Steel production processes typically dispose large volumes of coke gas and other speciality gases that, until recently, had been mainly used to produce steam in gas fired boilers. The utilization of speciality gases as engine fuel to generate mainly electrical power is certainly more efficient. However, a special engine design is required in order to use such gases with different compositions and thus different calorific values and combustion behaviours.
Steel production processes and the resulting waste gases
The different types of waste gases created during various steel production processes are shown in the figure opposite.
Several factories involved in the steel industry in northern Spain are serving as a global showcase for pioneering Jenbacher gas engine/cogeneration systems that utilize waste gases to support onsite operations, while increasing the factories’ energy efficiency and reducing their emissions levels.
GE Energy supplied two Jenbacher cogeneration units to Industrial Química del Nalón (IQNESA), the largest Spanish producer of high quality foundry coke, for the combined production of heat and power, utilizing coke gas as engine fuel. The power plant is located in Sama de Langreo, about 450 km north of Madrid.
Industrial Química del Nalón, a developer and manufacturer of chemical products with a focus on coal and its derivatives, owns both Jenbacher JMS 620 GS-S/N.L modules commissioned in December 2003 and supplies its excess coke gas to the power plant, which generates power and heat to support other factory onsite energy needs, such as for steam and products treatment processes.
Coke gas is a by-product of industrial coke production from pit coal by means of high temperature dry distillation of coking coals in the absence of oxygen. It consists mainly of hydrogen (50-60 per cent). Its high hydrogen content makes its combustion process very fast, with danger of knocking for the engine.
To avoid such risk, the engine control has to be able to fuel the engine with a very lean mixture, simultaneously providing the capability to react very quickly to variations in the engine load.
Commissioned in December 2003, each Jenbacher JMS 620 GS-S/N.L unit provides an electrical output of 1970 kW and a thermal output of 1120 kW. Both Jenbacher plants are equipped to run either on coke gas from the factory’s 22 coke ovens, natural gas or on a mixture of both – down to 30 per cent natural gas.
The coke factory creates more than 120 000 tons of coke annually. Utilizing 70 per cent of the released coke gas in the Jenbacher units generates 58 million kWh annually, which is equivalent to about 13 million m3N of natural gas.
Unique gas supply
The gas supply is provided by a unique gas train. The gas trains are designed for a gas flow of approximately 1200 m3N/h with DN 200 connection, including a flame arrester. This gas train is able to run on all three types of gases (coke gas, natural or a mixture of both) and is equipped with special materials for the coke gas application. Additionally, the gas train involves trace heating, avoiding the tar condensation within the gas train.
Two mixing stations, one per engine, provide the customer with the possibility of running each engine on different mixtures at the same time.
The Jenbacher units have two separate cooling circuits. One is a high-temperature circuit (HTC with 100 per cent coke gas/1.150 kW per engine), the other a low-temperature circuit (approximately 350 kW per engine). Part of the heat generated in the high-temperature circuit is recovered through a plate heat exchanger to pre-heat water for the boiler. The rest is dissipated in two radiators. All the heat generated in the low-temperature circuit is dissipated in two open cooling towers.
The exhaust gas heat is recovered individually through two exhaust air/gas-heat exchangers, allowing a safer use of the exhaust gas heat. The exhaust heat of one of both installed Jenbacher units is used for steam production, which is further utilized within some sub products’ treatment processes (e.g. cleaning) as well as for the plant’s central heating system. The exhaust heat of the second installed engine is used for drying processes of fine particles, reusing them afterwards within the coke production process.
Both exhaust systems are equipped with a safety valve. Electricity is produced in medium-voltage Leroy Somer synchronous alternators, directly connected to the engines. Each alternator has a power output of 2442 kVA at 6300 V. The generated electrical power is partly used for the coke plant’s power requirements; the remaining electricity is fed into the public grid.
A dozen of GE’s Jenbacher engines (type JMS 620 GS-S/N.LC) are supporting a unique cogeneration project utilizing LD-converter waste gas created during production at the Aceralia steel factory in Avilés, about 470 km north of Madrid. GENESA (Generaciones Especiales, S.A.), a developer of renewable and combine heat and power (CHP) power systems, proposed the installation of Jenbacher engines at the Aceralia factory as a solution for taking advantage of the existing Linz Donawitz steel production process (LD-converter) gas. Installed in 2004, the units are delivering approximately 20 MW and 12 MWth output.
Engines gearing up
The 12 engine modules each produce 1700 kW electrical output, for a total power output of 20.4 MW. The Jenbacher units are compact modular units, formed by the engine and alternator mounted on base frames with gas train, cooling, lubrication and start-up systems.
Three of the 12 modules can burn either LD converter gas or natural gas. Alternative natural gas operation is made possible by a separate gas train that is in parallel to the four LD-converter gas trains. The Jenbacher gas engines operate with separate parameter settings to ensure optimal operation despite the huge heating value difference between the gases. The use of natural gas ensures operation in the case of a reduction in the fuel supply from the steelworks.
The gas engines are equipped with Diane, the Jenbacher gas engine management system that consist of the powerful central industrial controls that handle open loop, closed loop and monitoring of the gas engine as well as the visualization. The control is also equipped with Profibus, which enables direct communication with the superordinate control system of the whole plant.
The gas supply to the engines consists of separated gas trains for LD converter gas and natural gas. For the LD converter gas, four parallel gas trains of DN200 size are used to cope with the high gas flow (approximately 2000 m3N/h) due to the low heating value of the gas. Furthermore special materials for the filters and instruments have been used in order to maintain a long component lifetime despite the high humidity in the gas.
The natural gas train is of much lower dimension designed for a gas flow of approximately 400 m3N/h.
As CO is a highly toxic gas that conjoins with the haemoglobin in the blood and inhibits the absorption of oxygen, which can lead to suffocation, it is a vital issue to evacuate CO from the whole generating set. This is made possible by flushing the gas leading parts with nitrogen. The nitrogen can be used to purge CO from the gas engine or the four parallel LD-converter gas trains of each generating set. As a result, maintenance can be carried out safely.
The Jenbacher units have two separate cooling circuits. One is a low-temperature circuit for the second stage of the intercooler and the other a high-temperature circuit for oil, intercooler first stage and jacket water cooling. Part of the heat generated in the high-temperature circuit is recovered in two 4417 kW plate heat exchangers to pre-heat the boiler water.
The heat not used in this circuit, along with what is produced in the low-temperature circuit but not utilized, is dissipated in a 16 000 thermal kW open cooling tower. The exhaust gas released at the outlet of each generating set enters a Y-branch pipe that leads it to an emergency outlet fitted with a safety valve in case of backfiring.
Under normal conditions the exhaust gas from each engine is sent to a separate duct passing an exhaust gas silencer, a catalytic converter and a three way valve, which leads the gas into the atmosphere or to the common exhaust gas boiler via a double exhaust gas flap with sealing air ventilator. By these means a backflow from hot exhaust gas from one engine to another at standstill is inhibited.
The tube type heat-recovery boiler is designed for a maximum flow of 16 engines working together. The boiler is equipped with an economizer, evaporator and superheater. It is arranged with a main chimney and a by-pass stack for start up and shut down of the generating sets. The boiler is also fitted with a reheater, capable of increasing the steam production up to 35 t/h or 20 t/h when the generating sets are at standstill.
Natural gas is utilized in the plant as an auxiliary fuel both for the generating set engines and the boiler burners. Electricity is produced in medium-voltage Leroy synchronous alternators, directly connected to the engines. Each alternator has a power output of 2407 kVA at 10 500 V. By means of an ABB 22/29-MVA booster transformer, the voltage is stepped up to 132 kV for export to the grid.
The superheated steam raising plant is specifically designed to burn coke, LD-converter and/or natural gas in its boilers. Coke gas is the preferential fuel and natural gas will be employed whenever necessary. By using the low-calorific value gas for 1.7 MW power generation per engine, the plant avoids using other fossil energy resources while keeping the plant’s NOx emissions below 500 mg/m3N.
In late 2005, Productos de Fundición S.A. (Profusa), one of Spain’s leading coke producers, marked the 10th anniversary of its coke oven gas fueled power plant at the company’s coke factory in the town of Bilbao. Profusa’s coke gas plant features a dozen Jenbacher generator sets (type JGS 316 GS-S/N.L). The power plant supplies an estimated average of 6 MW of electri-city, depending on the fuel composition, while a special engine control system allows for maximum flexibility in the gas composition.
Profusa coke oven gas plant, Bilbao, Spain
There is, therefore, the possibility of increasing power generation, depending on gas availability and electric and thermal power needs. The engines are also capable of running on different mixtures of coke oven gas/natural gas, 100 per cent coke oven gas or 100 per cent natural gas.
GE’s Jenbacher waste-gas plants in northern Spain are serving as reference models for steel factories around the world at a time when a number of the industry’s leading companies have joined together to identify key strategies and technologies to further reduce their environmental impacts and increase energy efficiency to help address climate change concerns.