Currently, municipal waste is mainly used a) in incinerators and b) in biogas plants. A better solution, from both economic and ecological standpoint, is to gasify the waste to produce synthesis gas, or syngas. Converting into syngas allows to utilize over three times more available energy from a kilogram of waste than through conventional incineration.

Such installations will not be feasible without superalloys resistant to high temperatures and corrosion. Let's discuss how alloy HR 160 overcomes these challenges and what cheaper alloys can be used.

Production of Syngas from Biomass and Low-Quality Coal

Syngas (synthesis gas) can be used in the chemical industry as a substitute for natural gas. It is also technically feasible to burn syngas to generate electricity and heat.

Coal/Biomass gasification installations enable the conversion of solid fuels into valuable synthesis gas.

Interesting Fact - Not Just Coal!

Initially, syngas production was based on coal gasification – and coal-based solutions are hardly the future. However, to improve economic indicators and environmental impact, newer syngas production systems use various ratios of low-quality coal mixed with biomass (municipal waste, sewage sludge, etc.).

Challenges for Nickel Alloys

All coal/biomass gasification technologies require a heat exchanger between the gasifier and the gas purification system. The hot gas, reaching temperatures of 500-600°C, creates a highly corrosive environment for the metal heat exchanger, both during operation and downtime. The hot gas is a mixture of the following corrosive agents:

• carbon monoxide CO (15-64%)
• carbon dioxide CO₂ (5-15%)
• hydrogen H₂ (10-30%)
• nitrogen N₂ (0-50%)
• methane CH₄ (2-4%)
• hydrogen sulfide H₂S (300-12000 vppm)
• ammonia NH₃ (200-5000 vppm)
• hydrogen chloride HCl (50-1000 vppm)

Different syngas production technologies and different fuels (biomass vs. coal) alter the percentage composition of these gases in the mixture, but generally, the nickel alloy must withstand the effects of all these gases simultaneously.

Solution: HAYNES® HR-160®

The HAYNES® HR-160® alloy is a nickel-cobalt-chromium-silicon alloy with excellent resistance to sulfidation. It performs excellently in corrosive environments resulting from burning low-quality fuels, such as sewage waste, biomass, and low-quality coal. Additionally, the alloy is easy to form and weld. The silicon alloy addition helps form a protective oxide layer in low-oxygen corrosive environments, as evidenced by the sulfidation resistance test results below:

The value of the HR-160 alloy is confirmed by the report "Materials for Gasifier Heat Exchangers" issued by the Power Generation Technology Centre at Cranfield University in the UK. It states:

“Sanicro 28, HR 160, and Haynes 556 performed the best. All performed well in gas environments with high and low H₂S content at 450°C and with low H₂S content at 550°C. At the higher temperature of 550°C and high H₂S content, the results varied, but were still better than for other tested alloys. Overall, the most resistant to high-temperature gas corrosion (HTGC) was the HR 160 alloy…”

Other Alloys to Consider

Sanicro 28 is a nickel-chromium-iron-molybdenum alloy with copper as an important alloying element. According to the above study, it performs nearly as well as HR 160 alloy and is noticeably cheaper.

Haynes 556 is an iron-nickel-chromium-cobalt alloy with similar but slightly lower performance at a lower cost.

Alloy 690 / 2.4642 - a nickel-chromium-iron alloy with good resistance to sulfidation and nitriding, used in older types of coal gasification installations.