Incineration

Technologies: Anaerobic Digestion, Bioethanol Production, Incineration

Introduction

The incineration of waste is on of the oldest disposal strategies with evidence of waste from the Stone Age having been incinerated. In recent times however incineration of waste has been used as an energy source. Leaders in the use of this technology have implemented systems where their municipal (and clinical/some hazardous) waste is incinerated, reducing the volume of waste by greater than 90%[1], pollutants and particulates are removed from flue gases, electric energy is produced and supplied to the National Grid and/or heat is supplied to through a district heating grid. Scotland has recently implemented incineration as energy from waste with two facilities one providing power to Dundee and another supplying heat to homes in Shetland.



The Process

The process is similar to the combustion of coal to generate power in that waste is combusted to generate heat to create stream and drive an turbine. Incineration of waste is the thermal degradation of combustible components of MSW including food, plastics, garden, paper and card waste. Residual waste will be comprised of ash and non-combustible portion of the waste including metals and glass particles. The components of the waste which are burned generate flue gases, ash and heat. The heat of the flue gases will be used to heat water to drive steam turbines.



Application to Municipal Waste

One of the benefits of incineration is that there is little need to segregate waste into its components. Combustion can occur readily in a mixed state and separation of the clinker from the residue ash will allow recycling of metals. Ideally the use of incineration would, however occur with a segregated collection system whereby metals are removed for recycling at the household level.

Collection of the waste can continue as for landfill however the collection vehicles would deposit the waste at incinerator facilities. At the facility the waste is input to the incinerator furnace. The furnace is fed with fuel and oxygen and generates flue gas at temperatures of around 1100oC. The flue gas enters the heat exchange where it heats water to around 400oC, this is used to drive the turbines in the generator .The gas is now cooled below 200oC and treated for contaminates before being emitted to the atmosphere.

Incinerators generally comprise[2]
·    Waste feed
·    Primary Combustion Chamber
·    Secondary Combustion Chamber
·    Heat Recovery System
·    Air Pollution Control System
·    Fan and Stack
·    Ash Handling System
 
From the incineration process there are several outputs gases and solid waste and heat. A number of gases will be produced notably CO2 and sulphur oxides. There are public concerns over the environmental and health issues these releases could cause, however these have decreased with the improvement of flue gas purifying methods[3]. The solid waste is comprised of ash and clinker ( incombustible materials) which can be sorted to remove recyclables and the remainder will be sent to landfill. It is important to state that under 10% of the volume of material will be sent to landfill. The heat is the desired product of the incineration which will be used to generate power and supply heat to the public.

Current Research

The science of incineration itself is well known however the topic is still researched from the perspective of improving efficiencies within the system, treating ash residue and of measuring and decreasing the contaminants in the flue gases.

Particular study is dedicated to the removal of dioxins and the variation that can exist due to temperature and removal method. Dioxins are a group of chemicals that are known to increase the likelihood of cancer[4]. Removal methods include injected and fixed bed activated carbon and studdies have shown that effectively removal can be over 98%[5,6].

Modelling has been conducted of the incineration processes to evaluate the efficiency of waste combustion. Models and methods vary considerably from article to article and each must be taken on its merits.

Studies into ash treatment for MSW incinerators have focussed on characterisation of the ash and variances due to waste source and to treatment of heavy metals[6]. Novel research has been conducted into reuse of the ash as concrete aggregate[7] and blended with polyurethane[8].



 


References
1- Coalition of Responsible Waste Incineration can be viewed at http://www.crwi.org/textfiles/process.htm
2-Process of Incineration, Dept. Water Affairs and Forrestry, S.A. can be viewed at http://www.dwaf.gov.za/Documents/ Policies/WDD/Waste_Incineration_Processs39.pdf.
3- UK Parliment Post Note 149 Dec 2000 can be viewed at http://www.parliament.uk/post/pn149.pdf 

4- BBC Medical Notes 3rdJune 1999 http://news.bbc.co.uk/1/hi/health/medical_notes/ 358889.stm#top
5- Chi, K H; 2006, Evaluation of PCDD/F partitioning between vapor and solid phases in MWI flue gases with Temperature Variation, Journal of Hazardous Materials 138 (3), Elseveir
6- Wang, H C; 2007, Formation and removal of PSDD/Fs in a municipal waste incinerator during different operating periods, Chemosphere 67 (9)
7- Qiao, XC; 2008, Production of lightweight concrete using incinerator bottom ash, CONSTRUCTION AND BUILDING MATERIALS 22 (4)
8-Chow, JD; 2008, Recycling and application characteristics of fly ash from municipal solid waste incinerator blended with polyurethane foam, ENVIRONMENTAL ENGINEERING SCIENCE 25 (4)

Pictures courtesy of
1-Moving Grate Incinerator -www.slate.com/id/2181083/
2-Diagram of Rotary Kiln Waste Incinerator - www.kureha-eng.co.jp/.../actual_result.htm


Anaerobic Digestion Technologies Bioethanol Production

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