29 Advanced Thermal Treatment Technologies – Gasification

Dr. Yogalakshmi K. N

 

Objectives:

1. To understand the concept of advanced thermal treatment technologies with a special emphasis on gasification
2. To study about the gasification process and the products generated from them during solid waste thermal degradation
3. To understand the mechanism of gasification
4. To gain knowledge about the types of gasification process and various gasifiers involved in performing the gasification
5. To realize the merits and demerits of the process
6.   To differentiate gasification from incineration

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. Gasification is the thermal degradation of a waste materials in the limited supply of oxygen. This process uses temperatures between 900°C and 1400°C in order to break down waste materials. It results in the production of syngas, tar and other fertilizer value products.

2.0 Process description

Gasification is a process by which the solid and liquid waste materials are converted by the action of heat into gaseous material, ash and tar in the presence of limited supply of oxygen. It is a unique process by which the carbon in the municipal solid waste is converted into energy without burning them. The gases generated include carbon monoxide and hydrogen termed popularly as ‘syngas’. Other gases include carbon dioxide and methane. Air, pure oxygen and steam are used for gasification. Gasification occurs in a wide range of temperature (i.e) 800 – 1400 ⁰C. According to the use of the supplement for burning the waste, the calorific value of the gas varies and ranges from as low as 4 MJ/m3 (when air is used) to as high as 20 MJ/m3 (when steam is used). The calorific value of gases can be compared to natural gas which is about 37 MJ/m3.

The mechanism involved in gasification is that at limited supply of oxygen carbon molecules in the waste is oxidized into carbon monoxide. Carbon monoxide can also be formed from reduction of carbon dioxide and water present on the solid carbon surface. Gasification is always preceded by pyrolysis and hence char like organic material seems to be the feed for the gasification. Gases interact with the solid char interface to extract carbon off of the surface into gases such as methane, carbon monoxide and carbon dioxide. Gaseous H2O can interact with the char surface to be reduced into gaseous H2.

3.0 Types of gasification: Gasification is classified into following types

3.1 Air gasification: When air is used for gasification the temperature used is around 800-1100 ⁰C. Air gasification produces a fuel of low calorific value in the range of 4-6 MJ/m3.

The moisture present in the air produces hydrogen. In addition to other gases, nitrogen also occurs as one of the major component in addition to carbon monoxide, carbon dioxide and methane. Nitrogen reduces the calorific value of the gas.

3.2 Oxygen gasification: Pure oxygen is used for gasification. The temperature is relatively high and is around 1000 – 1400 ⁰C. The calorific value of the fuel is medium and is around 10-15 MJ/m3.

3.3 Steam gasification: Steam is usually used as a supplement to oxygen to control the temperature. It is an endothermic process and is operated at a temperature of 700-900 ⁰C. Steam gasification is also operated under pressure of 20 bar at temperature of 700-900 ⁰C. Under pressure, the steam gasification is a exothermic process. The calorific value of fuel is around 15-20 MJ/m3.

In high pressure steam gasification, certain additional reactions occur. They include

3.4 Gasification with hydrogen: Gasification with pure hydrogen also termed as hydrogasification is considered to be a better alternative for air gasification system. It cracks the tar in the gasification vessel and results in the formation of methane rich gas. Methane can later be converted to hydrogen or methanol.

3.5 Thermal depolymerization: In this process, high energy microwaves are used to burn the waste in nitrogen atmosphere. The waste absorbs the microwave and increases the internal temperature and energy of organic material thereby resulting in thermal decomposition of waste. Nitrogen inhibits combustion. Thermal depolymerization is carried out at temperature 150 to 350⁰C

3.6 Plasma gasification: Plasma gasification is carried out at very high temperature of 5500 ⁰C. Plasma is an ionized gas formed when the electric discharge is passed through a gas. In plasma, the electrical energy is converted into intense heat (i. e) thermal energy. Plasma destroys the hazardous waste. Even the ash generated in incinerators are destroyed or converted into non-leachable gas in plasma gasification. Plasma gasifier follows the same mechanism of other gasifiers. However, the rate of reaction is very fast due to the temperature generated by the plasma. The chemical bonds break down and results in the formation of synthesis gas. The inorganic wastes melt and fuse to form a slag. The energy or the internal heat supplied to the waste in the gasifier depends on the quantity of waste. Plasma gasification is generally used in places where the landfill cost and demand of renewable energy is high.

4.0 Types of gasifiers

• Gasifier are selected on the basis of
• available fuel quality capacity
• gas quality conditions.

Gasifiers are broadly classified into two types namely: fixed beds and fluidized beds. Usually gasification of municipal solid waste is done in larger capacity gasifiers. Variable fuel feed, uniform process temperatures, high turbulent, good interaction between gases and solids, and high carbon conversion make large gasifiers more advantageous than the small ones.

4.1 Fixed Beds

Fixed bed gasifiers are easy and simple to operate. They are commonly used in small and medium scale power generation. Fixed bed gasifiers are provided with grate to support the feed material and maintain a stationary reaction zone. Updraft and downdraft gasifiers are example of fixed bed reactors.

4.1.1 Updraft gasifiers: Updraft gasifiers are also called as counter current gasification. In this reactor, the fuel (i. e) waste is added from the top and the gasifying agent (i. e) air, steam or oxygen is injected to the system from the bottom. The waste and air flows in the opposite direction (i. e) counter current. The syngas produced during the process leaves the gasifier from the upper side. Tar and other volatile products also leave the gasifier along with the gas. Gasification occurs in fixed bed. Inside the gasifier a clear zonation is observed. Combustion occurs near the bottom of the gasifier, above which reduction reactions occur. Heating and pyrolysis occurs at the upper part of gasifier due to forced convention and radiation from the lower zones. Updraft gasifiers result in less thermal breakdown of tar because the tar, moisture and the gases generated do not pass through the char bed. Henceforth the product gas consists of relatively high amount of tar. At certain times, to increase the efficiency of the process, the tar can be collected, condensed and recycled to the gasifier for better efficiency. Tar that escapes through gas can be removed through cyclone separator or candle filter.

Source: http://www.indiamart.com/nilkanthengineers/gasifiers.html#updraught-gasifier

Figure 1 Updraft gasifier

Advantages: The major advantages include

• simple
• high waste burning and internal heat exchange resulting in low temperature exit gas high equipment efficiency.
• Varied feed stock can be burnt

Disadvantages:

• less thermal breakdown of tar dangerous
• might cause explosive situations requires automatic moving grate.

4.1.2 Downdraft gasifiers:

Downdraft gasifier is also termed as co-current gasifier. This system is designed to overcome the drawbacks of updraft gasifiers. Updraft gasifiers faces the problem of tar entrainment in product gas. Thus the gasifying agent (i. e) air is injected at or above the oxidation zone through a throat or narrow section in the gasifier. The waste and the air flows in the same direction (i.e)   co-currently. Unlike updraft gasifier, a complete breakdown of tar and hydrocarbons is observed in downdraft reactors as they move through high oxidation zones. Increased levels of hydrogen and light hydrocarbons are observed in the gasifiers.

Source: http://www.enggcyclopedia.com/2012/01/types-gasifier/

Figure 2 Downdraft gasifier

Advantages:

• Production of tar free gas
• high conversion of pyrolysis intermediates
• Downdraft gasifiers are not ideal for waste treatment because they require a low ash fuel

Disadvantages:

• Cannot be operated for different feedstocks
• Problems of flow and pressure drop is observed in downdraft reactors lower efficiency
• product gas has low calorific value.

4.2 Slagging fixed bed gasifier: These gasifiers are operated under high-pressure. High temperature breaks down the tar and hydrocarbons to light gas. The residue is also converted into molten slag. They have commercial applications.

4.3 Fluidized bed gasifiers: Fluidized bed reactors consist of a bed of materials namely coarse sand or limestone through which gasifying agent (i. e) air or steam is passes through in an upward direction. The gasifying agent acts as a fluidizing medium and oxidant for cracking the tar. Through an auger or feed chute, the waste is fed from the top of the gasifier. Good mixing, efficient reactions, high heat transfer, good operational control and compactness of the chamber are advantages of the fluidized bed gasifiers. Fluidized bed gasifiers are mainly used for generators of capacity 10 MW. Twin fluidized bed reactors are also used for gasification where the first bed is used to gasify the waste and the other is used for the combustion of char. Two types of fluidized beds are popularly used. They are: bubbling and circulating fluidized beds.

Bubbling Fluidized Bed (BFB) consist of a cylindrical or rectangular chamber and is operated at a temperature of 900° to 1000 °C. The chamber shape is designed to facilitate contact between gas and solid to facilitate drying and size reduction through attrition. Air or steam is injected into the chamber at high velocity to fluidize, expand, lift and bubble the bed material. During such condition, the organics in the waste is vaporized by pyrolysis and later combusted.

Circulating fluidized bed (CFB) uses high fluidization velocities. The solids are elutriated, separated and recycled back to the gasifier to improve the performance of gasifiers. No clear distinction of the dense solids zone and the dilute solids zone is observed. This reactor can be used for a variety of feedstocks.

Source: http://www.soi.wide.ad.jp/class/20070041/slides/04/42.html

Figure 3 Fluidized bed gasifier

4.4 Entrained flow gasifiers: In this type of gasifiers, steam or air is introduced along with the waste in a vertical chamber. The gasification reactions occur in the gas suspension. The gasifiers are operated under low residence time, high temperature and pressure. Both liquid and solid waste can be thermally degraded within this gasifier. The conversion rate of waste to gas is very high. Low tar gas is the characteristic of entrained flow gasifiers.

4.5 Rotary kiln gasifiers: Rotary kilns consists of an inclined cylinder rotating at a slow speed. The walls of the gasifiers are lined with ceramic. The waste is introduced from the top and is slowly gasified as it moves down the cylinder. The retention is longer when compared to other gasifiers.

5.0 Feed stocks used:

The feed stocks that can be used in gasification to produce energy include

• Wood waste such as sawdust, twigs and bark
• Agricultural waste – crop residue, corn stalks, spoiled food grains
• Biosolids of wastewater treatment plant Municipal solid waste
• plastic wastes
• Animal wastes such as stall wastes Blends of the various feedstocks

Inorganic waste or recyclables such as glass and metals can be burnt by the gasification process. Hence the feedstock needs pre-processing or screening to remove the recyclables and inorganic materials. Shredding of feed stock can improve the efficiency of treatment of municipal solid waste. Moreover, moisture removal also improves the efficiency of gasifiers. Thus, the waste or the feed stocks should be dried before being fed into the gasifiers.

6.0 Benefits of gasification:

• Requirement of landfill space is reduced
• Reduces the transportation cost of waste being taken to landfills
• Minimizes the methane emission from landfills during the decomposition of waste
• Reduces the risk of contamination of water bodies (surface and ground water)
• Syngas, a usable energy product is produced
• End product is a high commercial value fertilizers and chemicals
• Enhances existing recycling programs
• Reduces the use of fossil fuels
• Tar, a valuable product is produced
• The volume of the waste is reduced
• Plastics that cannot be recycled can be brunt in gasification process
• low oxygen environment limits the formation of dioxins and large quantities of SOx and NOx.
• Requires small and less expensive gas cleaning equipment due to less gas production
• lower gas volume also means a higher partial pressure of contaminants in the off-gas and hence favours complete adsorption and particulate capture

7.0 Disadvantages of Gasification

• The tars, heavy metals, halogens and alkaline compounds that are released in the gas often causes environmental and operational problems.
• Tar produced during the process is high molecular weight organic gases. It possesses potential to destroy the sulfur removal systems, ceramic filters etc. They also lead to formation of slag in the boilers and on the refractory surfaces of the gasifiers.
• Presence of alkalis in the waste might affect the turbines during combustion
• Likewise, halogens will corrode the walls of gasifiers and might lead to acid rain when emitted.

8.0 Gasification and incineration:

Difference between incineration and gasification is elaborated below

9.0 Summary

To summarize, in this module we have familiarized about

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

References

• Williams, Paul T. (2013) Waste treatment and disposal, John Wiley Publishers.
• http://www.enggcyclopedia.com/2012/01/types-gasifier/
• http://www.soi.wide.ad.jp/class/20070041/slides/04/42.html