The easy coking of straw pellets fuel during combustion is a common problem in its application, and it is closely related to the characteristics of the raw materials, processing techniques, and combustion conditions.

The essence of coking is that some components in the fuel melt, bond, and deposit on the furnace wall or heated surface, forming hard coke blocks. The specific reasons can be classified into the following categories:

1. The inherent characteristics of the raw materials

High ash content and special composition: The ash content of straw (such as corn stalks, wheat stalks) is usually higher than that of wood chips and other raw materials (generally 5%-10%, some can reach 15% or more), and the ash contains silicon, potassium, sodium and other elements. These elements are prone to form low-melting-point compounds (such as potassium silicate, sodium silicate) at temperatures above 800°C, with a melting point as low as 600-700°C. They can melt into liquid state during combustion and solidify into hard coke blocks upon cooling.

Difference in the ratio of cellulose and lignin: Straw has a high cellulose content and low lignin (lignin is a natural adhesive that can enhance the hardness and combustion stability of the pellets). During combustion, the structure is prone to rapid disintegration, resulting in local heat concentration, and the ash is more likely to melt.

Presence of impurities in the raw materials: During straw harvesting, impurities such as soil and sand may be mixed in, and these inorganic impurities form non-combustible residues during combustion, accelerating the deposition of coke.

2. Unreasonable processing techniques

Insufficient pre-treatment before pellets formation: If the straw raw materials are not fully crushed or screened, the pellets contain longer fibers or impurities, which may cause incomplete local combustion during combustion, and unburned carbon pellets combine with the ash to form coke blocks.

Inappropriate moisture control: If the pellets moisture content is too high (over 15%), it will cause water vapor to be produced during combustion, reducing the local temperature, and preventing the ash from being fully burned and leaving residues; if the moisture content is too low (below 8%), the pellets become more brittle, and combustion is prone to cracking, resulting in excessive fly ash and indirectly promoting coking.

Uneven pellets density: Insufficient forming pressure or wear of the ring mold / flat mold will lead to low pellets density and a loose structure, resulting in rapid combustion speed and a sudden increase in local temperature in the furnace, intensifying the melting of the ash.

3. Incompatible combustion conditions

Excessive furnace temperature: The suitable combustion temperature for straw pellets is 600-800°C. If the furnace temperature exceeds 800°C (such as excessive air supply or dense fuel accumulation), the low-melting-point elements in the ash will melt prematurely, forming a viscous coke layer.

Insufficient air supply: Incomplete combustion, unburned carbon pellets combine with the ash to form black coke blocks;

Excessive air supply: Excessive cold air leads to a sudden drop in local temperature, causing the molten ash to solidify prematurely and adhere to the furnace wall to form coke blocks.

Incompatible furnace type: The ash characteristics of straw pellets are different from those of wood chips pellets. If a boiler specially designed for wood chips (such as a compact furnace structure and dense heat exchange surfaces) is used, it is prone to coking due to the poor discharge of ash.

These are the reasons summarized by TONY for the easy coking of straw pellets fuel.