Coal Gasification

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Coal gasification is a process that converts coal into syngas rather than burning it directly. In this process, coal undergoes controlled partial oxidation or thermal decomposition in a high-temperature (up to ~2600 °F) and high-pressure (up to ~1200 psig) gasifier, where it reacts with steam and limited amounts of oxygen or air. The resulting syngas, which is typically a mixture of carbon monoxide (CO), hydrogen (H₂), carbon dioxide (CO₂), methane (CH₄), and nitrogen (N₂), can be used to generate electricity, produce purified hydrogen, or serve as a feedstock for fuels and chemicals.

Source: Coal and biomass gasification process diagram, U.S. National Energy Technology Laboratory.

Feedstock Preparation

The characteristics of coal can influence both the syngas composition and the efficiency of the process prior to entering the gasifier. Coal is commonly classified into four main ranks according to carbon content and heating value:

• Lignite: Also called brown coal, this is the youngest form of coal with the lowest energy content, containing about 25-35% carbon.
• Sub-bituminous: Contains 35-45% carbon, offering higher energy content than lignite.
• Bituminous: Contains 45-86% carbon and provides a high heating value, making it one of the most widely used coals in industry.
• Anthracite: Contains 86-97% carbon. Despite being the hardest coal, it has a slightly lower heating value than bituminous coal.

Source: Types of coal

The term “low-rank” coal is often used to collectively refer to lignite and sub-bituminous coals. These types have the lowest heating values and highest moisture content, but they are generally less expensive and more abundant than “higher-rank” coals. Feedstock preparation includes coal handling and receiving, unloading, conveying, and storing the coal, as well as feed preparation, where the coal is crushed, screened, sized, and conveyed to the pressurized gasifier. Most of this equipment is standard in the coal industry. However, gasification requires specialized, technology-specific feeding systems, generally divided into slurry feeding and dry feeding:

• Slurry Coal Feed: Coal is wet-ground into fine particles and mixed with water to form a stable slurry, which is then pumped into the gasifier at high pressure. This system is reliable and proven, but the added water lowers the operating temperature and increases oxygen consumption, reducing overall efficiency.
• Dry Coal Feed: Coal fines are introduced through a lock hopper system, where batches are pressurized with nitrogen before being discharged into the gasifier. While mechanically more complex, this approach avoids the efficiency losses associated with water in slurry systems.

Gasifying Stages and Key Reactions

As the feedstock enters the gasifier, it undergoes a sequence of transformations:

• Dehydration: Moisture in the feed evaporates, producing steam.
• Pyrolysis: Rising temperature breaks weak chemical bonds, releasing volatile gases (tar vapors, CH₄, H₂) and forming solid char.
• Combustion: A fraction of the char and volatiles reacts with oxygen to generate CO, CO₂, and heat.
• Gasification: The remaining char reacts with CO₂ and steam to yield CO and H₂.
• Secondary gas-phase reactions: Water-gas-shift and methanation occur simultaneously, adjusting the CO/H₂ ratio depending on conditions.

Source: Coal gasification chemical reactions, U.S. National Energy Technology Laboratory.

Within the gasification process, the major chemical reactions are those involving elemental carbon (C), carbon monoxide (CO), carbon dioxide (CO₂), hydrogen (H₂), oxygen (O₂), water (H₂O, as steam) and methane (CH₄). Gasifying agents (O₂, H₂, CO₂, steam) influence the reaction balance and the final syngas composition and byproducts:

• Combustion (heat generation):
  • Partial oxidation: C + ½ O₂ ↔ CO
  • Complete oxidation: C + O₂ ↔ CO₂
• Gasification with steam:
  • C + H₂O ↔ CO + H₂
• Water-Gas Shift (adjusts H₂/CO ratio):
  • CO + H₂O ↔ CO₂ + H₂

The Gasifier

Most commercially available gasifiers can be grouped into three main types based on design and operation: fixed-bed gasifiers, entrained-flow gasifiers, and fluidized-bed gasifiers with each type differing in flow pattern, temperature distribution, and feedstock handling.

Source: Coal Gasifier Diagram, Pepperl+Fuchs: Coal Gasification White Paper.

Operating Parameters

Gasification reactions are strongly influenced by operating conditions including temperature, pressure, and gasifying agent ratios. Higher gasification temperatures increase carbon conversion and syngas production, promoting endothermic reactions such as the Boudouard reaction and steam reforming. Higher pressures can favor methane formation and affect tar and char yields. Low-pressure gasifiers typically favor CO and H₂ production.

The oxygen-to-fuel ratio determines the extent of partial oxidation. Higher ratios increase temperature and CO₂ production, while lower ratios favor CO and H₂. Gasification is typically operated below stoichiometric oxygen levels to maximize syngas production. Another critical metric for gasification reactions is the “residence time,” or the amount of time that the feedstock remains in the reactor. Longer residence times allow more complete gasification of char and volatiles, improving carbon conversion and syngas quality. Shorter residence times may lead to incomplete gasification and lower yields.

Products and Application

Coal gasification produces syngas and solid by-products such as slag and char:

Syngas

Syngas, produced from coal gasification, is a mixture primarily of hydrogen (H₂) and carbon monoxide (CO) and is used as an intermediate for power generation, chemical synthesis, and carbon capture applications. Raw syngas exits the gasifier at high temperatures and contains contaminants including fine particulates, sulfur compounds, ammonia, chlorides, mercury, and other trace heavy metals, which are removed through filtration, scrubbing, and other cleanup processes. High-temperature syngas is cooled via radiant or convective heat exchangers or direct quenching, generating steam for in-plant power or process heating, and then further cooled to low temperatures for final cleanup. Proper conditioning ensures syngas is suitable for electricity generation in integrated gasification combined cycle (IGCC) power plants, chemical synthesis (e.g., methanol, ammonia), or carbon sequestration applications.

Solid By-products

Coal gasification produces significantly different solid residues compared to conventional coal combustion, mainly slag and char, with minimal fly ash. Slag is a glassy, inert material formed from melted mineral matter in high-temperature gasifiers. It encapsulates toxic metals, making it non-leachable and safe for disposal. Slag can be marketed as a by-product for construction or industrial uses, reducing disposal costs. Char is the finer fraction of unreacted carbon and minor ash from the gasifier. It can be recycled into the gasifier to improve carbon efficiency or used as a low-cost adsorbent for mercury (Hg) and nitrogen oxides (NOx).

Source: Coal Gasifier Diagram, Pepperl+Fuchs: Coal Gasification White Paper.

Advantages and Challenges

Coal gasification transforms coal into a versatile fuel and intermediate feedstock compared to direct combustion, allowing for production of electricity, chemicals, or synthetic fuels. It also generates solid by-products like slag and char, which can potentially be reused to reduce waste disposal issues. Gasification requires high capital investment and sophisticated technology, making it more complex than conventional coal power plants. Its performance depends on coal quality and operating conditions, and it still produces CO₂ that must be captured or managed to meet climate targets. Operational safety and handling of residuals also need careful management.