The Electrical Arc Furnaces Engineering Essay

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Nava Bharat Ventures Limited, Paloncha has Electrical arc furnaces for production of Ferro alloys (Manganese, Silicon and Chromium alloys), which are essential inputs for manufacture of steel. Manganese and Silicon alloys impart strength and hardness and act as powerful deoxidizing agents, Chromium alloys make steel corrosion resistant and heat resistant.

The fume from the furnace is diluted with ambient air and cleaned by the gas cleaning plant before exhausted to atmosphere. One of the furnaces is equipped with radiant gas cooler. It is intended to save the heat, which is dissipated from the radiant cooler. It is proposed to install a double pass heat exchanger (LPHX-low pressure heat exchanger and HPHX-high pressure heat exchanger), that transfer the waste heat to Main condensate and feed water before the main condensate and feed water are preheated in the LP & HP heaters of 32MW Captive Power Plant.

Ferro Alloy Plant arrangement:

The plant consists of the Submerged Electric Arc Furnace (SEAF), Bag filter, Radiant Gas Cooler, ID fan and Stack.

The submerged arc furnace is a semi open furnace with a capacity of 27.6 MVA, in which the ores are melted. The off gas is emitted from the furnace. The quantity of the gas varies depending on the raw material. The dilution air is drawn from the openings around the furnace by the effort from ID fan, which is located at the downstream of the radiant gas cooler.

There are two chimneys directly connected to water-cooled furnace hood. These two chimneys are provided with butter fly dampers, which are normally kept under closed condition. Each chimney is provided with the branch connection to radiant gas cooler below the butter fly dampers. Under normal circumstances the gas goes through the radiant gas cooler.

Radiant gas cooler is an arrangement of gas carrying pipes. The heat from the gas is cooled by the natural radiation and convection to ambient air.

The gas after cooling is drawn by the ID fan and sent to the bag house to remove the dust in the gas stream. The bag house is provided with reverse air fan to clean the dusty bags. The bag house is with eight compartments out of which one compartment will be under cleaning at any time. The bag material is fiberglass.

Manufacturing Process:

The above alloys, known as bulk Ferro alloys, are manufactured by charging pre-determined quantities of raw materials consisting of ores, reductants and fluxes into submerged electric arc furnaces. The mix of raw materials depends on the specification of Ferro alloy to be produced.

High currents at low voltage are passed through the three electrodes of the furnace and the charge of raw materials. The resistance offered by the raw materials to the flow of electricity creates immense heat, resulting in smelting of the raw materials charged into the furnace and the consequent metallurgical reactions takes place.

Carbon in the reductant reacts with the oxides in the ores. The metallic content of the ore forms a Ferro alloy while the other gangue materials become slag. Both the Ferro alloy and the slag are in liquid form because of the high temperature in the furnace bath. Due to difference in densities the alloy and slag are separated. The density of slag is lower than the liquid metal, slag floats to the top.

At periodic intervals, the molten metal and slag are tapped out from the furnace bath through a tap hole. The liquid slag is granulated by impingement against a jet of water in case of Ferro chrome and Silico Manganese. The Ferro Manganese slag is reused for the production of Silico Manganese due to its high Manganese and low Phosphorous content.

The granulated slag is used for manufacture of fly ash bricks and concrete rings.

The liquid metal (i.e., the Ferro alloy produced) is collected in a ladle and cast into moulds in a continuous casting machine or powder beds as a cake. These cakes, after cooling, are broken down to the size specified by the customer, depending on the metallurgical practices followed by the customer. The sized material is packed in bags of 50 kilograms for domestic markets and one tone or loose for export markets in general and dispatched to the customers.

Thermal Power Plant arrangement:

Thermal Power Plant mainly consists of Boiler, ESP, Steam Turbine, Generator, Condenser, LP heater, HP heater, Boiler Feed Pump, Condensate Extraction Pump, Deaerator, Cooling Tower, CW & ACW pumps, Water treatment plant, Coal and ash handling plant.

Process of Power Generation:

The Thermal Power Plant employs steam turbine based power generation, which is the most widely used method for production of electricity from coal. In this system, water is used as a working fluid and is heated in a Boiler by burning coal, to produce steam, which, on further heating, becomes superheated steam having a high temperature of 530OC and a high pressure of 93 kg/cm2. This superheated steam runs the turbine, which converts heat energy into mechanical energy and drives an electrical generator coupled to it. The generator converts the mechanical energy into electric power.

The auxiliary system includes circulating water system, ash collection and ash handling system, coal handling system, electrical switchgear, transformers, etc.

The Boiler is of Eco-friendly FBC (Fluidized Bed Combustion) design. The required coal is crushed and screened in the Coal Handling Plant with the help of crushers & screens. This crushed coal is transported to boiler bunkers through conveyors. From the bunkers, the coal is fed into the boiler furnace.

Necessary air for combustion is pumped into the furnace by Primary and secondary air fans. Primary and Secondary air are heated in Air Pre-Heater by utilizing waste heat in fuel gases before fed to furnace for improving the Boiler efficiency and the flue gases are exhausted to atmosphere through ESP & Chimney.

Ash collected after combustion of coal in two different locations. One is under bed and the other is at ESP. Ash collected under bed is called as bottom ash or bed ash, which is conveyed to ash pond through slurry system and Fly ash at ESP is conveyed pneumatically to ash storage silo.

Required feed water in Boiler for steam generation is pumped from water treatment plant with the help of Boiler Feed Pump.

The steam turbine is a totally condensing type. After passing through the turbine, the steam is condensed to water in the condenser and pumped back to the boiler. This cycle is repeated. There are no discharges from this system except a very minor quantity of steam blown out of the steam circuit to maintain the technical quality of boiler feed water.

The steam Turbine has three extractions, one is for HP heater to heat the feed water, second is for Deaerator to deaeration process and third is for LP heater to heat the main condensate.

The cooling water system for condensing the steam is of circulating type. From the cooling tower sump, cool water is pumped to the turbine condenser where it picks up heat while condensing the steam and is pumped back to the cooling tower for cooling.

Waste Heat Recovery System

The company (NBV) has installed a heat exchanger to utilize the heat available in the furnace flue gases, for heating the boiler feed water and main condensate of 32 MW Captive Power Plant located in the same premises. The heat exchanger is of cross flow type with two passes. The first pass is called as HPHX and the second pass is called as LPHX. The flue gases are passed vertically downwards over bundles of horizontal water tubes in HPHX and upwards in LPHX. Feed water and main condensate are heated in HPHX and LPHX respectively. The flue gas coming from the furnace passes over HPHX first, later over LPHX and finally enters

Gas Cleaning Plant (GCP) through an ID fan.

WHRS is meant for utilizing the waste heat available in Submerged Electric Arc Furnace (SEAF) flue gases. It is installed in the down stream of furnace. This system consists of Low Pressure Heat Exchanger (LPHX) and High Pressure Heat Exchanger (HPHX) to heat the main condensate and feed water.

Constructional Features:

WHRS mainly consists of ducting from SEA furnace to heat exchanger, Double pass Heat exchanger, Ducting from Heat exchanger to GCP ID fan, Feed water & Main condensate lines from Power Plant.

Ducting from furnace to Heat exchanger: Submerged Electric Arc Furnace (SEAF) is having two chimneys of diameter 2000mm each with 65Mtrs height. A tap off (size 2000mm) dia from each chimney is taken horizontally at (+) 26.50Mtr elevation and made a common duct. After tapings, pneumatic operated isolation dampers are provided in two chimneys. The other end of common duct is connected to inlet of Heat exchanger. The diameter of common duct is 3000mm. One dilution damper of size 800mm is arranged in common duct to dilute the flue gas with fresh air. The chimney portion from furnace hood to isolation damper is made with SS304 of 5mm thick. Two branches & common duct are made with SS409M of 5mm thick. Complete ducting is provided with necessary expansion bellows (SS304) and 75mm thick insulation material. The operating & design temperatures of inlet duct are 380OC & 600OC respectively.

Double pass Heat exchanger: It is a two â€" pass cross flow type heat exchanger. First pass is called High Pressure Heat Exchanger (HPHX) and the second pass is called Low Pressure Heat Exchanger (LPHX). HPHX & LPHX consists of 3 modules each and having water tube coils inside the casing. Flue gases are passing over the water tube coils during operation. HPHX casing, tube coils & Headers are made with MS (except module â€" 1, i.e. SA387 GrII), SA210 GrA1 & SA106GrB material respectively. LPHX casing, tube coils & Headers are made with MS, SS304 & SA106 GrB material respectively. A by-pass duct (1000mm dia) with regulating isolation damper is provided in between HPHX & LPHX. Insulation material of 175 & 125mm thick is provided on HPHX & LPHX respectively.

Ducting from Heat exchanger to GCP ID Fan: It is 2200mm dia and made with MS material. LPHX bypass duct is connected to the outlet duct. Outlet duct is provided with one temperature transmitter & one draft transmitter. The ID fan inlet is having a multi louver damper for controlling the flow rate.

Feed Water line: A tap â€" off is taken from Power Plant feed water line after HP heater with a motor operated isolation valve (with bypass valve) and connected to HPHX. The outlet from HPHX is connected before the control valve in Power Plant feed water line with a motor operated isolation valve. One HPHX by-pass motor operated valve is provided near control valve. Insulation material of 75mm thick is provided over the complete piping.

Main Condensate line: A tap â€" off is taken from Main Condensate line of Power Plant after LP heater with a motor operated isolation valve (with bypass valve) and connected to LPHX. The outlet from LPHX is connected before the control valve in Power Plant Main Condensate line with a motor operated isolation valve. One LPHX bypass motor operated valve is provided near control valve. Insulation material of 50mm thick is provided over the complete piping.

Process :

Feed water coming from the Boiler Feed Pump with a temperature of 155OC is heated in HPHX with 380OC of flue gas coming out from SEA furnace, then the feed water temperature increases to 225OC and the flue gas temperature comes down to 226OC. This feed water line is connected to the Boiler for steam generation.

Main Condensate coming from Condensate Extraction Pump through Steam Jet Ejector & Gland Steam Condenser with a temperature of 89OC is heated in LPHX with a 226OC of flue gas, then the Main Condensate temperature will rise up to 123OC and the flue gas temperature will comes down to 160OC. This outlet of Main Condensate is connected to Deaerator inlet piping.

The flue gases are discharged to atmosphere, after utilizing the waste heat available in flue gases through chimney with the help of ID fan.

The extraction steam for HP & LP heaters will be cut off from Turbine i.e. HP & LP heaters are bypassed, while Waste Heat Recovery System is in service.

The gas under normal condition shall be taken through the HPHX & LPHX. When there are blows in the furnace the gas flow will increase, as the dilution air drawn by the ID fan will be more. This may result in high heat input to the WHRS. Hence the partially the gas has to be bypassed through the radiant gas coolers.

When the gas flow happens to be less at lower load operations, the LP heater may have to be partly bypassed on the gas side in order to keep the gas temperature at 160OC minimum.

As the heat duty is inadequate & minor fluctuations are expected in gas flow, it is proposed to put the heat exchangers in series with LP/HP heaters as shown in the scheme.

The control of the bypass dampers shall be located within the gas cleaning plant. The motor operated bypass valves will be actuated from this plant in the event of any leakage in HPHX / LPHX.

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