The calorific value, also known as the heating value, is a measure of the energy released when a substance undergoes complete combustion. For paper and cardboard, the primary combustible component is cellulose, which is derived from wood pulp. However, the exact calorific value is not static and depends on several characteristics, including the type of paper, its moisture content, and the presence of any additives or coatings.
The Average Calorific Value of Paper and Cardboard
Research indicates that the average higher heating value (HHV) of mixed paper and cardboard products is approximately 15.8 MJ/kg. Other studies report similar figures, with some waste-to-energy plants citing a net calorific value (NCV) of 15.0 MJ/kg for mixed paper/cardboard waste. It is important to distinguish between HHV and NCV: HHV includes the energy released when water vapor from combustion is condensed, while NCV does not. Therefore, NCV is typically lower and is often a more relevant metric for industrial applications.
Individual paper and cardboard types can have slightly different values. For example, a study examining different waste streams found cardboard to have an HHV of 15.2 MJ/kg, while newspaper had a higher value of 17.6 MJ/kg. Kraft paper, a component in some types of cardboard, has been found to have a lower calorific value (LCV) around 12.66 MJ/kg. Specialty papers can also vary significantly. Polymer-coated papers, for instance, have been measured with net calorific values from 16.29 to 22.98 MJ/kg, depending on their composition.
Factors Influencing the Calorific Value
The measured calorific value of paper and cardboard can fluctuate due to several key factors:
- Moisture Content: Moisture is arguably the most significant factor impacting calorific value. Water content directly reduces the energy-releasing potential of a fuel because energy is consumed to evaporate the water during combustion. A study on specialty waste paper notes that moisture negatively influences both calorific efficiency and the rate of energy release.
- Ash Content: High ash content, which consists of non-combustible materials like clay and titanium dioxide used as fillers in paper production, decreases the heating value. Glossy magazine paper, for example, can have a higher ash content than plain office paper, resulting in a lower energy output per unit mass.
- Additives and Coatings: The presence of additives, coatings, and inks can alter the calorific value. Some treatments, like certain de-inking processes, can lead to minimal changes, while others, like the addition of plastics, can significantly increase the heating value.
- Density: The overall density of the material affects the amount of energy that can be obtained from a given volume. For instance, paper dust briquettes are densified to ensure efficient handling and to improve gasification performance for waste-to-energy applications.
Paper vs. Cardboard vs. Other Fuels
While paper and cardboard are effective fuel sources, their calorific values differ from other common combustibles. This comparison is useful for understanding their role in waste management and energy production.
Comparison Table: Calorific Values (MJ/kg)
| Material | Typical Calorific Value (MJ/kg) | Notes |
|---|---|---|
| Paper (Mixed) | ~13.5 | Lower values for wet or high-ash paper. |
| Cardboard (Mixed) | ~15.2 to 17 | Higher values generally correspond to dry, uncoated corrugated cardboard. |
| Dry Wood | ~14.4 | A benchmark for many biomass fuels. |
| Coal | ~15 to 27 | Significantly higher and more variable depending on the type of coal. |
| Petrol (Gasoline) | ~45 to 47 | A much higher energy density, typical of fossil fuels. |
| PVC Plastic | ~41 | Very high energy content, though burning PVC releases toxic gases. |
Applications in Waste-to-Energy
Given its energy content, waste paper and cardboard are valuable resources in waste-to-energy (WtE) plants. In these facilities, mixed municipal solid waste (MSW) is incinerated to generate steam, which in turn drives turbines to produce electricity. Because the calorific value of unsorted MSW can be inconsistent, waste streams containing high energy materials like paper are often homogenized to ensure a stable and predictable energy output. The combustion of paper dust is also being explored to produce electricity and biochar, offering a sustainable alternative for waste management within the paper industry.
Furthermore, paper and cardboard waste can be densified into fuel briquettes, which can be co-fired with other biomass materials. Research has shown that mixing cardboard with sawdust can improve the compaction and combustion efficiency of the briquettes. The practice of thermal co-processing with biomass is a viable option for valorizing paper mill sludge and other paper wastes.
Conclusion
In summary, the calorific value of paper and cardboard makes it a significant source of stored energy, with typical values averaging around 15.8 MJ/kg. The value is influenced by moisture content, density, ash content, and the presence of additives, causing it to vary between different types of paper and cardboard. While its energy density is lower than fossil fuels, it is comparable to or greater than many forms of wood biomass. This makes it a valuable feedstock for waste-to-energy initiatives, though careful management and sorting are necessary to achieve optimal energy recovery. Understanding these properties is crucial for waste management strategies that aim to recover energy efficiently and reduce reliance on landfills. For more information on waste-to-energy processes, a helpful resource is the U.S. Environmental Protection Agency's overview of the topic. Waste to Energy (WTE) | US EPA.
How does the calorific value of paper compare to wood?
Both paper and wood are cellulose-based biomass, giving them similar calorific values. However, paper's value is often slightly lower (~13.5 MJ/kg) than dry wood (~14.4 MJ/kg), especially if the paper contains non-combustible fillers like clay. Dry, untreated cardboard can have a calorific value comparable to or slightly higher than dry wood.
Is the calorific value of paper and cardboard high enough to be a standalone fuel?
Yes, waste paper and cardboard have a high enough calorific value to be used as a fuel, particularly in waste-to-energy facilities or when processed into fuel briquettes. However, its energy density is lower than fossil fuels, meaning it may not be as efficient for certain applications without additional processing or co-firing with other materials.
What is the difference between HHV and NCV for paper and cardboard?
HHV (Higher Heating Value) includes the energy released from condensing the water vapor produced during combustion, while NCV (Net Calorific Value) does not. NCV is a more practical measure for many industrial applications because the latent heat of water vapor is often not recovered. The NCV for paper/cardboard is typically slightly lower than the HHV.
Does moisture affect the calorific value of paper and cardboard?
Yes, moisture content significantly decreases the effective calorific value. Energy is required to evaporate the water during combustion, reducing the overall heat released. Therefore, dry paper and cardboard will have a higher heating value than wet material.
Are there any environmental concerns with burning paper and cardboard for energy?
While burning paper for energy can reduce landfill waste, it can produce harmful emissions, especially if the material is treated with inks or coatings. Waste-to-energy plants must use specialized flue gas cleaning systems to capture pollutants and comply with emission standards. The potential for emissions is a key consideration when using paper and cardboard as a fuel source.
How does recycling paper compare to burning it for energy?
Both recycling and waste-to-energy offer environmental benefits. Recycling conserves energy used in manufacturing new paper and reduces greenhouse gas emissions from landfills. Burning paper for energy recovers the inherent energy content, preventing it from being wasted in a landfill. The optimal approach depends on the specific type of paper and the efficiency of local recycling and WtE infrastructure.
How does the type of paper affect its calorific value?
The type of paper, including its composition, fillers, and coatings, directly influences its calorific value. Plain office paper, composed of pure cellulose, will likely have a higher energy content than glossy magazine paper, which contains a higher percentage of non-combustible clay fillers. Specialty papers or those with plastic coatings can have different heating values.
