9
1. INTRODUCTION
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. (Puig-Arnavat, et al., 2013)
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. In the recent years,
several studies have been performed to validate the design and to optimize the operation
conditions of coal and/or biomass gasification processes. (Aydar, et al., 2014)
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
bi oma s s w i t h a un i f or m s i z e a nd a moi s t ure ’ s c ont e nt no hi gh e r t ha n 20 pe r c e nt . 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.
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.
10
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 resource can be considered as organic matter, in which the energy of
sunlight is stored in chemical bonds. When the bonds between adjacent carbon, hydrogen and
oxygen molecules are broken by digestion, combustion or decomposition, these substances
release their stored, chemical energy. Biomass has always been a major source of energy for
mankind and is presently estimated to contribute for a big share of the world's energy supply.
Fluidized bed reactors are important for thermo-chemical decomposition of biomass because
of high rate of heat and mass transfer, and the ability to separate the solid products from the
volatile components produced during the operation. Biomass particles, due to their peculiar
shapes, sizes and densities, it can ’t be uniformly mixed without a fluidizing medium such as
sand in a fluidized bed reactor. (Sharma, et al., 2013) The sand acts as a heat vector,
distributing the necessary heat; the continued movement of this material ensures the
maintenance of isothermal conditions of the bed, preventing the formation of hot spots.
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.
The 4th Report of the Intergovernmental Group of scientific experts on Climate Change
published in states that to ensure climate stability long term is necessary to limit the increase
of global temperature below 2 °C above pre-industrial levels. In this sense it is necessary
industrialized countries reduce of 40% their emissions of greenhouse gases by 2020 below
1990 levels and at least 85% by 2050. So the potential threat posed by climate change, due to
high emission levels of greenhouse gases (CO2 being the most important one), has become a
major stimulus for renewable energy sources in general. When produced by sustainable
means, biomass emits roughly the same amount of Carbon during conversion as is taken up
during plant growth. The use of biomass therefore does not contribute to build up of CO2 in
the atmosphere.
Moreover fossil fuels are a finite resource. It seems possible that cheap oil (on which our cars
and lorries run) and cheap gas (with which we heat many of our buildings) will run out in our
lifetime. So we seek alternative energy sources. Indeed given that fossil fuels are a valuable
resource, useful for manufacture of plastics and all sorts of other creative stuff, perhaps we
should save them for better uses than simply setting fire to them.
11
2. BIOMASS GASIFICATION PLANT
Figure 1 Biomass gasification plant
The whole plant is composed by a storage hopper where the biomass is sent to the reactor, a
cyclone where a certain percentage of impurities is eliminated thanks to the elliptical motion
of the gas, two heat exchangers where the gas is cooled thanks to the water going into some
pipes, another filter where condensed tar is deposited in, one more filter composed first by
water and after by biomass chips of different dimensions that captures other particles still
remaining into the gas, a gas lung where an amount of gas is stored into and finally a vacuum
pump and a torch or an engine.
STORAGE REACTOR CYCLONE
HEAT
EXCHANGER
HEAT
EXCHANGER
FILTER
(big)
FILTER
(fine)
Tar
storage
I.C.ENGINE TORCH
Figure 2 Scheme of the plant
3. STORAGE SYSTEM
3.1. Hopper
The function of the hopper is to stock biomass while it's going into the reactor.
Next it will be explained how the volume of the hopper, the mass and the consumption time
both for woodchip and pellet have been calculated.
12
3.1.1. Calculation Procedure
3.1.1.1. Total Volume of the hopper
To calculate the total volume of the hopper, it has been ideally divided in two parts: a prism
and a cube.
To calculate the prism volume, it has been used the following formula:
3
Where:
- V
prism
: Volume of the prism
- h
prism
: Height of the prism
- b : Width of the hopper
- l : Depth of the hopper
To calculate the volume of the cube, it has been used the following formula:
3
Where:
- V
cube
: Volume of the cube
- h
cube
: Height of the cube
The total volume of the hopper will be:
3
3.1.1.2. Mass of woodchip/pellet
To calculate the mass of woodchip and pellet it has been used the following formula:
Where:
m
wc
: Mass of woodchip
m
pellet
: Mass of pellet
13
ρ
wc
: Density of woodchip
ρ
pellet
: Density of pellet
3.1.1.3. Consumption time
The consumption time can be calculated using the following formulas:
Where:
t
wc
: consumption time for woodchip
t
pellet
: consumption time for pellet
W
out
: Biomass consumption rate (Wet)
3.1.2. Results
The following table resumes the obtained results:
Total volume 4,59 m
WCs Pellet
Mass 918,11 kg 3213,40 kg
Consumption time 14,35 h 50,21 h
Table 1 Obtained values for the Hopper
3.2. Gear motor
The characteristics of the gear motor are resumed in the following table:
Gear Motor IIE
Nominal Engine velocity (RPM) 1.500
Transmission relation enter/exit 100
Screw velocity at 25 HZ 7,50
Screw velocity at 50 HZ 15,00
Screw velocity at 60 HZ 18,00
Table 2: Gear Motor IIE
14
3.3. Screw
Successful feeding is critical to biomass utilization processes, but difficult due to the
heterogeneity, physical properties and moisture content of the particles. A critical problem in
all cases is how to feed biomass into reactors. Feeding problems often impede smooth
operation. Such properties as mean particle size, size distribution, shape, particle surface (e.g.,
smooth, rough or sharp edges), density, moisture content, compressibility and other fuel
properties (e.g., strength of large particles, consolidation over time) can all affect the ability to
feed the material. Hopper-screw feeders are common in biomass applications. Lack of flow is a
common and serious solids handling problem.
Screw feeders are volumetric devices, with delivery depending on the screw ’s outside and
inside (shaft) diameter, pitch (distance between adjacent flights) and fullness. If the solids are
compressible (e.g., sawdust, hog fuel), the mass delivered per unit time varies. The velocity of
conveyed solid material is a vector having an angle to the direction of rotation. As the screw
rotates, particles move in helical paths of direction opposite to that of the screw. The frictional
effects of the solids on the screw flights and on the casing surface, together with the
configuration of the screw, determine the efficiency of the feeder.
3.3.1. Calculation procedure
The main point to keep in consideration to design a Screw is the inverted frequency at
maximum flow (Max 60 Hz) and the inverted frequency at minimum flow (Min 25 Hz) have to
range between about 25 and 60 Hz.
To design the screw it has been preceded through an iterative process.
Ricevi informazioni sui nostri servizi, sulle offerte e non perdere news e consigli su università e lavoro.
