler used, produces relatively small amounts of particles compared to other incinerating plants. A positive result is that the separation of small harmful particles is efficient. The particles can simply be removed from the system with a filter. No observable damage of the absorption solution was possible to establish during the experiments. The heat recovery from the flue gas increased the heat production from the plant by about 40% when wet fuels were used. The increased electrical input is estimated at 2.2 kW and not included in the comparison. A commercial unit will use significantly less electricity. Acknowledgments This work was carried out thanks to funding from the Swedish National Energy Administration (STEM), Swebo Bioenergy and the County Administrative Board of Norrbotten. The particle measurements were performed by the Department of Applied Physics and Electronics at Umeå University. References [1] Messerer A, Schmatloch V, Pöschl U, Niessner R. Combined particle emission reduction and heat recovery from combustion exhaust – a novel approach for small wood-fired appliances. Biomass Bioenergy 2007;31:512–21. [2] Ghafghazi S, Sowlati T, Sokhansanj S, Bi X, Melin S. Particulate matter emissions from combustion of wood in district heating applications. Renew Sust Energy Rev 2011;15:3019–28. [3] Ronnback M, Jones F. Avskiljning av stoft med rökgaskondensering anpassade till biobränsleeldning <10 MW. SP Rapport 2010:81; 2010 [Swedish only]. [4] Riffat SB, Zhao X, Doherty PS. Application of sorption heat recovery systems in heating appliances – feasibility study. Appl Therm Eng 2006;26:46–55. [5] Westerlund L, Dahl J. Open absorption system: experimental study in a laboratory pilot plant. Appl Energy 1991;38:215–29. [6] Westerlund L, Dahl J. Absorbers in the open absorption system. Appl Energy 1994;48:33–49.
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[7] Westerlund L, Dahl J, Hermansson R. Heat and mass transfer simulations of the absorption process in a packed bed absorber. Appl Therm Eng 1998;18:1295–308. [8] Westerlund L, Dahl J. Use of an open absorption heat-pump for energy conservation in a public swimming-pool. Appl Energy 1994;49:275–300. [9] Johansson L, Westerlund L. An open absorption installed at a sawmill. Description of pilot plant used for timber and bio-fuel drying. Energy 2000;25:1067–79.
[10] Gröhn A, Suonmaa V, Auvinen A, Lehtinen K, Jokiniemi J. Reduction of fine particle emissions from wood combustion with optimized condensing heat exchangers. Environ Sci Technol 2009;43(16):6269–74. [11] Kim HT, Jung CH, Oh SN, Lee KW. Particle removal efficiency of gravitational wet scrubber considering diffusion, interception and impaction. Environ Eng Sci 2001;18(2):125–36.