WO2003031662A1

WO2003031662A1 - Process and apparatus for the treatment of contaminated fumes

Info

Publication number: WO2003031662A1
Application number: PCT/NO2002/000362
Authority: WO
Inventors: Geir Hilding
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 2001-10-10
Filing date: 2002-10-04
Publication date: 2003-04-17
Anticipated expiration: 2004-04-10
Also published as: NO20014924L; NO20014924D0

Abstract

Process and apparatus for the treatment of contaminated fumes from a furnace for melting contaminated scrap metal, in particular remelting and recycling of waste aluminium metal, the furnace including at least one melting chamber (1) with burners for heating the chamber (1) and melting the metal. The fumes from the melting chamber are led to a secondary chamber (2), including a mixing se ction (4) and incineration section (5), where the fumes are respectively mixed and treated under controlled temperature, oxygen content and moisture before being passed to a regenerator (6), further to a filter system (8) and finally to a stack (10). In a preferred embodyment ambient air is preheated by the regenerator (6) before being passed partly to burners (9) for the melting chamber (1) and an air heater (7) for the heating and supply of air to the secondary chamber (2)

Description

Process and apparatus for the treatment of contaminated fumes The present invention relates to a process and apparatus for the treatment of contaminated fumes from a furnace for melting contaminated scrap metal, in particular remelting and recycling of waste aluminium metal, the furnace including at lest one melting chamber with burners for heating the chamber and melting the metal.

In remelting and recycling operations a mixture of clean scrap metal and contaminated scrap metal has often been treated as clean metal. The contamination of the scrap metal may be oil, grease, paint, lacquer, thermal break insulation, plastics and organic matter gathered in the recycling chain.

However, as the legislation on incineration of waste is becoming more stringent with tighter limit values on toxic effluents, there is a demand for improved melting furnace systems with controlled process conditions and predictable effluents as well as improved operational procedures at the recycling operations.

Various melting furnace configurations are in operation for remelting and recycling of waste metal. Among the most common types of furnaces are reverbatory furnaces, dry heart furnaces, various configurations of two and multi chamber furnace systems such as side well furnaces and closed well furnaces, with and without liquid metal pumping systems. The selection of furnace system for a specific operation is often based on the type or format of the scrap materials to be melted. Generally heavy gauge materials can stand direct firing at higher temperatures, while thin gauge materials needs limited heat load and may be melted by liquid metal supplied by pumping systems to prevent unacceptable dross formation.

The heating systems for these various furnace systems include cold air burners, burners with combustion air preheating by means of recuperation and regeneration. In some cases oxygen enrichment or oxyfuel burner are in use.

Some operations are using a separate furnace system where the organic contamination is removed by heat treatment at temperatures below the melting point of the metal being treated. This process is complicated and must be designed to a narrow range of scrap metal formats and incoming scrap must often be shredded before the heat treatment can take place. The heat treated scrap can then be melted in furnaces designed for ordinary clean metal.

Many operators are considering the expenses and complications with a separate pre-treatment plant to be prohibitive and are seeking solutions to combine the removal of the contamination with the melting of the metal in the same furnace.

When combining thermal pre-treatment and melting in the same furnace one may encounter several contradicting requirements.

First of all are the requirements imposed by legislation regarding process operating conditions and toxic compound limit values in the effluent gases. Melting furnaces for scrap aluminium recycling are known to have high values of dioxins in the effluent gases and one may expect to have strict conditional requirements laid down in the operation permits for future recycling plants.

The requirements proposed in the new EU Directive on incineration of waste with respect to dioxins are two folded;

- First: When thermal treating of halogenated waste there is a set of process conditions that must be fulfilled; the incineration atmosphere shall contain at least 6 % (vol.) of oxygen, the temperature shall be at least 1100 degrees C and the residence time at these conditions shall be at least 2 seconds.

- Second: Maximum allowable average values of dioxins and furans in effluents at worst case operational conditions is 0.1 ng/Nm3 (O2 content 6%).

Furnaces of conventional design with cold air burners or a regenerative burners are very difficult to operate at the required 6 % oxygen content in furnace atmosphere at all times and maintaining the temperature at 1100 degrees C because most conventional burner systems are designed for operation fairly close to stoichiometric combustion conditions. When charging scrap metal with organic contamination at these process conditions will create an extremely fast release of volatile components that will result in furnace over pressure, extreme flames and smoke release. The controlled incineration conditions are lost for a considerable amount of time. A two chamber furnace where the thermal treatment of the scrap metal is performed in the well chamber at controlled temperature and the incineration of the volatile components are performed in a main heating chamber will also be difficult. The fumes from the pyrolysis reactions and products of combustion from the main heating burners are mixed and the oxygen content must be raised to 6% for the total mixture while maintaining at least 1100 degrees C. The consequences are large heat consumption and increased risk of unwanted dross formation due to high oxygen content. There is also very difficult to obtain homogenous distribution of the oxygen to the total gas volume and maintain the required residence time.

With the present invention a process and apparatus which aims at fulfilling possible coming official requirements on the incineration of waste, particularly process requirements regarding incineration of halogenated waste and the corresponding emission limits for dioxins, while utilising best available technology for heat recovery.

The process is characterised in that the fumes from the melting chamber are led to a secondary chamber, including a mixing section and incineration section, where the fumes are respectively mixed and treated under controlled temperature, oxygen content and moisture before being passed to a regenerator, further to a filter system and finally to a stack, as defined in the accompanying claim 1.

The Apparatus is further characterised by a secondary chamber for the treatment of the contaminated fumes being connected with the melting chamber, the secondary chamber including a mixing section for the mixing of heated air and the contaminated fumes and an incineration section combustion of the mixed gasses, as defined in the accompanying claim 5.

The invention will be further described by means of examples and with reference to the attached drawing showing a flow chart of the process.

This invention is using a different approach to solve the problems described for furnaces with combined thermal treatment of scrap metal and scrap melting. The base case, shown in the drawing consists of a melting chamber 1 which may be of conventional type. The main melting chamber 1 is connected to a secondary incineration chamber 2. A fume duct 3 between the chambers 1 and 2 has a damper that may be used to shut off the flow of fumes to the secondary chamber 2.

The secondary chamber 2 has two sections, an inlet section (mixing section) 4 and an incineration section 5.

The incineration section 5 is designed to obtain the required residence time of the flue gases at maximum design flow rate.

The inlet section 4 is a mixing section designed for good mixing of inflowing fumes to be incinerated and the air required to raise the oxygen content to 6%. The reason for using a defined mixing section 4 is to distribute the oxygen evenly in the gas stream to prevent channelling of partly un-reacted organic material through the incinerating section 5 that might otherwise occur because of the highly viscous atmosphere. Diluted air supplied to the mixing section 4 is heated in a regenerative heat recovery equipment 6. For the purpose of controlling the temperature in the incineration section 5 more energy is added to the air stream. The energy input is provided by combustible fuel (either gas or oil) burning at contact with the hot air in an air heater 7. Together with the combustible fuel, atomised water is also supplied to the heated air in the air heater 7, and thereby effectively supplying superheated steam to the mixing section 4. The reason for injecting water is to control the water vapour pressure in the incineration chamber 5 and thereby increase the combustion speed for aromatic hydrocarbons generated by the pyrolysis reactions in the melting chamber 1. Water may also be injected to the preheated combustion air to the main burners.

The following main control loops are required to control the secondary chamber 2:

- Water addition requirement is calculated from the mass flow leaving the secondary chamber 2 taking into account the hydrogen content of the combustion fuel.

- Hot air supply is controlled by oxygen analyses in the incineration section 5.

- Combustible fuel supply to the mixing section 4 is controlled by the temperature in the incineration section 5.

From the outlet of the incinerating section 5 the hot flue gas is transported in a duct to the inlet of the regenerator system 6 to rapid cool flue gas to an outlet temperature well below 200 degrees C. The rapid cooling of the flue gas will limit the recombination of trace quantities of aromatics and free chlorine to form dioxins and furans. The flue gas is further filtered in a conventional filter plant 8 with lime primed filter bags to limit dust emissions.

Ambient air is passed through the regenerator system 6 and will collect energy given off by the flue gas. The heated air is, as previously stated, used as dilution air in the secondary chamber 2 as well as combustion air to the main burners 9 in the melting chamber 1.

During charging of scrap metal to the main chamber 1 the duct 3 between the main chamber 1 and the secondary chamber 2 is shut off as long as the main charging door (not shown) is open. This is done to prevent cooling of the secondary chamber 2 and avoid disturbance of the process control. A certain minimum flow of dilution air is maintained in the secondary chamber 2 at all times.

When the main door in the melting chamber is being closed after charging of scrap metal, the damper 3 opens to connect the main chamber 1 to the secondary chamber 2. Hot air and combustible fuel are supplied to one burner 9 in the melting chamber 1 to maintain open flames for safety reasons should the temperature in the chamber go below 750 degrees C. Other burners 9 in the melting chamber are supplying hot air and water vapor to the ongoing pyrolysis reactions.

Supplied heat and hot air to the main melting chamber 1 during the pyrolysis period is controlled to give the desired reaction speed and progress. Estimation of amounts and properties of scrap contamination is performed based on mass and energy balance calculations and the transients measured in the secondary chamber 2 and can be used to control optimal amounts of scrap metal in subsequent charging to the main melting chamber 1 as well as heat and air supply to have a continuos transition from the pyrolysis phase in the main melting chamber 1 to ordinary melting. The invention as defined in the claims is not limited to the example shown in the drawing and described above. Thus the method and equipment may be used for any type of furnaces for melting aluminium scrap having one or more melting chambers.

Claims

Claims.

1. Process and apparatus for the treatment of contaminated fumes from a furnace for melting contaminated scrap metal, in particular remelting and recycling of waste aluminium metal, the furnace including at lest one melting chamber (1) with burners for heating the chamber (1) and melting the metal, characterized in that the fumes from the melting chamber are led to a secondary chamber (2), including a mixing section (4) and incineration section (5), where the fumes are respectively mixed and treated under controlled temperature, oxygen content and moisture before being passed to a regenerator (6), further to a filter system (8) and finally to a stack (10).

2. Process according to claim 1, characterized in that ambient air is preheated by the regenerator (6) before being passed partly to burners (9) for the melting chamber (1) and an air heater (7) for the heating and supply of air to the secondary chamber (2).

3. Process according to claims 1 and 2, characterized in that the air is heated by combustion of fuel supplied to the the air heater (7).

4. Process according to claims 1-3, characterized in that atomised water is supplied to the air heater (7) and the burners (9).

5. Apparatus for the treatment of contaminated fumes from a furnace for melting contaminated scrap metal, in particular remelting and recycling of waste aluminium metal, the furnace including at lest one melting chamber (1) with burners (9) for heating the chamber and melting the metal, characterized in a secondary chamber (2) for the treatment of the contaminated fumes being connected with the melting chamber (1), the secondary chamber including a mixing section (4) for the mixing of heated air and the contaminated fumes and an incineration section (5) for combustion of the mixed gasses.

6. Apparatus according to claim 5, characterized in that a regenerator (6) is connected with the secondary chamber (2) to utilise the heat from the combustion gasses for heating ambient air being supplied partly to the main burners (9) and partly to a air heater (7).

7. Apparatus according to claims 5 and 6, characterized in that the air and or water mist is supplied to the air heater (7) and burners (9) through separate supply means (11,12).

8. Apparatus according to claims 5-7, characterized in that a damper (3) is provided between the melting chamber (1) and the secondary chamber (2).

9. Apparatus according to claims 5- 8, characterized in that a filter device (8) is provided after the regenerator (6).