La mujer-naturaleza perversa y enigmática

The production of hydrogen from biomass is also being studied as it would be environmentally benign. Hydrogen is either manufactured from fossil fuels such as natural gas, naphtha and coal, or from nonfossil energy resources like water electrolysis, photolysis, and thermolysis (Momirlan and Vziroglu 1999). Differ-ent thermochemical routes of hydrogen production from biomass are described below.

3.4.1 P^yrolySiS

Conventional pyrolysis methods (Section 3.3) produce hydrogen in amounts that are not significant. Catalytic pyrolysis could be a useful method in this regard, which could be achieved by (1) the catalytic steam reforming of pyrolysis liquids to pro-duce hydrogen, (2) pyrolysis at 700°C, with removal of the tar content of the gas and improving the quality of the product gas, and (3) pyrolysis at a lower temperature (<750°C) and incorporation of catalyst in the same reactor. Some types of reactor used in this process are Waterloo fast pyrolysis unit, free fall reactor, reactor for RhKeO2/SiO2 catalyst, and dual bed gasifier reactor.

3.4.2 SuPercritical w^ater e^xtraction

Biomass can be converted into fuel gases rich in hydrogen by supercritical water extraction. The water is attractive as a potential medium for industrial chemical reactions because it is environmentally benign. The supercritical water acts as a homogeneous, nonpolar solvent of high diffusivity and high transport properties, able to dissolve organic compounds and gases (Feng et al. 2004). In such a process, hydrogen can be produced at thermodynamic equilibrium.

tAble 3.3

charcoal proximate and elemental Analysis (wt%)

laboratory scale pilot plant scale proximate Analysis

Volatile matter 18.9 15.4

Fixed carbon 74.4 79.1

Ash 6.7 5.5

elemental Analysis

C 85.6 81.5

H 2.9 3.1

N 1.3 0.8

S <0.1 <0.1

O + ash (by difference) 10.2 14.6

3.4.3 GaSification

Gasification converts biomass into a combustible gas mixture (CO, CO2, CH4, H2, and H₂O) by the partial oxidation of the biomass at high temperatures, typically in the range of 800 to 900°C. The main reaction steps in biomass gasification are:

Heating and pyrolysis of the biomass, converting biomass into gas, char,

•

and primary tar.

Cracking of the primary tar to gases and secondary and ternary tars.

•

Cracking of secondary and tertiary tars.

•

Heterogeneous gasification reactions of the char formed during the

pyroly-•

sis and the homogeneous gas phase reactions.

The combustion of char formed during pyrolysis and oxidation of

combus-•

tible gases.

In a gasification process, the solid fuels are completely converted (except the ashes in the feed) to gaseous products having different compositions. The gasifica-tion process is attractive because of the producgasifica-tion of cleaner gaseous fuel as well as almost complete conversion of biomass. The gasification processes can be catego-rized as below:

1. Air gasification. Air gasification is the most widely used technology, as a single product is formed at high efficiency and without requiring oxygen.

Temperatures of 900 to 1100°C are achieved.

2. Oxygengasification. This gives a better quality gas of 10 to 15 MJ/ Nm³ and temperatures of 1000 to 1400°C are achieved. However, this process requires an O₂ supply, with concomitant problems of cost and safety.

3. Steam gasification. Steam gasification of biomass results in the conversion of carbonaceous material to permanent gases (H2, CO, CO2, CH4, and light hydrocarbons), char, and tar (Kim 2003).

For thermochemical processes, different reactor designs are needed. One way to characterize reactor types is based on the method of transport of fluids or sol-ids through the reactor. The four main types are quasi-non-moving or self-moving feedstock, mechanically moved feedstock, fluid-moved feedstock, and special reac-tors. Some of the reactors used include multi-stage circulating fluidized-bed reactors, pressurized fluidized-bed air-blown gasifiers, two-stage gasification reactors, atmo-spheric bubbling fluidized-bed reactors, countercurrent fixed-bed gasifiers, open top reburn downdraft gasifiers, down draft fixed-bed, etc. (Dasappa et al. 2004; Narvaez et al. 1996; Padban et al. 2000; Saxena et al. 2007). There is no significant advantage to using either the fixed-bed or the fluidized-bed reactor (Warnecke 2000).

3.5 conclusIon

Visualizing the present energy requirements worldwide and concerns for environ-mental problems, it is necessary to develop a process that can convert biomass into

useful energy products, especially to liquids that can be good substitutes for the depleting fossil fuels. Thermochemical conversion methods like pyrolysis have received a significant amount of interest. The fast and the flash pyrolysis processes give higher yields of liquid product as compared to slow pyrolysis. Catalytic pyroly-sis, which is another way to upgrade the quality of the products, is also a process of interest. It may also be applied to improve the quantity and the quality of the gaseous products to make them more useful for obtaining energy.

Apart from liquid fuels, hydrogen from biomass also is a potential fuel that may meet the energy requirement without creating problems for the environment. Vari-ous thermochemical conversion technologies can be applied for conversion of renew-able biomass into hydrogen-rich gas.

AcKnoWledgments

The authors gratefully acknowledge the continuous encouragement given by the Direc-tor, Indian Institute of Petroleum, Dehradun during the preparation of this chapter.

references

Adhikari, D. K., Seal Diptendu, R. C. Saxena, and H. B. Goyal. 2006. Biomass based fuel/

energy. Journal of Petrotech Society 3(1): 28–42.

Asadullah, M., T. Miyazawa, S. Ito, K. Kunimori, and K. Tomishige. 2002. Role of catalysis and its fluidisation in the catalytic gasification of biomass to syngas at low temperature.

Industrial Engineering & Chemistry Research 41: 4567–4575.

Bridgwater, A. V. 1994. Catalysis in thermal biomass conversion. Applied Catalysis A: Gen-eral 116: 5–47.

Bridgwater, A. V. 1999. Principles and practice of biomass fast pyrolysis processes for liq-uids. Journal of Analytical and Applied Pyrolysis 51: 3–22.

Chen, G., J. Andries, and H. Spliethoff. 2003. Catalytic pyrolysis of biomass for hydrogen rich fuel gas production. Energy Conversion and Management 44: 2289–2296.

Dasappa, S. P. J. Paul, H. S. Mukunda, N. K. S. Rajan, G. Giridhar, and H. V. Sridhar. 2004.

Biomass gasification technology: A route to meet energy needs. Current Sciences 87:

908–916.

Demirbas, A. 2001. Biomass resources facilities and biomass conversion processing for fuels and chemicals. Energy Conversion and Management 42: 1357–1378.

Demirbas, A. 2004. Hydrogen-rich gas from fruit shale via supercritical water extraction.

International Journal of Hydrogen Energy 29: 1237–1243.

Diebold, J. P. 2000. A review of the chemical and physical mechanism of the storage stability of fast pyrolysis bio-oils. Golden, CO: National Renewable Energy Laboratory.

Feng, W., J. Hedzer, K. Vander, and A. Jakob de Swan. 2004. Phase equilibria for biomass conversion process in subcritical and supercritical water. Chemical Engineering Jour-nal 98: 105–113.

Goyal, H. B., A. K. Gupta, A. K. Bhatnagar, P. S. Verma, and K. V. Padmaja. 2005. Liquid fuels from biomass. Paper presented at 6th International Petroleum Conference and Exhibition (Petrotech-2005), January 16–19, India.

Goyal, H. B., Seal Diptendu, and R. C. Saxena. 2006. Bio-fuels from thermochemical con-version of renewable resources: A review. Renewable & Sustainable Energy Reviews, Available online at www.sciencedirect.com.

Kim, H. Y. 2003. A low cost production from cabonaceous waste. International Journal of Hydrogen Energy 28: 1179–1186.

Manuel, G. P., Abdelkader Chaala, and C. Roy. 2002. Vacuum pyrolysis of sugarcane bagasse.

Journal of Analytical and Applied Pyrolysis 65: 111–136.

McKendry, P. 2002. Energy production from biomass. II. Conversion technologies. Biore-source Technology 83: 47–54.

Momirlan, M. and T. Vziroglu. 1999. Recent directions of world hydrogen production. Renew-able and SustainRenew-able Energy Reviews 3: 219–231.

Narvaez, I., A. Orio, M. P. Aznar, and J. Corella. 1996. Biomass gasification with air in an atmospheric bubbling fluidized bed. Effects of six operational variables on the quality of the produced raw gas. Industrial Engineering & Chemistry Research 35: 2110–2120.

Ozbay, N., A. E. Putun, B. B. Uzun, and E. Putun. 2001. Biocrude from biomass: pyrolysis of cottonseed cake. Renewable Energy 24: 615–625.

Padban, N.; W. Wang, Z. Ye, I. Bjerle, and I. Odenbrand. 2000. Tar formation in pressurised fluid bed air gasification of woody biomass. Energy and Fuels 14: 603–611.

Saxena, R. C., D. K. Adhikari, and H. B. Goyal. In press. Biomass based energy fuel through biochemical routes: A review. Renewable & Sustainable Energy Reviews. Available online at www.sciencedirect.com.

Saxena, R. C., Seal Diptendu, Kumar Satinder, and H. B. Goyal. In press. Thermochemical routes for hydrogen rich gas from biomass: A review. Renewable & Sustainable Energy Reviews. Available online at www.sciencedirect.com.

Warnecke, R. 2000. Gasification of biomass; comparison of fixed bed and fluidized bed gas-ifier. Biomass and Bioenergy 18: 489–497.

Williams, P. T. and N. Nugranad. 2000. Comparison of products from pyrolysis and catalytic pyrolysis of rice husks. Energy 25: 493–513.

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4 Production of Biofuels