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Design of a 50 kWE fluidized bubbling bed biomass gasification plant
Language: English This thesis is written in English
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Stefano Silvestrini, Università degli Studi di Napoli - Federico II, 2015-16
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Stefano Silvestrini
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Abstract
The present work describes the procedure to calculate the most relevant parameters to design a 50 kW fluidized bed biomass gasification plant.
It assumes considerable importance considering that climate change is one of the most serious environmental problems that humanity faces. Global warming is threatening the world's ecosystems, the sustainable development and the welfare of mankind. Scientific studies show that the planet will face irreversible human and natural disaster if atmospheric concentrations of CO2 continues above 350 parts per million.
Gasification process has been used for different application areas such as power generation, gaseous and liquid fuel production or chemical production. But the production of gas having high calorific value, high H2 and CO content together with high fuel conversion ratio and gas efficiency are the main targets to be realized in the design and operation.
Gasification is a thermo-chemical process that converts biomass into a combustible gas called contains Carbon Monoxide, Hydrogen, Methane, Water vapor, Carbon Dioxide, Nitrogen, tar vapor, Carbon and ash particles. Gasification produces a medium or high heating value gas, depending mainly on the gasifying used medium. Syngas contains from 70 up to 90 percent of the energy originally present in the biomass feedstock. The syngas can be burned directly by heating or drying, or it can be burned in a boiler to produce steam or hot water. Syngas can be converted into bio-fuel by mean of the fischer tropsch process. Cyclones, heat exchangers and filters remove tars and particulate matter contained in the syngas. The clean syngas is suitable for use in an internal combustion engine, gas turbine or other application requiring a clean and high-quality gas. Electric power generation is possible by a gasification plant with an internal combustion engine, a gas turbine or a fuel cell. Use of producer gas in a fuel cell requires reforming clean syngas into Hydrogen ions and carbon monoxide. Fuel cells produce electricity and thermal energy from hydrogen through an electrochemical conversion process. Nowadays gasification technology is still in the development stage. Optimal gasification requires dry biomass with a uniform size and a moisture’s content no higher than 20 percent. Biomass gasification is a multi-stage process, divided by steps: in the first stage, called drying, water steam is evaporated, dry biomass is pyrolised; in the second step called pyrolysis, heat vaporizes the volatile components of biomass in the absence of air at temperatures ranging from 300° to 500° C. Pyrolysis gas is composed by Carbon Monoxide, Hydrogen, Methane, volatile Tars, Carbon Dioxide, water and moreover from 10 to 25 percent of the original fuel mass is converted into charcoal; the thirds step is the combustion, part of charcoal is burned and it produces the heat required by the all other processes, this process is carried out at temperatures in the range of 600 to 1400 ºC; finally there is one last stage called char conversion or reduction when the charcoal residue from the pyrolysis stage reacts with carbon dioxide and water steam, producing mainly carbon monoxide and hydrogen.
Biomass is a term for all organic material that stems from plants (including algae, trees and crops). It is produced by green plants converting sunlight into plant material through photosynthesis and includes all land and water - based vegetation, as well as all organic wastes.
The biomass gasification is a clean process able to produce synthesis gas with a LHV and higher output per unit of feedstock. The energy from biomass has solved two fundamental problems that plague other forms of renewable energy such as solar and wind power, the difficulty of storage energy and the capacity of produce energy when it is needed. Energy can be easily storage on and the continuity of supply is guaranteed by the fact energy from biomass can be adjusted and interruptible at any time.

One of the advantages of biomass gasification is the ability to turn raw material resources and waste into a useful fuel without having to rely on specialized crops and not subtract therefore land of basic agriculture, being able to take advantage of the use of uncultivated land or areas of low agricultural production value.