Biomass pellet fuel refers to a new type of environmentally friendly solid fuel. It is made from agricultural and forestry wastes such as wood chips, sawdust, wood shavings and branches, which are processed through crushing, drying, blending, compression and other procedures into pellets of various specifications. This fuel can directly replace coal in boiler combustion, and its exhaust gas emissions after combustion are far lower than international standards. As a new type of clean energy, biomass fuel has gained widespread recognition due to its advantages of high calorific value, low ash content, high energy efficiency and no pollution. Meanwhile, it not only has economic benefits but also environmental advantages, which is in line with the requirements of China’s sustainable development. However, coal is still the most commonly used fuel at present. So what are the differences between these two types of fuels? Let’s take a look:
Advantages of Biomass Energy
- Provides low-sulfur fuel
- Supplies low-cost energy (under certain conditions)
- Reduces environmental pollution by converting organic matter into fuel (e.g., waste-derived fuel)
- Involves fewer technical challenges compared with other non-traditional energy sources
Disadvantages of Biomass Energy
- Plants can only convert a tiny fraction of solar energy into organic matter
- The energy density of organic matter per unit land area is relatively low
- Shortage of land suitable for cultivating energy plants
- Organic matter has a high moisture content (ranging from 50% to 95%)
Advantages and Disadvantages of Coal
1. Advantages
1.1 Good Environmental Benefits of Underground Coal Gasification (UCG) Technology
The ash and slag generated by underground coal gasification remain underground. With the adoption of filling technology, surface subsidence is greatly reduced and no solid waste is discharged. Therefore, UCG minimizes the land area occupied by waste and fly ash disposal, and reduces damage to the surface environment—an advantage unmatched by other clean coal technologies. The coal gas produced by underground gasification can be centrally purified to remove tar, sulfur, dust and other harmful substances, which can eliminate SOx and NOx pollution, and significantly reduce other pollutants such as mercury, particulates and sulfur-containing compounds.
Compared with traditional underground coal mining combined with surface combustion, UCG can reduce carbon dioxide emissions and facilitate carbon capture and storage (CCS). After CO in the gas is converted on the surface, CO2 can be separated and stored or utilized for other purposes through separation technologies, thereby producing clean coal gas. Therefore, underground gasification technology is conducive to solving the problem of air pollution.
The hydrogen content in the coal gas produced by underground gasification is more than 40%, and hydrogen of various purities can be obtained through separation. Hydrogen is the most ideal clean energy for human beings today. It has good storability and transportability, and is not only a high-energy fuel but also can be used as an intermediate energy carrier. It features flexible conversion, convenient use, cleanliness and hygiene, and forms a natural water-hydrogen-water cycle in nature. Thus, hydrogen energy is a renewable energy source that meets the needs of human sustainable development.
1.2 Improved Coal Resource Utilization Rate with UCG Technology
UCG technology can greatly improve resource recovery rate. Underground coal gasification carried out before coalbed methane extraction can recover more than 75% of the coal’s calorific value, while the recovery rate can still reach 70% if gasification is conducted after coalbed methane extraction. In addition, it enables the exploitation of coal resources that are difficult to access with traditional processes, including deeply buried coal, marginal coal seams, coal under “three types of surface structures” (rivers, bridges and buildings), protective coal pillars left by abandoned or soon-to-be-abandoned mines, and high-sulfur, high-ash low-quality coal that is prohibited from mining according to environmental protection regulations.
Coal is a fundamental industry supporting China’s national economic development. However, restricted by the technical level of traditional underground mining, a large number of mines have been or are about to be abandoned with the gradual increase of mining intensity. According to statistics, 297 mines were abandoned from 1953 to 1989, and another 244 mines will be decommissioned from 1990 to 2020. Up to now, the reserves of abandoned coal resources have exceeded 30 billion tons, which mainly consist of coal pillars, thin coal seams, low-quality coal seams and high-gas coal seams left by underground mining (a mining method where workers go underground to extract resources, as opposed to open-pit mining; underground recoverable coal only accounts for 11.43% of the total coal resource reserves). The development and application of UCG technology provides an effective way to utilize these resources. It is estimated that about 50% of China’s abandoned coal resources can be reused through UCG technology. This technology can also be applied to exploit coal seams that are difficult or economically and technically unfeasible to mine with traditional underground methods, such as thin coal seams, deep coal seams, coal under “three types of surface structures”, high-sulfur, high-ash and high-gas coal seams, as well as shallow offshore coal seams. Therefore, underground gasification can significantly improve the utilization rate of coal resources.
1.3 High Safety
UCG technology enables unmanned and equipment-free coal gas production underground, thus ensuring high safety and avoiding accidents such as mine collapse, water inrush and gas outburst that are common in traditional coal mining.
1.4 Low Investment and Good Economic Benefits
The investment required for constructing an underground coal gasification plant is 2.5 times lower than that for building a mine and a coal yard, and is also significantly lower compared with surface gasification projects.
1.5 High Labor Productivity
The labor productivity of UCG is as high as that of open-pit coal mining, which is four times that of underground mining. The product cost is equivalent to that of open-pit mining and is much lower than that of underground coal mining.
1.6 Elimination of Coal Transportation and Loading/Unloading Procedures
This eliminates fuel loss and pollutant emissions such as coal dust during transportation, and reduces the corresponding costs.
2. Disadvantages
The large-scale industrial promotion of underground coal gasification still faces many challenges that require substantial R&D investment to overcome. Despite its numerous advantages, the technology is not yet fully mature and has several limitations:
- Potential for significant environmental impacts: pollution of underground aquifers and surface subsidence. With current technological knowledge, structures can be designed to avoid or mitigate such risks.
- Although UCG may be technically feasible for many coal resources, the number of coal deposits suitable for this technology is subject to far more restrictions. This is because some deposits have geological and hydrogeological characteristics that may raise environmental risks to unacceptable levels.
- The control over underground coal gasification cannot reach the level achieved in surface gasification. Many process variables, such as water injection rate, reactant distribution in the gasification zone, and cavity expansion rate, can only be estimated by measuring temperature and the quality and quantity of product gas.
- There is great uncertainty regarding its economic viability until a sufficient number of power plants based on UCG technology are constructed and put into operation.
- Underground coal gasification is essentially an unsteady-state process, resulting in fluctuations in the flow rate and calorific value of product gas over time, as well as unstable gas composition.