Introduction
Pyrolysis has become one of the most attractive options for converting carbonaceous biomass into bio-oil or biochar. This study explores a novel solar pyrolysis process intended to produce both bio-oil and biochar, thereby improving carbon efficiency. Aspen Plus and SolarPILOT were used to model a 10 MW biomass pyrolysis plant thermally sustained by hot particles from a falling-particle solar tower receiver. A yearly analysis was carried out for three configurations to estimate the annual production of oil and biochar.
The results showed that the hybrid plant, combining solar receiver and biochar backup combustor, leads to the lowest cost of bio-oil (18.7 € per GJ, or 0.29 € per kg) and a carbon efficiency of 83%. Whereas, the plant fully sustained by solar power achieves a carbon efficiency of 90%; however, it results in a significantly higher cost of bio-oil (21.8 € per GJ, or 0.34 € per kg) due to the larger size of particle storage and a lower capacity factor of the pyrolysis plant. In comparison, a conventional pyrolysis plant with no biochar production yielded the most expensive option in terms of the cost of produced bio-oil (27.5 € per GJ) and features the lowest carbon efficiency (74%).
The pyrolysis process is endothermic and requires an external source of energy to heat up the feedstock and break down the molecular structure of the carbonaceous material. This thermal input can be supplied by various sources, e.g. direct combustion of its by-products, electrical resistive heating or solar thermal heating. The limit of conventional pyrolysis processes sustained by combustion is the loss of part of the biogenic carbon as CO2. Supplying the process heat via electric or solar heating allows for the reduction of CO2 emissions and improves the carbon efficiency and the amount of bio-based products.
Concentrated Solar Power Integration
Concentrated solar power (CSP) systems use a series of mirrors that concentrate the solar radiation towards a receiver where solar energy is converted into thermal energy, which can either be used directly or to produce electricity using a thermodynamic cycle. The possibility of storing heat in a low-cost thermal energy storage (TES) system for later use allows for decoupling the availability of solar radiation from thermal power production. In contrast to solar photovoltaics (PV), CSP systems use almost the entire spectrum of solar radiation to produce heat typically in the range of 400–2000 °C, which may be used to drive chemical reactions such as pyrolysis.
The integration of CSP into conventional pyrolysis may help fulfil the energy requirements of the endothermic pyrolysis reactor. Previous studies on CSP-based pyrolysis predominantly focus on small-scale laboratory experiments involving direct irradiation on the reactor. The solar-driven pyrolysis using direct radiation faces several challenges, including uneven heat distribution within the reactor, thermal stress caused by variations in solar flux, and lower oil yields due to the slow pyrolysis of biomass. Furthermore, there is no control over the temperature and heating rates, and the process stops working in the absence of sunlight.
To address these issues while maintaining the concept of fast pyrolysis using a fluidized bed reactor, indirect solar CSP heating was adopted through solid particle heat carriers (PHC). This solution also offers thermal storage for longer periods in the absence of sunlight, ensuring the smooth operation of pyrolysis. There is a notable absence of studies examining CSP-based pyrolysis with solid PHC falling particle receivers on an industrial scale. Moreover, there is a significant gap in the literature regarding the techno-economic analysis of such industrial-scale solar-assisted pyrolysis plants. This work aims to address this gap and serves as a starting point for future projects in this area.
Process Description
This study explores for the first time the process integration of an industrial scale biomass pyrolysis plant with a falling particle solar tower system using the same solid particle heat carrier (PHC) for the biomass pyrolysis reactor and as a heat carrier in the solar receiver. The main purpose of this study, which is conducted as part of the EU Pysolo project, is to perform a techno-economic assessment of 10 MWth fast pyrolysis plants for bio-oil and biochar production integrated with a falling particle solar tower system, starting from existing industrial scale models for the pyrolysis process and CSP system.
The following three cases are assessed and compared through techno-economic indicators:
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Conventional pyrolysis process: reference configuration, where the heat for the pyrolysis process is supplied through the combustion of a fraction of the pyrolysis products (char and pyro-gases).
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Solar-based pyrolysis process: heat for the pyrolysis process is provided only by solar heat produced using the CSP system equipped with a Thermal Energy Storage (TES) system, and all the produced biochars is exported as products.
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Hybrid pyrolysis process: heat for the pyrolysis process is supplied either from the CSP system or, when no solar heat is available, from the combustion of a fraction of the pyrolysis products, resulting in the export of most of the biochar produced.
Woody biomass of poplar tree that grows abundantly in Europe, Canada and South America is considered as the biomass feedstock. The plant is sized to convert a biomass input of 50 dry t per h (10 MWth on a lower heating value basis).
Conventional Pyrolysis Plant
A block diagram of the conventional pyrolysis section is depicted in Fig. 1A. Biomass is initially dried using hot flue gases obtained from the combustion of pyrolysis products. Then, it is fed to a fast pyrolysis fluidized bed reactor (FBR) together with hot PHC and used as a heat carrier. The solid products and the PHC exiting the pyrolysis reactor are then sent to a combustor, where the combustion of char increases the PHC temperature before it is recirculated to the FBR. The gaseous products exiting the FBR are then cooled and separated into non-condensable pyro-gases, partly recirculated to the reactor and partly combusted to thermally sustain the process.
Solar-Driven Pyrolysis Plant
A schematic of the solar-driven pyrolysis plant is reported in Fig. 1B. In this configuration, the solid PHC and char exiting the FBR are separated. The PHC particles are sent back to the CSP section of the plant, and the biochar is extracted from the plant as an additional product. The CSP section includes the falling particle receiver on top of the power tower and the TES system. In the receiver, the solid particles acting as PHC are directly irradiated to raise their temperature and then sent back to the FBR.
Hybrid Pyrolysis Plant
The hybrid pyrolysis plant involves a reduction in the biochar output but allows for increasing the capacity factor of the pyrolysis plant, possibly improving the economic KPIs. When hot PHC is not available from the receiver or form the high-temperature PHC storage, sludge and almost 75% char are sent to the combustor to provide an alternative heat source for the PHC.
Process Modeling and Techno-Economic Analysis
The biomass considered in the model is hybrid poplar wood, with thermochemical analysis data presented in Table 1. For ambient air, summer conditions are assumed (32.2 °C, 10132 bar, 75.5% relative humidity).
The pyrolysis plant energy conversion efficiency ηpyro plant is calculated by applying the following equation:
ηpyro plant = (ṁprod,i × LHVprod,i) / (ṁbiom × LHVbiom + PAux / ηel,ref + PHC)
where ṁprod,i and LHVprod,i are the mass flow rate and Lower Heating Value (LHV) of the ith product, respectively. The pyrolysis products are biochar, sludge, bio-oil and some gaseous products.
The CSP plant solar-to-thermal efficiency ηsol–th is calculated by applying the following equation:
ηsol–th = ηopt × ηth,rec
where rec and PHC,rec are the solar power incident on the receiver and thermal power delivered to PHC, respectively. DNI is the direct normal irradiance and Ah is the total heliostat area.
Carbon efficiency εC is calculated using the following equation:
εC = Σ(yC,prod,i × ṁprod,i) / (yC,biom × ṁbiom)
where yC,prod,i and yC,biom are the carbon content in the ith product and biomass, respectively.
The thermal energy usage efficiency of the CSP-based pyrolysis plant ηth,use is calculated using the following equation:
ηth,use = (PHC,rec – def) / rec
where def is the total solar power loss through defocusing.
Emission to oil ratio (ETO) is calculated using the following equation:
ETO = E / Oil
where E and Oil are the CO2 emissions and oil production in kg and GJ, respectively, from the pyrolysis plant.
Considering that biogenic CO2 emissions are climate neutral, CO2 emission credits associated with biochar production are also computed as net negative emission to oil ratio (ETOnet) using the following equation:
ETOnet = ETO – (yC,char × ṁchar × 44/12) / Oil
Process Sections
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Biomass pretreatment: The first section comprises the preparation of biomass before the pyrolysis process via drying and grinding.
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Pyrolysis: This section consists of a fluidized bed pyrolysis reactor, where dried biomass is converted by contact with hot PHC and fluidizing gases.
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Solid removal: It encompasses the cyclone filter, which is responsible for the separation of volatile products from solids entrained from the reactor at high temperatures.
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Bio-oil recovery: In this section, quenching columns are used for the condensation and collection of bio-oil along with other auxiliaries and bio-oil filters.
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Combustion: This section includes the combustor block, where biochar, a portion of pyrolytic gases, and retentate of the filter are burnt to heat the circulating PHC.
Solar Thermal Integration
The main changes in the solar-based and hybrid pyrolysis plants compared to the conventional pyrolysis plant are:
- Introduction of CSP block to replace the biochar combustor.
- Introduction of an additional combustor for the combustion of excess gases for biomass drying.
- Introduction of biochar cooling and recovery unit.
The design and simulation of the solar field are carried out using SolarPILOT. Five solar field designs are considered by varying the solar multiple (SM), which characterizes the relative size of the CSP plant with respect to the pyrolysis plant.
Two types of particle heat carriers (PHCs) are considered, namely CARBO ACCUCAST ID 50 (spherical sintered-bauxite particles) and sand. The performance of the receivers is simulated by varying the DNI and thus the incident thermal power on the receiver from 0.2 to 1.15 of the nominal value while tuning the PHC flow rate to maintain a fixed temperature.
The actual SM values are obtained by dividing the design thermal power of the receiver by the thermal power required by the pyrolizer. The obtained thermal efficiency values for Carbo ID 50 are slightly higher than those reported in the literature for similar average heat flux on the receiver, which is consistent with the lower receiver operating temperatures considered in this work (434–609 °C) compared to particle receivers in the literature, which are mainly designed for electricity generation purposes (operating at 550–750 °C).
The electrical power consumption for particle lift is calculated using the following equation:
Plift = 0.0027 × ṁp × Hlift / ηlift
where Hlift is the height that the particles need to be lifted, and ηlift is the efficiency of the lift system, considered as 0.8.
Economic Analysis
The techno-economic assessment is based on the following assumptions:
- The Net Present Value (NPV) is set to zero at the end of the plant life (30 years) with a fixed discount rate (i).
- The total installed cost Cinst is estimated by multiplying the component cost by the installation factor.
- The cost of the pyrolizer, biomass pretreatment, utilities, and auxiliary components are scaled from literature data to the desired reference size and year.
- The Fixed Capital Investment (FCI) of the CSP section is computed by summing up the heliostat field, receiver, tower, TES, particle and particle elevator costs, computed with correlations and reference values adapted from the literature.
- The fixed operating costs include wages, insurances, maintenance, taxes, and other benefits and overheads.
Results and Discussion
Conventional Pyrolysis Plant
The energy conversion efficiency of the conventional pyrolysis process is 73.9%, demonstrating that even a conventional pyrolysis process can retain a substantial portion of the input biomass energy in the form of produced bio-oil and pyro-gas. A carbon efficiency of 74.1% indicates that most of the carbon in the biomass is retained in useful products rather than being emitted as CO2.
Solar-Based Pyrolysis Plant
The solar-based pyrolysis can achieve over 90% carbon efficiency, resulting from about 70% of the inlet biogenic carbon retained in the bio-oil and about 20% of the carbon in the biochar. The additional energy input from concentrated solar power (CSP) helps to achieve this gain in efficiency. The emission to oil ratio significantly decreases from 36.5 to 13.1 kgCO2 GJoil−1, demonstrating a reduction in the carbon footprint of the produced bio-oil. Moreover, the net negative emissions of −27.5 kgCO2 GJoil−1 indicate that the process can act as a carbon sink, removing more CO2 from the atmosphere.
Hybrid Pyrolysis Plant
The hybrid plant results in a carbon efficiency of 83%. Owing to the carbon stored in biochar, the hybrid plant achieves net negative emissions of −22.3 kgCO2 GJoil−1. For the hybrid plant, with an assumed biochar selling price of 1.89 € per kg, a minimum bio-oil selling price of 18.68 € per GJLHV was obtained, vs. 27.53 € per GJLHV of the reference pyrolysis process.
Techno-Economic Analysis
The cost of bio-oil produced in the CSP-based plant is higher than in the hybrid case (21.79 € per GJLHV vs. 18.68 € per GJLHV) despite the higher char yield due to the higher impact of Capex caused by the lower yearly capacity factor (82% vs. 100% of the hybrid case). Therefore, an economic-environmental trade-off exists between CSP-based and hybrid configurations, driven by minimum fuel selling price (i.e., fuel production cost) and overall process carbon efficiency.
The sensitivity analysis showed that the variation of the discount rate, plant availability, biomass cost, biochar selling price and pyrolyzer CAPEX exhibits the highest impact on the minimum fuel selling price (MFSP). However, the type of particle heat carrier (sand or Carbo ID50 PHC) has a minor impact on the MFSP.
The breakeven biochar selling prices for CSP-based and hybrid cases that can make them economically competitive with conventional pyrolysis are 1.24 € per kg and 0.64 € per kg, which correspond to carbon credit values of 407 € per tCO2 and 210 € per tCO2, respectively.
Conclusion
This study explores the techno-economic feasibility of integrating an industrial-scale biomass pyrolysis plant with a falling particle solar tower system. The key findings are:
- Solar-based pyrolysis can achieve over 90% carbon efficiency, resulting in net negative emissions of −27.5 kgCO2 GJoil−1.
- The hybrid plant, combining solar receiver and biochar backup combustor, leads to the lowest cost of bio-oil (18.7 € per GJ) and a carbon efficiency of 83%.
- The sensitivity analysis showed that the variation of the discount rate, plant availability, biomass cost, biochar selling price and pyrolyzer CAPEX have the highest impact on the minimum fuel selling price.
- The breakeven biochar selling prices for CSP-based and hybrid cases are 1.24 € per kg and 0.64 € per kg, respectively, to be economically competitive with conventional pyrolysis.
This work serves as a starting point for future projects on the integration of CSP systems with industrial-scale biomass pyrolysis plants, providing valuable insights into the techno-economic feasibility and environmental benefits of such systems. Further research is needed to develop detailed pyrolysis reactor models and experimental validation of PHC-char separation techniques to refine the process design and economic analysis.
