23 Advanced Thermal Treatment Technologies – Pyrolysis

Dr. Yogalakshmi K. N

Objectives:

To understand the concept of advanced thermal treatment technologies with a special emphasis on pyrolysis
To study about the pyrolysis process and the products generated from them during solid waste thermal degradation
To understand the mechanism of pyrolysis process
To gain knowledge about the types of pyrolysis process and various reactors involved in performing the pyrolysis
To realize the merits and demerits of the process

1.0 Introduction

Waste is an unwanted material that is of no use to the owner and hence is disposed off in open area. Disposal onto the land or water body is not the solution as they create huge problem to the environment and health of the human beings. Moreover, the polluter pay principle of environmental protection act has forced the industries to treat the waste prior to disposal. Advanced thermal treatment technologies (ATT) are waste treatment technologies that use high temperature to degrade or destruct the waste. They are designed to thermally degrade the waste and recover energy in the form of heat or electricity. Its main purpose is to reduce the impact of residual waste on the environment. Pyrolysis and gasification are examples of advanced thermal treatment technologies. Pyrolysis is the thermal degradation of a waste materials in the absence of oxygen. This process uses temperatures between 300°C and 850°C in order to break down waste materials.

2.0 Process description

Pyrolysis is defined as a process in which the organic waste is thermally degraded in the absence of oxygen to produce products like char, oil and combustible gases. Pyrolysis is a flexible process and has received great attention in recent times due to its ability to produce solid, liquid and gaseous products at varying levels by altering the operating parameters such as temperature or heating rate. Pyrolysis also transforms low-energy density waste to bio-fuels of high-energy density and other high value chemicals. Pyrolysis is a waste to energy process where waste is converted into valuable products. The formation of end products depends on the temperature and the rate of heating. Oxygen plays an important role and differentiates Pyrolysis from combustion and gasification. Combustion occurs in the presence of oxygen while gasification and pyrolysis occurs in the limited supply and in the absence of oxygen respectively. The temperature range used for pyrolysis is 400℃- 800℃ which is relatively lower than incineration. When compared to all other thermal degradation process, pyrolysis works at low temperature and emits less air pollutants.

The use of this technology for thermal degradation of waste is very recent development. The end products of pyrolysis are mainly the char and oil. They are explored for fuel applications, chemical feed stock and other energy extraction process.

3.0 Mechanism involved

The solid waste consists of various types of complex materials like paper, plastic, metals, glass, cardboards, leaf litter, biodegradable material, hazardous chemicals and many more. The large complex polymeric organic molecules such as cellulose, hemicelluloses and lignin, when pyrolysed, result in the formation of short chain molecules. These short chain molecules are broken down into oils and other gases. Biomass, forest waste and plastic also follow similar mechanism.

4.0 Products of pyrolysis

4.1 Char

Char is the solid end product of pyrolysis. Maximum char is produced when the pyrolysis process is done at low temperature with very slow heating rate. Pyrolysis of wood at slow heating rate and moderate temperature maximizes the char yield. Moderate heating in the range of 20-100 ⁰C per minute at a maximum temperature of 600 ⁰C results in equal proportion of oil, char and gases. This method is referred to as conventional or slow pyrolysis. The char formation is reduced during flash pyrolysis. The solid char produced from slow pyrolysis has been used as charcoal. The percentage of char production depends upon pyrolysis process condition. Pyrolysis of tyre results in maximum production char of around 49.5% with an ash content of about 10%. Municipal solid waste yields around 35% char with a high ash content upto 37%. Pyrolysis of wood by batch process results in the production of 23% char. Char possesses a relative higher calorific value. Around 19 MJ/kg, 29 MJ/kg and 33MJ/kg calorific value are derived from char produced from pyrolysis of municipal solid waste, tyre and wood respectively. Char can be used as medium grade soil fuel. The value of char for fuel production minimizes due to the ash content. Higher the ash content lower is the fuel value. A mentioned earlier, char derived from wood can be suitable for fuel purpose as they contain less than 2% of ash unlike char derived from tyre and municipal solid waste. Steam activation can upgrade the pyrolysis char to activated carbon. However, before stem activation de-ashing is very essential. Similar to activated carbon char produced from waste pyrolysis needs physical and chemical activation. Chemical activation can be done with by impregnation of zinc chloride followed by carbonization using pyrolysis process. Likewise, physical activation is done by steam or carbon dioxide gasification. The initial surface area of tyre (60m2/gm) increased to 650m2/gm after steam activation. During steam activation the pores are opened while the carbon reacts with steam to produce carbon monoxide, carbon dioxide and hydrogen. Likewise, flax textile waste char surface area was increased from 5m2/gm to 900 m2/gm post activation. According to research reports, chemical activation results in high surface area generation. E.g. the surface area of flax textile waste was increased to 2000 m2/gm after chemical activation.

4.2 Oil

The oil generated from pyrolysis process contains high energy density per unit weight. Similar to char oil has direct application in fuel. The oil generated from the pyrolysis process is very complex in composition and contains a wide variety of chemicals like organic acids, phenols, alcohols, aldehydes, ketones and furans. Tyre and oil contains alkanes, alkenes, monoaromatic and polycyclic compounds. During pyrolysis of single plastic, a waxy liquid is produced with a basic structure of original plastic. Polyvinyl chloride produces aromatic oil during pyrolysis. Maximum oil (70%) production is obtained from flash pyrolysis process. The oil generated from pyrolysis process cannot be directly used as fuel and hence require modifications. Oil is generally described as liquid. Depending upon the feedstock used in the pyrolysis process they can be categorized as true oil, oil/aqueous phase separated oil and aqueous phased. Some waste feedstocks result in a waxy material. Sometimes the oil can be mixed with petroleum refinery stocks and upgraded to premium quality using catalysts. Oil is derived from pyrolysis of biomass contains high oxygen of 35% by weight. This is due to the cellulose, hemi-cellulose and lignin content present in the biomass. Likewise, municipal solid waste especially paper, cardboard and wood also generate oil with high oxygen content. Waste degraded in flash pyrolysis process results in oil of viscosity thereby reducing their calorific value. Oil generated from slow pyrolysis process records the maximum viscosity. The calorific value of the oil generated from municipal solid waste ranges from 25 MJ/kg to 40 MJ/kg. Oil derived from municipal solid waste are not used directly as fuel due to presence of solid char particles, viscosity and acidity. The pyrolysis oil is also used as chemical feedstock. The compounds such as methyl phenol, methoxy phenol furfural and methoxy propenyl phenol are produced during wood pyrolysis are used in food, paint and pharmaceutical industry. Tyre oil that contains DL-limonene is used as solvents and adhesives in industries. They also replace chlorofluoro carbon and used as a cleaning agent of electronic circuit boards. Benzene, toluene, xylene and styrene are used in chemical and pharmaceutical industry. The waxy oil generated during mixed pyrolysis is used in catalyst cracking to produce gasoline.

4.3 Gases

The gas generated during the pyrolysis process is composed of H2, CO, CO2, methane and hydrocarbon gases. High levels of carbon dioxide and carbon monoxide are derived from cellulose, hemicelluloses and lignin. High levels of hydrogen, methane and other hydrocarbon gases are formed during the pyrolysis of tyre and mixed plastics. These gases have medium to high calorific value. Conventional pyrolysis of municipal solid waste produces gas of calorific values of 18 MJ/m3 unlike wood that produces 16 MJ/m3 High heating rate and temperature breaks down the pyrolysis oil to gas. The maximum calorific values of 40 MJ/m3 was observed in gas produced during tyre pyrolysis. Higher gas yields are generally found in pyrolysis process where the products are kept in the hot zone for a long period of time. Gas yields are higher even at temperature above 750℃. Original feed stock yield around 70% of gas.

5.0 Types of pyrolysis process

5.1 Slow pyrolysis

It is also referred as conventional pyrolysis. In this type of pyrolysis, heating is done at very slow pase and low temperature. It results in the production of maximum amount of char. When heating is done at the temperature of 600 ℃ and heating rate of 20℃ to 100℃ per minute, the oil, char and gas production becomes equal. Since the process involves slow heating rates and slow removal of products from the pyrolysis unit, the formation of secondary reactions is very frequent.

Slow pyrolysis set-up for the batch production of biochar: (1) nitrogen gas supply (2) flow control (3) gas preheater (4) electric tube furnace (5) pyrolysis reactor (6) sintered base plate (7) packed biomass bed (8) biomass locked hopper (9) condenser (10) condensate/gas separator (11) cotton filter (12) diaphragm flow meter (13) gas vent and (14) bio-oil recovery

Figure 1 Slow pyrolysis process

5.2 Fast/ flash pyrolysis

This process involves a temperature below 650℃ and very high heating rate of 100℃ to 1000℃ per second. This process results in liquid yield of 70% with minimum amount of char and gas. The gas production is minimised due to rapid quenching of the liquid generated during the process. Likewise, high reaction rate also minimises char formation. Wood and tyre are pyrolyzed through flash pyrolysis.

Figure 2 Fast pyrolysis process

5.3 Vacuum pyrolysis

Unlike other pyrolysis process, vacuum pyrolysis is carried out at very low pressure of about 5kPa. Short residence time and low decomposition temperature reduces the occurrence and frequency of secondary reactions. Vacuum pyrolysis is used for different types of wastes such as household wastes, wood, plastics, tyre and circuit boards. This type of pyrolysis is not popular among other types.

6.0 Types of pyrolysis reactors

The design of the pyrolysis unit is very important due to the involvement of huge amount of heat transfer for waste degradation. Construction of an economically viable pyrolysis unit with maximum productivity is the need of the hour. Fixed bed reactors, batch or semi-batch reactors, rotary kilns, fluidized bed reactors, microwave assisted reactors and plasma or solar reactors are commonly used pyrolysis reactors.

6.1 Fixed bed reactors

Fixed bed reactors are the simple reactors commonly used in laboratories. Their design is simple and easy to be constructed. The reactor is generally made of stainless steel which is heated externally through electric furnace. Before the process, the unit is flushed with N2 or Ar gas to maintain inert condition and anaerobic condition. The waste is placed at the bottom of the reactor. Fixed bed reactor work under the principle of slow pyrolysis where low heating rate is involved. The major drawback of this process is lack of uniform heating due to non-movement of waste within the reactor. Fixed bed reactors can be used for large scale applications provided uniform heat transfer and distribution is ensured through pipelines. The gases generated during the process are removed everyday while the char is collected at the end of the process.

6.2 Batch reactors

In this type of reactor, the waste is loaded, sealed and operated. No addition of reactants or removal of products is done during the operation of batch reactors. High conversion rate is observed in this type of closed reactors. In semi batch reactors, addition of reactants and removal of products is allowed. However, the end pyrolysis product is not uniform for every batch which is the major disadvantage of semi batch rectors.

6.3 Fluidized bed reactors

The waste material is fluidised within the reactor. Fluidisation results in better mixing and good distribution of heat throughout the waste mixture. The characteristic feature of fluidized bed rector is high heating rate. It works under the principle of fast pyrolysis and generates around 70% of oil. Municipal solid waste especially polymers and plastics are pyrolyzed by these reactors. Polymer possesses a characteristic feature of low thermal conductivity and high viscosity. High heating efficiency and heat distribution in fluidised bed reactors facilitate the cracking of polymers. These reactors suffer from two main problems. Char separation from the reactor is difficult and better fluidisation is achieved with the particles of smaller size. Henceforth, the waste requires pre-treatment before pyrolysis. After collection, the material should be sorted, dried and crushed into small pieces before loading into the reactor. Plastics and paper can be sized to 5 mm while biomass can be powered to a size not more than 1 cm. the fluidised bed reactors are difficult to be scaled up and hence used in laboratories.

6.4 Spouted bed reactor

Spouted bed reactors are operated under continuous mode. It follows flash pyrolysis method and is popularly used for polymer pyrolysis. Polymers like polystyrene, polyethylene, polypropylene and polyethylene terephthalate are thermally degraded in spouted bed reactors. low bed segregation and lower attrition makes this reactor more advantageous over fluidised bed reactors. The polymers melt during the addition of plastics into the reactor and provide a uniform coating around the sand particles. Spouted bed reactors are highly versatile and show excellent heat transfer rate, good gas flow at short retention time and faces minimum defluidisation problems. Like fluidised bed reactors, spouted bed reactors also operate with smaller size feedstock. Irregular texture, fine particles, sticky solids and those with a wide size distribution can also be pyrolyzed in spouted bed reactors.

6.5 Rotary kiln reactor

Rotary kiln reactors use slow pyrolysis for the thermal degradation of municipal solid waste. It is the only type of reactors that has been installed at industrial level. The waste material is broken down at a temperature of 500°C at a short residence time of 1 h. like spouted bed reactors, minimum pre-treatment is also required for rotary kiln reactor. Waste is sorted, shredded and used as feedstock in the reactor. Rotary kilns are operated both in batch and continuous mode. They show better heat transfer ability than fixed bed reactors. Moreover, they are less complicated than fluidised bed reactors. Residence time is one important factor which determines the efficiency of pyrolysis in the rotary kiln reactors. Energy received by the charge at a given heating rate is determined by the residence time which is the function of volumetric flow and rotational speed of the kiln. A slow rotational speed of the kiln enables better waste mixing, thereby resulting in uniform products.

6.6 Microwave assisted reactors

Waste pyrolysis is also carried out by microwaves in reactors. The wavelength of microwave range between 1 mm to 1 m with corresponding frequencies between 300 GHz and 300 MHz, respectively. However, in microwave assisted reactors frequency of around 915 MHz and 2.45 GHz are used to degrade waste. Microwave assisted reactor are supplemented with numerous advantages such as uniform and fast heating, rapid start up and shut down, good controllability and high energy efficiency. These reactors do not require agitation. The major disadvantages of microwave assisted reactors include (1) the organic vapours generated during the process should be removed immediately to avoid cracking of products, (2) the process require particle of smaller size, (3) high operating cost as they are energy intensive process. These disadvantages limit the application of microwave assisted reactors on commercial scale. Paper, biomass and plastic waste are pyrolyzed using microwave assisted reactors.

6.7 Plasma reactors

Plasma is an ionized gas with negatively and positively charged ions generated by subjecting the gas to strong electromagnetic field. Plasma known as the fourth state of matter is distinguished into two types, i.e. the high temperature or fusion plasmas and the low temperature plasmas or gas discharges. Thermal plasma generation can be done by various methods such as radio frequency induction, microwave discharge, alternating and direct current. Plasma burns the waste rapidly, releases the volatile matter and the waste is cracked to hydrogen and hydrocarbons such as methane and acetylene. Plasma pyrolysis produces a combustible gas (H2, CO, C2H2, CH4, and C2H4) and solid residue. The heating value of these gases range between 4 to 9 MJ/Nm3. Plasma pyrolysis processes is easy to manage and it works effectively at low power. According to reports, the solid residue produced from polypropylene pyrolysis have application as high surface area catalysts, carbon adsorbent or electronic applications such as super capacitors.

6.8 Solar reactors

A transparent Pyrex balloon reactor with a graphite crucible insulated with black foam and located at the focus of a 1.5 kW vertical-axis solar furnace is used for wood pyrolysis. The reactor is operated under inert condition by supplying argon gas. The temperature of the unit is between 600 °C and 2000 °C. it does not require any additional heating sources and the process yielded 14% of char at 600 °C with a heating rate of 50 °C/s.

7.0 Advantages of pyrolysis

The advantages of pyrolysis solid waste include:

It degrades the biodegradable wastes and plastics using high temperatures
Energy is recovered as heat or electricity from waste. When compared to mass burning, energy and other value added products are recovered by advanced thermal treatment process, the pyrolysis.
High temperature involved in pyrolysis kills the pathogens and other harmful organisms in the solid waste
The water volume of the waste content is reduced
The gas produced during the thermal degradation of waste is used as fuel. It minimizes the use of external fuel if the producer fuel is used for pyrolysis operation
Biochar produced during pyrolysis can be used as a soil amendment or as activated carbon. Biochar sequesters carbon and increases the fertility of soil.
The bio-oil can be combusted directly in boilers and engines
It degrades the complex pollutants present in the solid waste
It favours the recycling of mixed plastics, soiled plastics (agricultural plastics, mulch/silage/greenhouse films and dripper/irrigation tube), plastic laminates, packaging materials that are difficult to be treated by traditional recycling methods
Minimizes the volume of waste and residual waste sent to the landfills
Pyrolysis plants are flexible due to their modular nature. The number of pyrolysis units can be added or reduced according to the requirement. Moreover, these units are easy to be built.
Pyrolysis units produce low air pollutant emission levels unlike incineration

8.0 Disadvantages of solid waste pyrolysis

The potential disadvantages of pyrolysis unit include

Pyrolysis unit can be used only for residual waste, failing which it will undermine the advantages of recycling and composting.
Feedstock of pyrolysis needs pre-treatment or pre-processing.
High capital and operational cost as they are energy intensive
Requires skilled operators for operating the units
Air pollution control devices are required to control air emissions
The ash generated during the thermal degradation might contains heavy metals and hence need safe disposal.

9.0 Summary

To summarize, in this module we have familiarized about

The concept of pyrolysis and its process description Mechanism of pyrolysis
Products produce from pyrolysis
Types of pyrolysis and various reactors used for pyrolysis Merits and demerits of pyrolysis

References

Williams, Paul T. (2013) Waste treatment and disposal, John Wiley Publishers.
George Tchobanoglous, Frank Kreith (2002) Handbook of Solid Waste Management, Second Edition, Mc Graw-Hill publication.
TV Ramachandran (2009), ‘Management of Municipal Solid Waste’. Centre for Ecological Sciences, IISc Karnataka research foundation.
www.nptel/Municipal solid waste management.com