Origin of clastic partings and their impact on ash yield in mined lignite: A case study from middle Miocene of central Poland

Abstract

The investigated first Mid-Polish lignite seam (MPLS-1) was formed during the middle part of the Mid-Miocene. It covers ca. 70,000 km(2) of the Polish territory and has been mined exclusively from the Konin Basin on an industrial scale for eighty years, that is, since 1942. This seam with a thickness >10 m was exploited for decades, although the interbedded clastic sediments were rather thin (<0.3 m). Unfortunately, the exploitation of increasingly thinner lignite beds (<10 m on average) began in recent years due to the depletion of deposits. The currently mined MPLS-1 near Konin (central Poland) is locally split into 2-3 lignite benches by layers of sand and/or clay, which are 0.3-5.3 m thick. Such relatively thick interseam clastics affect the quality of the lignite that is sent to power plants to produce electricity. Therefore, for the first time in the case of Polish deposits, the quantitative impact of these clastic partings on ash yield of the exploited lignite seam was calculated and presented in this study.

The presence of sand and clay strata in the MPLS-1 is very interesting in terms of their origin. The interseam beds of sand we interpret as representing crevasse-splay sediments, while the layers of clay as formed in lakes existing on the surface of Mid-Miocene mires. On the other hand, these clastic partings complicate the planning and process of lignite exploitation. In brief, the interseam sands and clays significantly increase the ash yield of the entire MPLS-1. As a result, larger amounts of ash also have a more negative impact on the environment, because these ashes have to be deposited in water (filling the post-mining depressions), which leads to a strong alkalization of the reservoirs. Therefore, we proposed an original formula on the basis of which the ash yield in the entire exploited lignite seam (MPLS-1) was calculated. Such a scientific approach is innovative in lignite mining, not only in Poland and the Czech Republic. As a result, our calculations along the geological cross-sections and/or mining fronts show that the clastic partings, mined together with the lignite benches, increase the ash yield by 2.5-65.0 wt% to even 74.7 wt% on a dry basis. Such lignites do not meet the Polish energy sector suitability criteria, which are defined as a maximum of 40 wt% of ash. In summary, mining less clastics together with lignite could make electricity production in lignite-fired power plants somewhat cleaner.