Co-production of Pyrolysis Oil in District Heating Plants: Systems

Aug 20, 2013 - The use of excess heat from the pyrolysis production in the district ... The potential for use of energy from pyrolysis vapor condensat...
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Co-production of Pyrolysis Oil in District Heating Plants: Systems Analysis of Dual Fluidized-Bed Pyrolysis with Sequential Vapor Condensation Christer Gustavsson* and Lars Nilsson Department of Engineering and Chemical Sciences, Karlstad University, SE-651 88 Karlstad, Sweden ABSTRACT: Flash pyrolysis of biomass yields a liquid applicable as a fuel oil substitute and as a basis for production of chemicals and fuels. Biomass, being a renewable resource, is foreseen to be in increasing demand. An increased usage may lead to scarcity of biomass and emphasizes the need for high-efficiency conversion processes. In this study, the efficiency and capacity aspects of an integration of pyrolysis oil production with a district heating plant by means of dual fluidized-bed technology has been modeled. Further, fractional condensation of the pyrolysis vapors has been applied, enabling part of the condensation energy to be recovered. The concept shows potential for significant pyrolysis oil production while keeping the delivered power and heat constant. The use of excess heat from the pyrolysis production in the district heating net results in a 10% higher overall efficiency than production without heat supply to the district heating net.

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INTRODUCTION Because of depletion of fossil resources and climate concerns, biomass, being a renewable resource, is foreseen to be in increasing demand. Over time, new industries, such as the chemical, petroleum, and petrochemical industries, may gradually base an increasing share of their production on renewable raw materials. Such a future bioeconomy may result in a competitive situation for biomass and emphasizes the need for high-efficiency conversion processes. Flash pyrolysis is a thermochemical conversion route that has attracted considerable interest for the production of fuel oil substitute and subsequent upgrading to transport fuels and chemicals.1,2 The technology is characterized by high efficiency3 and results in a densified product, pyrolysis oil (bio-oil, pyrolysis liquid, etc.), potentially enabling cost- and energy-efficient transfer of biomass from existing biomass-handling sites to the new sectors. For flash pyrolysis, a wide variety of biomass types can be used. Forest residues are an interesting raw material because they allows for large-scale production without competing with established value chains for sawmilling and the pulp and paper industry. Pyrolysis of forest residues has been investigated, e.g., at VTT in Finland.4 Several concepts have been proposed and tested for pyrolysis oil production, e.g., ablative pyrolysis, rotating screw pyrolysis, and auger reactor pyrolysis.5 Commercial plants realized thus far have mainly used circulating fluidized-bed (CFB) and bubbling fluidized-bed (BFB) reactor technologies. Significant research has been carried out, aiming to increase efficiency by means of optimization of the liquid yield.3 Another approach to further enhance the overall energy efficiency is to integrate the pyrolysis plant with a boiler plant used for heat generation and, thereby, use waste heat from the pyrolysis plant. Research6 has demonstrated that a pyrolysis reactor can be integrated with a fluidized-bed (FB) boiler with communicating beds that enable heat for pyrolysis to be supplied from the boiler to the pyrolysis reactor. This novel technology enables simultaneous production of pyrolysis oil, noncondensable gas, heat, and power. © 2013 American Chemical Society

Combined heat and power (CHP) plants using BFB solid fuel biomass boilers are numerous and represent an interesting platform for large-scale distributed production of pyrolysis oil, potentially enabling significant investment savings and logistic benefits compared to stand-alone units. Furthermore, the use of the BFB solid fuel biomass boilers in the district heating (DH) systems typically varies over the year. Energy demand for DH is typically considerably lower during summer, and hence, there is a broad interest to find ways to increase the asset use, especially during the summertime. Integration of CHP plants and biomass pyrolysis in the DH network has previously been examined7 by applying a simplified pyrolysis model, where 90% of the biomass energy content formed bioslurry (pyrolysis oil + char). Furthermore, use of an indirectly heated fluidized-bed reactor rather than communicating fluidized beds was assumed. Other research has demonstrated that pyrolysis oil composition can be controlled by the condensation temperature8,9 and that a thermodynamic model can be established, enabling prediction of vapor−liquid equilibrium versus temperature as well as condenser heat load.10 An increased condensation temperature can enable the heat of vaporization to be recovered at a useful temperature level, thereby improving overall efficiency for the pyrolysis plant. In addition to the positive effect in terms of energy efficiency, the sequential condensation results in a fractionation of the produced pyrolysis oil, yielding two fractions with significant differences in composition and properties. Such a fractionation has been used as an initial separation step in a process for upgrading of pyrolysis oil to fuels and chemicals.11 This paper uses previous research results and commercial operation parameters to model and evaluate a flash pyrolysis plant with sequential vapor condensation integrated with a DH CHP plant. The aim of the study is to evaluate the impact from Received: June 19, 2013 Revised: August 13, 2013 Published: August 20, 2013 5313

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resulting in a heat-transfer coefficient of 2800 kW/K that was used for further calculations. On the basis of the heat-transfer coefficient and the annual power/heat ratio, the average turbine isentropic efficiency was calculated to be 79.8%. This value was kept constant in further calculations. In the “pyrolysis case” simulation model, the flash pyrolysis plant was integrated with the CHP plant, as described in Figure 2. Hot sand from the existing BFB is fed to a CFB pyrolysis reactor, where it is mixed with dried biomass. In the CFB pyrolysis reactor, where sand is fluidized by recirculated noncondensed gas from the pyrolysis condenser train, biomass decomposes and forms vapor and char. In the subsequent cyclone, char and sand are separated from the vapor and returned to the BFB boiler, where the char is assumed to be combusted in the bed.12,13 The vapor is fed to two sequential spray condensers, where cooled recirculated condensed bio-oil is used to condensate bio-oil. The recirculation flow of bio-oil is assumed to be significantly higher than the vapor flow, thereby enabling the condensation to be considered isothermal. In the model, drying of biomass for pyrolysis is assumed to be accomplished with a bed dryer, which has been found to be a feasible method for low-temperature drying.14 The drying air is heated in three serial batteries, AH 1−3, where heat is transferred from pyrolysis condenser 1 (AH 1), DH return water flow (AH 2), and DH supply water flow (AH 3). For heat integration, a general temperature difference of 10 °C between hot and cold media was used. Consequently, the supply air to the bed dryer was 10 °C colder than the DH supply water for each month. Two air coolers, C 1 and C 2, enable removal of heat from the pyrolysis condensers at all conditions and were used to balance any excess condenser heat that could not be used in the pyrolysis case. For setting up and solving the material and heat balances of the pyrolysis reactor, boiler, and condensers, the database connected to CHEMCAD was extended with three new components; biomass, char, and bio-oil. Table 3 contains some basic data and properties used when creating those components. With regard to compositions, heating values, and specific heat of biomass and char, there are ample experimental data in the literature supporting the assumptions made when creating these two components.15,6,16 Also, commercially available data support these assumptions.17 Naturally, experimental data for compositions and heating values of bio-oil are obtained for water-containing bio-oil, but the component introduced here represents water-free bio-oil, allowing us to model bio-oil with different water contents depending upon the condensation temperature. Correcting for the hydrogen and oxygen contents of the water, the composition data used here are in excellent agreement with compositions measured in two studies.15,3 The heating values in Table 3 are not readily comparable to measured data, but when the waterfree bio-oil is combined with water so that the water content is 25.73%,15 the lower and higher heating values will be 16.2 and 17.9 MJ/kg, respectively, which are in rather good agreement with literature values15,3 for bio-oil with comparable water contents. There are fewer available data regarding the average specific heat and average heat of condensation of bio-oil; however, some support of the assumed values in Table 3 can be found.18 The assumptions regarding the pyrolysis reactor are that the incoming biomass has a moisture content of 10% (wb) and that the yields of bio-oil, water, char, and noncondensable gases are 52, 22, 14, and 12%, respectively, based on the total incoming biomass, corresponding to a bio-oil water content of 31%. Such yields have been demonstrated in pilot-scale pyrolysis plants.6 Out of the water leaving the pyrolysis reactor, 45% comes from the biomass and 55% is produced in the pyrolysis reactions. The composition of the noncondensable gases produced during pyrolysis was assumed to be 52 wt % carbon dioxide, 41 wt % carbon monoxide, and 7 wt % methane. This composition is reasonable based on available experimental data,19,16 and it further allows us to close the component balances of carbon, hydrogen, and oxygen for the system. The assumptions outlined here regarding component properties and reaction yields result in a heat of pyrolysis of 1.74 MJ/kg dry substance, which is in good agreement with available data.20,21 Overall,

pyrolysis integration on the plant in terms of pyrolysis oil production, power generation, biomass consumption, and overall energy efficiency. The potential for use of energy from pyrolysis vapor condensation is estimated, and a sensitivity analysis for the condenser temperature is carried out. Furthermore, capacity restrictions in the boiler affecting pyrolysis oil production are assessed and discussed.

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METHOD AND MODELING

Installation of a pyrolysis plant using dual fluidized-bed technology has been examined for a municipal DH CHP plant with a BFB solid fuel biomass boiler, situated in central Sweden. The plant is designed for combustion of wet biomass and is, hence, equipped with a flue gas condenser (FGC). The present plant configuration is illustrated in Figure 1. The CHP plant, using forest residues as fuel, is in operation 9

Figure 1. Present (base case) plant configuration. months/year. Main boiler features are given in Table 1. The present actual monthly production features of the DH CHP plant are summarized in Table 2.

Table 1. Main Boiler Data maximum boiler power (fuel LHV) minimum boiler power (fuel LHV) fuel dry content boiler flue gas temperature before FGC steam pressure steam temperature primary air fan capacity (1 bar, 25 °C)

86 MW 17 MW 50% 170 °C 94 bar 480 °C 9.2 m3/s

The process of evaluating the impact from integrated flash pyrolysis was started by establishing a “base case”. The DH CHP plant production was modeled in CHEMCAD 6.4.1, and “total energy to DH” (Table 2) was fulfilled. In the base case model, the fuel moisture content and boiler flue gas temperature were set constant at design values. For the FGC, the temperature difference between exiting flue gas and DH water feed temperature was adjusted to reach a good agreement with present energy flows from the turbine condenser and the FGC. Good conformance was achieved at a value of 2 °C. For the turbine condenser, a minimum temperature difference of 8 °C between the steam and water side was assumed at present design load, 5314

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Table 2. Main Present DH CHP Plant Data present situation

Jan

Feb

March

April

May

June

July

Aug

Sept

Oct

Nov

Dec

ambient temperature (°C) DH flow (m3/s) DH return temperature (°C) DH supply temperature (°C) flue gas condenser power (MW) turbine condenser power (MW) total energy to DH (MW) gross electric power production (MW)

−2.4 0.45 47 80 17.4 46.8 64.2 17.6

−5.5 0.45 49 85 18.4 51.5 69.9 20.5

1.4 0.50 45 75 17.7 48.1 65.8 19.1

9.5 0.46 43 72 7.9 49.4 57.3 19.3

11.9 0.39 44 72 3.4 45.0 48.4 18.1

17.8

19.4

17.9

13.5 0.16 44 72 1.2 17.9 19.1 6.0

7.6 0.46 42 72 14.3 45.9 60.3 18.1

5.1 0.42 43 72 13.2 41.4 54.6 16.0

1.3 0.47 46 75 15.7 48.9 64.6 17.4

Figure 2. Pyrolysis case plant configuration.

Table 3. Basic Data and Properties for Pyrolysis Fractions component-specific parameters

composition (wt %)

molecular mass heating value (MJ/kg) specific heat (kJ kg−1 K−1) heat of condensation (MJ/kg)

biomass

char

bio-oil

52.0% C 6.0% H 42.0% O 23.09 18.9 (lower) 20.2 (higher) 1.3 (s)

83.0% C 4.0% H 13.0% O 100.00 29.6 (lower) 30.5 (higher) 1.3 (s)

59.2% C 6.3% H 34.5% O 20.31 22.7 (lower) 24.0 (higher) 3.0 (l) 3.0 (v) 0.79

experimental data for this process are available in the literature.8,10 To validate the present model, the data in the first of these two studies were used, because in that study, the bio-oil is treated as a single liquid component, which is the same assumption as taken here. When modeling the condensation process in CHEMCAD, the Wilson

the experimental support of all assumptions made in combination with the good agreement when closing the material and heat balances of the pyrolysis reactor lends credibility to the mathematical model. To model the heat integration of the two condensers in the system, it is essential to obtain the best possible estimates for the condensation behavior, including the water content of the produced bio-oil. Relevant 5315

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ṁ c LHVc + ṁ w hw + Q̇ S + Q̇ surplus = ṁ w hs + ṁ v hv + ṁ php

thermodynamic model was used, assuming the vapor pressure (Pa) of bio-oil as a function of the temperature (K) to be 0 Pbio ‐oil

⎛ 2559.17 ⎞⎟ = exp⎜16.5067 − ⎝ T ⎠

(3) Applying the design values for the boiler plant to eq 2 enables the heat supply from bed surroundings QS to be calculated. The calculated value for the present design case (6.1 MW) was assumed to be constant and was used for calculation of surplus heat in the bed for the pyrolysis case (eq 3). The pyrolysis simulation was carried out using CHEMCAD. The simulation results from CHEMCAD for each month of a year were imported to Excel, where the heat integration to the DH net was simulated. The pyrolysis case temperatures in all positions of the DH net were calculated on the basis of the base case flows and energy supply. For the pyrolysis case, the temperature in pyrolysis condensers 1 and 2 were set to 90 and 35 °C, respectively. A sensitivity analysis was carried out for April, where the temperature in the primary pyrolysis condenser was varied between 50 and 110 °C. April was chosen as a basis for the sensitivity analysis because the ambient temperature for this month is close to the average ambient temperature on a yearly basis.

(1)

Setting the interaction parameters between bio-oil (1) and water (2) as A12 = 0.4 and A21 = 0.1, we obtained yield predictions in good agreement with experimental data for the amount of condensed bio-oil at different temperatures (Figure 3) as well as relatively accurate

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RESULTS AND DISCUSSION Table 4 shows the main features of the simulated CHP plant for “base case” and with pyrolysis oil co-production, “pyrolysis case”. Figures for pyrolysis oil production include oil from both condensers. A total of 48% of the pyrolysis oil is produced in condenser 1, and the remaining 52% is produced in condenser 2. Because the production capacity is considered to be limited by the char combustion capacity in the fluidized bed, the production capacity is significantly higher during low-load season when char load from biomass combustion is low. The total pyrolysis oil production is close to 72 000 tons/a, corresponding to 300 GWh/a (LHV). The change in power production resulting from an integration of pyrolysis oil production is moderate, and a major difference is found for the period of June−August only. The increase in power production during the summer is a result of the boiler being in operation during this period when it is stopped according to the base case conditions. Co-production of pyrolysis oil with heat integration as described in this study results in a somewhat increased total boiler load (fuel chips + pyrolysis char + pyrolysis gas) to the BFB boiler compared to the base case because of the heat demand for biomass drying. It further results in a slightly higher turbine condenser pressure because of the reduced flue gas condenser capacity. Increased boiler load and increased turbine condenser pressure counteract each other in terms of power generation. The resulting effect is a somewhat higher power production also during wintertime. The ratio between power production and heat from the turbine condenser exhibits variation over the year, because of the varying DH water temperature, which causes varying temperature/pressure in the turbine condenser. The seasonal variations in the power/heat ratio are not greatly affected by the co-production of pyrolysis oil. Figure 5 illustrates the overall performance of the CHP plant for the two cases. Overall plant efficiency for the base case, expressed as the ratio between sold heat + power and purchased biomass, clearly exceeds 100% during operation time, with an annual average of 112%. This is a result of the high moisture content of the fuel, which lowers the fuel LHV and simultaneously increases energy recovery in FGC. For the pyrolysis case, the fuel consumption will be rather constant over the year. The fuel consumption will increase by 387 GWh/ a. Pyrolysis oil production is calculated to be 302 GWh/a.

Figure 3. Organic loss from the condenser as a function of the condenser temperature. predictions for the water content of the bio-oil produced (Figure 4). It should be emphasized, though, that the sole motivation for choosing this specific thermodynamic model is that it gives good agreement with available data.

Figure 4. Water content in pyrolysis oil as a function of the condenser temperature.

In the pyrolysis case model, the pyrolysis oil production was maximized with respect to two constraints: (i) energy supply to DH net to be unaltered in comparison to the base case and (ii) char load on the boiler BFB to be a maximum of 3380 kg/h corresponding to the maximum combustion capacity enabled by the primary air fan of the boiler. The heat power balance for the boiler BFB at design load with 800 °C bed temperature in the base case can be expressed as ṁ c LHVc + ṁ w hw + Q̇ S = ṁ w hs + ṁ v hv

(2)

Whereas the heat power balance for the boiler BFB in the pyrolysis case can be expressed as 5316

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Table 4. Simulated DH CHP Plant Data for the Base Case and Pyrolysis Case Jan

a

Feb

March

flue gas condenser power (MW) turbine condenser power (MW) gross electric power production (MW) wet biomass consumption (kg/s)

15.6 48.6 18.2 9.0

16.0 53.9 19.6 9.9

16.8 49.1 18.8 9.2

flue gas condenser power (MW) turbine condenser power (MW) gross electric power production (MW) pyrolysis oil production (kg/s) wet biomass consumption (kg/s) energy efficiencya (%) energy efficiencyb (%)

15.4 49.3 18.4 0.8 38.5 83.4 85.0

16.0 54.2 19.7 0.3 37.9 82.4 84.2

16.6 49.7 19.1 0.7 38.4 82.8 84.6

April

May

Base Case 15.2 12.6 42.1 35.7 16.6 14.2 7.9 6.7 Pyrolysis Case 14.7 11.6 43.2 37.2 16.9 14.7 1.4 2.0 39.2 40.0 83.7 84.6 85.3 86.0

June

July

Aug

Sept

Oct

Nov

Dec

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

5.0 14.1 5.7 2.7

16.3 44.0 17.3 8.3

14.6 40.0 15.8 7.5

16.1 48.5 18.6 9.1

2.0 10.7 4.4 4.8 43.5 73.5 77.9

2.1 10.7 4.4 4.8 43.5 73.5 77.9

2.1 10.7 4.4 4.8 43.5 73.5 77.9

3.0 16.8 6.8 4.2 42.6 84.9 86.3

15.8 44.9 17.5 1.2 39.0 84.8 86.1

14.2 41.4 16.3 1.6 39.4 83.4 85.1

15.9 49.2 18.8 0.7 38.4 82.3 84.2

Pyrolysis oil production (LHV)/Δbiomass (LHV). b(Pyrolysis oil production (LHV) + electric power)/Δbiomass (LHV).

Figure 6. Use of energy from pyrolysis condenser 1 (PC 1) and 2 (PC 2).

condensation energy can be used, but during the summertime, the use rate falls dramatically. The average excess heat from the two condensers in the summer period (June−August) is 14 MW. Over the year, 48 GWh/a out of the available 80 GWh/a released from pyrolysis condensers can be used. The effect of a varying temperature in condenser 1 was examined for April. The condenser temperature was altered between 50 and 110 °C. Figure 7 shows the effect on the pyrolysis oil production capacity and the efficiency for coproduction of pyrolysis oil. A steep change in efficiency and oil production can be observed between 50 and 60 °C. This reflects the threshold value where the heat of condensation from condenser 1 can be transferred to the DH net. For condenser temperatures below the threshold value, the energy must be supplied by additional firing in the boiler, which, in turn, reduces the potential for pyrolysis oil production. A slight decrease in efficiency and oil production can observed when the condenser temperature is increased above the threshold value, but this effect is minor.

Figure 5. Overall plant efficiency for the base case and pyrolysis case.

Power production will increase by 12 GWh/a. The total efficiency on a yearly basis for the added pyrolysis oil production will thereby be 81% [=(302 + 12)/387]. As shown in Table 4, the total efficiency for the added pyrolysis oil production will vary over the year. During the wintertime, the total efficiency is in the range of 85%, whereas during the summertime, the total efficiency is clearly below 80%. The lower total efficiency during the summertime is due to the fact that the heat from pyrolysis condensers cannot be fully used; i.e., the available heat from the pyrolysis vapor condenser, flue gas condenser, and turbine condensor is higher than required for biomass drying. To maintain energy balance, part of the power from the pyrolysis condenser needs to be released to the atmosphere. Figure 6 illustrates the available and used energy from the two pyrolysis oil condensers. For most of the year, the total

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CAPACITY LIMITATIONS A constant char load, assumed to burn within the bed, combined with a varying flow of water and volatiles because of a varying flow of wet biomass to the CFB results in a varying heat balance for the bed. In Figure 8, the calculated energy surplus (eq 3) is shown. With the primary air temperature constant at a design value of 228 °C and the bed temperature kept constant at a design value of 800 °C, a maximum heat surplus of 9 MW occurs during the summer when no firing of wet fuel takes place. Such a heat surplus would result in an increase of the bed temperature of several hundred degrees, 5317

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Figure 7. Effect from the varying temperature in pyrolysis condenser 1 (April).

In addition to the char combustion capacity and bed energy balance, a main restriction for boiler and pyrolysis capacity is the flue gas flow. Figure 9 illustrates the calculated flue gas flow

Figure 8. Heat surplus in a BFB boiler at 800 °C for the pyrolysis case.

which would significantly increase the risk of bed agglomeration and bed malfunction. To reduce the energy surplus in the bed and counteract an increased bed temperature, a first measure would be to decrease the primary air temperature. Figure 8 also shows the heat surplus without preheating of primary air from April to September. As can be seen, to cease the preheating of the primary air is an insufficient action to maintain the design bed temperature and further cooling of the bed would be required. This could, in principal, be achieved, e.g., by means of cooling coils in the BFB, water injection in the bed, or probably more feasible, introducing a cooling circuit, where sand is taken out from BFB, cooled, and returned to the BFB or the sand silo. To maintain the desired bed temperature of 800 °C, a sand circulation flow for cooling of approximately 12 kg/s is required during summer. The magnitude of the heat surplus calculated within the scope of this paper shall be considered indicative because the heat transfer to the bed as a result of radiation and convection from the bed surface and the free board is affected in a highly complex way by an altered ratio of volatiles/non-volatiles in the boiler fuel. Furthermore, there is an uncertainty about the assumption that char burns in the bed, because the char from the biomass pyrolysis reactor enters the boiler BFB as a fine powder that might partly elutriate from the bed and burn in the boiler freeboard. However, the calculations clearly show the risk of an increasing heat surplus with increased pyrolysis oil production.

Figure 9. Boiler flue gas flow.

from the boiler in the base case as well as in the pyrolysis case. The calculations show that integration of pyrolysis oil production will result in an increased maximum flue gas flow, but the increase is not significant. In the studied example having the highest flue gas flow in February, the increase in maximum flow is 1%. During periods with low boiler load, when pyrolysis oil production is higher, the increase of flue gas flow is significant compared to the base case for the same period, but the flow is clearly within the boiler and flue gas fan capacity range. If hot air from a bed cooler is added to the exhaust gases from the boiler, the total exhaust volumes increase, as also illustrated in Figure 9. Such a use of the warm air from the sand cooler would not be feasible in the studied case because this would only increase the amount of non-used heat from the pyrolysis condenser 1.

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CONCLUSION The simulation of an integration of pyrolysis oil production with DH CHP shows a potential for significant oil production. The potential for pyrolysis oil production is rather limited at periods with high energy demand in the DH net (high boiler load), whereas in periods of low energy demand in the DH net, the production potential is significantly higher. Although, to 5318

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(3) Bridgwater, A. V. Review of fast pyrolysis of biomass and product upgrading. Biomass Bioenergy 2012, 38, 68−94. (4) Oasmaa, A.; Kuoppala, E.; Gust, S.; Solantausta, Y. Fast pyrolysis of forestry residue. 1. Extractives on phase separation of pyrolysis liquids. Energy Fuels 2003, 17, 1−12. (5) Ringer, M.; Putsche, V.; Scahill, J. Large-Scale Pyrolysis Oil Production: A Technology Assessment and Economic Analysis; National Renewable Energy Laboratory (NREL): Golden, CO, 2006; NREL/ TP-510-37779. (6) Solantausta, Y.; Oasmaa, A.; Sipilä, K.; Lindfors, C.; Lehto, J.; Autio, J.; Jokela, P.; Alin, J.; Heiskanen, J. Bio-oil production from biomass: Steps toward demonstration. Energy Fuels 2012, 233−240. (7) Kohl, T.; Pambudi, N. A.; Laukkanen, T.; Fogelholm, C.-J. Improved primary energy efficiency of district heating networks by integration of communal biomass fired combined heat and power plants with pyrolysis. Proceedings of the 12th International Symposium on District Heating and Cooling; Tallinn, Estonia, Sept 5−7, 2010; pp 168−176. (8) Westerhof, R. J. M.; Kuipers, N. J. M.; Kersten, S. R. A.; Van Swaaij, W. P. M. Controlling the water content of biomass fast pyrolysis oil. Ind. Eng. Chem. Res. 2007, 46, 9238−9247. (9) Pollard, A. S.; Rover, M. R.; Brown, R. C. Characterization of biooil recovered as stage fractions with unique chemical and physical properties. J. Anal. Appl. Pyrolysis 2012, 93, 129−138. (10) Westerhof, R. J. M.; Brilman, D. W. F.; Garcia-Perez, M.; Wang, Z.; Oudenhoven, S. R. G.; van Swaaij, W. P. M.; Kersten, S. R. A. Fractional condensation of biomass pyrolysis vapors. Energy Fuels 2011, 1817−1829. (11) Jarboe, L. R.; Wen, Z.; Choi, D.; Brown, R. C. Hybrid thermochemical processing: Fermentation of pyrolysis-derived bio-oil. Appl. Microbiol. Biotechnol. 2011, 91, 1519−1523. (12) Scala, F.; Chirone, R.; Salatino, P. Combustion and attrition of biomass chars in a fluidized bed. Energy Fuels 2006, 20, 91−102. (13) Scala, F.; Chirone, R.; Salatino, P. The influence of fine char particles burnout on bed agglomeration during the fluidized bed combustion of a biomass fuel. Fuel Process. Technol. 2003, 84, 229− 241. (14) Johansson, I.; Larsson, S.; Wennberg, O. Drying of Bio Fuel Utilizing Waste Heat; Värmeforsk: Stockholm, Sweden, 2004; ISSN: 0282-3772. (15) Oasmaa, A.; Kuoppala, E.; Solantausta, Y. Fast pyrolysis of forestry residue. 2. Physicochemical composition of product liquid. Energy Fuels 2003, 17, 433−443. (16) Wang, X.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M. Biomass pyrolysis in a fluidized bed reactor. Part 2: Experimental validation of model results. Ind. Eng. Chem. Res. 2005, 44, 8786−8795. (17) Dynamotive Energy System, Inc. Dynamotive CQuest BioChar. Information Booklet; Dynamotive Energy System, Inc.: Richmond, British Columbia, Canada, 2011; Vol. 115, p 24. (18) Shihadeh, A.; Hochgreb, S. Impact of biomass pyrolysis oil process conditions on ignition delay in compression ignition engines. Energy Fuels 2002, 16, 552−561. (19) Bridgwater, A. V. Fast Pyrolysis of Biomass: A Handbook; CPL Press: Newbury, U.K., 1999; Vol. 22. (20) Daugaard, D. E.; Brown, R. C. Enthalpy for pyrolysis for several types of biomass. Energy Fuels 2003, 17, 934−939. (21) Kornmayer, C. Verfahrenstechnische untersuchungen zur schnellpyrolyse von lignocellulose im doppelschneckenmischreaktor. Ph.D Thesis, Karlsruhe Institute of Technology, Karlsruhe, Germany, 2010.

harvest this, potential excess heat from the BFB must be removed to maintain energy balance and avoid overheating, which could lead to bed agglomeration and jeopardize the BFB function. The maximum heat surplus that needs to be removed from the boiler bed is difficult to determine without experimental research. Further research within the field of elutriation of small-sized, pyrolysis origin, char from a BFB boiler would be valuable to make more precise prediction of potential pyrolysis oil production capacity. The sequential oil condensation with the first condenser operating at an elevated temperature, as applied in this study, enables a substantial increase of plant efficiency compared to a single condenser design. The temperature in condenser 1 can be varied significantly with maintained high efficiency and can thus be chosen on the basis of considerations related to oil composition. The system design, as outlined in this study, with pyrolysis condenser 1 placed prior to the turbine condenser, is accordingly beneficial for flexibility in terms of pyrolysis oil quality control, because it allows for a low condenser temperature with the maintained ability to transfer heat of condensation to the DH net.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +46-70-584-46-68. Fax: +46-54-100-249. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Associate Professor Roger Renström for valuable comments during the preparation of the manuscript and Gävle Energi AB for supplying data for their combined heat and power plant. This study was financially supported by VIPP, “Values created In fibre-based Processes and Products”, a multidisciplinary industrial research school at Karlstad University, and Pöyry Sweden AB.

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NOMENCLATURE P0bio‑oil = vapor pressure of bio-oil (Pa) T = temperature (K) ṁ c = total char flow (kg/s) LHVc = char heating value (with exhaust gas reference temperature at 800 °C) (kJ/kg) ṁ w = water flow to BFB (kg/s) hw = water enthalpy (kJ/kg) Q̇ S = heat supply from bed surroundings (kW) hs = steam enthalpy (kJ/kg) ṁ v = mass flow of volatiles (kg/s) hv = enthalpy of volatiles (kJ/kg) Q̇ surplus = excess heat in bed (kW) ṁ p = mass flow to pyrolysis (kg/s) hp = enthalpy for pyrolysis (kJ/kg)

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REFERENCES

(1) Butler, E.; Devlin, G.; Meier, D.; McDonnell, K. A review of recent laboratory research and commercial developments in fast pyrolysis and upgrading. Renewable Sustainable Energy Rev. 2011, 15, 4171−4186. (2) Mohan, D.; Pittman, C. U.; Steele, P. H. Pyrolysis of wood/ biomass for bio-oil: A critical review. Energy Fuels 2006, 20, 848−889. 5319

dx.doi.org/10.1021/ef401143v | Energy Fuels 2013, 27, 5313−5319