Filling agricultural waste into coal mine goafs : a potential carbon sequestration in China

China is a significant atmospheric carbon dioxide producer. Burning of agricultural waste in China is also a problematic issue, raising environmental and carbon-emission-related concerns. Furthermore, the coal-dependent economy is accompanied by the formation of large coal mine goaf areas, particularly in Shanxi Province. In this context, the idea of filling crop residues into the coal mine goafs is proposed. This concept addresses multi-functions: (1) carbon sequestration, (2) an alternative disposal method of crop residue in rural areas, and (3) coal mine goaf remedy.

The anthropogenic activities have led to a significant increase in Earth's atmospheric carbon dioxide levels, from about 280 ppm in the pre-industrial time to about 353 ppm in November 1990, and about 404 ppm as of March 2016 (http://co2now.org).2][3][4] Therefore, an urgent step forward to decrease carbon dioxide level is critical.
Due to the rapid economic expansion, China has become one of the biggest producers to the present global greenhouse gas. 3,5 n this context, the Chinese government has initiated a series of policies (the twelfth five-year plan in particular) to move toward a carbon-neutral and sustainable economy.Carbon trading with Chinese characteristics is an important element of the national plan to fight climate change. 6Up to date, a more effective integrated portfolio of strategies is still highly desirable to cut carbon emissions.

PATH FORWARD FOR CARBON SEQUESTRATION
Besides huge efforts in improving the energy efficiency and increasing the use of renewable energy, China's investments in carbon sequestration are also on the rise.][9][10][11] Terrestrial ecosystems are natural carbon reservoirs.In this sense, afforestation and enhanced soil carbon storage will definitely continue to be China's environmental-friendly strategies.
Another important concept of carbon sequestration is to capture the carbon dioxide generated during combustion and store it in a suitable place. 12Typically, this carbon capture and storage (CCS) concept features in carbon dioxide separation/capture at coal-fired power/thermo plants and its subsequent storage in porous sedimentary rocks deep below ground.A key advantage associated with the CCS concept is that it can provide a rapid and effective approach to carbon emission mitigation.The CCS technology can also be combined with enhanced oil recovery. 13] Looking into the future, an integrated application of various technologies in China for efficient carbon sequestration is highly desired.Still, there remains an ongoing need to search for alternative strategies that will help to win the battle against climate change.

PROBLEMS ASSOCIATED WITH BURNING OF AGRICULTURAL WASTE
Agriculture has always been a key element of China's economy.A large amount of crop residues are produced each year.As one category of bio-based resources, crop residues are mainly rich in lignocelluloses. 16The utilization of these residues has been the center of the Central Government policy, and many products/processes have been commercialized.However, a large amount of the collectable crop residues are still not used at all.For example, in 2012, about 26% of the collectable crop residues, which amounted to about 210 million tons, were treated as wastes. 17

Opinion
These unused wastes have posed serious disposal problems.In many parts of China, including Henan, Shandong, Hebei, Jiangsu, and other provinces, the burning of crop residues (e.g., wheat/rice straws) by farmers has been common (see Fig. 1). 18For instance, in such provinces as Henan and Shandong, the farmers tend to burn the residues after the completion of wheat harvesting so as to plant corns as early as possible.However, during burning, many pollutants (including cancer-causing compounds) are released into the air, resulting in significantly deteriorated air quality.This also contributes to increased atmospheric greenhouse gas (mainly carbon dioxide) levels.Furthermore, the burning of crop residues may significantly increase the possibility of fire disasters, causing safety threats to the people in rural areas.Therefore, the mitigation of the negative impacts of crop residue burning is an urgent task.The current situations call for appropriate disposal of the unused crop residues.
Fig. 1 Air pollution and increased atmospheric carbon levels caused by burning of crop residues in a farmland in Jiangsu Province, China.

PROBLEMS ASSOCIATED WITH COAL MINE GOAF AREAS
China is one of the largest consumers and producers of coal in the world.Based on the current situations, coal will continue to be China's major energy source in the foreseeable future.
Coal mining is accompanied by the formation of large goaf areas.In Shanxi Province, the coal mine goaf areas are about 20 thousand square kilometers, currently accounting for 1/8 of the total area of the province.In many parts of China, Shanxi Province in particular, there are serious dangers and damages caused by the collapse of rural/urban areas.The situations are even more pronounced under harsh weather conditions (e.g., heavy rainfalls/snowfalls).Although measures have been taken to solve these problems, up to now, the mitigation of the negative impacts of goaf areas has been far from satisfactory.

POSSIBILITIES OF FILLING AGRICULTURAL WASTE INTO COAL MINE GOAFS
][21][22] This concept may be specifically tailored for the disposal of unused agricultural crop residues in China (see Fig. 2).A feature is that crop residue utilization and coal goaf filling can be well combined.In this concept, a considerable flux of carbon dioxide is constantly assimilated/ captured via photosynthesis during the growth of crops, and its subsequent burial in the coal mine goaf areas, cuts off its return pathway.Thus, an effective carbon sink can be formed.

Fig. 2 Proposed strategies for disposal of agricultural crop residues
China has large coal mine goaf areas.In particular, the total goaf volume in Shanxi province is about 6.3 billion cubic meters. 23The bulk density of crop residues is in the range of 60 to 450 kg/m 3 . 24Based on this bulk density range, the direct filling (i.e., loose packing) of coal mine goaf with the currently unused annual crop residues of about 210 million tons, 17 would roughly take up 0.525 to 2.1 billion cubic meters, indicating that just Shanxi province alone would have coal mine goaf volume equivalent to 3 to 12 years of unused annual crop residues.Based on the average carbon content of agricultural crop residues of roughly 40%, 22 filling with 210 million tons of unused crop residues annually can result in the sequestration of 84 million tons of carbon, which is equivalent to carbon dioxide sequestration of 308 million tons.This indicates the potential of carbon sequestration by filling crop residues into the coal mine goaf areas.
Besides carbon sequestration, the implementation of this concept in China may lead to mitigated safety problems associated with the coal mine goaf.As an option, mechanical and/or chemical pretreatments (e.g., crushing, compressing and carbonizing) of the crop residues can be conducted to increase their packing density inside the goaf, so that the allowable filled amount for a given goaf volume can be increased.Meanwhile, filling unused crop residues into coal mine goaf can be a potentially effective disposal method of crop residues.
In essence, the above proposed concept fits China's current situations very well.For instance, Shanxi is a coalrich province, while its adjacent provinces, including Henan, Hebei, and Shandong, have huge crop reside reserves, particularly wheat/rice straws.In this context, the Chinese government may consider making policies/legislations and providing funds to put this concept into practice, so that it may be integrated into the portfolio of various carbon sequestration strategies.
A further noteworthy point is that in addition to the use of agricultural crop wastes, planting fast-growing carbonsequestrating crops (particularly those adaptable to harsh conditions, such as Miscanthus grasses), 25 and subsequently filling them into the coal mine goaf areas, may be considered.