Article pubs.acs.org/EF
Technoeconomic Assessment of a Fast Pyrolysis Bio-oil Production Process Integrated to a Fluidized Bed Boiler Kristin Onarheim,* Jani Lehto, and Yrjö Solantausta VTT, P. O. Box 1000, 02044 VTT, Finland ABSTRACT: The integration of a fluidized bed fast pyrolysis process producing bio-oil to an existing fluidized bed boiler combined heat and power (CHP) plant is presented. The purpose of this work is to assess the cost and performance of the integrated fast pyrolysis bio-oil production compared to a stand-alone fast pyrolysis bio-oil production plant. The reason for integrating bio-oil production into a fluidized bed boiler is to increase overall energy efficiency and profitability and to decrease the production costs of the bio-oil. In the integrated fast pyrolysis concept hot sand from the fluidized bed boiler is used for heating the fast pyrolysis reactor. Simultaneously, fast pyrolysis process byproducts such as char and noncondensable gases are cofired in the CHP boiler together with the primary forest residue boiler fuel. The assessment shows that the integration decreases the primary fuel requirement of the boiler. The integration causes changes in the net power and heat output of the CHP plant, but the integration can still be more profitable than a stand-alone fast pyrolysis process. The differences in pyrolysis feedstock characteristics are important when comparing integration to stand-alone bio-oil production. In this work pine sawdust and forest residue feedstock were evaluated, of which only the forest residue proved to be economically advantageous for integration to a CHP boiler. The advantage is evaluated as a reduction in bio-oil production cost compared to a stand-alone fast pyrolysis process for bio-oil production. For implementation of the integrated process, three potential industrial strategies for boiler operation in combined heat and power plants were assessed. These include keeping the superheated steam mass flow constant, keeping the boiler flue gas mass flow constant, and keeping the electricity output constant. The total integrated process efficiency was 87% for the case of sawdust and 86.2% for the forest residue. Sensitivities were studied for variations in the cost of forest residue boiler fuel, cost of heat, and cost of electricity and for the variations in the capital expenditure benefits obtained due to the integration. It was shown that the advantage of integration is highly sensitive to the cost of heat, primarily because of the energy-intensive pyrolysis feedstock drying. The feedstock cost is also a major factor in estimating the advantage of integration. Utilizing forest residue as feedstock for fast pyrolysis proved to be advantageous in all cases evaluated in this work, whereas the break-even price for a competitive integration utilizing sawdust feedstock is 25 €/MWh. The most beneficial operational integration strategy for the boiler would be to maintain a fixed flue gas flow (exhaust) from the boiler, resulting in an advantage of integrated oil production (reduced bio-oil production cost) compared to stand-alone bio-oil production of 7.9 €/MWh bio-oil. The integrated sawdust case, which is shown to be less competitive in this study, would lead to an additional production cost compared to the stand-alone bio-oil production cost of 1.6 €/MWh bio-oil. The results obtained in this study are independent of process scale.
1. INTRODUCTION
In the integrated concept a fast pyrolysis process is integrated into a fluidized bed boiler process such as a combined heat and power (CHP) plant. A CHP plant is a thermal power plant producing heat in addition to electricity. The produced heat is typically used for residential district or industrial heating purposes. Integrating a fast pyrolysis process into a fluidized bed boiler has significant benefits. The fast pyrolysis process contributes to the boiler performance by generating combustible byproducts such as noncondensable gases (mainly CO and H2) and carbon-rich char. In an existing CHP facility the integration enables the production of an additional profitable product, bio-oil, through the exploitation of low-value woody residues. This integration enables a more cost-efficient bio-oil production compared to stand-alone pyrolysis, as the energy contained in the process byproducts is utilized with higher efficiency compared to a stand-alone pyrolysis process.
Research and development of fast pyrolysis processes for bio-oil production gained ground already after the global oil crisis in the beginning of the 1970s. Fast pyrolysis of biomass is a thermal decomposition process taking place at atmospheric pressure around 500 °C in an oxygen-free environment. The main product, bio-oil, can be used as a renewable substitute for heavy fuel oil. Noncondensable gas and char are important byproducts from biomass fast pyrolysis. The idea was, and still is, that the pyrolysis oil would first replace mineral fuel oil from fossil sources and eventually also become suitable for higher value utilization modes such as transportation fuels and chemicals production, thus decreasing the current heavy dependence on fossil fuels.1 However, as the economy stabilized and the oil price decreased over the next decade bio-oil proved too expensive for commercial markets and most research and development was discontinued. In the 1990s Sipilä2 showed that it would be possible to cut the production costs of bio-oil by integrating the fast pyrolysis process into a fluidized bed boiler. © XXXX American Chemical Society
Received: June 15, 2015 Revised: August 21, 2015
A
DOI: 10.1021/acs.energyfuels.5b01329 Energy Fuels XXXX, XXX, XXX−XXX
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fast pyrolysis process presented by Onarheim et al.19 The fast pyrolysis model presented is largely based on experimental data from VTT’s small pilot unit, which has been shown to give results comparable to the industrial pilot by Valmet.13 The model can be used to calculate the mass and energy balances and constitutes a comprehensive tool for feasibility and technoeconomic studies of the integrated fast pyrolysis concept. The model enables process analyses such as assessing the impacts of the pyrolysis process on the heat and power outputs of an existing CHP plant and offers a valuable tool for process optimization of integrated fast pyrolysis. The model also enables the comparison of a stand-alone pyrolysis process to an integrated bio-oil production plant. In this work the advantage of integrating a fast pyrolysis biooil production process to a CHP fluidized bed boiler has been assessed and compared to a stand-alone bio-oil production process. The advantage is calculated as a reduction in bio-oil production costs compared to the cost of producing bio-oil in a stand-alone fast pyrolysis process. The produced bio-oil is an additional product in the integrated concept and will be transported to replace mineral fuel oil in various boilers.
Furthermore, the investment and operational costs of an integrated fast pyrolysis plant into an existing CHP plant are lower compared to those of stand-alone fast pyrolysis bio-oil production, even when taking into account costs of modifying the existing boiler to accommodate the integration.2 Please note that a stand-alone pyrolysis plant always needs a boiler to generate energy for the pyrolysis process and to combust fast pyrolysis byproducts char and noncondensable gases. It has also been shown that fast pyrolysis bio-oil can replace heavy fuel oil and natural gas in industrial-scale combustion tests in district heating applications.3 A potential next step is to develop bio-oil as a suitable fuel for diesel engines and gas turbines.4 Ultimately, with proper bio-oil upgrading and deoxygenation processes bio-oil could be blended into mineral oil refineries in order to produce transportation fuels and other higher-value-added chemicals.5−9 A pilot-scale project for demonstrating a technically and economically viable approach for producing sustainable transport fuels from biomass via pyrolysis, catalytic hydroconversion, and co-processing with vacuum gas oil has been ongoing since 2009.10 The integrated fast pyrolysis concept has been patented and developed by VTT.2,11 This integrated process was scaled up to a 2 MW feedstock pilot plant in a cooperation project between Valmet, Fortum, UPM Kymmene, and VTT starting in 2007.12−14 As a result of successful piloting, the world’s first integrated fast pyrolysis demonstration plant was built in Joensuu, Finland. The plant is integrated into Fortum’s existing CHP plant producing electricity and district heating for the city of Joensuu. The demonstration plant is designed to produce 50000 tons of bio-oil annually (30 MW oil capacity), mainly from forest residues and sawdust.15 A study has been made on the market potential of replacing fossil fuels with pyrolysis bio-oil in industrial boilers in Europe.16 The study identified a potential for 50 pyrolysis processes integrated to fluidized bed boilers in the pulp and paper sector alone. The bio-oil potential was estimated to be 7% of the primary fossil energy supply of the European pulp and paper industry and 130% of the fossil fuel consumption in lime kilns, utilizing 14% of the total European forest energy wood potential (forest energy wood potential includes stump wood and felling residues). Hitherto, only limited work has been published on modeling of a fast pyrolysis process integrated to a fluidized bed boiler. Kohl et al.17 investigated the effects of integrating a biomass fast pyrolysis process to a CHP plant. The study included an evaluation of the effects of varying energy and environmental performance under both full- and part-load operation of the CHP plant. The idea of indirect heat exchange presented in Kohl et al. is less efficient than direct use of boiler sand as a heat source. It is also more costly, as it requires additional process equipment. Majanne et al.18 developed a dynamic simulation of a fast pyrolysis process integrated in a fluidized bed boiler. The model analyzed fluctuations and disturbances in the integrated process and their impacts on the boiler operation. On the basis of the results, an automatic control system stabilizing the boiler performance with the pyrolysis process was developed, ensuring a stable boiler operation. This work focused on the technical aspects of integration and did not take into account any economic aspects. The purpose of the present study was to develop a model for simulating an integrated fast pyrolysis process with Aspen Plus. This is a continuation of the modeling work of a stand-alone
2. PROCESS DESCRIPTIONS 2.1. Pyrolysis. The quality of bio-oil as a fuel suffers from high water content. Fast pyrolysis feedstock moisture increases bio-oil−water content. For this reason, the fast pyrolysis feedstock is dried in a belt dryer from 50 to 8 wt % moisture content. The dryer uses steam and hot water from the combined heat and power steam cycle. After drying, the feedstock is ground to a size of less than 5 mm and fed to a circulating fluidized bed (CFB) fast pyrolysis reactor. The fast pyrolysis reactions generate pyrolysis vapors, noncondensable gases, water, and char. Char is separated from the pyrolysis vapors in cyclones immediately after the reactor and sent to the fluidized bed boiler for combustion. The pyrolysis vapors are cooled and condensed into bio-oil in a scrubber. Noncondensable gas exits the scrubber and is partly recycled as fluidizing medium to the pyrolysis reactor while the remaining gas is combusted in the fluidized bed boiler. Condensed bio-oil is used as quenching medium in the scrubber. 2.2. Combined Heat and Power. Combined heat and power plants are co-generation facilities, which simultaneously produce heat and power (electricity). The main sections of a combined heat and power plant are a boiler, for example a fluidized bed boiler, and a steam cycle including a steam turbine and a generator. A typical fluidized bed boiler consists of a combustion chamber with sand as a heat transfer medium, which is fluidized by air. As a result, fuel is mixed with combustion air in a high-turbulence and high-temperature bed, which enhances the combustion reactions and heat transfer in the furnace. Combustion heat from the boiler is used to produce steam for the steam turbine that generates electricity in a Rankine water−steam cycle. Electricity production, however, involves a significant heat loss in the form of low-pressure steam exiting the steam turbine. The loss of heat of condensation of steam can be turned into an advantage in the combined heat and power plant when it is used for production of hot district heating water, or steam in industrial co-generation. After the steam is condensed, it will be returned to the boiler for reheating via low- and high-pressure economizers to complete the steam cycle. In a combined heat and power plant there are usually a number of intermediate steam extractions, bleed B
DOI: 10.1021/acs.energyfuels.5b01329 Energy Fuels XXXX, XXX, XXX−XXX
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The fluidized bed boiler produces steam for the steam cycle, which is generating electricity. Hot water is also produced, when steam is condensed after the turbine. Steam, hot water, and heat from bio-oil condensation are used for fast pyrolysis feedstock drying. Figure 2 shows a simplified block diagram of the integrated fast pyrolysis process as modeled in Aspen Plus.
streams, and recycle streams in order to optimize the heat transfer and energy efficiency of the plant. Typically, hot water is used for district heating in residential and commercial buildings in Nordic and Eastern European countries, but increasingly also in other countries. Combined heat and power plants are economically well suited for relatively small installations in cold climate areas and can typically be fueled with a variety of fuels, such as biomass, coal, gas, and industrial and municipal waste. The main difference between a combined heat and power plant and a condensing power plant is the utilization of the waste heat after the steam turbines. In condensing steam turbine power plants waste heat is condensed, enabling an efficiency of typically 35−45%. The overall efficiency of a CHP plant is typically around 90%; see Figure 1.
Figure 2. Simplified block diagram of the fast pyrolysis process integrated into a CHP plant.
Modifications to the fluidized bed boiler due to the integration of a fast pyrolysis process include systems to extract part of the sand from the boiler and an additional system for returning sand and char to the boiler. For combusting the noncondensable gases a system for feeding the gas to the boiler is needed in addition to a gas burner. There are also synergies between fast pyrolysis and CHP processes in the fuel procurement and handling. As feedstock volumes increase at the site level, the logistics become more cost-efficient and quality control for fast pyrolysis feedstock becomes easier (i.e., the most suitable fractions of the feedstock can be used in the fast pyrolysis process while the rest are utilized in the CHP process).
Figure 1. Typical energy flows for condensing and combined heat and power plants.
3. METHODS
2.3. Integrated Fast Pyrolysis. Integrating a fast pyrolysis process into a fluidizing bed boiler is mutually advantageous. The pyrolysis process utilizes the hot sand from the boiler, but on the other hand the forest residue fuel feed to the boiler may be reduced because the fast pyrolysis process byproducts char and noncondensable gas replace part of the boiler fuel feed.11 Char is the most valuable byproduct from the fast pyrolysis process. The yield and energy content of fast pyrolysis char depends mainly on the feedstock type. The higher the oil yield, the less byproduct char is produced. Forest residue produces more char during fast pyrolysis than pine sawdust. Sand at around 800−900 °C is extracted from the fluidized bed boiler, providing heat for the pyrolysis reaction which takes place at around 450−500 °C. Heat transfer inside the fast pyrolysis reactor is a combination of solid−solid and solid− gas−solid heat transfer based on conductive heat exchange from sand particles and gas to the biomass particles, and conductive heat exchange from sand to fluidizing gas. The sand to biomass ratio depends on type and properties of feedstock, moisture content, and reaction temperature, but is typically around 8−12. Sand is separated after the pyrolysis reactor in the cyclones together with the char, and both are led to the fluidized bed boiler, where sand is reheated and char is used as fuel.
3.1. Model Development. Modeling of a stand-alone fast pyrolysis process for bio-oil production with Aspen Plus was described in detail by Onarheim et al.19 The work presents detailed information on feedstock types, pyrolysis yield, component selection, bio-oil composition, and property methods for fast pyrolysis modeling. In this work the integration of a fast pyrolysis bio-oil production process to a fluidized bed boiler has been compared to a stand-alone fast pyrolysis process. The preferred type of fluidized bed boiler configuration for this integration would be a CHP plant in the case when there is a heat sink available. First, two integrated fast pyrolysis base cases are evaluated: one using pine feedstock and one using forest residue feedstock; see Figure 2. The feedstock composition is given in Table 1. In these two base cases, total fuel capacity to the fluidized bed boiler is fixed to 120 MW. This includes the pyrolysis byproducts char and noncondensable gases; see Table 2. This fluidized bed boiler size is typical for a combined heat and power plant in the Nordic countries. Regardless of the feedstock type utilized in the fast pyrolysis process, the boiler is in all cases fueled with forest residue at 50 wt % moisture as received, which is typical fuel for a Nordic CHP plant. Bio-oil production in both integrated cases is 30 MW (LHV, wet). Both types of feedstock are assumed to contain 50 wt % moisture as received. The fast pyrolysis feedstock is dried to 8 wt % moisture prior to feeding it to the pyrolysis reactor. The combined heat and power plant model consists of a combustion section and a steam cycle section. The physical property C
DOI: 10.1021/acs.energyfuels.5b01329 Energy Fuels XXXX, XXX, XXX−XXX
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to the economizer. Another extraction is made in the intermediatepressure section. Part of the intermediate-pressure steam is used for controlling the temperature of the condensate deaerator in order to preheat the condensed steam. The rest of the intermediate-pressure steam extracted is used for pyrolysis feedstock drying. In reality, a combined heat and power plant might consist of a significantly higher number of extraction points for steam cycle optimization purposes. However, this simplification of the steam cycle is not considered to undermine the model applicability. After the turbine, low-pressure steam is condensed with water, which may be used for district heating. Part of the hot water is also used for pyrolysis feedstock drying together with the intermediate pressure steam extracted. The feedwater flow in the steam cycle is a function of the rate of boiler fuel feeding and is regulated by a design specification that controls the final flue gas stack temperature. The temperature of the emitted flue gas was kept constant at 145 °C in all cases evaluated to avoid condensation and corrosion issues in the stack. The main specifications for the combined heat and power plant used in the model are given in Table 3.
Table 1. Ultimate and Proximate Analysis (wt %, Dry) and Heating Value of Feedstock19 element ash carbon hydrogen nitrogen oxygen sulfur chlorine feedstock heating value, LHV MJ/kg, wet MJ/kg, dry
pine
forest residue
0.2 50.9 6.3 0.1 42.5 0 0
2.6 51.4 6.3 0.3 39.4 0 0
8.3 19.1
8.5 19.5
moisture VM FC ash
pine
forest residue
0 83.9 15.9 0.2
0 80.0 17.4 2.6
Table 2. Fluidized Bed Boiler Feed (LHV, Wet) for the Nonintegrated CHP and the Two Integrated Base Casesa feed pyrolysis feed boiler feed (forest residue) char noncondensable gas total feed to boiler a
unit MW MW MW MW MW
CHP
integrated pine
integrated forest residue
120.0
41.5 105.4
51.7 94.5
120.0
12.2 2.4 120.0
21.9 3.6 120.0
Table 3. CHP Specifications
Bio-oil production corresponds to 30 MW (LHV).
value
°C bar bar bar bar °C
535 110 4.7 2.5 0.6 85
⎛Q + MWe,net + MWth,net ⎞ ⎟ × 100 ηint = ⎜⎜ bio‐oil ⎟ Q boiler + Q pyro ⎝ ⎠
(3)
The efficiency for the combined heat and power plant was calculated according to eq 4.
⎛ MWe,net + MWth,net ⎞ ⎟⎟ × 100 ηCHP = ⎜⎜ Q boiler ⎠ ⎝
(4)
3.2. Economic Parameters. The integrated fast pyrolysis concept has been evaluated and compared to a stand-alone fast pyrolysis process. The prices of electricity, heat, and feedstock types used in this evaluation are all listed as typical market prices in Finland. Table 4 summarizes the assumptions made for the economic analysis. The interest rate of 5.0% is a typical rate currently used in industry. The annual operating hours of 6500 hours annually might be bordering the upper limit. The typical operational time for a CHP plant is around 5500 hours annually. The reason is that a CHP plant cannot be operated as many hours annually as a condensing power plant because there is not enough annual heat load available (there is
(1)
Heat from the combustion chamber is used to preheat feedwater and generate superheated steam. The feedwater is heated in several stages until reaching the final steam conditions. An economizer heats the feedwater up to about 300 °C at 110 bar. Saturated steam is vaporized and then superheated to the final steam temperature. The steam turbine is modeled as a three-stage back-pressure turbine. The efficiency (η) of each turbine stage is calculated on the basis of the actual volumetric flow rate through the specific stage, taking into account any condensation taking place at the turbine interface. Equation 221 shows the calculation for the efficiency of each individual turbine stage:
⎛ q + q2 ⎞ (2 − x1 − x 2) ⎟ + 0.75 − ηs = 0.024 ln⎜ 1 ⎝ 2 ⎠ 2
unit
The efficiencies (ηint and ηCHP) of the integrated concepts were calculated according to eq 3, where Qbio‑oil is the bio-oil capacity in MW (LHV), MWe is the net electric output, MWth is the net heat output, QCHP is the capacity of the boiler fuel and Qpyro is the capacity of the fast pyrolysis feedstock.
methods applied in the combined heat and power model are STEAMNBS for the steam cycle and RK-SOAVE for the combustion section. The STEAMNBS property method was chosen over the ASME steam table correlations as STEAMNBS is a more recently developed property method in Aspen Plus. The STEAMNBS is based on correlations from the NBS/NRC steam tables for thermodynamic properties and the IAPWS correlations for transport properties.20 Thermodynamic properties in the RK-SOAVE property method are calculated based on the Redlich−Kwong−Soave cubic equation of state. The fluidized bed boiler is modeled with an RStoic reactor block. The boiler is operating at atmospheric pressure, and the combustion air mass flow is controlled by a design specification that ensures 3.0 vol % oxygen in the wet flue gas. The heat radiance loss from the boiler is specified according to the DIN 1942 standard for industrial kilns and boilers: Q radloss = 0.0315(Q th)0.7
specification steam temperature steam pressure high-pressure bleed intermediate-pressure bleed low-pressure bleed district heating water
Table 4. Assumptions for Economic Analysis
(2)
where q1 and q2 are the actual volumetric flows of the vapor phase at the turbine inlet and outlet, respectively, and x1 and x2 are the mass fractions of the vapor to the total flow at the turbine inlet and outlet, respectively. At the high-pressure turbine stage a small fraction of the steam is extracted and returned upstream to heat the feedwater prior D
assumption
unit
value
price of electricity price of heat forest residue feedstock pine feedstock annual operating hours interest rate service life
€/MWh €/MWh €/MWh €/MWh h/a % years
40.0 25.0 20.0 22.0 6 500 5.0 20
DOI: 10.1021/acs.energyfuels.5b01329 Energy Fuels XXXX, XXX, XXX−XXX
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pyrolysis process to a fluidized bed boiler, different strategies regarding the boiler operation should be considered for maximizing the operational income of the plant. In this work, three industrially feasible integration strategies were compared: (1) keeping the superheated steam flow constant, (2) keeping the fluidized bed boiler (CHP) flue gas constant, and (3) keeping the net electricity output constant. The effects of the integrated fast pyrolysis plant on the fluidized bed boiler economics taking into account existing boiler design restrictions and output requirements for each operation strategy are presented in Table 6. All cases are reported for both pine and forest residue pyrolysis integration. Table 6 presents the results of integration as the difference (Δ) compared to the CHP plant. The difference of advantage of integrated pyrolysis compared to stand-alone pyrolysis is calculated as the total boiler difference (disadvantage of less electricity and heat that could otherwise be sold and advantage from reduced primary boiler fuel consumption) over annual produced bio-oil. The boiler fuel input is the primary forest residue fuel needed in the boiler in addition to char and noncondensable gas from the pyrolysis process. Feeding fast pyrolysis char and noncondensable gases to the boiler allows for a reduction of primary fuel to the boiler, thereby decreasing the fuel input by 11.9 and 25.5 MW in the two base cases. Capital expenditure benefit (CAPEX benefit) is the reduction of investment costs of an integrated fast pyrolysis plant to an existing boiler compared to a stand-alone fast pyrolysis plant for bio-oil production. Investment costs for a stand-alone fast pyrolysis plant are higher than for an integrated fast pyrolysis plant when no investment costs are assumed for the existing boiler/CHP plant. Capital expenditure benefits have been translated into an annual cost saving with a capital recovery factor (CRF) of 0.1. The amount of capital expenditure benefit is based on VTT in-house data. The capital expenditure is higher for forest residue pyrolysis because more feedstock needs to be processed in order to produce the same amount of bio-oil, and this requires larger front end equipment such as a dryer and grinder. Labor costs have not
no demand for heat produced during the warm season). In the integrated concept, however, heat from the CHP plant is used in the fast pyrolysis feedstock dryer, which increases the heat load. As a result, the annual operating hours for the CHP plant can be extended. The investment expenses associated with the CHP plant as a result of integration include boiler modifications only, since we assume integration of fast pyrolysis into an existing CHP plant. Investments for the fast pyrolysis plant include feedstock receiving station, drying, milling, pyrolysis reactor, pyrolysis scrubber, bio-oil tanks, piping and ducting, and a delivery station for tank trucks.
4. RESULTS AND DISCUSSION In this study the economic potential of a fast pyrolysis plant for bio-oil production integrated to a fluidized bed boiler (in this case a CHP) has been compared to a stand-alone fast pyrolysis plant. Results from the stand-alone fast pyrolysis process used as comparison for the integrated concepts can be accessed in Onarheim et al.19 The bio-oil elemental composition is given in Table 5. Table 5. Elemental Composition of Bio-oil (wt %, Dry)19 element
bio-oil
carbon hydrogen nitrogen oxygen sulfur
54.5 7.2 0.1 38.2 0
For the integrated comparison two base cases were established first: one for pine fast pyrolysis and one for forest residue fast pyrolysis, in which the total fluidized bed boiler feed capacity was fixed to 120 MWth (LHV); see Table 2. No other criteria were taken into account for integration of the base cases. The base cases for fixed boiler capacity are presented in Table 6. Integrating a fast pyrolysis process to a fluidized bed boiler the way it is done in the base cases is, however, not the most technically feasible method of integration. This is due to design limitations in the existing CHP plant. When integrating a fast
Table 6. Results of Fast Pyrolysis Integration to a Fluidized Bed Boilera base cases: fixed boiler capacity
strategy variable steam flow steam flue gas flow electricity output district heat output boiler fuel input
CHP units
CHP
unit
kg/s MWth kg/s MWe k€/a MWe k€/a MW k€/a
40.9 109.8 72.8 33.5 8 721 76.0 12353 120 15606
CAPEX benefit bio-oil production total boiler savings difference of integration efficiency a
%
91.3
strategy 1: fixed steam flow
strategy 2: fixed flue gas flow
strategy 3: fixed electric output
pine
forest residue
pine
forest residue
pine
forest residue
pine
forest residue
Δkg/s ΔMWth Δkg/s ΔMWe Δk€/a ΔMWe Δk€/a ΔMW Δk€/a Δk€/a GWh/a Δk€/a
−1.0 −2.8 −0.4 −1.9 −497 −10.0 −1 630 −11.9 −1 544 240 195 −344
−0.9 −2.5 −3.3 −2.0 −513 −11.6 −1 879 −25.5 −3 320 360 195 1 287
0 0 1.5 −1.0 −268 −8.1 −1 320 −8.8 −1 142 240 195 −206
0 0 −1.6 −1.2 −306 −9.8 −1 597 −22.7 −2 956 360 195 1 413
−0.8 −2.2 0 −1.7 −448 −9.6 −1 564 −11.2 −1 457 240 195 −314
0.9 2.5 0 −0.4 −107 −8.2 −1 328 −20.1 −2 608 360 195 1 534
1.2 3.3 3.8 0 0 −8.6 −1 403 −5.2 −672 240 195 −492
1.4 3.8 0.9 0 0 −9.7 −1 571 −18.6 −2 419 360 195 1 209
Δ €/MWh
−1.8
6.6
−1.1
7.2
−1.6
7.9
−2.5
6.2
bio-oil %
87.0
86.2
87.0
85.1
87.0
84.1
85.3
83.6
Results are given as the difference (Δ) compared to the combined heat and power (CHP) plant. E
DOI: 10.1021/acs.energyfuels.5b01329 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels been taken into account, although it is expected that an integrated concept requires less manpower compared to the stand-alone unit (benefits are expected from boiler and pyrolysis operation and maintenance). A more detailed evaluation of overall production costs of stand-alone fast pyrolysis can be found in Onarheim et al.19 Costs and savings are expressed on an annual basis. Mass and energy balances for the evaluated strategy 2, keeping the boiler flue gas constant, are illustrated in Figures 3 and 4. The energy balance is based on lower heating value (LHV). The carbon balance is illustrated in Figure 5.
Figure 5. Carbon balance of the integrated fast pyrolysis plant using forest residue as pyrolysis feedstock (strategy 2).
electricity and heat output due to integration. This is true even though the forest residue pyrolysis process needs approximately 25% more feedstock than the process fed with pine in order to be able to produce the same 30 MW (LHV) of bio-oil. The annual cost of the fast pyrolysis feedstock is higher for the process using forest residue, but the higher rate of char production and the lower unit cost of the feedstock offset the increased feedstock expenditure. Although pine pyrolysis has a lower feedstock requirement, the integration is not economically viable due to a higher feedstock unit cost and production of far less char. A sensitivity analysis showed that the results presented in Table 4 are independent of the size of the pyrolysis plant and the size of the CHP plant. An additional integration strategy to those presented in Table 4 would be to keep the same net heat output from the CHP as prior to integration. However, this integration strategy would probably be very unlikely in practice since the corresponding steam flow and flue gas flow would increase approximately by 10−15%. In many industrial applications an increase of this magnitude would be too much for the boiler (the steam turbine is typically a limiting factor). In cases where it would be possible to maintain a fixed net heat output in the plant, however, this integration strategy would offer the most advantageous overall integration result, with an additional 11− 13% net electric output, depending on the pyrolysis feedstock type. The cost of heat and the cost of boiler fuel are the most important factors affecting the advantage of integration. The main reason is that a significant amount of the heat produced in the boiler is used in the fast pyrolysis feedstock dryer. The results of varying the cost of heat for strategy 2 (fixed flue gas flow) are illustrated in Figure 6. The break-even price of heat in this case is 20 €/MWh for pine and 54 €/MWh for forest residue pyrolysis integration. The extraction of steam from the CHP steam cycle has a significant effect on the overall benefit of integration. Figure 6 shows the importance of cost-efficient drying of the pyrolysis feedstock. It also depicts that the extraction of steam from the CHP steam cycle for the dryer is a major drawback of the integrated process. In order to minimize the steam extraction and to improve the overall integration advantage, the use of secondary heat sources (if available) for feedstock drying would be beneficial. It might even pay off to invest in another type of dryer, such as for instance a flue gas rotary kiln dryer or a flue gas scrubber. McKeough et al.22 calculated the integrated process efficiency with two different dryers: a flue gas dryer and
Figure 3. Mass balance for the overall integrated forest residue bio-oil production considering strategy 2, in which the boiler flue gas has been kept constant.
Figure 4. Simplified energy balance (based on LHV) for the overall integrated forest residue bio-oil production considering strategy 2, in which the boiler flue gas has been kept constant.
The process efficiency drops from 91.3% for the CHP plant to between 83 and 87% for the various integrated concepts. The main reason for this significant drop is the energy consumption of the fast pyrolysis feedstock dryer. It can be seen from the results presented in Table 4 that only integration of the forest residue pyrolysis process would have a beneficial effect on the bio-oil production cost. The most advantageous operational strategy would be to maintain a fixed flue gas flow in the boiler, offering a reduction of bio-oil production cost of 7.9 €/MWh compared to a stand-alone fast pyrolysis process. In the case of maintaining a fixed steam flow the integration benefit is also significant at 7.2 €/MWh. The added value for the forest residue integration comes from greater savings in boiler primary fuel compared to the losses in F
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Figure 8. Difference in bio-oil production costs due to integration as a function of the cost of electricity. The cost of heat was 25 €/MWh, and the cost of boiler fuel was 20 €/MWh.
Figure 6. Difference in bio-oil production costs due to integration as a function of the cost of heat. The cost of electricity was 40 €/MWh, and the cost of boiler fuel was 20 €/MWh.
in Figure 9. The forest residue integration remains beneficial even in the cases in which capital expenditure would be in favor
a steam dryer. They concluded that the entire drying heat required in the integrated process can be provided by waste heat from the final flue gases of the CHP boiler, but that the overall process efficiency for the integrated concept depends on the size of the CHP boiler. For large CHP boilers (>100 MWth) flue gas dryers provide a higher efficiency, whereas for smaller CHP boilers (∼30 MWth) that have less waste heat available, steam dryers would offer a better efficiency. McKeough et al. also concluded that the steam dryer is only economically feasible when there is a demand for district heat. The price of boiler primary fuel affects the integration, as illustrated in Figure 7. As the cost of boiler fuel increases, the
Figure 9. Difference in bio-oil production costs due to integration as a function of savings in capital expenses. The cost of heat was 25 €/MWh, the cost of electricity was 40 €/MWh, and the cost of boiler fuel was 20 €/MWh.
of a stand-alone pyrolysis process. On the other hand, pinefueled cases are not competitive with stand-alone concepts. The bio-oil quality from different feedstock types has not been addressed specifically in this work. Please see Lehto et al.3 for further reading on the effect of bio-oil quality on the combustion processes.
Figure 7. Difference in bio-oil production costs due to integration as a function of the cost of boiler fuel. The cost of electricity was 40 €/MWh, and the cost of heat was 25 €/MWh.
5. CONCLUSIONS A technoeconomic assessment was made for the integration of a fluidized bed fast pyrolysis process to a bubbling fluidized bed boiler for power production. The integration of a fast pyrolysis process to a fluidized bed boiler provides advantages for both processes. The byproducts from the fast pyrolysis process, char and noncondensable gases, are used as fuel in the fluidized bed boiler, thereby decreasing the fuel feed requirement. In addition, the CHP plant gets an additional product: bio-oil. Two feedstock types were used for the fast pyrolysis process: sawdust (pine) and forest residue. Integrating a fast pyrolysis process to a fluidized bed boiler producing heat and power will affect the net output of the CHP plant. In this evaluation, the integrated pine process, in which the total feed to the boiler is 120 MW, has an efficiency of 87% whereas the process based on forest residue has an efficiency of
integration of the fast pyrolysis process to the CHP plant becomes more and more beneficial compared to stand-alone pyrolysis. The forest residue feedstock benefits the integration above 10 €/MWh, while the pine feedstock will bring a disadvantage at less than 25 €/MWh, which is also the current situation with a pine cost of 22 €/MWh (ref. Table 4). Variations in the price of electricity have only negligible effects on the total integration advantage for the forest residue pyrolysis integration. The pine integration case is more sensitive to an increase in the cost of electricity and is not able to provide any advantage to the integrated concept. The effects of the cost of electricity are illustrated in Figure 8. The savings in capital expenses due to integration have a significant impact on the overall benefit of integration as shown G
DOI: 10.1021/acs.energyfuels.5b01329 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
CHP = combined heat and power CRF = capital recovery factor (ratio of constant annuity to present value over a period of time) LHV = lower heating value h = hour HXTR = heat exchanger IAPWS = International Association for the Properties of Water and Steam k = kilo mm = millimeter MW = megawatt MWh = megawatt hour PDU = process development unit Q = energy (MW) q = volume flow (m3/s) VTT = VTT Technical Research Centre of Finland Ltd. wt = mass x = mass ratio € = euro η = efficiency
86.2%. Nevertheless, integration makes sense if the boiler operator has access to low-cost feedstock. It is also a prerequisite that there is a user for the bio-oil so the additional product can be sold with revenue, which means the bio-oil must be cost-competitive with mineral fuel oil. The integration will be advantageous in terms of reduced bio-oil production cost compared to stand-alone fast pyrolysis when the pyrolysis process utilizes forest residue as feedstock. On the basis of the assumptions used in this evaluation, integration of a pine-fueled fast pyrolysis process into a fluidized bed heat and power boiler will not be beneficial unless the pyrolysis feedstock cost exceeds 25 €/MWh. The reason forest residue fast pyrolysis is more beneficial is mainly due to the higher flow rate of carbon-rich char from the pyrolysis process to the fluidized bed boiler. The energy content of the char exceeds that of the higher pyrolysis feedstock rate and the associated increase in energy demand for the pyrolysis feedstock dryer. Thus, the differences in feedstock costs are critical when evaluating the overall advantage of integration. Other factors affecting the advantage of integration are cost of heat, cost of electricity (limited effect), and oil yields associated with the different feedstock types. The cost of heat is a crucial factor due to the high energy demand in the fast pyrolysis feedstock dryer. In the integrated pine process the break-even price of heat is already at 20 €/MWh, whereas for the forest residue integration the break-even price approaches 50 €/MWh. The effect of the cost of electricity is not significant in the forest residue case, whereas it gives a slight disadvantage in the pine case, provided that the feedstock dryer is not heated electrically. Industrial optimization is often limited by existing boiler design. Typical limitations in the boiler are steam flow and flue gas flow. Three integration strategies were evaluated: keeping the flue gas flow fixed, keeping the steam flow fixed, and producing the same net electric output. Of these, the most beneficial way to integrate a fast pyrolysis process with the assumptions made in this study would be to keep a fixed flue gas flow in the boiler. The resulting advantage of integration, 7.9 €/MWh bio-oil, is slightly higher than when keeping the steam flow fixed, which would give an advantage of 7.2 €/MWh bio-oil. Correspondingly, the integrated sawdust case would lead to an additional cost of 1.6 €/MWh bio-oil and 1.1 €/MWh bio-oil for the respective integration strategies. The results obtained in this study were found to be independent of both fast pyrolysis and fluidized bed boiler scales.
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Subscripts
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bio-oil = bio-oil capacity (MW) boiler = boiler fuel capacity (MW) CHP = combined heat and power plant e = electric int = integrated pyro = fast pyrolysis fuel capacity (MW) radloss = radiation loss (MW) s = turbine stage th = thermal
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AUTHOR INFORMATION
Corresponding Author
*E-mail: kristin.onarheim@vtt.fi. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Authors would like to thank VTT and the Finnish Funding Agency for Technology and Innovation, Tekes, for funding this work. Product Manager Joakim Autio, Valmet, is acknowledged for providing data.
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NOMENCLATURE a = annual ASME = American Society of Mechanical Engineers CFB = circulating fluidized bed H
DOI: 10.1021/acs.energyfuels.5b01329 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.5b01329 Energy Fuels XXXX, XXX, XXX−XXX
