Natural gas and its Usage in Iron and Steel Industry
Natural gas and its Usage in Iron and Steel Industry
Natural gas (NG) is an environmentally friendly non-renewable gaseous fossil fuel which is extracted from deposits in the earth. It is a clean and green fuel with a high efficiency and plays a major role in helping many industries cut emissions and improve the overall air quality. It is normally supplied as (i) piped natural gas (PNG), (ii) compressed natural gas (CNG), and (iii) liquefied natural gas (LNG).
Natural gas is a mixture of hydro-carbons consisting primarily of methane (CH4), generally in a percentage of over 85 % by volume. Other hydro-carbons in NG include varying amounts of various higher alkanes such as ethane, propane, and butane etc. It also contains water vapour (H2O) at varying degrees of saturation, or condensed water. It may also contain some small percentage of nitrogen (N2), carbon dioxide (CO2) and hydrogen sulphide (H2S) and helium (He) etc.
NG burns with a clean blue luminous flame when mixed with the requisite amount of air and ignited. It is considered one of the cleanest burning fuels. On burning, it produces primarily heat, CO2, and water vapour.
NG is a fuel found in deposits in its gas phase. It is colourless and odourless, non-toxic, and lighter than air. It does not contain olefins (hydrocarbons produced during the process of destructive distillation or reforming). It is a highly flammable and combustible gas. Its CAS number is 8006-14-2 and UN number is 1971.
Quantities of natural gas are measured in normal cubic meters (corresponding to 0 deg C and 1 atmosphere pressure) or standard cubic feet (corresponding to 16 deg C and 14.73 pounds per square inch absolute pressure). The higher heat value of one cubic meter of natural gas varies from around 9500 kcal to 10,000 kcal. Its density is around 0.85 kg/cum.
NG is transported normally to long distances (upto 5000 kms) through a pipeline network. The pressure of NG in the pipeline depends on several factors which include (i) quantity of gas to be transported, (ii) diameter of the pipeline, (iii) the distances involved, and (iv) the safety of the gas pipeline and environment. NG supplied to the consumer through pipe is PNG. The pipeline pressure at the consumer end is generally less than 16 atmospheres.
CNG is a form of natural gas which undergoes compression (200 atmospheres to 250 atmospheres) into containers wherefrom it is relayed to consumers who, due to geographic and other reasons are incapable of connecting into the NG pipeline. CNG is storable. Unlike NG conveyed via pipeline and immediately consumed (similarly to electricity), CNG can be used for storage and for discontinuous utilization. NG compression into containers raises risk levels.
LNG is made by cooling natural gas to a temperature of minus 162 deg C. At this temperature, NG becomes a liquid and its volume is reduced by 600 times. LNG gas is easier to store than the gaseous form since it takes up much less space. LNG is stored at atmospheric pressure in designed vessels and is easier to transport to the users. NG is normally transported in LNG form where the distances of the destination is generally above 5000 kms.
Uses of NG in iron and steel industry
The uses of the NG in iron and steel industry (Fig 1) include (i) as a reductant, (ii) a source of heat or as a fuel, (iii) in power generation, and (iii) in cutting and welding application.
Fig 1 Uses of natural gas in iron and steel industry
Natural gas as a reductant
Natural gas as a reductant is mainly used in ironmaking. For using NG for direct reduced iron (DRI) production in gas based DRI production processes, the NG is needed to be reformed into a usable reducing gas that is high in hydrogen (H2) and carbon monoxide (CO) content. More than 90 % of the global DRI plants use NG. In case of ironmaking in blast furnace (BF), NG is normally injected in the BF along with hot air blast at the tuyere level.
Use of NG in DRI production
NG reforming is an advanced and mature production process. The standard process for the reforming of NG to produce reducing gas is a thermal process and is known as the steam-methane reforming (SMR) process. It is a process in which high-temperature steam (700 deg C to 1,000 deg C) is used to produce reducing gas consisting of H2 and CO. In the SMR process, CH4 reacts with steam (H2O) under 3 atmospheres to 25 atmospheres pressure in the presence of a catalyst to produce H2, CO, and a relatively small amount of CO2. Steam reforming is an endothermic process which means that heat is to be supplied to the process for the reaction to progress. The SMR reaction is CH4 + H2O (+ heat) = CO + 3H2.
In the DRI production process, NG is reformed in a reformer which is a gas tight refractory lined furnace containing alloy steel tubes filled with a catalyst. The feed gas to the reformer is the fresh NG blended with the top gas from the shaft furnace which is being recycled. This blended mixed gas is heated and passed through catalyst filled tubes. Reformed gas is produced due the catalytic reactions taking place inside the catalyst filled tubes. The newly reformed gas containing around 90 % to 92 % of H2 + CO (on dry basis) is then fed hot directly to the shaft reduction furnace as the reducing gas.
DRI production process generally uses a solid catalyst for the gas phase reaction. Alumina (Al2O3) or magnesia (MgO) is the carrier material which gives the catalyst its shape and strength. The active ingredient of the catalyst, which increases the speed of the reaction, is normally nickel (Ni). Cobalt (Co) has also been used in some cases. Sulphur (S) and halogens are the most common reforming catalyst poisons.
The main steam reforming reaction which is taking place in a reformer is CnH(2n+2) + n H2O = (2n+1)H2 + nCO. During the steam reforming there is possibility of the cracking of heavy hydrocarbon. The cracking reaction is CnH(2n+2) = (n+1)H2 + nC. This is also known as carbon (C) deposition reaction. In actual practice, during the DRI production, since the top gas from the shaft furnace is recycled, it contains CO2 and oxygen (O2). These gases also take part in the reforming reaction of NG. The reactions of reforming CH4 which takes place to produce reducing gas consisting of CO and H2 are given below.
CH4 + CO2 = 2CO + 2H2
CH4 + H2O = CO + 3H2
2CH4 + O2 = 2CO + 4H2
CO + H2O = CO2 + H2
CH4 = C(S) + 2H2 (Carbon deposition reaction)
The reformer and catalyst design is to be such that it promotes the reforming reactions without permitting the C deposition reactions to take place. For steam reformer, NG gas needs to be desulphurized.
The DRI production process generally uses stoichiometric reformer. In this reformer stoichiometric ratio is an important parameter. The stoichiometric ratio is simply the molar or volume ratio of oxidants to hydro-carbons which generally results in the consumption of the hydro-carbon with no oxidant left over if the reaction proceeds to completion. In steam reformer, it is the steam to carbon ratio.
The energy consumption in NG based DRI production is well known and established to be 10.4 giga joules per ton of DRI. NG based DRI production also leads to lower the CO2 emissions ranging from 0.77 tons of CO2 per ton of steel to 0.92 tons of CO2 per ton of steel, depending upon the type of electricity used.
Injection of NG in BF
NG is injected in the tuyeres of the BF as an auxiliary fuel. It is injected along with the O2 enrichment of the hot blast air. The purpose of injection of NG as an auxiliary fuel is to have reduction in the specific consumption of coke. The replacement ratios of coke being achieved with NG gas injection in the BF are in the range of 1.3 to 1.4. The injected NG in the BF provides reducing gases consisting of H2 and CO to the furnace which moves up the furnace shaft and takes part in the reduction reactions of the iron oxides.
Significantly higher NG injection rates (through the tuyeres) are not feasible because injection of NG in the tuyeres has an endothermic effect on the raceway adiabatic flame temperature (RAFT). This strongly limits the quantity of NG which can be injected through the tuyeres. To compensate for this reduction in the flame temperature, the hot blast air needs to be enriched with O2. However, the increase in the O2 content of the blast reduces the N2 content, which in turn causes the top gas temperature to drop. The top gas temperature Is to be always above the dew point to prevent any undesirable condensation in the upper part of the BF.
Mass and energy balance calculations show that injection rate of the NG at the BF tuyere cannot exceed around 150 kilograms per ton of hot metal (kg/tHM). This is shown by calculating allowable levels of O2 enrichment of the hot blast, for different rates of NG injection, such that the conditions of top gas temperature greater than the minimum, and RAFT greater than its minimum, are both satisfied. These two conditions define an ‘operating window’ for the BF. Hence, a minimum RAFT (typically in the range of 1700 deg C to 1900 deg C) and a minimum top gas temperature (typically higher than 100 deg C on average) is to be maintained to ensure stable functioning of the BF.
Because of the dual constraints of flame temperature and top gas temperature, the amount of NG which can be injected at the tuyeres gets limited. A different approach is necessary if the utilization of NG in BF is to be increased. As per this approach, NG is required to be injected higher in the shaft of the BF, above the tuyeres. The injection of NG in the shaft of the furnace is expected to result in cracking of CH4 over iron/iron oxide, which is known to act as a catalyst for the C deposition. Direct injection of NG into an iron oxide shaft is currently utilized in the HYL direct reduction process. The HYL zero-reforming (ZR) process (also known as Energiron process) optimizes its overall energy efficiency by combining a high reduction temperature (above 1,050 deg C) and ‘in-situ’ reforming inside the shaft furnace.
Injection of NG into the shaft of the BF is expected to result in the cracking of CH4 at elevated temperatures which is usually above 800 deg K (Kelvin). Additionally, the presence of CO2 and H2O in these regions is expected to reform the CH4 and produce CO and H2. The concentrations of CO, CO2, H2 and H2O are expected to be controlled by the water-gas shift reaction. The expected reactions on injecting NG into the BF are as given below.
Methane cracking – CH4 = C + 2H2, Delta H at 298 deg K = +75.6 kilo Joules per mole (kJ/mol) of CH4
CO2 reforming of methane – CH4 + CO2 – 2CO + 2H2, Delta H at 298 deg K = 247 kJ/mol of CH4
Steam reforming of methane – CH4 + H2O = CO + 3H2, Delta H at 298 deg K = 206 kJ/mol of CH4
Water gas shift reaction – CO + H2O = CO2 + H2, Delta H at 298 deg K = -41.1 kJ/mol
The H2 and CO gases generated by the above reactions moves up the BF shaft and takes part in the reduction reactions. It is to be noted that the water gas shift reaction is kinetically favoured at high temperatures and thermodynamically favoured at low temperatures as the equilibrium constant of the reaction decreases with increasing temperature. Since there is no change in the volume from the reactants to the products, the reaction equilibrium is unaffected by change in pressure.
Similarly, shaft injection of reducing gas is among the options being considered in the European ‘Ultralow CO2 Steelmaking (ULCOS) program’. The top-gas recycling BF process (ULCOS-TGRBF) (which has been tested at the pilot level) removes CO2 from the top gas of the furnace, and re-injects the remaining CO and H2 at the tuyeres and the lower shaft region of the BF.
Natural gas as a fuel in the furnace
NG is one of the principal sources of energy and can be used for meeting the heating requirements in the iron and steel industry. It is an extremely important source of energy for reducing pollution and maintaining a clean and healthy environment. The use of NG also offers a number of environmental benefits over other sources of energy, particularly other fossil fuels.
Essentially, industrial applications which require energy, particularly for heating, use the combustion of fossil fuels for the energy. Because of its clean burning nature, the use of NG wherever possible, either in conjunction with other fossil fuels, or instead of them, can help to reduce the emission of harmful pollutants.
Composed primarily of CH4, the main products of the combustion of NG are CO2 and water vapour. The process of combustion of CH4 consists of a reaction between CH4 and O2. When this reaction takes place, the result is CO2, H2O, and a great amount of energy. The reaction showing the combustion of CH4 is represented by the equation CH4[g] + 2O2[g] = CO2[g] + 2H2O[l] + 891 kJ.
NG, as the cleanest of the fossil fuels, can be used in many ways to help reduce the emissions of pollutants into the atmosphere. Burning NG in the place of other fossil fuels emits fewer harmful pollutants, and an increased reliance on NG can potentially reduce the emission of many of these most harmful pollutants. The combustion of NG, on the other hand, releases very small amounts of sulphur oxides (SOx) and nitrogen oxides (NOx), virtually no ash or particulate matter, and lower levels of CO2, CO, and other reactive hydro-carbons.
Natural gas based power generation
Power plants based on NG generate electricity by burning NG as their fuel. There are many types of NG based power plants which all generate electricity, but serve different purposes. Natural gas power plants are cheap and quick to build. They also have very high thermodynamic efficiencies compared to other power plants. NG can be used to generate electricity in a variety of ways as given below.
Conventional boiler based generation – The most basic NG fired electric generation consists of a steam generation unit, where NG is burned in a boiler to heat water and produce steam which then turns a turbine to generate electricity. NG can be used for this process, although these basic steam units are more typical of large coal or nuclear generation facilities. These basic steam generation units have fairly low energy efficiency. Typically, only 33 % to 35 % of the thermal energy used to generate the steam is converted into electrical energy in these types of units.
Combined cycle units – Many of the new NG gas fired power plants are known as ‘combined-cycle’ units. In these types of generating facilities, there is both a gas turbine and a steam unit, all in one. The gas turbine operates in much the same way as a normal gas turbine, using the hot gases released from burning NG to turn a turbine and generate electricity. In combined-cycle plants, the waste heat from the gas-turbine process is directed toward generating steam, which is then used to generate electricity much like a steam unit. Because of this efficient use of the heat energy released from the NG, combined-cycle plants are much more efficient than steam units or gas turbines alone. In fact, combined-cycle plants can achieve thermal efficiencies of upto 50 % to 60 %.
Centralized gas turbine based power plants – All these plants use gas turbines and combustion engines to generate electricity. In these types of units, instead of heating steam to turn a turbine, hot gases from burning NG are used to turn the turbine and generate electricity. Gas turbine and combustion engine plants are traditionally used primarily for peak-load demands, as it is possible to quickly and easily turn them on. These plants have increased in popularity due to advances in technology and where there is the availability of the NG. However, they are still traditionally slightly less efficient than large steam-driven power plants.
Use of NG in gas welding and gas cutting operations
NG gas depending on its availability is being used in the gas welding and the gas cutting operations because of its high CV. For oxy-fuel gas cutting of steels, NG is being used in manual gas cutting torches as well as in automatic machine mounted gas cutting units. It is also being used in the gas cutting equipments used in billet, bloom, and slab continuous casting machines.