1 Introduction

1.1 Overview

Waste is generated in every facility. The quantity of waste generated increases as the number of activities increases. As the number of occupants increases for residential apartments, the waste also generated increases. Waste might be in the form of solid, liquid, gas or a mixture of any three. The generated waste may also be organic or inorganic.

Waste generated in a facility must be appropriately disposed of. The conventional disposal method engaged is to use the waste for landfills. Due to the pollution, alternative means of treatment, recycling and reusing wastes have continuously been developed, including waste incineration.
This article aims to give a general overview of waste incineration, focusing on non-hazardous waste. Typical components of fixed hearth incinerator will be discussed.

This article aims to give a general overview of waste incineration, focusing on non-hazardous waste. Typical components of fixed hearth incinerator will be discussed.

Figure 1: Waste Dump

2 Waste

The definition of waste can vary depending on the context. Waste can be summarily described as a substance unsuitable for its original use. Materials and substances intended to be recycled may be termed waste by the holder, while the recycler may term them as materials to produce other materials.

Waste is generally categorized as hazardous and non-hazardous waste.

2.1 Hazardous Waste

Waste that has the potential to cause harm to the environment and human are termed hazardous waste. Hazardous waste is mostly generated from industrial production and activities. They require special handling and different treatment from non-hazardous waste.

The treatment and recycling of these types of waste are a function of their physical and chemical properties. The main characteristics of hazardous waste are toxicity, corrosiveness, ecotoxicity, flammability and explosiveness.

Hazardous liquid and powder waste require special treatment, handling and packaging to avoid dispersal of the waste.

A possible method of treating hazardous waste includes incineration or high-temperature treatment, recovery and recycling, Chemical treatment and safe storage. The focus of this article is on waste incineration. Below are some of the hazardous waste

2.1.1 Medical waste

These types of waste usually consist of medicines, pharmaceuticals, chemicals, bandages, body parts, body fluids, etc. Medical waste is very dangerous and can be infectious, containing harmful microorganisms such as bacteria, viruses, and fungi, including organisms that may be drug-resistant. These types of waste are usually incinerated at a very high temperature beyond which no microorganism can survive. Medical waste incineration is not performed inhabitable areas; they are usually incinerated in dedicated isolated areas to reduce contact with the waste.

2.1.2 E-Waste

E-waste may also be referred to as E-scrap, electronic waste. These terms describe all wastes associated with used electronics that have reached or neared the end of their useful life. They are usually appropriately discarded or recycled. Most electronics waste is supposed to be recycled, reused or refurbished to reduce the chances of ending up in landfills or improperly disposed of.

Unsafe handling of E-waste can harm human health and the environment. For example, open disposal and burning of electronic materials and components can result in harmful materials leaching into the environment and exposing people to high-level contaminants, including lead, mercury, arsenic, and cadmium, leading to various degrees of health complications, including cancer.

2.1.3 Hazardous Industrial Waste

These are waste resulting from industrial activities such as production activities. These types of waste are very common in oil and gas production and storage facilities. Usually, they contain a high quantity of hydrocarbon, which can be very harmful to people and the environment. A typical example of this type of waste is sludge recovered from liquid hydrocarbon storage tanks. This waste is recovered in large quantities and must be appropriately disposed of according to established regulations and procedures.

2.1.4 Radioactive Waste:

Radioactive waste is one of the most dangerous waste with the ability to cause complicated medical and genetic effects. The treatment, handling, and storage of radioactive waste are more complicated and, therefore, not covered in this article.

2.2 Non-Hazardous Waste

All waste that has not been classified as hazardous is called non-hazardous waste. These waste may include organic waste, beverage cans, plastics, papers etc. it should be noted that solid non-hazardous waste can have health and environmental impact if not properly collected, stored, treated and recycled. The major bulk of non-hazardous waste is categorized under municipal waste, which includes the following.

2.2.1 Organic Waste

A higher percentage of non-hazardous waste is organic waste.

Organic waste is waste resulting from biodegradable materials and comes from either a plant or an animal. Organic waste can be broken into carbon dioxide, methane or simple organic molecules. Typical examples of organic waste include food waste, food-soiled papers, green waste, wood waste etc. Most organic wastes are usually incinerated.

2.2.2 Packaging Waste

This waste results from packaging materials. The most common source of this is single-use food packaging, including plastic bottles, takeaway packs, disposable cups, disposable carrier bags, etc. A high volume of the materials is made of inorganic compounds. Packaging waste constitutes a menace in our environment, blocking drainages, water channels, etc. There has been a significant effort to reduce the number of packaging waste by encouraging recycling and reusing these materials.

Though packaging waste can be incinerated, the most efficient way of managing packaging waste is the reuse and recycling of packaging materials

2.2.3 Other Materials

Other waste non-hazardous waste include metals, glass etc. these materials though might t is not harmful to health they, constitute an environmental menace if not properly stored or disposed

3 Waste Incineration

Incineration is a waste treatment process that entails combusting waste material at a very high temperature. Incineration and other waste treatment processes that entail high temperature are also termed thermal treatment of waste. At the end of the combustion, the waste is converted into ash, flue gas and heat. The ash may be in the form of solid lumps or carried as particulates in the flue gas.

Before flue gas are released into the atmosphere, the gaseous and particulate pollutants must be removed before being disposed into the atmosphere. International and local regulations guide the installation and control of emissions from incinerators and gas exhaust sources. Some of these guidelines include EPA (the United States Environmental Protection Agency), Nigeria Federal Environmental Protection Agency Standard, Nigeria Federal Ministry of Environment Guidelines etc.

The heat energy from incineration can be recovered and used to generate electricity or heating. The conversion of the heat generated from incineration to other forms of energy is termed waste to energy.

At the end of the waste combustion, the solid mass of the waste may be reduced by about 80% to 85%, and the volume may be reduced to about 95% to 96% of the originally compressed volume by the garbage trucks. After combustion, leftover ash implies that incinerators do not automatically replace landfilling; it only significantly reduces the volume of waste.

Though there have been concerns and arguments about the environmental effect of incinerators resulting mostly from the exhaust gasses, modern incinerators have been equipped with emission treatment and monitoring units to minimise the actual amount of harmful gasses released into the environment. The emission monitoring and reduction system are also complemented by implementing waste segregation by ensuring that waste with a tendency to release more harmful chemicals is removed from the materials to be incinerated.

The benefits of incineration are even more for treating medical waste, which may contain pathogens and microorganisms that can cause health issues. These types of waste cannot be used in landfills and must be burnt in a controlled environment provided by incinerators.

Figure 2: Model of a Small Size Incinerator

3.1 Activities Associated with Waste Incineration

Before burning the waste in the incinerator, other activities are performed to achieve an efficient combustion process and reduce the release of harmful gasses into the atmosphere. The below section describe typical activities that are performed:

3.1.1 Waste Transportation

The waste is collected from all the facilities and transported to a dedicated segregation facility. In some cases, the waste might be segregated from the source, thereby eliminating transportation to segregation facilities. The waste is transported directly to the incinerating plant.

Figure 3: Waste Trucks Equipped with Compactor

3.1.2 Waste segregation

Depending on the strategy employed by the incinerator operators, waste segregation is performed to remove materials that should be recycled, reused, or released harmful chemicals into the atmosphere. Segregation might also be performed to separate waste by the amount of moisture contained, the possible amount of heating value etc. Liquid waste should be separated from solid waste. Most medical waste and harmful waste are segregated from sources to minimise multiple waste handling, which might expose workers to more harm.

3.1.3 Waste Compaction

Compaction is performed to reduce the volume of the waste. Most garbage trucks are equipped with a compressor which significantly reduces the volume of the waste. Also, at the incinerator site, compactors may further reduce the volume of the waste before they are fed into the incinerator.

3.1.4 Waste Incineration

Incineration is the actual combustion of the waste generating ash, flue gas and heat as already described.

3.2 What Type of Waste Can be incinerated

Below is a list of waste that can be incinerated; however, note that not all hazardous waste can be incinerated; some require special disposal, e.g. radioactive waste.

  • Animal Carcasses
  • Medical Waste
  • Hazardous Waste
  • Mining Waste
  • Camp Waste
  • Plastic
  • Waste Water

3.3 Types of Incinerators

Over the years, different types of incinerators have been built; however, the incinerators built in recent times from 2000 are more advanced than the previous. Though incinerators can be built and tailored to client needs, below are the types of incinerators

  • Rotary Kiln.
  • Fluidized Bed.
  • Moving Grate.
  • Liquid Injection.
  • Multiple Hearth.
  • Catalytic Combustion.
  • Waste-Gas Flare.
  • Fixed Grate / Direct-Flame.

Among the listed types of incinerators above, the rotary kiln incinerator is very popular in municipal waste treatment.

3.4 Components Description of an Incinerator

This section will briefly describe the components of a small to medium-sized fixed hearth incinerator suitable for camps, residential apartments, hospitals etc. These units can handle waste treatment up to 400 kg/hr.

Fixed hearth incinerators are controlled air, starved air, or pyrolytic incinerators. As the rotary kiln incinerator, they employ a two-stage combustion process.

They can utilise natural gas, diesel or appropriate liquid hydrocarbon as fuel and can be operated continuously or intermittently. They may also be equipped with an automatic feeder system, however, due to their small size, most systems are manually fed with waste.

Note that some of the components described here are optional and may not be found in all units; also, process parameters stated here might vary depending on requirements and vendor offered technology.

Below are the components of a standard incinerator.

Figure 4: Schematics of Waste Incineration System/Unit with Exhaust Abatement System

3.4.1 Primary Combustion Chamber

Combustion of the waste material occurs in the primary combustion chamber. Controlled amount of air and fuel is introduced into the chamber to commence combustion of waste and ensure low environmental impact.

The combustion temperature can be over 800oC to ensure the waste is completely reduced to ash. The primary chamber is mostly manufactured from steel plates and steel bracing of appropriate thickness.

The chamber may be provided with a simple access door, pneumatic or hydraulic ram loader, etc., through which the waste is introduced.

The chamber may be equipped with a single or double burner. The primary combustion chamber design temperatures may exceed 1400oC

The chamber is lined with the appropriate thickness of castable refractory material.

3.4.2 Secondary Combustion Chamber

After combustion in the primary chamber, the smoke passes into the heavy-duty secondary combustion chamber fitted with burners. The secondary chamber is mounted on top of the primary chamber. Complete combustion of the smoke and emission from the primary chamber is performed in the secondary chamber. To ensure complete combustion, the secondary combustion chamber operating temperature may be over 1100oC, while the design temperature may exceed 1500oC. Note operating the chamber at high temperature may be limited to about 1300oC to minimize nitrogen oxides production and lengthen equipment life. On the lower limit the chamber should be operated at a minimum temperature of about 850oC to achieve stabilized reaction rate needed to complete the combustion process.  Note burners may be natural gas-fired or liquid fuel-fired, the same for the primary combustion chamber. The temperature of the chamber is controlled by controlling the quantity of air injected into the system. The chamber should be designed to ensure that the residence time for the smoke and emission in the secondary chamber may range from 0.5 to 2 seconds, depending on the statutory regulation. Like the primary combustion chamber, the secondary chamber is mostly made of steel of appropriate thickness with appropriate refractory material lining.

3.4.3 Refractory materials

The refractory material can withstand temperatures over 1500oC. The typical thickness of refractory material used for primary and secondary combustion chambers is about 100mm. The refractory material used must be chemically and physically stable at high temperatures.

Common materials used in the manufacturing of refractories depending on the service temperature are the oxides of aluminium (Alumina), magnesium (Magnesia) and Silicon (Silica). Amongst the mentioned, Alumina is very prominent and used in many incinerators. It can withstand temperatures exceeding 1400oC.

The refractory material lining serves the following purpose

  • Thermal barrier between a hot medium and the steel plate wall of the chamber
  • Ensure efficient combustion
  • Withstand physical stresses and prevent erosion of steel plate due to the hot combustion temperature
  • Protecting against corrosion
  • Providing thermal insulation

3.4.4 Burners

Each chamber is equipped with a burner that may be gas-fired or liquid fuel-fired. Also, some systems may have more than one burner depending on the thermal capacity required, the type of waste and the nature.

The burners are automatically ignited to commence waste combustion after turning on the system. Some of the components of a typical gas burner include a monoblock with (filter, governor, gas pressure switch, pressure gauge, safety valve etc.) relay and UV supervisor, ionization electrode, gas burner control, burner motor, air pressure switch, ignition transformer and solenoid valve.

3.4.5 Fuel Source

Gas-fired incinerators must be connected to a gas supply source. Note the supplied gas pressure must not exceed that required by the incinerator. Some incinerators may be ordered with an inbuilt gas pressure regulator, or the customer shall make this provision before the gas is connected to the equipment. Incinerators using liquid fuel should be provided with fuel tanks connected to the burners. The customer shall provide all fuel piping to the incinerator.

3.4.6 Combustion Air Blowers (Forced Draft)

For combustion to occur, oxygen is required to complete the fire triangle. The connected FD centrifugal fans provide the required quantity of combustion air in the primary and secondary chambers. The combustion air supplied to the chambers is controlled to aid proper combustion in the two chambers and minimize particulate entrainment and carryover.

3.4.7 Ash Door

The primary and the secondary combustion chamber should be provided with an ash door to remove ash from the system.  An auto de-ashing system may also be provided for larger units where continuous operation is desired

3.4.8 Observation Port with Protective Glass

Internal combustion activities in the incinerator can be observed using this port. The observation port is mostly provided for the primary combustion chamber and may also be provided for the secondary combustion chamber.

3.4.9 Chimney

After combustion, the flue gas is sent into the atmosphere through the exhaust stack or chimney. Typical chimney height is a function of the regulatory requirements and building height within the vicinity of the incinerator. The stability of the chimney is also a factor to consider because small size incinerators are equipped with self-supported chimneys. A typical 100kg/hr incinerator may have a chimney of about 15m in height. The chimney should be considerably higher than the tallest building close by; otherwise, exhaust gas may find its way into the building. Also, medical and hazardous waste incinerators have higher chimneys than incinerators used for non-hazardous waste.

Chimney installed on a system with wet scrubbers should be lined with appropriate materials such as hard thick natural rubber or refractory to avoid corrosion due to oxygen and acids in the flue gas.

3.4.10 Optional Components

The optional components are provided to automate, safeguard the system, and ensure that exhaust gas quality does not exceed the values stipulated by the required regulations. Emergency Stack

An incinerator with exhaust abatement devices installed may be equipped with an emergency stack to exhaust hot gas directly into the atmosphere when a power failure occurs. This ensures that the hot gas bypasses the gas scrubber preventing it from being damaged. The stack should be refractory lined with appropriate material. Also, water from an emergency water tank may be directed simultaneously into the scrubbers to cool down the temperature as the hot gas flows through the emergency stack. Gas Scrubbing System

Depending on the emission regulation and the quality of exhaust gas desired, there are different gas scrubbers or flue gas treatment equipment. There are dry, semi-dry and wet scrubbing units. These units are usually tailored to requirements or proposed by the incinerator vendor. A typical flue gas scrubbing system may have the following units. Saturator

The saturator is the first scrubbing device. Flue gas from the secondary chamber travels into the saturator, reducing the gas temperature to about 250oC and also performing initial gas scrubbing. Wet Venturi Scrubber

The flue gas exits the saturator and enters the venturi scrubber, using water and caustic soda as a scrubbing medium. Particulate matter is removed, and the venturi scrubber also removes pollutants like HCL and SO2. Also, the temperature of the flue gas is brought down to about 80oC Mist Eliminator

The Mist eliminator removes moisture from the scrubbed gas from the wet Venturi scrubber. It is essential to ensure the moisture is removed and not carried into the induced draft fan. Tanks

There are various optional tanks associated with the scrubbing system.

These tanks include

  • Caustic soda tank
  • Recirculation water tanks
  • Emergency water tank. Scrubber Water Pump

Wet scrubbing systems should have at least two pumps; usually, centrifugal installed for water re-circulation. One is running while the other is on standby. Induced Draft Fan

The induced draft fan, usually centrifugal, draws out the scrubbed gas into the chimney and discharges it into the atmosphere. Auto De-Ashing System

This unit allows the incinerator to be operated for indefinite periods without requiring a shutdown for manual cleaning. The introduction of this unit reduces the frequency of start-up and burn-down cycles and associated increased use of fuel. Heat Recovery System

This optional unit, when installed, recovers heat from waste gasses and use for pre-heating. This recovery unit increases fuel efficiency and ensures complete combustion due to more stable high temperatures, with decreased fuel usage. Emission Monitoring System

Due to regulations requirements, all gases expelled from the incinerator are continuously monitored by installing an emission monitoring system. The emission constituent expelled must not exceed those stipulated by regulations. Gasses and properties monitored include Carbon monoxide (CO), smoke opacity etc.

When ordering an incinerator, request the Vendor the guaranteed emission limits and review against the regulatory requirements. Alternatively, you can provide the Vendor with the emission limits required.

Some of the emission limits to be provided by the Vendor include

Table 1: Emission Limits to be requested from Vendor Electrical and Control System

Modern incinerators are automated. Parameters such as combustion temperature, the quantity of fuel and air can be automatically adjusted. Also, the incinerator can automatically shut down when combustion is complete. Key automation in a modern incinerator detects an unwanted quantity of flue gas, generating an alarm and shutting down the system.

5 References

FEPA / FMENV S.1.15: National Environmental Protection Management of Solid and Hazardous Wastes Regulations, 1991.

Wikipedia: https://en.wikipedia.org/wiki/Incineration