Extending gasification runtime with antifoulant
When fouling is observed in the syngas cooler, an antifoulant programme can provide a gasification unit with an extended runtime
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Gasification is a mature process for converting hydrocarbons into electric power, clean synthesis gas, fertilizers, fuels and chemicals with minimum environmental impact. Powerful antifoulants can improve runtime and profitability when fouled syngas coolers or soot scrubbers are causing bottlenecks.
Gasification is an old technology and the process has been developing for over 200 years. It is an exothermic, non-catalytic reaction between hydrocarbons and a limited amount of oxygen in a highly reducing atmosphere. It was primarily used to produce town gas from coal for heating and lighting purposes. Oxygen and steam flowed through a bed of coal. A fraction of the coal was burned, producing heat to maintain the operating temperature. The carbon in the remaining coal formed a mixture of carbon monoxide and hydrogen gases by the so-called steam reactions:
CnHm + n/2 O2 → n CO + m/2 H2 + Heat
CnHm + n H2O → n CO + (n + m/2) H2 - Heat
CO + H2O → CO2 + H2 + Heat
In the 1950s, the importance of coal gasification declined because of natural gas exploration. But there was still a need for synthesis gas, and the demand for ammonia as a nitrogen fertilizer grew exponentially. In the late 1940s, the Texaco gasification process was developed and commercialised, and in 1950 commissioned for the production of ammonia. The first Texaco gasifier for oil feedstocks was introduced six years later to the market. The Shell gasification process was developed in the early 1950s, and the first gasifier for heavy fuel oil went into operation in 1956.
Meanwhile, there have been a number of technical developments with different designs and a broad range of reactor types. In most cases, the reactor type can be grouped into one of three categories:
• Moving-bed gasifiers
• Fluid-bed gasifiers
• Entrained-flow gasifiers.
The gasification process takes place in a temperature range of 800-1800°C. The exact temperature depends on process design and the characteristics of the feedstock. The produced gases are called synthesis gas (syngas). The detailed composition of syngas may differ, depending on feedstock qualities and the applied gasification process.
Figure 1 shows the principle of gasification. Coal, natural gas, oil, refinery residual oils, petroleum coke (pet-coke), biomass or municipal waste are typical feedstocks for gasification. More than 250 units with the Texaco or Shell gasification process are installed worldwide. Shell’s gasification process is designed for fluid feedstock gasifiers and the Shell coal gasification process for solids such as coal, petroleum coke and lignite.
Air-blown or oxygen-blown gasifier designs are used to provide oxygen to the partial oxidation process. Air-blown gasifiers are larger in size and the syngas contains significant amounts of nitrogen. Oxygen-blown gasifiers produce syngas with a large proportion of combustible gas. They operate at very high temperatures and can produce syngas with high purity.
The cleaned syngas can be supplied to a combined cycle gas turbine. Integrated gasification combined cycle (IGCC) technology is one of the most promising technologies to meet the most stringent emissions limits, and provides an alternative way of producing electricity, steam and hydrogen for hydroprocessing facilities. The produced syngas can be used to fuel the gas turbine. This is a very efficient and economical way to produce electricity.
Figure 2 shows a typical flow scheme of the Shell gasification process. The partial oxidation of hydrocarbons takes place in the entrained-flow gasifier reactor. The oxidant is preheated and mixed with steam. The specially designed burner and reactor geometry are constructed so that the oxidant/steam mixture is intensively mixed with the preheated feedstock. The produced raw gas has a temperature of about 1300-1400°C. The reactor effluent is routed to the syngas cooler (proprietary design), where the raw gas is cooled to about 320-340°C by generating high-pressure steam.
The Texaco reactor is an empty, refractory-lined vessel with a water-cooled burner design. Steam and oil are routed through a circularly slit surrounding the oxygen pipe. Texaco offers direct quenching with water or a syngas cooler to generate steam. The syngas cooler mode is used when a high CO concentration is required. The quench removes the main part of the solids in the gas, which are extracted as soot-water slurry or “black water”. The gas is scrubbed in a Venturi scrubber and in a packed column to remove traces of soot. The raw gas is then routed to downstream units for CO shift and acid gas removal.
The produced syngas has high levels of purity with a large proportion of combustible gas.
The hot raw gas leaving the gasifier reactor has to be cooled before the gas can be treated for use. There are a number of syngas cooler designs available for oil gasification. Water-tube boilers or fire-tube boilers are principal designs. Syngas coolers are designed so that formed ash and soot particles are transported with the process stream and do not usually deposit in the cooler. A shorter runtime or a temperature or differential pressure increase are clear indicators of existing syngas cooler fouling. Typical elements found in the solids are Fe, Ni, V, Na, Ca, K, Mg, Al and Si, because in the high-temperature regions the carbon fraction of the particles is gasified, while the inorganic components remain as deposits. The molten V, Al, Mg, Cr and Si do not initiate the typical hydrocarbon fouling on the boiler tube as no liquid hydrocarbon compounds are formed, but they are still fouling contributors by themselves. Severe fouling of the syngas cooler may result in an unplanned shutdown of the unit. The typical cleaning time is two to four working days, with production losses and loss of high-pressure steam. Beside additional cleaning costs, the shutdowns weaken the metallurgy when hot equipment is frequently cooled down and heated up again.
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