There is a lot of potential for energy saving in the incineration of hazardous waste
16. July 2020
Steam engines and turbines

How to turn hazardous waste into electricity

By Serap Dogan
Readtime 6 min.
Keywords #steam compression #environment #heat pump #process heat #energiy efficiency

Table of contents

  • Incineration of hazardous waste at above 1000°C
  • This ensures a complete burnout
  • Steam engines or steam turbines are used to generate electricity
  • Project example from France with enormous savings potential

Incineration of hazardous waste at above 1000°C

A particular challenge is the incineration of hazardous waste. Hazardous waste includes all combustible, explosive and toxic waste generated in industry and in normal households, for which high temperatures are necessary, as for these organic compounds: Pesticides, solvents, paints, acids, batteries, infectious hospital waste, animal carcasses or medicines. The disposal of these environmentally harmful wastes usually takes place by incineration at higher temperatures well above 1000°C. Incineration reduces the volume of the waste and, above all, the substances are rendered harmless. Hexachlorobenzene, fluorinated hydrocarbons (CFCs) or polychlorinated biphenyls (PCBs) are particularly stable. If these are heated to over 1100 degrees Celsius for more than two seconds, the molecules literally fly apart and are rendered harmless.
So-called rotary kilns are used in hazardous waste incineration plants - a proven technology whose beginnings go back 40 years and which is designed to incinerate more than 2 tons of hazardous waste per hour. In these tubes, the waste is thoroughly mixed and enriched with atmospheric oxygen, which is how the particularly high temperatures are achieved. In this technique, the waste is fed into the upper end of a sloping and slowly rotating pipe of huge dimensions. The length of such a rotary kiln for waste incineration can be up to 12 metres, while the diameter is between four and five metres. The inclined position makes it easier to transport the waste and slag.

This ensures a complete burnout

Classically, a hazardous waste incinerator with rotary kiln plant consists of separate waste storage, the feeding devices for all types of waste, the rotary kiln, an afterburner chamber, a waste heat steam generator and a waste gas cleaning system. The hazardous waste is fed into the rotary kiln at the front end of the rotary kiln either via a classic waste feed hopper, via an automatic drum lift for containers or via a feed lance for gaseous, liquid or pasty waste directly into the rotary kiln. Depending on suitability or calorific value, liquid and gaseous waste can also be fed into the afterburner chamber.
In the downstream afterburner chamber, the combustion gases and thus residual organic matter, entrained solid particles, are completely afterburned. Depending on their suitability, liquid waste and gaseous pollutants can be fed directly into the afterburner chamber at its beginning. This also ensures complete burnout here.

The process of hazardous waste incineration holds a potential for energy generation that is often unused: For example, the cooling process of the exhaust gases produces steam. The waste heat of the steam is often discharged unused into the atmosphere via air-cooled steam condensers.

Steam engines or steam turbines are used to generate electricity

Spilling has two interesting solutions to offer to plant operators: Instead of operating a multi-stage turbine with the available saturated steam, which often has a rather moderate steam quality, e.g. with regard to moisture content, which is a real challenge, Spilling steam engines or Spilling single-stage steam turbines have proven themselves.
With over 50 years of experience with saturated steam applications, Spilling combines the power plants with the necessary peripherals, such as suitable steam dryers, as required. In this way, the plants operate extremely reliably and help considerably in the generation of green electricity and the reduction of CO2 emissions.
Since the plants are also usually operated as backpressure plants, in which the steam is still available at above 100°C at the outlet from the power plant, the remaining heat in the exhaust steam can often be used for operational heating purposes

Project example from France with enormous savings potential

At a hazardous waste incineration plant north-east of Paris, 16.5 tonnes of saturated steam per hour at 30 barg are produced from the waste gas heat by means of a steam boiler. This high-pressure steam is expanded by a steam engine to an outlet pressure of
0.5 barg, which allows an electrical output of 1250 kW(el) to be generated.
16.5 t/h of saturated steam corresponds to a heat output of about 10,800 kW(th). The French hazardous waste incineration plant achieves an electricity output of approx. 8,750 MWh/a at approx. 7,500 annual operating hours and almost permanent full load operation.

Profitability analysis of steam engine in hazardous waste incineration plant with excess steam:

Characteristics for steam engine from 30 to 0.5 barg, 16.5 t/h steam flow rate     
   
Kenndaten für Dampfmotor von 30 auf 0,5 bara, 16,5 t/h Dampfdurchsatz      
electrical power output, max. 1.25 MW
assumed hours of full use 7,000 h/year
Investment costs    
   Steam engine with accessories: 980,000 Euro
   Periphery (buildings, pipeline construction etc.): 550,000 Euro
Operating costs steam engine    
   Maintenance costs 70,000 Euro/year
   Energy costs (heat)*: 0 Euro/year
Operating cost savings    
   Reduced costs of electricity purchases: 700,000 Euro/year
   Savings CO2 tax:  126,000 Euro/year
Total annual savings 756,000 Euro/year
Total annual CO2 savings 4,200 tons/year
Payback period 2.0 years
Basis:
Costs: steam: 30 Euro/MWh; electricity: 80 Euro/MWh; CO2: 30 Euro/t
   
CO2 emissions: Steam (with gas boiler): 0.22 t/MWh; Electricity:
0.48 t/MWh
   
*if excess steam is involved, the value can be set to zero. This results in significantly higher CO2 and cost savings.    

Our experts also have suitable solution proposals for your application. You can contact us here.


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