IntroductionThe plant has two nos. low resistance submerged arc smelting furnaces where the electrodes are deeply buried in a conductive charge mix, with micro arcing from the tips to a floating coke bed. The resistance to immersion depth relationship provides the basis for regulating the furnace power. The transformer secondary tap is set at the desired value and the electrodes are raised or lowered to maintain the set point resistance or impedance. Control of load resistance essentially controls furnace power. The electric arc furnace (EAF) is a major consumer of electrical energy. Reduction of energy consumption has therefore always been regarded as a vital priority. The laws of thermodynamics, which govern the reactions used, limit the reduction of energy necessary for the smelting process. The reduction of the overall energy consumption is therefore in most cases only possible by recovering the energy content of the hot furnace off-gas or optimizing the auxiliary utility equipments.

Concerning the energy usage, the disadvantage of the smelting furnaces used without energy recovery is the high amount of energy lost as CO in the off gas and as waste heat. For instance by producing ferro-silicon and silicon metal only about 32 % of the energy consumed is chemical energy in the product, that means about 68 % of the energy is lost as heat in the furnace off-gas.

Process integrated measures

Following the definition, best available technique shall mean the most effective and advanced stage in the development of activities and their methods of operation. In practical terms this refers to emission reduction and other environmental beneficial techniques that include both end-of pipe techniques and process integrated measures.

Process integrated measures are technical or operational solutions that can be introduced directly in the production to reduce the environmental impact of a process at the source. To identify such techniques the core process should be examined according to its input and output mass streams.

Figure 2 Main input and output mass steams to an electric arc furnace

The reduction of both the amount of dust and fume emitted and the energyconsumed by the process are linked as shown above by the smelting furnace as the heart of the process.

3.1 Process integrated measures to reduce dust emissions

Reducing the emission of dust and fume at source means reducing the off-gas volume. For example, the ferro-alloy industry still uses open furnaces and retrofitting with an appropriate hood in order to change the open furnace into a semi-closed furnace will reduce the off-gas volume. By applying a nearly closed hood it is possible to limit the infiltration of air, but at the same time supply enough air to combust the CO generated in the furnace. This will then lead to the following effects:

Reduced off-gas volume to be cleaned and consequently less dust emitted to air, which also means reduced specific emission factor for dust.

Reduced energy demand for the bag - filter plant. Reduced capital and operational costs for the bag - filter plant Increased off-gas temperature up to 500 ºC and with that the possibility

torecover the energy content from the hot off-gas.

3.2 Process Integrated measures to reduce the energy consumption

The operation of open furnaces leads to huge amounts of ambient air sucked into the furnace to burn the CO, which is generated by the smelting process. This consequently results in a very large volumetric flow of waste gas, which does not

allow the recovery of its energy content because the temperature level is low (100 – 150 ºC) and the flow rate large to build technically and economically efficient heat exchangers. To recover as much as possible from the process energy the off-gas volume needs to be reduced. This can be done as already described by installing a nearly closed hood to the furnace. A furnace hood reduces not only the off-gas volume, it increases also the temperature, which then makes energy recovery by using a waste heat boiler possible.

The positive effects are:Recovered energy can be used to produce electricity that can be used onand off site. Recovered energy can be used as steam on-site, for chilled water generationThe furnace hood can be integrated as superheater in the recovery boiler.The overall energy consumption of the plant will be reduced.Due to the physical properties of the raw material some ferro-alloys like ferrochrome and ferro-manganese can also be produced in totally closed furnaces. This reduces even more the off-gas volume, but generates off-gas that contains a high amount of CO. After dedusting the CO can be used as high quality fuel for a variety of purposes, which then combines the reduction of dust and the use of energy in a very efficient way.

4 BAT for process integrated measures

Taking into account the advantages mentioned before, providing the furnace with a nearly closed hood or depending on the raw material, closing the furnace completely, are regarded as process integrated BAT measures in the Non-Ferrous Metals Industries BREF for the production of ferro-alloys. Due to the increased offgas temperature and in the case of the closed furnace, the presence of CO, both allow the operation of an efficient energy recovery system or utilisation of the energy content of the CO. A combination of the BAT process integrated measures for the furnace and the energy recovery is presented in the following table.

Table no. Process integrated BAT measures to reduce dust emissions and energy consumption

4.1 Requirements for implementation of process integrated BAT measures

As shown in the previous part of the presentation, the process integrated BATmeasures for smelting furnaces and energy recovery go hand in hand. Otherwise, providing a furnace with a nearly closed hood without recovering the energy content from the high temperature off-gas requires an additional gas-cooling system, where then the main advantage of a semi-closed furnace has been lost. In the case of a closed furnace without energy recovery, the CO should be flared off, which indeed is a waste of energy.According to the different furnaces (semi-closed, closed and blast furnaces) the metals produced and the infrastructure of a plant there are several options to recover and use the energy from the off-gas. Taking into account the considerations given by Annex IV of the IPPC-Directive especially the cost and benefits of the measures, there are a couple of BATs considered to recover and use the energy.By implementing the above integrated BAT measures it is important to bear in mind that changing an open furnace into a semi-closed furnace or replacing the open furnace by a closed furnace and installing an energy recovery system means a large financial investment. This might probably be the most expensive financial investment a company can take. For the installation of a waste heat boiler we are talking about several million Euro. Due to the potentially high costs and the important technical impact in the production process, a window of

opportunity should be used to introduce such BAT measures. A smelting furnace can be operated without a major interruption for several years, and the time when the furnace needs to be significantly changed or replaced is the right moment to consider major changes. Also regular maintenance intervals or investment cycles might be used to introduce high costly investments.As already mentioned, for the installation and operation of an energy recovery system several million Euro needs to be spent. The payback time for such an investment is then dependent on factors like local energy prices, times of operation and the presence of potential customers for the steam and hot water produced. Beside the cost of investment, the economic assessment should also take these factors into account, especially the price of electrical energy is the critical point in the timing of introducing these process integrated BAT measures.

ENERGY SAVING OPPORTUNITY IN BAG FILTERS

The plant has installed three nos. battery of bag house filters with a dedicated one for each furnace and one new bag house common for both furnaces. The earlier installations are with a FD fan for the bag filters and the new common unit has an ID fan.

F1 Furnace bag filters:

The bag house filters have been provided with a FD fan of capacity 170 HP, 80000 m³/hr and sucks in a mixture of ambient air, charge dust and flue gases from the furnace. The dust laden air is filtered through the bag house filter via a cooler (operated in summer) and recirculated back to the chimney. Measurements on air flow static pressure indicate 650 mm WC at discharge of fan before bag filters and 70 mm WC after bag filters. Air flow estimations indicate about 25000 Nm³/hr flow after bag filters. Actual power consumed by FD fan motor is 51 kw. This indicates that the pressure drop across the bag house filters is about 580 mm WC which is very high. Similar readings were taken for bag house filters and table below gives the average values.

F1 Furnace F2 Furnace Common for F1 & F2

It can be seen from the above values that the pressure drop across the bag filters is exceptionally high. This is because the bag filters are choked with dust particles and collection efficiency is lower. This leads to increased restrictions for sucking flue gas and dust from the furnace and their escape to the ambient surroundings causing safety and environmental exposure risk that takes place for the people working on the operating floor.

One of the reasons for frequent choking of bag filters is that the compressed air used by plant is not dried and hence carries a lot of moisture along with it. This moisture condenses on the filter elements and forms a thick cake of dust particles which are then difficult to dislodge thus causing the plant to replace the filter. Attempts to open the compressed air receiver drain valve lead to lot of accumulated water being drained. It is recommended to immediately re-commission the compressed air dryers and maintain required dryness fraction of the compressed air. This will help reduce the cake formation, reduce resistance to air flow and extend the useful life of the bag filters.

Opportunity for Energy Saving by replacing Bag Filters with Electro Static Precipitators:

The plant has installed a set of bag filters for each furnace and an additional bag filter set common for both the furnaces. An ID fan / FD fan sucks in air and flue gases through the furnace and the bag filters and discharges the same to the chimney. Total fan power connected on the pollution control devices is about 457 KW. Based on measurements conducted on the fan motors actual power consumption is bout 166 KW. The pressure drop offered by the bag filters is given in the below table.

Bag filter F1furnace

F2furnace

Common for both furnaces

1 Mode F.D. F.D. I.D.1.1 Connected fan motor; KW 127 150 1801.2 Actual fan power consumed; KW 47 51 682 Pressure drop across bag filter;

mm WC580 360 NA (~13.5 + 2.5)

= 160 mm3 Total head developed by fan;

mm WC650 380 550

4 Volume of air handled; m³/hr 22500 22300 NA5 Quantity of ash collected; kg/day 56

The bag filters limit the flue gas emission to about 135 mg/Nm³. Also, the bag filters are prone to frequent choking thus requiring expensive filter replacements and breakdowns. The choking of bag filters causes insufficient suction pressure at the furnaces openings and inefficient removal of the gases causing dusty atmosphere at and above the furnace-operating floor. Table below gives the comparative norms for bag filter versus Electro Static Precipitator (ESP). The pressure drop across a bag house filter is about 200 mm WC and across an ESP about 25 mm WC. As per fan empirical laws the fan power varies as square of the discharge head. Thus, it is evident that fan power required for the same volume of air handling will be far less in an ESP. Although, the replacement cost of bag house filters with an ESP is very high the option can be evaluated by the plant management as a first cost measure for their other upcoming new installations or whenever they are considering replacing of the old filters.

Below table gives the savings potential by replacing these bag filters with an ESP and save fan power

1 Total air handled by all three no. fans of pollution control bag house filters

= 75000 m³/hr.

2 Dust collected by all bag filters (@ 7 kg/day per individual filter)

=

=

(7X5)+(7X6)+(7X5) 245.5 kg/hr.

3. Total power consumed by all three no. fans of pollution control bag house filters

= 166 KW

4 Pressure drop across bag filters = 360 & 580 mm WC5 Pressure drop across E.S.P. = 25 mm WC6 Estimated power consumed by fan

for common ESP=

=

75000 x 10 x 2.72 x 10 -5 0.632 KW

7 Power consumed by rectifiers of ESP = 15 KW 8 Total power consumption at new

(ESP) pollution control equipment= 47 KW

9 Savings potential in fan power ===

166 – 47 119 KWRs.33 lacs p.a.

10 Approximate cost of installing an ESP

= Rs. 50 lacs

11 Simple payback period = 18 months

Opportunities for saving in D. M. Water circulation for furnace :

The plant circulates D.M. water from a battery of 5 pumps to each furnace for internal cooling needs. Total flow measured for each furnace is about 50m 3/hr with a power consumption of about 43 KW for 5 nos. pumps. Thus, total power consumed for DM water pumping is about 86 kw in 10 nos. pumps. This gives an overall pump house efficiency of about 14%. The total discharge available at the plant header is not the sum of rated discharge capacities of all pumps because the pumps are pumping in parallel to a common header.

If the same flow required for both furnaces (i.e. about 100 m³/hr) is delivered by one single pump it is estimated that the plant will save about 70 KW of pumping power equivalent to Rs.19.4 lacs per annum. The existing 10 nos. pumps will act as stand by.

Below Table gives the brief savings calculation for saving in DM water pumping :

1 D.M. Water circulation through F1 furnace 50 m³/hr2 D.M. Water circulation through F2 furnace 50 m³/hr3 Total D. M. Water circulation 100 m³/hr4 Power consumed by 5 nos. pumps for F1 furnace 42 KW5 Power consumed by 5 nos. pumps for F2 furnace 44 KW6 Total pumping power 86 KW7 Actual pumping power required by one no. pump

for delivering 100 m³/hr flow at 45m head (pump efficiency 85% and motor efficiency 90%)

= 45 x 100 x 1000___ 3.67 x 105 x 0.85 x 0.9= 16.0 KW

8 Savings potential in pumping power = 86 – 16 = 70 KW9 Annual savings potential =70 x Rs.3.5/unit x 7920 hrs

p.a.= Rs.19.4 lacs

10 Approximate cost of pump set and piping cost upto header

= Rs. 2 lacs

11 Simple payback period = 1 to 2 months

Opportunities for savings in Cooling Tower make water :

Saving potential in DM water circulation network :

The plant has installed D. M. water Cooling Tower for cooling the D. M. water in circulation. Total return water to the cooling tower from both furnaces and transformer oil coolers is about 170 m³/hr (including about 40 m³/hr overhead tanks overflow). Thus, at a 2% evaporation and carryover loss from the cooling tower, the total makeup D. M. water to the cooling tower is about 81 m³/day, and the same also tallies with the D. M. plant regeneration log sheet. The total D. M. water generation cost is about Rs.50/m³, thus giving an annual D. M. plant operating expense of Rs. 12 lacs.

It is proposed to cool this D. M. water in circulation by indirect contact with raw water in a plate type heat exchanger. The raw water will pick up heat from the D. M. water and in turn will be cooled in the existing cooling tower. Additional raw material pumping power of about 12 KW equivalent to Rs. 3.3 lacs p.a. will be incurred. Net savings potential is to the tune of Rs. 8.7 lacs p.a. with an approximate investment cost of Rs. 4 lacs for installing a raw water pump set, necessary piping, plate type heat exchanger and miscellaneous supports etc. giving a simple payback period of 5.5 months.

Opportunities for savings in D.M. water circulation to Overhead storage tanks for Transformer Oil Cooling

The plant circulates D. M. water through individual overhead tanks to both the transformer oil coolers. Return hot water is cooled in common cooling tower and returned back to the overhead storage tanks (OST). Total D. M. water in circulation is about 70 m³/hr (35 m³/hr each from two nos. pumps for both the transformers) out of which about 40 m³/hr (20 m³/hr from each OST) was measured as overflow and balance through the oil coolers. Also, at times based on process conditions the plant needs to operate both the oil coolers on line for individual transformers. The overhead storage tanks are kept full incase of sudden tripping of grid supply to supply cooling water for furnace internal requirements till the time the stand by DG sets are brought on line. However, this overflow from both the tanks can be minimised by sharing of feed water pumps.

It is proposed to operate only one pump to feed both the overhead storage tanks and save on extra water overflow. Estimated savings by stopping one no. delivery pump is about 10 KW equivalent to an annual savings of Rs.2.8 lacs p.a. with a minor investment in piping modifications cost.

Alternatively, the plant can install auto level indicator cum controllers for both the overhead tank pumps to ensure the tanks are full as & when required.

Insulating Refractory lining

The term "freeze lining" refers to the refractory system's ability to maintain a temperature profile that is low enough to freeze a layer of process material on its hot face, which insulates the refractory and prevents direct contact with molten metal and slag. In doing so, the common wear mechanisms found in the submerged arc furnace - chemical attack, erosion, and thermal stress - can be prevented. These wear mechanisms are all related to high temperature; thus, they are prevented by maintaining low temperatures.

The expected minimum campaign life of the freeze lining is fifteen years as compared to the ten years of the conventional linings. 

The success of the freeze lining is very critical to the quantum and direction of heat flow through the lining that control the freeze protection. Under freeze protection can lead to reduced lining life and over freeze protection, on the other hand, can lead to operating difficulties and loss of smelting efficiency.

The lining, together with the electrodes, forms the heart of the operation of a submerged arc furnace.

The amount of water vapour generated during the combustion process has a significant impact on thermal efficiency because both the physically and chemically bond water must be evaporated. All water in the fuel leaves the furnace in vapour forms at between 150 °C and 300 °C and is also one cause of choking of bag filters. The water vapour condenses when it reaches the bag filters and sticks to embedded dust particles leading to cake formation.