What are processed or secondary fuels?

Explanation

**Step-by-Step Solution:**

1. **Definition of Processed or Secondary Fuels:**
Processed or secondary fuels are fuels that cannot be used in their raw form as they are found in nature. They require some form of processing before they can be utilized effectively.

2. **Need for Processing:**
The raw forms of these fuels are not suitable for direct use due to various reasons such as impurities, inefficiency in combustion, or the need for specific properties to be effective as fuels.

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