How to Choose the Right Grate Bar Material Based on Actual Requirements?

The order for choosing grate bar material is combustion temperature first, then fuel type (sulphur, ash, corrosiveness), then the mechanical duty of the grate structure, and finally budget. Working through this order item by item narrows the material range.

Selection by furnace temperature

Below 700℃, for small heating boilers, atmospheric hot-water boilers and low-load intermittent coal boilers, HT200 grey cast iron or malleable cast iron can be used at a lower price, but they cannot withstand long high-temperature service and crack easily under frequent start-stop. 700 to 850℃ is the main temperature range for industrial coal and biomass boilers, covering about 90% of conventional boilers on the market. This range suits 4t, 6t and 10t chain grates, reciprocating grates and biomass pellet boilers, where RTSi5 silicon 5 heat-resistant cast iron is the first choice.

At 850 to 920℃, high-temperature combustion duty such as pulverised-coal blending, high-load continuous boilers and hot-air furnaces can use RTCr1 and RTCr2 low-chromium heat-resistant cast iron. Above 900℃, incineration furnaces such as municipal waste incineration, heavy straw firing and hazardous waste incinerators take RTCr16 high-chromium cast iron or heat-resistant stainless cast steel as the first choice.

Selection by fuel

With ordinary raw coal (medium-low sulphur coal) the flue gas is only mildly corrosive, and silicon 5 (RTSi5) is a cost-effective choice. With high-sulphur coal, poor coal or coal gangue, sulphur dioxide attack is severe and silicon 5 has only moderate sulphur resistance, so RTCr1 or RTCr2 low-chromium cast iron should be used. Chromium resists sulphide corrosion and reduces perforation by the flue gas.

With biomass such as wood chips, straw and rice husk, straw fuel is rich in potassium and sodium alkaline ash that cokes and sticks to the grate at high temperature, so silicon 5 is preferred over low-chromium cast iron and ordinary grey cast iron is not advised. For municipal waste, sludge and hazardous waste incineration, where acid and alkaline flue gas alternate, corrosion is strong and temperature swings are large, RTCr16 high-chromium cast iron, heat-resistant cast steel or stainless steel castings must be used.

Selection by grate structure and mechanical duty

Step 3 looks at grate structure and mechanical duty. The passive bars of a chain grate carry almost no tensile or squeezing load, so silicon 5 and grey cast iron both do. The active bars take pin loads and repeated pulling friction, so ductile iron or silicon 5 is preferred, and brittle ordinary grey iron is not used. A reciprocating grate pushes and pulls back and forth with large impact, and the bars are continually squeezed, struck and warped, so the material must be tough: carbon cast steel ZG230-450 and ductile iron are the main choices, while purely brittle heat-resistant cast iron is unsuitable.

Cross-beam grates and the grates of heavy large-capacity boilers carry high loads with small thermal deformation requirements, so low-chromium cast iron or cast steel can be used. Fixed static grates have no mechanical stress and only need high-temperature oxidation resistance, for which silicon 5 is enough.

Selection by start-stop frequency

Production boilers running continuously for 24 hours have stable temperature and little thermal shock, so silicon 5 and chromium-series heat-resistant cast iron both suit. Intermittent boilers such as heating boilers alternate hot and cold frequently with large thermal shock, so ordinary grey cast iron should be avoided and silicon 5 or ductile iron is preferred, for better toughness and thermal shock resistance.