Why Are Pellets Black and Burnt in vertical ring die pellet machine？

Mar 27, 2026

Blackening or charring of biomass pellets produced by rice straw pellet making machines is a typical high-temperature failure in pellet production. This usually indicates overheating, frictional carbonization, or unstable feeding, which can seriously damage product quality and shorten equipment life.

This guide provides a structured diagnosis of pellet burning issues from moisture, feeding, die condition, clearance, and impurities.

1. Core Phenomenon of Burnt Pellet Failure

Table: Burnt Pellet Symptoms

Item	Description
Surface color	Yellow → dark brown → black
Odor	Strong burnt smell
Internal structure	Carbonized / brittle
Discharge temperature	>120°C
Equipment status	Smoke, overheating alarm
Die condition	Accelerated wear

👉 This is a classic frictional overheating + material carbonization failure mode.

2. Low Moisture → Frictional Overheating

Table: Moisture Risk Analysis

Moisture Level	Result
<10%	Severe friction, overheating, burning
13–15%	Stable plasticization
>18%	Steam effect, weak structure

Failure Mechanism

Low moisture reduces lubrication inside the die →
👉 friction increases → temperature spikes → material carbonizes.

Solutions

Adjust moisture to 13–15%
Maintain stable feed rate (avoid idle friction)
Control discharge temperature: 80–100°C

3. Insufficient Feeding → Idle Overheating

Table: Feeding Failure Chain

Stage	Result
Hopper blockage	Material shortage
Bridging	Intermittent feeding
Converter failure	Feed instability
Idle running	Dry friction → burning

Symptoms

The wood fuel pellet making machine runs but produces little output
Chamber temperature rises rapidly
Burnt smell appears suddenly

Solutions

Clean feeder system
Repair frequency converter
Install vibration device to prevent bridging
Use level sensors for continuous monitoring

4. Die Hole Blockage → Material Carbonization

Table: Blockage Risk Effects

Issue	Effect
Partial blockage	Local overheating
Material retention	Carbon buildup
Shutdown residue	Sintering inside die

Mechanism

Material stays in high-temperature die holes too long →
👉 thermal decomposition → blackened pellets

Solutions

Clean die with needle + steam
Before shutdown: run oil + fine sawdust flushing (5 min)
Regular die maintenance schedule

5. Die–Roller Clearance Too Small → Excess Friction

Table: Clearance vs Risk

Clearance	Condition
<0.1 mm	Severe friction, burning risk
0.1–0.3 mm	Optimal range
>0.3 mm	Poor compression

Failure Mechanism

Too tight → metal-to-metal contact → heat spikes → pellet burning.

Solutions

Recalibrate using feeler gauge
Maintain standard gap: 0.1–0.3 mm
Inspect every production cycle

6. Raw Material Impurities → Localized Overheating

Table: Impurity Impact

Impurity Type	Effect
Iron nails	Spark + friction heat
Sand	Abrasive wear
Plastic	Melting + blockage

Symptoms

Random black spots on pellets
Local burning marks
Uneven temperature distribution

Solutions

Install magnetic separator
Add vibrating screen system
Strict raw material cleaning standard

7. Quick Fault Diagnosis Flow (Engineering Checklist)

👉 If pellets are burning, check in this order:

Moisture level (13–15%)
Feeding continuity (no idle running)
Die blockage condition
Roller clearance (0.1–0.3 mm)
Raw material impurities
Discharge temperature (≤100°C recommended)

8. Maintenance Prevention System

Table: Burn Prevention Plan

Item	Frequency
Moisture testing	Each batch
Die cleaning	Daily/shift
Feeding system inspection	Weekly
Roller gap calibration	Every 500 hours
Impurity filtration system check	Continuous

9. Engineering Summary

Pellet burning is primarily caused by frictional overheating + material stagnation
Moisture below 10% is the most dangerous threshold
Idle running is a hidden high-risk failure mode
Die blockage causes localized carbonization
Impurities create point overheating + uneven wear