The hazards of hydrogen

Hydrogen is a colorless, odorless, water-insoluble combustible gas with a density only 1/14 that of air. Under standard conditions, its density is 0.089 g/L. Hydrogen ignites at 574°C and has an explosive range of 4.1%-74.1%. Within sulfuric acid plants, hydrogen accumulates in the headspace of equipment and can explode upon contact with open flames.

Causes of hydrogen formation in sulfuric acid plants

Dilute sulfuric acid reacts with iron to produce hydrogen gas, as shown by the equation: Fe + H?SO? → FeSO? + H?↑. During plant operation, water leakage into sulfuric acid or acid leakage into water creates large amounts of dilute sulfuric acid. This dilute acid reacts with iron to generate hydrogen. During maintenance, concentrated sulfuric acid absorbs moisture from the air, diluting into weak sulfuric acid. This weak acid then reacts with iron to produce hydrogen. Due to equipment sealing or dead zones at the top, hydrogen cannot be effectively vented. It accumulates until reaching explosive limits, triggering explosions upon contact with open flames.

Which equipment in the sulfuric acid plant may accumulate hydrogen and precautions should be taken

Liquid sulfur areas: Residual hydrogen ions in liquid sulfur float on its surface. Equipment in these areas is predominantly carbon steel. Contact between hydrogen ions and carbon steel generates hydrogen gas, which accumulates at the top of equipment. Upon reaching the explosion limit, it can ignite upon contact with an open flame. Therefore, molten sulfur tanks and filter tanks are lined with acid-resistant bricks, and heating coils are made of stainless steel—or at least the surfaces contacting liquid sulfur use stainless steel coils—to prevent equipment corrosion and hydrogen generation. Vent ports are installed at the top of all tanks. Liquid sulfur storage tanks are made of carbon steel. Under normal conditions, sulfur melting processes strictly control the acidity of liquid sulfur. However, trace hydrogen ions still contact the tank, producing small amounts of hydrogen gas. This hydrogen must be vented into the atmosphere through the tank's top vent.

Acid Piping: During maintenance of sulfuric acid plants, sulfuric acid is drained from pipelines to prevent personnel exposure. However, trace amounts of acid remain on pipe walls. This residual acid absorbs moisture from the air, gradually forming dilute acid over time. Dilute acid corrodes the passivation layer on pipe walls and reacts with the piping to produce hydrogen gas. If this hydrogen cannot be released, it accumulates until reaching explosive limits, potentially igniting upon contact with an open flame. Therefore, sulfuric acid pipelines that remain idle for extended periods require maintenance. This can be achieved by periodically circulating 98% concentrated sulfuric acid or filling the pipeline with concentrated sulfuric acid to prevent air ingress. This approach protects equipment from corrosion while minimizing hydrogen generation.

Acid Towers: The lower sections of the drying and secondary absorption tower demisters feature brick-lined structures, while the upper spaces are stainless steel. The high-temperature absorption tower uses special-grade stainless steel. Under normal operating conditions, diluted acid does not form. However, improper water addition can reduce acid concentration. If acid-water heat exchangers leak, water entering the acid creates diluted acid and generates hydrogen gas. During maintenance, when acid is completely drained from the tower, residual concentrated sulfuric acid on the tower walls absorbs moisture, forming dilute acid that also generates hydrogen. If the flue gas outlet of some towers is not positioned at the top of the equipment, a dead zone may form at the tower top where hydrogen can accumulate. Similarly, certain flue gas ducts may have a dead zone apex where hydrogen can also accumulate, reaching explosive limits and potentially igniting upon contact with an open flame. Therefore, strict control of acid concentration is essential during plant operation. Acid-water heat exchangers must be monitored, with immediate action taken upon detecting leaks. Even idle acid towers require maintenance, such as periodically circulating 98% concentrated sulfuric acid.

Flue Gas Piping: During operation, flue gas piping inevitably accumulates acid sludge. After shutdown, this sludge absorbs moisture from the air, forming dilute acid that reacts with the piping to generate hydrogen. This hydrogen accumulates in the dead corners at the apex of the flue gas piping, reaching explosive limits and potentially igniting upon contact with an open flame.

Feedwater Equipment: During production, to enhance thermal efficiency, demineralized water is heated via acid water heat exchangers before entering deaerators. Equipment such as the demineralized water tank, deaerator, economizer, and boiler are all made of carbon steel. If a leak occurs in the acid water heat exchanger, large amounts of acid enter the water, turning it into dilute acid. This acid corrodes all equipment through which the water flows, producing hydrogen gas. If this hydrogen gas cannot be effectively vented, it accumulates in a confined space, reaching explosive limits and igniting upon contact with an open flame. Therefore, monitoring the acid water heat exchanger is critically important. Any detected leakage must be addressed immediately to prevent equipment damage and avert safety incidents.