Direct Solar-Powered Biomass Gasification Using Low-Temperature Steam

Direct Solar-Powered Biomass Gasification Using Low-Temperature Steam

Flavio Manenti^*, Zohreh Ravaghi-Ardebili

Politecnico di Milano

Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”

Piazza Leonardo da Vinci 32

20133 Milano, Italy

*Corresponding author. Phone: +39 (0)2 2399 3287; Fax: +39 (0)2 7063 8173; Email: flavio.manenti@polimi.it

ABSTRACT

This work is aimed at coupling the concentrating solar technology with the biomass gasification. The main issue is that the biomass gasification occurs at temperatures of the order of 800-900°C, whereas the available technology in concentrating solar power plants using thermal fluids allows to achieve temperatures of 550-600°C. Different elements are needed to be effective: (1) the detailed characterization of biomass pyrolysis and the successive gas phase reactions; (2) the selection of appealing biomass gasification technologies (i.e., traveling grate, countercurrent); (3) the integration with discontinuous energy sources (concentrating solar); (4) the definition of proper thermal energy storage; and (5) the study of effective operational procedures to achieve direct solar powered biomass gasification.

KEY-WORDS

Biomass gasification; concentrating solar; biomass pyrolysis; energy integration; thermal energy storage

INTRODUCTION

Biomass is one of the most promising feedstock able to satisfy the increasing demand for renewable energy, biofuels, and green chemicals (Cucek et al. 2010, Klemeš et al. 2010, Lam et al. 2010a, Lam et al. 2010b, Vaccari et al. 2005). Unfortunately, biomass conversion to bioproducts is tough to be industrially scaled-up due to the complexity of chemical and transport phenomena as well as to be integrated with other renewable sources that can support for energy and steam generation the gasification process. Thus, two critical steps are:

1.            the development of mechanistic models capable of describing transport phenomena and reaction kinetics for a better understanding of biomass pyrolysis;

2.            the integration of these models at the process scale to develop novel process solutions.

For the former one, detailed chemical mechanisms are needed both for biomass pyrolysis and for the successive gas phase reactions, since they are still unavailable even for major products released such as levoglucosan, hydroxymethylfurfural, and phenolic species (Ranzi et al. 2013). Chemical mechanisms need to be integrated into particle model accounting for transport phenomena, which are critical in predicting global reactor performance. Developing these models is challenging because of the biomass complexity as well as the multi-phase and multi-scale nature of the conversion process (Mettler et al. 2012). For the latter step, the use of two-tanks direct thermal energy storage in concentrating solar plant is an established and appealing technology (Vitte et al. 2012, Manenti and Ravaghi-Ardebili 2013).

A brief overview of these two main topics is given hereinafter.

BIOMASS GASIFICATION

Combustion, gasification, and biomass-to-liquid pyrolysis are some of the main thermo-chemical conversion routes, which can convert an abundant and well distributed feedstock into energy, syngas, bio-oil, and chemicals. One of the main problems when studying this type of feedstock is the intrinsic variability of the biomass composition. As a consequence, it is necessary to properly characterize the biomass, preferably on the basis of few lumped components, which are typical for all the possible feedstock.

The kinetic model adopted is explained in detailed elsewhere (Ranzi et al. 2013) and is based on a multi-step devolatilization and decomposition of the three key-biomass reference species: cellulose, hemicelluloses and lignin. One of the main features of this model is its ability to provide detailed information on yields composition of gas, tar, and solid residue. The kinetic model also involves the char gasification and combustion reactions, with steam and/or air or oxygen, as well as the secondary homogeneous gas phase reactions of the released gas and tar species. The multistep kinetic model has been validated on the basis of thermo-gravimetric data of fine particles, with negligible resistances. Next, the model has been extended to the reactor scale with the analysis of a countercurrent biomass gasifier.

ARCHIMEDE PLANT

Archimede concentrating solar plant uses linear parabolic streams and direct two-tanks thermal energy storage where the molten salt thermal fluid flows inside (Vitte et al. 2012). Energy storage is a major process design and control issue in concentrated solar plants (Manenti and Ravaghi-Ardebili 2013). The intrinsic discontinuous nature of solar energy forces to install units able to store energy under favourable conditions, then to release energy during night or unfavourable conditions. Accordingly, smoothened operations and continuous energy production can be guaranteed. Several technologies are today available for storage; tanks of molten salts, steam accumulators, high thermal capacity solids are among the most important.

SOLAR GASIFICATION

According to the brief descriptions above, the idea is to supply the biomass gasification using the steam generated by the concentrating solar plant. To do so, it would be necessary to produce steam at 900°C, whereas the existing molten-salt solar technology allows to achieve about 600°C. Nevertheless, it is possible to select specific configuration of gasifier that may suit the achievement of the solar-powered biomass gasification. Actually, the use of countercurrent gasifier allows to exploit the high temperature of ashes slowly moving to the bottom of the unit to increase the temperature of the steam entering from the bottom (Figure 1). By doing so, the work is aimed at demonstrating that the lower limit for the inlet steam temperature to keep the “hot” condition of the biomass gasifier is not higher than the steam temperature obtained from the concentrating solar plant, making it feasible. Oxygen supply is needed to sustain the endothermic reaction of gasification by means of in situ combustion of a portion of biomass. Specific operational procedure is needed to operate the integrated plant. Also, the study will analyse the possibility of cogeneration before supplying the biomass gasifier to further intensify the solution energetically (Sikos and Klemeš 2010, Zhu et al. 2000). The existing GasDS tool for biomass characterization and gasification (Ranzi et al. 2013) is adopted for the dynamic simulation of the countercurrent gasifier and it is integrated in DynSim™ suite for process dynamic simulation by Simulation Science, Invensys.

Figure 1. Qualitative layout.

REFERENCES

Cucek, L., Lam, H. L., Klemeš, J. J., Varbanov, P. S. and Kravanja, Z. (2010) 'Synthesis of regional networks for the supply of energy and bioproducts', Clean Technologies and Environmental Policy, 12(6), 635-645.

Klemeš, J. J., Varbanov, P. S., Pierucci, S. and Huisingh, D. (2010) 'Minimising emissions and energy wastage by improved industrial processes and integration of renewable energy', Journal of Cleaner Production, 18(9), 843-847.

Lam, H. L., Varbanov, P. and Klemeš, J. (2010a) 'Minimising carbon footprint of regional biomass supply chains', Resources, Conservation and Recycling, 54(5), 303-309.

Lam, H. L., Varbanov, P. S. and Klemeš, J. J. (2010b) 'Optimisation of regional energy supply chains utilising renewables: P-graph approach', Computers and Chemical Engineering, 34(5), 782-792.

Manenti, F. and Ravaghi-Ardebili, Z. (2013) 'Dynamic simulation of concentrating solar power plant and two-tanks direct thermal energy storage', Energy, (doi: 10.1016/j.energy.2013.02.001).

Mettler, M. S., Vlachos, D. G. and Dauenhauer, P. J. (2012) 'Top ten fundamental challenges of biomass pyrolysis for biofuels', Energy Environ. Sci., 5, 7797-7809.

Ranzi, E., Corbetta, M., Manenti, F. and Pierucci, S. (2013) 'Kinetic Modeling of the Thermal Degradation and Combustion of Biomass', submitted to Chemical Engineering Science.

Sikos, L. and Klemeš, J. (2010) 'Reliability, availability and maintenance optimisation of heat exchanger networks', Applied Thermal Engineering, 30(1), 63-69.

Vaccari, G., Tamburini, E., Sgualdino, G., Urbaniec, K. and Klemeš, J. (2005) 'Overview of the environmental problems in beet sugar processing: Possible solutions', Journal of Cleaner Production, 13(5), 499-507.

Vitte, P., Manenti, F., Pierucci, S., Joulia, X. and Buzzi-Ferraris, G. (2012) 'Dynamic Simulation of Concentrating Solar Plants', Chemical Engineering Transactions, 29, 235-240.

Zhu, X. X., Zanfir, M. and Klemeš, J. (2000) 'Heat Transfer Enhancement for Heat Exchanger Network Retrofit', Heat Transfer Engineering, 21(2), 7-18.

 ADDIN EN.REFLIST