Gasifying coke to produce hydrogen in refineries
Coke gasification to produce hydrogen for hydroprocessing is more capital intensive than steam methane reforming, but not so vulnerable to natural gas price variations
Tek Sutikno and Kevin Turini
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One way a petroleum refinery can meet the hydrogen demand of hydroprocessing is to gasify petroleum coke. It can be especially attractive to a refinery that includes a coking facility such as a delayed or fluid coking unit producing petroleum coke. Refineries have demanded more hydrogen in the past two to three decades as more extensive hydroprocessing (hydrotreating and hydrocracking) is required for the processing or upgrading of heavy crudes and the removal of sulphur and nitrogen to meet the more stringent specifications of transportation fuels.
To enable the processing of heavier crudes to maximise yields of valuable distillates or light hydrocarbons, some refineries have expanded their complexity by adding an upgrading unit such as a hydrocracker, hydrotreater or a thermal cracking unit like a delayed coker, in addition to a fluid catalytic cracking (FCC) unit. All of these units enable the refinery to process heavier or more sour crude oils, which typically have a low cost advantage over the lighter crude oils, and to maximise the conversion of heavy crude fractions to more valuable, lighter distillate products. The addition of hydrotreating or hydrocracking units increases the hydrogen demand of the refinery, and the thermal cracking units generate coke and other valuable distillates or light hydrocarbons. To meet the increased demand for hydrogen, refinery managers often need to select a scheme or schemes for generating or supplying more hydrogen from a number of available alternatives. These may vary from one refinery to another, but typically include the following:
• Recovering hydrogen from refinery fuel gas
• Expanding existing catalytic reformer throughput to increase hydrogen generation, if available and viable, to meet the maximum benzene limit of gasoline
• Expanding the steam reforming unit (or hydrogen production unit), if available
• Installing a new steam reforming unit, which normally uses fuel gas or naphtha
• Over the fence (OTF) hydrogen import
• Gasifying coke from the existing or new coking unit to generate hydrogen.
Among these alternatives, gasifying coke from the delayed coker or a thermal cracking unit is probably the least common scheme implemented to date for expanding hydrogen generation. However, gasifying coke to hydrogen can be an economically attractive option for certain refineries given the favourable market conditions. Availability of coke and the price fluctuation of alternative raw materials should be considered and evaluated before finalising the selection of a new hydrogen production scheme for the refinery. This article presents an overview of coke gasification for generating hydrogen in refineries and discusses design options for optimally integrating the coke-to-hydrogen plant within the refinery’s operation.
Hydrogen yield from coke
Gasification of coke or coal typically involves licensed processes with different supplementary, reactant feed schemes and varying gasifying reaction conditions. For example, air is fed directly to air-blown gasifiers, while other gasifiers are fed with high-purity (about 99.5%) oxygen from an air separation unit. Steam is also fed directly to some dry feed gasifiers, while others are fed with coal or coke in a slurry with water. Reaction conditions typically vary in pressure from 430 psig to about 1200 psig (30-80 bar) and in reaction outlet temperature up to about 2700°F (1480°C). Product compositions vary, depending upon the selected gasification technology and the characteristics of the petroleum coke. For the gasification of petroleum coke, the gasifier generally produces syngas with typical H2-to-CO molar ratios of less than one. CO2, H2O and CH4 are the secondary product components at lower concentrations relative to those of H2 or CO. In addition, the syngas is contaminated with H2S, CO2, COS and other sulphur compounds. Raw syngas from the gasifier can be cooled through a steam generation system or a quench and scrubbing section, where the raw syngas becomes saturated with water at a target temperature adequate for converting CO and H2O to H2 and CO2 in the downstream shift reaction unit.
For a rough estimate at the conceptual evaluation stage, about 27 scf of hydrogen can be generated per pound of coke. While the amount of coke a typical refinery could generate depends on a number of factors, such as crude oil characteristics, refining schemes and others, a refinery generates an approximate average of 12.5 tons of coke per 1000 barrels of crude oil. Coke yield varies, depending on crude properties such as Conradson carbon content, and generally increases as crude feed becomes heavier.
As an example, a 200 000 b/d refinery generates approximately 2500 t/d of petroleum coke that can be gasified, generating approximately 135 MMscfd of hydrogen. If LPG is used to generate the hydrogen from the conventional steam reforming process, about 8000 b/d (excluding fuel requirement) of LPG would have been required as the raw material, and steam reforming requires a significant amount of fuel in addition to the raw materials.
Gasifying coke to hydrogen
Obviously, cost competitiveness, technological maturity and operating reliability need to be evaluated when considering a gasification scheme. Another important aspect to evaluate is the optimal design integration of the process units in the new coke gasification facilities with the existing or planned refinery operation. Optimal integration will maximise the economic benefits from gasifying coke for hydrogen generation and enhance the competitiveness of the overall refinery operation. Examples of integrating issues are discussed next.
Figure 1 shows a typical flow scheme for gasifying coke to produce hydrogen. Oxygen-blown gasifiers are typically required for generating hydrogen from coke. However, the air separation unit for separating oxygen from ambient air constitutes a significant part of the total capital cost. Air-blown gasifiers directly utilising oxygen in air generate syngas with high concentrations of nitrogen, and the resulting syngas is more suitable for electricity generation using gas turbines than for producing hydrogen.
For gasifiers where coke is fed as a pumpable slurry, water instead of steam is fed to the gasifier. Water or steam serves as a reactant to generate hydrogen and a quenching medium for the gasifier. Syngas from the gasifier requires cooling and scrubbing to remove particulate matter before entering the shift conversion unit, where CO reacts with H2O to produce H2 and CO2.
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