Introduction expanding their industrial sector and aIntroduction expanding their industrial sector and a

Introduction & Objectives

Waste-to-Energy: Assessment of fluidization conditions for optimum
waste conversion

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Yousef El Delbi
Supervisor: Dr Materazzi
    
     

 

 

Objectives

This research project attempts to determine and analyse the
optimum physical conditions of a fluidized bed when converting waste to energy.
To be more accurate the actual process taking part in the fluidized bed is the
gasification of a waste-derived fuel called R.D.F which stands for
refuse-derived fuel and is composed of municipal waste which has been modified
to an extent and compressed. The product produced is Syngas, which can be used
in a variety of modern devices to produce heat and/or electricity, an example
of which is fuel cells or a gas-engine.

A systematic study will be undertaken to reveal the
mechanisms of single particle devolatilization specifically and another study
to describe the segregation profile for a waste particle changing in density,
this will be undertaken using advanced X-ray imaging technology available at
UCL. Experimental data gathered will be integrated and compared with real data
from a pilot plant owned and operated by Advanced Plasma Power Ltd. Extensive
research and articles have been written on fluidized bed gasification of coal
and biomass but not on waste (specifically R.D.F). Details on how different
operating conditions affect the syngas product from waste are not widely
available, the reason being is the wide variance of waste composition, unlike
biomass and wood. This project will attempt to provide qualitative and
quantitative data regarding the two important phenomena’s mentioned above.

Introduction

Energy: Supply & Demand

World energy consumption has been
rising from the early 1960’s, current forecasts predict a rise of 28% between
2015 and 2040. This is partially due to developing countries such as India and
China expanding their industrial sector and a growing middle class in
developing countries consuming more energy than in previous decades.  Most of this energy consumed will still be
supplied by hydrocarbons in the form of liquids or natural gas, however, renewables
have been on the rise and are set to be the fastest growing source compared to
other types of fuels. Figure 1 below from the U.S Energy Information
Administration shows the rise of renewables compared to other fuels from 1990
through to 2040 1.

 

 Figure 1: World energy consumption by energy source

As energy demands increase we may
need to find new sources of energy. As most of the current sources of energy
come in the form of fossil fuels and are sourced largely from the Middle East
the question of energy security arises. The IEA (international energy agency)
defines energy security as the uninterrupted availability of energy sources at
an affordable price. This can refer to either reliability/uninterruptible
supply or affordable/competitive supply or Accessible/available supply. As the
situation in the Middle East continues to deteriorate and more countries are
being engulfed in war and tragedy, energy security becomes more of a prevalent
issue. Renewables have been put forward as a candidate to fill some of the gaps
in our energy demands, however, more work needs to be done in this regard.
Energy efficiency will also play a part in any future decisions made regarding
what fuel to use to generate energy, here when we talk about energy efficiency
we need to think beyond fuel density and caloric value, we also have to include
carbon footprint, GHG emissions environmental regulations etc. if the
efficiency is below standards then demand will not be sufficiently met. 

Energy is defined as renewable if
its source is naturally replenished such as wind, sunlight, tide etc and it
does not directly contribute to greenhouse gas accumulation. In the European
Union, renewables account for 80% of new capacity as policies continue to
support renewable electricity worldwide 2. Renewables are projected to
generate roughly around 11 trillion KWH of electricity in 2040 compared to 6
trillion KWH in 2015. That is 31% share of the electricity generation market in
2040 which is the same as coal 1. This may seem as irrelevant information
because this project is focused on converting R.D.F which is waste derived from
municipal waste, however, according to the EPA (U.S. Environmental Protection
Agency) municipal waste can be classified as renewable. Also, the
Intergovernmental Panel on Climate Change (IPCC) and the world economic forum
agree the entire waste stream should be considered as renewable as sending it
to a landfill is more damaging to the environment. So clearly a connection
exists between sources of energy termed renewable and waste which can be
converted by various methods to energy directly or indirectly by converting to
an intermediary product such as Syngas such as in the case of this project.

Waste: Source of energy?

The UK generated approximately
209 million tonnes of total waste in 2014. Over half of this (59.4 percent) was
generated by construction, demolition, and excavation, with households
responsible for a further 13.7 percent. Per capita 485kg of waste was generated
in the UK in 2015, multiply this by the population of the UK and this will show
how much municipal waste is produced 3. From Figure 2 below it can be seen
that over a fifth of this waste is deposited in landfills that is 38.5 million
tonnes is sent to landfills in the UK alone. Assuming all incineration is used to
produce electricity then only 4.5% of all waste is being used for energy
recovery in one form or another. It can be concluded from the data provided
that a lot of waste is wasted so to speak. Another factor to take into
consideration when sending waste to landfill sites is the increase in the
landfill tax, it is currently £80 per tonne which is a very large increase from
2004/05 when it was a more affordable £15 per tonne 4.  If advance waste to energy techniques are to
be improved and perfected in the future then the raw material will be available
in abundance, and waste being sent to landfills will be dramatically reduced as
it will be cheaper to convert to RDF or pay companies that specialise in
waste-to-energy conversion to remove the waste.

 

Figure 2: Waste split by final
treatment method, UK 2014 3

 

Refuse Derived Fuel (R.D.F)

R.D.F is prepared from municipal
solid waste or other types of waste by removing recyclable materials, drying
and shredding the remaining material then pelletizing if required by the
client. As R.D.F is typically produced from municipal waste the composition and
calorific value of the fuel will vary widely from one sample to another. This
has been a setback in utilising RDF for energy generation. However, using
fluidized bed gasification may be an answer to this particular problem,
historically fluidized bed gasification was mainly used for coal and biomass
which is a more simpler fuel compared to RDF which can contain a range of
plastics, fibre, textiles, putrescible materials, rubber, metallic components,
and dust, some of which are considered hazardous in nature.  

R.D.F can be analysed using two
distinct methods, the first of which is called ‘Ultimate analysis’ which gives
a breakdown of each element present in the sample as well as the ash content.
The other analysis is termed ‘Proximate analysis’, this will give us the
R.D.F’s moisture content, combustibility as well as its ash content. Each
analysis is useful in its own right depending on what final treatment the R.D.F
will be put through. In this research project, the reactor used will be a
fluidized bed and the process will be gasification with air.

Fluidized bed gasification: An
overview

‘Fluidization is defined as the
operation through which fine solids are transformed into a fluid-like state
through contact with a gas or liquid’ 5, this concept was first introduced by
Fritz Winkler of Germany in 1921. In the early 1960’s the idea of using a
fluidized bed to burn coal was promoted by Douglas Elliott. Since then major
developments in the technology have taken place and now fluidized bed reactors
are used in various industries such as petrochemical processing to the
pharmaceutical industry. Fluidized bed reactors work by having a solid bed,
either composed of sand, limestone or gravel depending on the application, the
bed is transformed into a fluid-like state through suspension in a gas normally
air or steam. This allows for extremely good mixing and heat transfer from the
solid to the fluid.

As good mixing and heat transfer are
important for gasification the fluidized bed makes for an excellent reactor for
this process. Also, another major advantage of using a fluidized bed gasifier
is the fuel flexibility of the system, as mentioned above R.D.F can have
various compositions as it is heterogeneous in nature, therefore this feature
will be essential to avoid problems when feed variation occurs. The fuel
flexibility of the system is due to the hydrodynamics in the bed which allows
for excellent gas-solid and solid-solid mixing, because of the excellent mixing
the fuel particle is quickly dispersed in the large mass of bed solids which
quickly raise the temperature of the fuel particle without there being a
significant drop in the temperature of the overall bed 6.

Gasification is one of the
earliest application of fluidized beds on a commercial level 7. Gasification
is a process that converts organic (or fossil fuel based material) into CO, H2,
and CO2. This is achieved by reacting materials at high temperatures with a
controlled amount of O2 and or steam, the resulting product is syngas (H2/CO).
This differs from combustion as the oxygen supplied is not insufficient for
stoichiometric completion of the reaction. The overall process of gasification
occurring in the bed can be roughly split into four parts;

1.       
Dehydration:
This occurs at a temperature of up to 160°C the resulting steam is mixed into
the gas flow and may be involved with other chemical reactions. 8

2.       
Pyrolysis/de-volatilisation:
occurs at around 200–300 °C volatiles released, and char is produced.

3.       
Combustion:
volatile products and char react with oxygen to form primarily carbon dioxide.

4.       
Gasification:
The char reacts with steam to produce CO & H2 (syngas)

 

The dehydration stage may
possibly take part even before the R.D.F enters the reactor and/or bed as the
piped feeding the fuel may reach these temperatures, as a result, the fuel will
dry and eventually shrink. This speed of this process depends on the reactor
temperature, the fuel particle size, and its porosity because the moisture is
evaporating out from within the particle. Pyrolysis or devolatilization
separates all the remaining water vapor, organic liquids and non-condensable
gases from the remaining solid carbon content and ash content of the fuel. This
involves a series of complex physical and chemical processes, the product gas
produced is a function of temperature, pressure and gas composition during
devolatilization 9. The remaining solid carbon after devolatilization is
termed char, and it is this char which is combusted with oxygen from the
fluidizing fluid to produce carbon dioxide and to generate the thermal energy
needed for the previously mentioned reaction in devolatilization which is
endothermic. Gasification of the char is an endothermic reaction supported by
the thermal energy produced in the combustion stage. A series of reactions
occur producing the final product which is syngas a mixture of CO and H2, the
main reactions which occur are;

1.           
Water-gas
reaction: partial oxidation of carbon by steam to produce hydrogen and carbon
monoxide.

2.           
Boudouard
reaction: carbon dioxide present in the gasifier reacts with carbon to produce
carbon monoxide.

3.           
Shift
conversion: carbon monoxide reacts with steam to produce carbon dioxide and
hydrogen, this is highly desirable as syngas with a higher composition of
hydrogen will have a higher heating value. 

4.           
Methanation:
methane can be formed by the reaction of carbon with two hydrogen molecules.

 

An overview of all the reactions
which take place in a typical gasification process is shown below in Table 1.0
please note the data was obtained from biomass gasification, not R.D.F.,
however, the similarities are significant.

Table 1.0: Gasification reactions
and energy produced/needed. 10

 

Depending on the certain requirements
such as the grade of syngas desired, different gasifying mediums can be used
such as oxygen, air or steam. The medium will also affect the heating value of
the product gas.

There are other parameters which
need to be considered when studying the behaviour of the fluidized bed, such as
the composition of the R.D.F, this includes its moisture content and ash
content, the feed location of the fuel, equivalence ratio, tar production and
composition as well as particle attrition and elutriation, all these factors
will be looked at in more detail in the literature review section.

In this research project, a close
study of the fuel particle will be made while devolatilization is occurring,
this will identify more clearly what occurs with the R.D.F particle while in
the fluidized bed.

Advanced imaging techniques
available at University College London will be used for this purpose, as well
as having access to a pilot plant in Swindon which belongs to Advance Plasma
Power Ltd (APP) a world leader in waste to energy and advanced fuels
technology. The plant will be used to validate the results obtained in the lab.
Also, a study of the ash segregation will be made, this is the ash which is
produced in the gasification process and which settles at the bottom of the bed,
this is important because if the ash agglomerates it can cause blockage and
eventually de-fluidise the bed which can be an economic setback in commercial
settings.

References:

1          EIA, “International Energy Outlook 2017 Overview,” U.S.
Energy Inf. Adm., vol. IEO2017, no. 2017, p. 143, 2017.

2          IEA, “World Energy Outlook 2017 – Executive summary,” Int.
Energy Agency, 2017.

3          DEFRA, “Digest of Waste and Resource Statistics – 2017
Edition,” Dep. Environ. Food Rural Aff., no. March, 2017.

4        A. Seely, “Landfill tax?: introduction
& early history,” House of Commons, p. 19, 2009.

5       P. Basu, Combustion and
gasification in fluidized beds. Boca Raton, Flor.: Florence, 2006, p. 21.

6       P. Basu, Combustion and
gasification in fluidized beds. Boca Raton, Flor.: Florence, 2006, p. 11.

7      “Coal Gasification: Striking While
the Iron is Hot”, Power-eng.com, 2017. Online. Available:
http://www.power-eng.com/articles/print/volume-109/issue-3/features/coal-gasification-striking-while-the-iron-is-hot.html.
Accessed: 29- Dec- 2017.

8     M. de Souza-Santos, “Comprehensive
modelling and simulation of fluidized bed boilers and gasifiers”, Fuel,
vol. 68, no. 12, pp. 1507-1521, 1989.

9       P. Basu, Combustion and
gasification in fluidized beds. Boca Raton, Flor.: Florence, 2006, p. 65.

10    Biomass
gasification design handbook. Amsterdam: Academic, 2010.