Experimental investigation of thermal conversion of solid waste under high
temperature agent (steam/air)
Roger Gołombek
Degree Project, Second Level SoM EX 2013-19
Stockholm 2013
___________________________________________________________
KTH, Royal Institute of Technology
TABLE OF CONTENTS
Abstract ... 7
1. Introduction ... 8
1.1. Global energy consumption ...8
1.2. Biomass ...9
1.3. Municipal Solid Wastes...10
1.4. Automotive Shredder Residue ...11
1.5. Rofire ...12
1.6. Biocoal...12
1.7. Gasification process ...13
1.8. HTAG gasification ...15
1.9. Waste gasification ...15
1.10. Municipal and industrial solid waste treatment ...16
2. Objectives ... 17
3. Lab-scale test unit ... 18
3.1. Description of the testing reactor ...18
3.2. Experimental part ...20
3.2.1. Fuel characterization ... 20
3.2.2. Mass loss - experimental procedure ... 21
3.2.3. Mass loss – results...22
3.2.4. Synthesis gas composition – procedure and results ... 32
4. Large scale facility ... 43
4.1. Description of the facility ...43
Preheater ……….45
Feed system ... 46
Updraft gasifier reactor ... 48
Afterburner ... 49
4.2. Investigation of biocoal ...49
4.2.1. Biocoal properties ... 50
4.2.2. Calibration of feeding system ... 51
4.2.3. Experimental conditions ... 51
4.2.4. Temperature distribution ... 52
4.2.5. Pressure drop ... 53
4.2.6. Synthesis gas composition ... 54
4.2.7. Flue gas composition ... 56
5. Conclusions ... 58
Acknowledgement ... 59
References ... 60
LIST OF FIGURES Fig. 1. World energy consumption by source of fuel, 1990-2030. ...8
Fig. 2. Scheme of the sHTAG system. ... 18
Fig. 3. A real picture of small HTAG system.. ... 19
Fig. 4. The gasifying reactor of the sHTAG ... 20
Fig. 5. Distribution of the temperature during heating up of the reactor ... 21
Fig. 6. Relative mass loss during gasification of straw biomass in different temperature regimes. ... 23
Fig. 7. Relative mass loss during gasification of biocoal in different temperature regimes. .... 23
Fig. 8. Relative mass loss during gasification of RDF in different temperature regimes. ... 24
Fig. 9. Relative mass loss during gasification of ASR in different temperature regimes. ... 24
Fig. 10. Relative mass loss during gasification of pure polyethylene in different temperature regimes. ... 25
Fig. 11 Relative mass loss and conversion rate during straw biomass gasification. ... 27
Fig. 12. Relative mass loss and conversion rate during biocoal gasification. ... 27
Fig. 13. Relative mass loss and conversion rate during RDF gasification. ... 28
Fig. 14. Relative mass loss and conversion rate during ASR gasification. ... 28
Fig. 15. Relative mass loss and conversion rate during pure polyethylene gasification. ... 29
Fig. 16. Percentage mass loss at each stage and quantity of residue of particular feedstock for different temperatures of a steam. ... 31
Fig. 17. An example of the microGC result during one specific sampling test. ... 33
Fig. 18. Percentage concentration of spices with respect to different time regimes at two
residence time intervals. ... 35
Fig. 19. Percentage concentration of spices with respect to different time regimes at two residence time intervals. ... 35
Fig. 20. Percentage concentration of spices with respect to different time regimes at two residence time intervals. ... 36
Fig. 21. Percentage concentration of spices with respect to different time regimes at two residence time intervals. ... 36
Fig. 22. Percentage concentration of spices with respect to different time regimes at two residence time intervals. ... 37
Fig. 23. H2/CO ratio with respect to different temperature regimes. ... 38
Fig. 24. Lower Heating Value variations at different temperature regimes during the first time interval (0-3 min)... 39
Fig. 25. Lower Heating Value variations at different temperature regimes during the second time interval (4-6 min). ... 40
Fig. 26. Comparison of syngas composition produced from the various kind of fuel within the first time interval at each temperature regime. ... 41
Fig. 27. Comparison of syngas composition produced from the various kind of fuel within the second time interval at each temperature regime. ... 42
Fig. 28. General profile of the HTAG facility. ... 43
Fig. 29. Scheme of the HTAG test facility in the up draft configuration. ... 44
Fig. 30. The High Temperature Air Combustion Preheater and the control panel. ... 45
Fig. 31. he electrical steam ... 45
Fig. 32. Preheater (HiTAC) by NFK ... 46
Fig. 33. The feeding system (the biomass feedstock hopper with a screw conveyor duct on the right and top part of the gasifier with second feeding tank on the left). ... 47
Fig. 34. The gasifier with a scheme of the sections spacing along the reactor. ... 48
Fig. 35. The Gas Combustor and one of the peepholes. ... 49
Fig. 36. Spacing of the S-type thermocouples along the gasifier. ... 52
Fig. 37. Temperature distribution inside the gasifier. ... 53
Fig. 38. Behaviour of pressure inside the gasifier. ... 54
Fig. 39. Composition of syngas and HHV in given time interval. ... 55
Fig. 40. Variation of syngas composition and fuel feed rate vs. time. ... 55
Fig. 41. Flue gas changes with respect to time. ... 57
LIST OF TABLES Table 1. Characterization of the tested fuel. ... 21
Table 2. An experimental conditions. ... 22
Table 3. The results of mass and conversion rate for gasification of different kind of the fuel. ... 30
Table 4. Gas composition results for steam pyrolysis/gasification of different kind of the fuel at initial temperature of: (a) 1000oC (b) 930 oC (c) 800oC ... 33
Table 5. Hydrogen to carbon oxide ratio with respect to different temperature regimes. ... 38
Table 6. Heating value of combustible gas components. ... 39
Table 7. Tested fuel characterization ... 50
Table 8. Ash composition ... 50
Table 9. Dependence of the feeder engine’s frequency on the feed rate of biocoal. ... 51
Table 10. . Average temperature at each thermocouple position. ... 53
Table 11. Variation of the tested results. ... 56
Table 12. Composition of afterburner’s flue gases with respect to time. ... 57
NOMENCLATURE
wi – Initial net weight,
w0 – Actual net weight,
A – Weight fraction of ash,
c – Conversion rate,
dx – Derivative of normalized ash free basis,
dt – Derivative of time,
zi – Volume fraction of combustible gas component, HVi – Heating value of combustible gas component,
H2
z – Volume fraction of hydrogen,
CH4
z – Volume fraction of methane,
zCO – Volume fraction of carbon oxide,
y xH
zC – Volume fraction of hydrocarbons, m&air – Mass of air,
m&fuel – Mass of fuel,
Abstract
Most of the problems with providing a continuous and sustainable energy supply for the worldwide society are negative consequences to the environment and its living habitants steaming from uses of conventional technologies. Those consequences should be minimized by developing and improving new technologies as well as by utilization of other type of feedstock than fossil fuels, such as biomass, industrial or municipal solid waste. Nowadays, gasification is the main technology for biomass conversion to energy and a great alternative for the thermal treatment of solid waste. The number of various applications for produced gas shows the flexibility of gasification and that is why allows it to be integrated with other industrial processes, as well as power generation systems.
The main objectives of this thesis were to present behavior of different kind of feedstock undergoing pyrolysis/gasification processes in reactors using highly preheated agents and additionally compare the compositions of produced gases. In this thesis two different systems were presented; the first is lab-scale gasifier for the treatment of biocoal, automotive shredder residue (ASR), refuse derived fuel (RDF), biomass (straw pellets) and plastic waste (polyethylene) and the second one is a large up-draft, fixed bed gasifier used for investigation of biocoal.
The thesis was divided into four main parts: beginning with theoretical introduction, subsequently showing outcomes from investigations carried out on lab-scale test unit, large HTAG facility and finishing on short conclusions.
1. Introduction
1.1. Global energy consumption
An electric energy is known as a condition of the world economic and civilization development. The dynamic and the level of the electric energy consumption in particular countries or regions of the world mostly depend on number of inhabitants, the level of economic and civilization development as well as structure and effectiveness of energy utilization. Within last century generation of the electric energy by conventional methods based on fossil fuels have been facilitated industrial, agricultural, transportation and telecommunication revolutions as well as the economic growth. They have been the primary source of energy and chemical feedstock contributing to about 82 percent of total energy use [1].
A primary energy is a form of energy found in nature that not undergoes to any transformation nor conversion process. This energy is contained in primary energy carriers which are directly leverage from non-renewable or renewable natural resources.
Fig. 1. World energy consumption by source of fuel, 1990-2030.
(Source: International Energy Outlook 2011, U.S. Energy Information Administration [2])
Utilization, mining and processing of the non-renewable resources (fossil fuels) have become one of the major factors which contribute to devastation of the natural environment.
That is why mankind has to deal with a problem of fossil fuel depletion to find alternatives such as renewable sources of energy to cover a still rising demand for energy. There are several sources of the renewable energy that may be used to replace them, i.e. wind, solar, hydro power, fission, fusion, chemical or electrochemical. Nowadays the humanity has to use and improve different kind of the technologies to provide the sustainable and continuous energy supply to the whole society and what goes along with that minimizing the negative effects on the environment and living habitants [3].
Beside all of those resources used for generation of the electricity there is another non-renewable source of energy and those are wastes which are stored in many places around the world. It remains a huge problem for mankind and its surroundings but at the present there are different thermal conversion methods which are being improved by engineers and in short future should replace the conventional technologies.
1.2. Biomass
Conventional energetic raw materials like coal, natural gas, crude oil are getting more and more expensive. Their finite amount and legal regulations which extort to take advantage of renewable energy force to exploring alternative sources of the energy. Biomass utilization is a prospect for power industry as well as for agricultural engineering. Utilization of the biomass for power engineering purposes has many advantages: reproducibility of the material, natural environment protection (limited emission of harmless gases: nitrogen and sulfur oxides), being independent from external energy carriers, growth of the national energy security, activation of agricultural economy which leads to decrement of unemployment in rural areas and many more.
Biomass is generally considered as one of the biggest inexhaustible energy resources in many countries around the world. It is also the only renewable resource that may be perceived as a fuel source and products based on coal. The biomass can be produced and utilized without huge technological investments. The energy contained in biomass pose the smallest capital intensity inexhaustible energy resource.
The biomass can be converted into any fuel physical state: solid, liquid or gaseous.
Energy resources of the biomass enclose two groups: the first are energy carriers in solid phase, suitable for combustion, pyrolysis or air-steam gasification to produce mixture of carbon oxide and dioxide, hydrogen and methane. The produced gas may be converted into electric energy and heat according to the type of technology, but also into methanol or engine fuel which is currently realized in commercial scale. The second group includes conversion of particular biomass components into liquid fuels or biogas, which is primarily a mixture, consists of 60% vol. methane and 40% of carbon dioxide. The biomass can be processed in three different ways: physically, thermo-chemically and biologically.
Following forms of the biomass may be used for energy purpose:
- Waste wood from a forestry or wood industry - Straw from grains, oil plants, leguminous and hay - Energy crops
- Organic wastes from agricultural-food industry: manure, sludge, scrap paper - Liquid biofuels for transportation purposes, i.e. vegetable oils, biodiesel,
bioethanol
- Biogas from dunghills, sludge and landfills etc.
1.3. Municipal Solid Wastes
The Municipal Solid Wastes (MSW) are commonly known as garbage, trash or refuse which is a waste type consisting of everyday items that are discarded by the public. The composition of the MSW varies greatly from country to country and changes significantly with the time. The MSW contains fraction of biodegraded biomass, plastics and inert in the form of glass, rubber, scrap, sand and ash, as well as water. The first two fractions are combustible and they decide about capability of the waste burning. An issue of the MSW combustion is a bit problematic for the environmental and social aspect. The combustion is enclosed by many legal restrictions and requirements. Therefore it is significant to select proper combustion technology in order to avoid social objections and that one which is acceptable for economic and environmental aspects.
In power industry using combustion methods of naturally occurring organic matters create a list of technological problems, mostly because biomass occurs in many forms, so it is
impossible to construct all-purpose furnace for every type and form. Thus for the many years scientists have been trying to invent new technologies or upgrade the existing ones to improve effectiveness of the waste utilization. One of them that may replace combustion process in conventional incinerating plants and seems to be prospective is gasification process.
1.4. Automotive Shredder Residue
The world-wide annual generation of wastes by automotive industry is about 50 mln tones. A well-developed infrastructure exists for the reuse and recycling of automotive parts and materials. Every vehicle at the end of its usefulness is disassembled into pieces which part of them is sold for reuse and the remains are mechanically processed. About 75% of vehicle’s total weight is recovered at the present, which are mostly metal elements (ferrous or non- ferrous). The remainder of the vehicle materials are processed (shredded) to a fraction called Automotive Shredder Residue (ASR).
The ASR is a complex mixture containing a variety of materials (glass, fiber, rubber, automobile liquids, plastics, dirt), some of which may be toxic or potentially harmful.
Treating this type of waste is extremely challenging, because the exact material or chemical composition is not easy to estimate and very often it varies [4]. In each book there are different compositions of the ASR. Day et al. reported that ASR comprises (20%) of rubber, (19%) of plastics, , (10%) of fiber materials and textiles and (2%) of wood where the rest are metals (8%), and oils (5%). ASR contains heavy metals like, aluminum, chromium, lead, cadmium, copper and others which should be recycled due to their usefulness for industry and harmful effects on the natural environment.. That is why, some countries have classified ASR as hazardous waste and have established legislative controls [5].
The ASR is one of the wastes which is classified as hazardous and according to the directives (2000/53/EC) can not be land-filling. Therefore that is the difficulty for human beings to find an appropriate waste management system to treat with those waste materials.
Thus, one of the ASR use can be as a feedstock to the gasification process.
1.5. Rofire
Refuse Derived Fuel (RDF) is a solid feedstock made of rejected materials from paper industry. It is a result of dried paper sludge manufacturing process in paper mills which separates the recycled paper into organic and small inorganic fractions. The new mill production unit processes combustible fraction (separated organic fractions such as: fabric fiber, plastic, wood and rope fiber) into solid fuel pellets using a multistage process. The feedstock consists of paper fiber mixed with other type of scrap materials such as fabric fiber, wood chips and plastic. About 55% of carbon has fossil origin (plastic) and the rest has biomass origin (wood chips, paper fabric). The RDF has relatively low content of fixed carbon compared to ordinary wood biomass but high content of ash and volatiles. In the production of Rofire only small fraction of wood chips is used because of a high plastic content which cause significant reduction of oxygen content with respect to pure biomass [5].
1.6. Biocoal
Hydrothermal carbonization (HTC) is an artificial coalification process which converts, under a high pressure, raw biomass into a coal-like product. The HTC is well suited for a many different types of a biomass such as: straw, corn stalks, palm fronds, waste from oil-producing plans, fruit industry waste, waste coffee ground, spent tea leaves, agricultural wastes, meat by-products and many other kinds of organic residues.
In general the HTC is a chemical process, which under heat (220oC) and pressure (22 bar) removes water molecules and converts biomass in an anaerobic environment into CO2- neutral biocoal. For that purpose, the biomass is heated up in a pressurized vessel as an aqueous solution and subsequently an exothermic process takes place. It means that no additional energy is required for the further operation of the plant because certain amount of energy is released by removal of water molecule from the biomass organic molecules.
Moreover the HTC is a self-contained process without odor or noise emissions and environmentally friendly (residual water is ecologically harmless), and has low investment and maintenance costs due to proven, tried-and-tested technology [6].
The HTC process is very robust and precise because uses all the existing carbon consisted in a biomass. Thus, the biocoal is similar to high quality brown coal and has a higher energy density than the normal biomass and is easier to process, transport and store.
1.7. Gasification process
Gasification is a process based on the reaction of carboneous materials contained in fuels with gas conveyed to a reacting system. In other words gasification is a conversion process of carbonaceous (solid or liquid, fossil, biomass or waste) materials into gas.
Gasification can be carried out using different types of fuel: hard coal, brown coal, peat, wood, biomass, industrial waste products, municipal solid waste or even heavy residues from distillation of a crude oil. The gas, which is the direct end-product of the gasification process, is called synthesis gas (syngas or producer gas). The synthesis gas may be used as a standalone fuel. The energy density of syngas is only about 50 percent that of natural gas and it is therefore mostly suited for use in producing transportation fuels and other chemical products. Moreover the syngas can be used as an intermediary building block for the final production (synthesis) of various fuels such as synthetic natural gas, methanol and synthetic petroleum fuel. The usual components of syngas are: CO, H2, CO2 and CH4. Depending on the aim of the syngas production, different compositions and properties of the syngas are desired.
Feeding fuel inside the gasifier reactor undergoes following processes:
1) Dehydration (drying) – vaporization of volatile matters from the feedstock for the further chemical reactions.
2) Pyrolysis – decomposition of organic material in lack of oxygen at elevated phase.
Heating the organic material leads to release of volatiles from the solid material, part of which will condense when cooled down. Thus pyrolysis products are: gas, liquids and solid residue. Pyrolysis phenomenon proceed at the temperature between 350- 850oC and additionally at this stage proceeds the biggest mass decrement.
3) Combustion - A carefully controlled burn using small amounts of air allows the volatiles and the char to react with the oxygen to create primarily carbon dioxide water and trace amounts of carbon monoxide. The heat created in the process is used in the gasification process.
- Exoergic and homogeneous reactions
- Forming, among others, carbon mono and dioxide
- The more volatiles matters, the harder complete combustion proceed
Combustion reactions CO O C+ 2 =
2
1 -111MJ kmol
2
2 2
1O CO
CO+ = -283MJ kmol
O H O
H2 2 2
2
1 =
+ -242MJ kmol
Water gas shift reaction
2 2
2O CO H
H
CO+ Û + -41MJ kmol
Steam Methane Reforming
2 2
4 H O CO 3H
CH + Û + +206MJ kmol
4) Gasification - char reacts with the carbon dioxide and the steam produced in previous steps to form carbon monoxide and hydrogen.
- Endoergic and heterogeneous reactions
- Main products formed at this stage: Hydrogen (H2), Carbon monoxide (CO) - The slowest reactions proceed
Boudouard reaction CO CO
C+ 2 Û2 +172MJ kmol Synthesis of methane
4
2 CH
H
C+ Û -75MJ kmol
5) Equilibrium - A chemical reaction known as the “water gas shift reaction” helps to balance the carbon monoxide, steam, carbon dioxide and hydrogen in the gasifier establishing a chemical equilibrium during the final step of the process.
Water Gas Shift Reaction
2
2O CO H
H
C+ Û + +131MJ kmol
1.8. HTAG gasification
High Temperature Air/Steam Gasification (HTAG) is a technique in which a preheated air or/and steam is utilized as a gasification agent. Preheating of an oxidizer (air or steam) is realized by means of the modern “High cycle regenerative Air/steam preheater”.
The preheated oxidizer boosts thermal decomposition of the feedstock (e.g. biomass or waste materials) by supplying additional quantity of energy into the gasification system. Moreover, this technique has shown promising properties in terms of product gas yield, heating value, gas composition, low tar content and “cold” gas efficiency. The effect of its high reaction temperature is to sustain the gas phase reactions that are dominant at elevated temperatures over 1000oC. Ordinary low temperature gasifier may process feedstock with moisture content only up to 50%, however HTAG may operate with higher level of the moisture content. It may be noticed also that the presence of moisture in the HTAG feedstock increases combustible gas yield and its heating value since the moisture takes part in the secondary reduction and steam reforming reactions that are responsible for the formation of more CO and H2 gases [7].
1.9. Waste gasification
From the one tone of wastes it is possible to receive ca. 1k m3 of the synthesis gas in air-steam method or even 2k m3 by using oxygen gasification system. It is at least five times less than amount of flue gases formed during conventional combustion technology. It means that cleaning of a gas before combustion takes place is more profitable than flue gas cleaning (because there is 80% less gas before burning than flue gases after after-burning). The syngas produced during gasification process is used to generate thermal and/or electrical energy. It is possible to replace conventional natural gas boilers with gasification systems, which simultaneously allow for utilization of the wastes and generation of the electricity. In gasification chamber (gasifier) wastes change into calorific gas, which is subsequently burn inside a combustion chamber. More over hot exhaust gases may be then utilize in different kinds of drying stoves or for water boilers (e.g. in municipal heat-generating plants). If the gas is properly cleaned it can be used directly in a generating engine, then instead of combustion chamber it is connected to syngas cleaning system and to generating engine. Gasification
allows for more effective utilization of different kinds of waste materials than in conventional combustion, resulting in reduction of waste mass at about 80% (very little ash remains). It is the process which may be effectively controlled following all the environmental standards [8].
Gasification and incineration are both thermal processes which could be used for the waste treatment but there are many advantages of gasification over the incineration [9]:
- Multidirectional application of produced gas (synthesis gas) which can be utilized for heat generation in form of heat, electrical energy or as a raw material for production of chemicals for high-value products or for producing liquid fuels.
- Reduction of harmful substances emission to the atmosphere, because during direct combustion of the waste dangerous carcinogenic compounds such as furans and dioxins are formed.
- The syngas is produced under controlled conditions and it is generated without formation of impurities associated with incinerator flue gas.
- Possibility of significant increase in efficiency of electrical energy generation - Possibility of application of different kind solid wastes in various forms
1.10. Municipal and industrial solid waste treatment
Waste material, refuses, by-products of human activity, which has no use at a certain area and time where they were produced are harmful and nuisance for the natural environment. There are two groups of the waste which can be distinguished assuming the origin of their formation:
- Municipal solid waste - formed in resident areas and related to human existence (domestic waste, public utility waste, street waste etc.)
- Industrial waste products – related to economic activity (different kind of by- products, waste from sewage treatment, animal farm waste etc.)
In terms of environmental protection, very important criterion of waste division is their harmfulness to the natural environment. Regarding to this classification they can be divided into three categories:
- Hazardous waste - introduction them to the environment, even in small quantities, cause permanent degradation (e.g. toxic, radioactive, combustible substances)
- Harmful waste - higher amount of those materials and prolonged impact cause degradation of the environment.
- Nuisance waste - not contain substances causing degradation, however they have adverse influence on environmental aesthetics.
Waste management is a complex phenomenon with a range of consequences for the involved stakeholders and the society. One of the parameters for evaluation is the environmental impact of different treatment options or technical solutions [10]. The most commonly used tool for assessment of environmental impact is life-cycle assessment (LCA).
The LCA studies the environmental aspects and potential impacts throughout a product’s life, from raw material acquisition through production, utilization and disposal. LCA is probably best known as a tool with which the life cycle impacts of physical products are assessed, but the same methodological framework allows also for the analysis of services such as energy systems or waste management. [11].
2. Objectives
The general objectives were:
- To compare behavior of different kind of waste during steam gasification at particular temperature regimes.
- To show the influence of temperature of gasifying agent on the mass decrement, conversion rate and gas composition during gasification of different kinds of municipal and industrial wastes
- To distinguish and describe different stages of the gasification process
- To present two different types of High Temperature Agent Gasifier (HTAG) systems - To show, on the basis of the obtained results, what is the most favorable utilization for
the produced synthesis gases
3. Lab-scale test unit
3.1.Description of the testing reactor
A small High Temperature Agent Gasifier (sHTAG) is a lab-scale batch type gasifier which is located in a laboratory of the Division of Energy and Furnace Technology at the Royal Institute of Technology (KTH).
The small scale unit consists almost in its entirety of a horizontal combustion chamber (Fig. 3.) with an inner diameter about 0.1m which is internally lined with refractory materials.
Besides the chamber there are some significant elements which form the whole system (Fig.
2.):
- Gas burner
- Ceramic honeycomb (regenerator) inside the chamber - Gas pass
- Steam generator
- GC with product gas sampling line - Computer and electronic scale
Fig. 2. Scheme of the sHTAG system.
Fig. 3. A real picture of small HTAG system..
An operation of the batch type gasifier is described based on the Fig. 4. shown below.
The whole operation starts with heating up the reactor by a gas burner (1), particularly a honeycomb (2) which receives a heat from the hot flue gases arose from combustion of the natural gas. Once the proper temperature is reached the fuel feed is shut off and the stored heat is subsequently used to preheat the gasifying agent for reactions which enters the system through one of the inlets. The utilized agent can be pure steam (at temperature of 180oC and pressure between 2-4 bars), pure air (at ambient conditions), mixture of air and steam or carbon dioxide. The gasifying agent is rapidly heated up to particular temperatures while passing through the ceramic honeycomb. Afterwards, the flue gases flow through the second part of the reactor and towards the facility’s outlet where the temperature of the produced gas is controlled by a thermocouple placed exactly inside the elbow (10). Moreover, there are two S-type thermocouples which measure and control the temperature inside the gasifying chamber: first is promptly after the regenerator (3) and the second one, which was inserted
only for some tests, could measure temperature next to the biomass basket or inside the pellet (6). On the top of the reactor there is a lid (7) where the basket is inserted to the rig through a small hole drilled in the screw cap. The basket is hanging on a thin platinum wire (8) and the second end of the wire is mounted to the digital scale. Additionally, under the hatch there is a small cooling chamber cooled by nitrogen which is injected from the bottle (9). The experimental stage started when the gas burner is switch off and the batch of a biomass is placed inside the reactor chamber from the above. Once the sample is in the gasifying chamber, it could be observed through a glass window (5). In this case to avoid unnecessary heat losses the glass window was tighten by a glass wool to increase thermal insulation.
Fig. 4. The gasifying reactor of the sHTAG
3.2. Experimental part 3.2.1. Fuel characterization
The experiments on the lab-scale equipment were performed using different kind of the feedstock which are listed below:
- Straw – pellets,∅6-7mm and length 10-15 mm - Biocoal – pellets,∅7-8mm, length 10-20 mm - RDF – pellets,∅6-7, length 10-15mm - ASR – shredded waste biomass - Polyethylene – granule,∅3mm
The proximate and ultimate analyses of the fuels used for the experiments are presented in Table 1.
Table 1. Characterization of the tested fuel.
Fuel C H O N S Ash Moisture Volatiles LHV
[%] [%] [%] [%] [%] [%] [%] [%] [MJ/kg]
Straw pellets 46.5 5.81 43.39 0.48 0.04 3.8 10.1 77.7 17.51 Biocoal
pellets 66.3 7 16.1 3.7 NA 6.7 11.6 71.1 28.9
RDF pellets 63.3 8.9 21.0 0.3 NA 6 2.9 84.4 26.7
ASR 38.76 2.67 3.0 3.58 1.18 49.74 2.34 27.70 14.98
Polyethylene
granule 85.62 14.38 - - - - NA NA 45.00
3.2.2. Mass loss - experimental procedure
Temperature was measured continuously in two main points inside the reactor which were already mentioned: behind the honey comb (T1) and at the outlet from the gasifier (T2).
Heating up of the chamber from the initial state to the desired temperatures (1050- 1100oC) usually took around 2-3 hours using 6kW gas burner with a constant methane flow rate adjusted at 0.7-0,8 m3/h. The temperature distribution along the reactor during warming up of the gasifier is presented on the Fig. 5
Fig. 5. Distribution of the temperature during heating up of the reactor
Aim of this experimental part was to compare mass loss of the different kind of fuels during pure steam gasification. Steam flow was set at a continuous level of 35 grams per minute (~2kg/h). Each sample of the feedstock put to the test was prepared in the same manner: weighted on the scale to about 10 grams (light ASR about 8 grams) and subsequently poured to a metal basket to make a thin bed. When the gasifier reached certain temperature the gas burner was switched off and the steam (at about 180oC) was introduced to the system.
When the temperature inside reached the desired value the basket, which was attached to the electronic scale, was lowered from the top on the platinum wire to the center of the reactor.
The biomass decrement was recorded on a computer connected with the scale. When the feedstock was completely burnt the basket was lifted up, cooled down by the nitrogen and replaced by the next one.
The gasification process was carried out in a various three temperature regimes measured on the thermocouple (T1) for each type of a feedstock: the first basket was inserted at initial temperature of approximately 1000 oC, the second one at 930 oC and the third at 800oC.
Table 2. Experimental conditions.
Fuel type Oxidyzing agent
Temperature of the agent Flow rate of the
agent Sample weight
oC oC oC g/min kg/h g
Straw steam 1000 930 800 35 2 10
Biocoal steam 1000 930 800 35 2 10
RDF steam 1000 930 800 35 2 10
ASR steam 1000 930 800 35 2 8
Polyethylene steam 1000 930 800 35 2 10
3.2.3. Mass loss – results
Experimental measurement results of particular kind of the fuel are presented on the graphs in two manners: percentage mass loss with distinguish stages and with conversion rate with respect to time. Figs. 5-9 show the relative mass loss with respect to time.
Straw pellets
Fig. 6. Relative mass loss during gasification of straw biomass in different temperature regimes.
Biocoal pellets
Fig. 7. Relative mass loss during gasification of biocoal in different temperature regimes.
Refuse-derived fuel
Fig. 8. Relative mass loss during gasification of RDF in different temperature regimes.
Automotive Shredder Residue
Fig. 9. Relative mass loss during gasification of ASR in different temperature regimes.
Polyethylene granule
Fig. 10. Relative mass loss during gasification of pure polyethylene in different temperature regimes.
In every temperature regime only polyethylene had the most stable and constant curves because of the composition which is only a pure plastic without any admixtures.
Different composition of each fuel kind is a reason of the various results which are shown in the table, but especially it is a cause of the differences between stage and overall mass losses.
The highest measured residue (ash and other noncombustible materials) after the certain time of the gasification in all temperature regimes had ASR which could be presumed judging from the constituent of the feedstock. Moreover, for each of the tested fuel the highest was the temperature of a steam the fewer residues were left after the process. Besides of the polyethylene which was in every case completely burned. The greatest mass loss in the first stage was observed during the straw pellets gasification which was associated with the longest period of the stage in comparison with rest of the fuels
Each of the graphs was divided into three stages because the mass loss and the derivative of mass loss show a similar behavior in every temperature regime. An exception is polyethylene because the curves are quite constant and there are no distinct boundaries. The distinguished stages comply with different phenomena:
- Stage 1 - the highest mass decrement ranged from 40 to 70% and lasts for about 80-140s. It is the fastest part of the process because of the maximum decomposition rate and it contributes to the pyrolysis.
- Stage 2 – in this stage gasification of char appears. Pyrolysis and gasification reactions proceed simultaneously and the conversion rate decreases. Duration of that stage is a bit different for each biomass type, about 60-90s but the decrement of sample’s weight is much lower in comparison with the first stage.
- Stage 3 – after the pyrolysis process is finished the last step appears as a slow process of char gasification that consumes remaining carbon from the sample. In this stage the mass loss decrease steadily and the conversion rate are constant regardless which feedstock is taken into the consideration.
The conversion of material was evaluated based on standard ash free basis and calculated accordingly to the following equation:
0 0
0
Aw w
Aw xi wi
-
= - (Eq. 1.)
Thus, the conversion rate can be written as:
%
´ 100
= dt
c
idx
(Eq. 2.)Figs . 11-15 show the conversion rate (right vertical axis) and the relative mass loss (left vertical axis) with respect to time. The areas under the curves correspond to the conversion of the solid fuel into pyrolysis/gasification products and the heights of the peaks show a maximum reaction rate of a given stage.
In all cases it is noticeable that the conversion rate curves oscillate because of some disturbances during the experiments which could be caused by swinging of the baskets inside the reactor because of high steam flow and also by influence of a heat on an electric scale which was released through the hole at the top of the lid. Moreover, similar trends of conversion rate with respect to a temperature of gasifying agent may be noticed for every kind of fuel.
Straw pellets
Fig. 11 Relative mass loss during straw biomass gasification with respect to time.
Biocoal pellets
Fig. 12. Relative mass loss during biocoal pellets gasification with respect to time.
Refuse-derived fuel
Fig. 13. Relative mass loss during RDF gasification with respect to time.
Automotive Shredder Residue
Fig. 14. Relative mass loss during ASR gasification with respect to time.
Polyethylene granule
Fig. 15. Relative mass loss during pure polyethylene gasification with respect to time.
The greatest influence of temperature on the conversion rates and more significant differences in mass loss were observed running the tests with the higher temperature of steam.
It can be explained by increased convective heat transfer between hot steam and a sample but also by the higher partial pressure of steam, which accelerates kinetics of the process. The biggest slope among all of the fuels had polyethylene because of the small granules size and very quick decrement of sample’s weight during gasification and as well it could be a reason of small oscillations.
The characteristic time intervals, mass losses, average and maximum conversion rates corresponding to each of discussed stages are presented in greater detail in the table 3.
Duration of the mass loss was comparable for straw, biocoal and ASR but it can be said that the third one had the longest decrement time because of the smallest amount of batch introduced for the process.
On the basis of data from the table the conversion rate in almost every case decreases with diminishing agent temperature. The highest average/maximum conversion rate of straw pellets is caused by the fastest and overall mass loss and also by the lowest ash content which was used in order to calculate conversion rates using Eqs.1-2.
Table 3. The results of mass and conversion rate for gasification of different kind of the fuel.
Time
inverval Unit Straw Biocoal RDF ASR Polyethylene
Initial
Temperature Stage 1 oC 1000 930 800 1000 930 800 1000 930 800 1000 930 800 1000 930 800
Duration of period
All
stages s 536 552 544 492 508 548 348 364 396 580 548 564 136 176 200
Stage 1 s 136 136 136 116 116 116 120 120 120 90 90 90 - - -
Stage 2 s 72 72 72 64 64 64 62 62 62 80 80 80 - - -
Stage 3 s 328 344 336 312 328 368 166 182 214 410 378 394 - - -
Mass loss
Stage 1 % 73,18 69,54 61,90 53,41 42,18 33,12 66,40 60,86 53,92 39,19 32,60 31,68 - - -
Stage 2 % 12,28 11,21 14,49 7,43 14,53 13,04 11,98 10,21 8,26 6,36 5,01 8,02 - - -
Stage 3 % 11,68 14,38 16,62 6,73 7,27 11,99 13,16 14,51 19,27 15,65 21,37 16,28 - - -
Overall mass
loss All
stages % 97,15 95,14 93,01 67,57 63,98 58,15 91,55 85,59 81,45 61,20 58,97 55,98 100 100 100
Maximum reaction rate
Stage 1 %/s 0,46 0,48 0,44 0,45 0,36 0,39 0,20 0,20 0,17 0,11 0,10 0,08
0,23 0,20 0,19
Stage 2 %/s 0,19 0,17 0,14 0,29 0,36 0,36 0,11 0,12 0,09 0,05 0,04 0,05
Stage 3 %/s 0,09 0,13 0,08 0,12 0,09 0,08 0,06 0,06 0,07 0,05 0,05 0,04
Average reaction rate
Stage 1 %/s 0,33 0,30 0,28 0,22 0,24 0,20 0,12 0,12 0,10 0,06 0,06 0,06
0,14 0,10 0,09
Stage 2 %/s 0,12 0,13 0,09 0,10 0,12 0,12 0,07 0,06 0,06 0,04 0,03 0,04
Stage 3 %/s 0,06 0,06 0,06 0,04 0,03 0,03 0,04 0,03 0,03 0,02 0,02 0,02
Fig. 16. Percentage mass loss at each stage and quantity of residue of particular feedstock for different temperatures of a steam.
The temperature of the oxidizing agent plays an important role for the quality and yield of syngas generated from biomass/wastes. In this process steam is used for two reasons:
as a reactant and as a heat carrier. Hence, the increment of steam temperature has two expected advantages: increase of conversion and heating rate. Both of these effects will favorable influence on the increments of conversion rate and so on the gas yield.
3.2.4. Synthesis gas composition – procedure and results
This laboratory test was carried out in order to analyze compositions of gas mixture produced during gasification of various type of biomass at different temperature regimes. All the samples were weighted and prepared in the same manner as it was for mass loss tests. The gasifying agent was pure steam at a continuous flow rate of 35 g/min.
The experiment was to sample a small quantity of gas at the outlet from the gasifier where the major portion of the flue gases was driven to the gas pass and subsequently to the chimney. The suction of the gas sample was induced by pump connected with a gas chromatograph (GC). The GC used was a Varian micro-GC CP4900 equipped with a thermal conductivity detector. Before the gas was analyzed in the GC it had to pass through the sampling line which was composed of four water traps and two coalescence filters. The water traps were made up of washing bottles immersed in a coolant liquid (-10oC) and they were used in order to cool down the produced gas to the temperature below 110 oC whereas the filters were meant to capture the tar content as well as remove any particle that might be present in the gas mixture. The GC uses two gas carriers (Helium and Argon) to move gas compounds in order to be analyzed interacting with the walls of the columns. Analysis results of each tested sample were converted and shown on four graphs (channels) by microGC software (Fig.17.). A quantity of each compound in the gas mixture corresponds to the area beneath the particular voltage peak.
Fig. 17. An example of the microGC result during one specific sampling test.
The GC was sampling the gas every 3 minutes and its analysis was used for detection and identification of various compounds, such as: H2, O2, N2, CO, CO2, CH4, C2H4, C3H6, C4H8, C3H6, C2H2, iC4H10, iC5H12, CH3C2H, nC6H14, C3H4, tr-2-C4H8, 1-C4H8, cis-2-C4H8, nC4H10, 1,3-C4H6.
Results from the gas chromatography analysis are presented separately for different temperature regimes in the table 4a-c.
Table 4. Gas composition results for steam pyrolysis/gasification of different kind of the fuel at initial temperature of: (a) 1000oC (b) 930 oC (c) 800oC
a)
Compounds Unit
Straw Biocoal RDF ASR Poliethylene
0-
3min 4-
6min 0-
3min 4-
6min 0-
3min 4-
6min 0-
3min 4-
6min 0-
3min 4-
6min
H2 % 18,16 22,92 19,02 40,83 11,22 30,28 20,78 26,10 10,33 20,66
CO % 40,25 6,29 17,24 9,61 18,18 2,83 17,41 6,96 4,70 3,63
CO2 % 23,12 27,09 15,70 9,50 12,08 13,43 27,49 19,78 8,01 13,32
CH4 % 0,00 2,61 0,14 2,95 0,00 2,94 0,00 1,55 0,00 2,71
CxHy % 6,19 0,19 19,36 1,15 27,39 0,48 13,04 0,47 36,62 8,69
HHV MJ/Nm3 11,45 4,86 16,73 8,01 21,41 5,46 12,27 4,92 24,83 9,99
LHV MJ/Nm3 10,81 4,32 15,42 7,12 19,88 4,78 11,30 4,37 22,90 9,02
b)
Compounds Unit Straw Biocoal RDF ASR Poliethylene
0-3min 4-6min 0-3min 4-6min 0-3min 4-6min 0-3min 4-6min 0-3min 4-6min
H2 % 22,60 25,02 19,91 34,46 18,94 28,64 31,50 29,77 25,67 17,74
CO % 34,80 8,53 18,45 8,76 18,11 3,80 13,38 3,30 5,64 4,24
CO2 % 19,41 21,26 15,73 7,67 13,30 15,75 26,21 19,76 6,88 14,06
CH4 % 0,31 2,97 0,00 3,78 0,00 3,47 0,50 2,16 0,00 3,78
CxHy % 5,13 0,06 17,47 1,62 15,81 0,35 1,60 0,01 19,43 1,29
HHV MJ/Nm3 10,93 5,32 16,64 10,16 14,67 5,92 6,71 5,13 16,44 5,16
LHV MJ/Nm3 10,25 4,75 15,48 9,03 13,67 5,21 6,06 4,48 15,09 4,63
c)
Compounds Unit Straw Biocoal RDF ASR Poliethylene
0-3min 4-6min 0-3min 4-6min 0-3min 4-6min 0-3min 4-6min 0-3min 4-6min
H2 % 23,00 28,60 11,63 44,88 18,53 26,86 22,50 26,88 26,07 22,45
CO % 39,02 7,54 20,09 10,72 15,40 3,73 13,09 10,14 4,96 4,66
CO2 % 20,97 26,24 18,53 13,99 12,24 15,66 21,85 18,04 12,17 11,58
CH4 % 0,42 3,28 0,00 4,84 0,00 3,90 0,16 1,61 0,00 3,50
CxHy % 6,26 0,13 21,91 1,14 13,89 1,04 4,57 0,19 13,25 0,84
HHV MJ/Nm3 12,33 5,97 18,51 9,72 12,89 6,00 7,52 5,38 12,05 5,22
LHV MJ/Nm3 11,59 5,30 17,27 8,64 11,98 5,32 6,91 4,82 10,99 4,64
The GC analysis was carried out three temperature regimes for five types of the fuel, the same as it was presented in the table 2. Every gas mixture produced from the pyrolysis and gasification of particular biomass was analyzed two times in a row which can be noticed from the table 4a-c. where each type of the feedstock is shown in two time intervals. Those intervals may correspond to the stages described in the subsection 2.2.3 (stages 1 and stage 3, respectively). Percentage concentrations of the main species (flammable compounds and carbon dioxide) without nitrogen and oxygen are presented more clearly on the Figs. 21-25 as a comparison of syngas composition during both time intervals at the different temperatures.
Straw pellets
Fig. 18. Percentage concentration of spices with respect to different time regimes at two residence time intervals.
Biocoal pellets
Fig. 19. Percentage concentration of spices with respect to different time regimes at two residence time intervals.
Refuse-derived fuel
Fig. 20. Percentage concentration of spices with respect to different time regimes at two residence time intervals.
Automotive Shredder Residue
Fig. 21. Percentage concentration of spices with respect to different time regimes at two residence time intervals.
Polyethylene
Fig. 22. Percentage concentration of spices with respect to different time regimes at two residence time intervals.
It is noticeable that the concentrations presented on these figures are compared in a manner that shows the differences of composition during gasification of particular fuel within both time intervals at different temperature regimes. The comparisons of time intervals clearly illustrate the similarities and variations in constituent of produced gas. Thus, on the basis of the bar charts it can be said that concentration of carbon oxide and hydrocarbons were higher during the first interval which corresponds mostly to pyrolysis. Lack of the methane during this interval may be caused by too short time between introducing the batch into the system and first running of the GC. That is why the hydrocarbons did had enough time to crack the hydrocarbons into CH4. On the other hand second time interval, which correspond only to gasification process, distinguish oneself with much higher concentration of hydrogen but also methane was obtained from each kind of feedstock which could happen by higher occurrence of steam methane reforming. The increasing of temperature increases the concentration of methane (stage 3) but also other hydrocarbons in the first stage. Moreover, in majority of cases higher amount of carbon dioxide was determined because of intensive water gas shift which produced a lot of CO2and H2.
If the produced gas would be used for chemical synthesis the hydrogen yield has to be as high as possible with H2/CO ratio above 2. The second intervals in every case meet required level of the ratio but if both of the intervals would be normalized and taken together into account the values would like as it is determined in table 5. Therefore, it can be said that for the highest temperature regime (1000oC) H2/CO ratio is the highest almost for every kind of feedstock. When it comes to particular type of fuel it is determined that except of straw biomass all of them could be used for chemical synthesis but the most favorable are wastes and polyethylene which is in fact counted also as a discard.
Table 5. Hydrogen to carbon oxide ratio with respect to different temperature regimes.
Temp. Straw Biocoal RDF ASR Polyethylene
H2/CO ratio
1000 oC 1,10 2,00 2,30 3,67 4,39
930 oC 1,02 1,81 2,17 2,13 4,57
800 oC 0,80 1,97 1,98 1,93 3,72
Fig. 23. H2/CO ratio with respect to different temperature regimes.
Subsequently, the lower and higher heating value for the gaseous fuels were calculated by the summation of the partial heating value of each individual combustible gas component in the mixture (nitrogen and oxygen free), which is mathematically expressed as:
å
×= zi HVi
HV (eq. 3)
The distinction between calculation of LHV and HHV is that there are different heating values for each combustible component which have to be assumed in accordance to the standards content in DIN 51 850:
y x y
xH C H
C CO CO
CH CH
H
H
LHV z LHV z LHV z LHV
z
LHV = × + × + × + ×
4 4
2
2 (eq. 4)
y x y
xH C H
C CO CO
CH CH
H
H
HHV z HHV z HHV z HHV
z
HHV = × + × + × + ×
4 4
2
2 (eq. 5)
Table 6. Heating value of combustible gas components.
Component H2 CH4 CO C2H4 C2H6 C3H8 C3H6 C2H2 iC4H10 iC4H8 iC5H12 MJ/Nm3 MJ/Nm3 MJ/Nm3 MJ/Nm3 MJ/Nm3 MJ/Nm3 MJ/Nm3 MJ/Nm3 MJ/Nm3 MJ/Nm3 MJ/Nm3
LHV 10,81 35,8 12,62 59,5 64,34 93 87 56 118,5 112 145,9
HHV 12,67 39,7 12,62 63,4 70 100 93 63 128 125 157
Fig. 24. Lower Heating Value variations at different temperature regimes during the first time interval (0-3 min).
Fig. 25. Lower Heating Value variations at different temperature regimes during the second time interval (4-6 min).
The characteristics of the produced gas for all cases are summarized in Tables 4a-c as well as calculated lower and higher heating values. An actual heating value of the syngas is lower than values which are presented because molar fraction of the produced gas was normalized, without nitrogen.
Using produced gas for energy production requires high value of LHV which could be increased by presence of gaseous hydrocarbons in the gas mixture. This requires lower influence of steam reforming reactions in a gas phase, which in fact reduces hydrogen contents in syngas.
There is another set of bar charts (Figs. 26-27) in order to compare more clearly the syngas composition during gasification/pyrolysis of all kind of fuels in the same temperature regimes and the same time intervals.
Fig. 26. Comparison of syngas composition produced from the various kind of fuel within the first time interval at each temperature regime.
Fig. 27. Comparison of syngas composition produced from the various kind of fuel within the second time interval at each temperature regime.
4. Large scale facility
4.1. Description of the facility
A High Temperature Air and Steam Gasification (HTAG) facility is also located in a laboratory of the Division of Energy and Furnace Technology at the Royal Institute of Technology (KTH). This updraft, fixed-bed gasifier works in continuous mode and may be used for different kind of fuels, such as: charcoal, coal or biomass (especially non-volatile fuels).
The entire HTAG system is composed of five integrated elements (instruments):
- Preheater - Feeding system - Updraft gasifier
- Afterburner (Gas combustor) - Steam boiler
And some additional appliances: flow meters, sensors, controlling-monitoring equipment, piping, valves etc. Schemes of the HTAG facility are shown on Figs.28-29.
Fig. 28. General profile of the HTAG facility.
Fig. 29. Scheme of the HTAG test facility in the up draft configuration.
Preheater
High Temperature Air Combustion preheater (HiTAC) provides the supply of high- temperature gasifying agent which may be an air, pure steam or a mixture of air and steam.
The regenerative preheater works in cycles and therefore it is capable to preheat a gasifying agent to the high temperature, up to 1200 oC, when the mass flow rate of the feed gas ranges between 50-150 kg/h. HiTAC preheater with controlling pad is shown on the Fig. 30. An electrical steam boiler (Fig. 31) equipped with a water preparation unit may preheat steam to the temperature of 180oC and the pressure about 2.5 bar which is introduced to the air-line.
Air is supplied to the system by a blowing fan. Manual regulation and monitoring of the relative flow of the steam and air is possible by a set of flow meters.
Fig. 30. The High Temperature Air Combustion Preheater and the control panel.
Fig. 31. he electrical steam generator.
Operation idea of the preheater working in cycles is following (Fig. 32.). While a regenerator (a honeycomb) located in the bottom of a combustion chamber is heated up by out coming flue gases, the highly preheated gasifying agent go to the gasifying reactor through another honeycomb in the second chamber. Heat storage and heat release by the regenerators are repeated periodically when combustion gases and low temperature gasifying agent are alternately provided by on-off action of switching valve located on the low temperature side.
The preheated gases are continuously discharged through the exit nozzle (on the left side) and subsequently taken to the gasifying chamber. Whereas combustion exhaust gases are discharged to a gas pass (on the right side).
Fig. 32. Preheater (HiTAC) by NFK Feed system
The feeding system is composed of the three main elements (Fig.33) :
(1) A biomass feedstock hopper of the capacity 2m3. It is a tank where solid fuel is stored before it is conveyed to the gasifier. The biomass is fed from the top of the hopper and their gravimetric fall down to a conveyor is controlled by a slide damper which is located on the bottom of the hopper.
(2) A feeding screw conveyor duct that carries the biomass feedstock from the hopper to the second feedstock tank located above the gasifier. The conveyor is powered by an electrical motor of 3 kW. The duct has a diameter of 100 mm and it is made of polypropylene.
(3) A second feeding tank distributes the biomass on the four vertical water-cooled feeding channels which subsequently supply the biomass, at uniform rate, to the gasifying chamber by using the four synchronized shakers located inside each channel.
All four are powered by two electric motors.
Fig. 33. The feeding system (the biomass feedstock hopper with a screw conveyor duct on the right and top part of the gasifier with second feeding tank on the left).
The whole system is connected to the control panel where the feed rate of the screw conveyor with the shakers are regulated by changing their working frequency or to switch the number of shakers that work at the same time.
