Brown coal. Gen­e­sis and dis­tri­b­u­tion.

Brown coal is a tran­si­tion­al form of sol­id com­bustible min­er­al of sed­i­men­ta­ry ori­gin from peat to coal. Its deposits are found in the Earth­’s inte­ri­or in the form of plate- and lentic­u­lar-shaped bod­ies with a low thick­ness com­pared to the area of dis­tri­b­u­tion. Brown coal is includ­ed in the list of min­er­als of nation­al impor­tance approved by the Res­o­lu­tion of the Cab­i­net of Min­is­ters of Ukraine dat­ed Decem­ber 12, 1994, No. 827, as sol­id com­bustible min­er­als.

Conditions of formation

Coal is divid­ed into:

• humo­lites, which are formed from the prod­ucts of the trans­for­ma­tion of dead high­er plants;
• sapro­pelites, which are formed from the prod­ucts of the trans­for­ma­tion of low­er plants and microor­gan­isms;
• sapro­hu­mo­lites, which are a tran­si­tion­al type between humo­lites and sapro­pelites.

Humo­lites are divid­ed into humites and lip­to­bi­o­lites. Sapro­pelites are divid­ed into sapro­pelites (bog­heads) and humite-sapro­pelites. Sapro­hu­mo­lites are divid­ed into ken­nels and kas­sian­ites.

Most fos­sil coals are humites. All oth­er types are lay­ers, lens­es in humite beds, rarely form­ing inde­pen­dent lay­ers and deposits.

Coal for­ma­tion requires favourable con­di­tions, name­ly:

• phy­to­log­i­cal (plant mate­r­i­al);
• cli­mat­ic (ensur­ing the scale of life and inten­sive decom­po­si­tion of the mate­r­i­al);
• geo­t­ec­ton­ic (con­di­tions of mate­r­i­al dis­pos­al);
• geo­mor­pho­log­i­cal (deter­min­ing the cir­cum­stances for the accu­mu­la­tion and preser­va­tion of plant mate­r­i­al).

A favourable envi­ron­ment for coal for­ma­tion is water bod­ies with stag­nant or slight­ly mov­ing water, where there is no free access to oxy­gen. As plant residues accu­mu­late, the pool grad­u­al­ly becomes water­logged. If the required humid­i­ty is main­tained and the lithos­phere is con­tin­u­ous­ly sub­merged, the swampy regime lasts for a very long time.
The process of coal for­ma­tion is divid­ed into three stages: bio­chem­i­cal, chem­i­cal and geo­log­i­cal.

The bio­chem­i­cal stage involves the accu­mu­la­tion and decom­po­si­tion of plant residues in bog con­di­tions, which ends in peat for­ma­tion.
The chem­i­cal stage involves a series of chem­i­cal reac­tions that con­tribute to the cre­ation of a homo­ge­neous col­loidal mass. These include heli­u­mi­sa­tion, fusifi­ca­tion, and bitu­min­i­sa­tion.
The geo­log­i­cal stage begins after the peat deposit is buried and trans­formed into a rock, which under­goes a series of grad­ual trans­for­ma­tions (car­bon­i­sa­tion) under the influ­ence of pres­sure and ele­vat­ed tem­per­a­ture: peat — brown coal — coal — anthracite — graphite. The car­bon­i­sa­tion process is accom­pa­nied by a decrease in vol­ume, changes in phys­i­cal prop­er­ties and chem­i­cal com­po­si­tion.

The gen­e­sis of the coal basins is based on the genet­ic clas­si­fi­ca­tion, which is cre­at­ed on the tec­ton­ic prin­ci­ple, tak­ing into account the geo­t­ec­ton­ic con­di­tions of accu­mu­la­tion of coal for­ma­tions.

Three types of basins are dis­tin­guished by the nature of tec­ton­ic regimes:

• Geo­syn­cline basins are char­ac­terised by a large thick­ness of coal-bear­ing deposits (10–15 km) with a clear and fre­quent alter­na­tion of rocks of dif­fer­ent com­po­si­tion, a sig­nif­i­cant num­ber of mod­er­ate thick­ness coal seams, their aging, lin­ear fold­ing, a devel­oped fault sys­tem, and the pres­ence of zona­tion of coal meta­mor­phism in the ver­ti­cal and in the area.
• Plat­form basins are char­ac­terised by a low thick­ness of the coal seam, the pres­ence of lig­nite, and a small num­ber of seams that are not very old and have dif­fer­ent thick­ness­es. The coal-bear­ing deposits are hor­i­zon­tal or gen­tly slop­ing, and the ampli­tude of fault­ing is low.
• Tran­si­tion­al basinsTran­si­tion­al basins are char­ac­terised by inter­me­di­ate val­ues of the thick­ness of the coal seam and coal seams, their quan­ti­ty and degree of aging. The tec­ton­ic struc­ture of such basins is char­ac­terised by the devel­op­ment of brachyantri­cline folds, includ­ing dome-shaped folds.
• There are also poly­ge­net­ic basins where sev­er­al types are devel­oped simul­ta­ne­ous­ly. For exam­ple, the Donet­sk basin, where a con­sis­tent change from the geo­syn­cline type to the plat­form type is observed from east to west.

Physical properties

Brown coal cor­re­sponds to the low­est stage of car­boniza­tion and dif­fers from peat in a high­er degree of trans­for­ma­tion of plant residues and a high­er car­bon con­tent. The bound­ary between peat and brown coal is tak­en accord­ing to the fol­low­ing indi­ca­tors: car­bon con­tent — not less than 64%, mois­ture — more than 14%, calorif­ic val­ue — 20.2 MJ/kg. Brown coal is divid­ed into three groups: earthy — B₁, mat­te — B₂ and shiny — B₃. Between brown coal and hard coal, there is also a tran­si­tion­al brown long-flame (BD).

The main phys­i­cal prop­er­ties of brown coal are:

• col­or — light brown;
• gloss (depends on the con­tent of car­bon-form­ing com­po­nents) — with a pre­dom­i­nance of com­po­nents of the vit­ri­nite group and an increase in the car­boniza­tion stage — gloss increas­es, and with an increase in the con­tent of iner­ti­nite and lip­ti­nite — it decreas­es;
• reflec­tiv­i­ty – 0.30–0.49% (in air);
• frac­ture – earthy and uneven;
• struc­ture (depends on the shape and size of the coal-form­ing com­po­nents);
• tex­ture – brit­tle with an earthy tex­ture;
• den­si­ty – 0.8–1.35 g/cm³;
• Mohs hard­ness – 2;
• micro­hard­ness – 6–16 N/mm²;
• elec­tri­cal con­duc­tiv­i­ty – 10–200 Ohm*m, brown coal behaves as a dielec­tric.

In addi­tion to the above prop­er­ties, the refrac­tive index and bire­frin­gence, frac­tur­ing and gen­er­al poros­i­ty, strength, elas­tic and acoustic prop­er­ties are stud­ied.

The most impor­tant prop­er­ties of brown coal are:

• humid­i­ty – 16–60%;
• ash con­tent – accord­ing to the ash con­tent, coal is low-ash (less than 10%), medi­um-ash (10–20%) and high-ash (more than 20%);
• volatile mat­ter yield – 67–41%;
• sul­fur con­tent – accord­ing to the sul­fur con­tent, coal is low-sul­fur (0.5–1.5%), medi­um-sul­fur (1.6–2.5%), sul­fur (2.6–4.0%) and high-sul­fur (more than 4%);
• spe­cif­ic heat of com­bus­tion – 25.5–32.6 MJ/kg.

Ele­men­tal (chem­i­cal) com­po­si­tion of brown coal:

• car­bon – 63–77%;
• hydro­gen – 4.0–6.3%;
• oxy­gen and nitro­gen – bal­last com­po­nents of coal, with an increase in the degree of car­boniza­tion their con­tent decreas­es;
• phos­pho­rus — belongs to harm­ful impu­ri­ties.

In pet­ro­graph­ic stud­ies of coal, micro- and macro­com­po­nents are stud­ied. The fol­low­ing groups belong to the micro­com­po­nents of coal: vit­ri­nite (geli­fied com­po­nents), fusi­nite, iner­ti­nite (fus­enized com­po­nents) and lip­ti­nite (lipid com­po­nents).
Among the macro­com­po­nents of coal, the fol­low­ing are dis­tin­guished: vit­rain, clarain, durain and fusain. They dif­fer from each oth­er in phys­i­cal prop­er­ties (col­or, lus­ter, struc­ture, tex­ture, frac­ture).

Distribution in Ukraine

Ukraine has sig­nif­i­cant reserves of brown coal. The main part of the deposits is con­cen­trat­ed in the Dnieper brown coal basin, much less in the Dnieper-Donet­sk depres­sion, the Transnis­tri­an, Pre­carpathi­an and Tran­scarpathi­an coal-bear­ing areas. The deposits of the Dnieper basin and the Tran­scarpathi­an coal-bear­ing area are being devel­oped.
There are 80 brown coal deposits in Ukraine, of which 3 are under devel­op­ment. Bal­ance reserves as of Jan­u­ary 2021 are esti­mat­ed at 2593 mil­lion tons in cat­e­go­ry A+B+C₁ and 300 mil­lion tons in cat­e­go­ry C₂. Off-bal­ance reserves of brown coal in Ukraine are esti­mat­ed at 586 mil­lion tons.

The Dnieper brown coal basin is locat­ed with­in the bor­ders of Zhy­to­myr, Cherkasy, Kirovohrad, Dnipropetro­vsk, Zapor­izhia, Vin­nyt­sia and Kyiv regions. The basin stretch­es from south­east to north­west for 650 km, with a width of 70 to 175 km. Brown coal min­ing began in 1871, and at the end of the 19th cen­tu­ry it almost stopped due to com­pe­ti­tion with Donet­sk coal. In the 50s of the last cen­tu­ry, min­ing was resumed and the max­i­mum pro­duc­tion fell on the 60s–80s of the 20th cen­tu­ry. Cur­rent­ly, min­ing has sig­nif­i­cant­ly decreased and only 5 enter­pris­es are oper­at­ing.

Coal-bear­ing rocks with­in the Dnieper Brown Coal Basin are Juras­sic, Cre­ta­ceous, Pale­o­gene and Neo­gene deposits. The most car­bon-bear­ing are the con­ti­nen­tal deposits of the Buchach series of the Pale­o­gene, which are spread over a sig­nif­i­cant area (from Belarus and the city of Rom­ny to the city of Meli­topol). The place­ment of deposits of this series is sub­or­di­nate to buried pale­o­val­leys of main­ly ero­sion­al-tec­ton­ic ori­gin, which are elon­gat­ed in the form of strips of sub­merid­ion­al exten­sion. This series is divid­ed into the low­er part — sub­coal and upper — coal, with a total thick­ness of up to 70 m. In the coal-bear­ing part of the series, three hori­zons are dis­tin­guished: low­er (sub­coal), mid­dle (coal) and upper (supra­coal).

In the coal-bear­ing stra­tum, one to three coal seams (I‑III) are traced, which are sep­a­rat­ed by rock lay­ers with a thick­ness of 0.5 to 6 m.
The main seam in the basin is the low­er seam (I) of sim­ple struc­ture, which has a work­ing thick­ness at the Korostyshiv­ske, Olek­san­drivske, Fas­tivske, Mironivske, Verkhniod­niprovske and oth­er deposits. The thick­ness of the seam is from 1 to 25 m, on aver­age 4–6 m.
The mid­dle lay­er (II) of sim­ple struc­ture has lim­it­ed devel­op­ment in the basin area, the work­ing capac­i­ty is record­ed at Ora­tivske, Morozivske, Krasnopilske, Synel­nykovskie and oth­er deposits. The thick­ness of the lay­er reach­es 12 m, the aver­age thick­ness is 3–4 m.

The upper lay­er (III) is char­ac­ter­ized by an unsta­ble thick­ness (0.1–3 m), a small spread over the area and increased ash con­tent. The work­ing capac­i­ty is record­ed at Morozivske, Bal­akhivske, Ban­durivske and Semenivske-Olek­san­drivske deposits.
The coal seams lie almost hor­i­zon­tal­ly with a slight slope to the axi­al parts of the pale­o­val­leys, have an unsta­ble thick­ness that increas­es from the periph­ery of the deposits to the axi­al part. The max­i­mum thick­ness of all three deposits is estab­lished at the Verkhniod­niprovske deposit (29 m).
The devel­op­ment of coal deposits is car­ried out by open (cuts) and closed (mines) meth­ods.

Brown coal deposits with­in the Dnipro-Donet­sk depres­sion are known at the Novod­mytriv and Zhytkovytske deposits.Novod­mytriv deposit, locat­ed in the Kharkiv region. Brown coal seams are part of the Kharkiv, Polta­va and Kyiv series of the Pale­o­gene and undi­vid­ed Neo­gene. They are com­bined into 5 saucer-shaped hori­zons, with a total thick­ness from 8 m on the periph­ery to 157 m in the cen­ter. Bal­ance reserves for cat. A+B+C₁ are 390 mil­lion tons. Zhytkovytske deposit, locat­ed in the Sumy region. It is con­fined to the Romen­ska depres­sion, which is locat­ed in the north­west­ern part of the Dnipro graben. The deposit has 4 brown coal seams, with a thick­ness from 0.7–0.9 m to 1.8–2.5 m, lying at a depth of 40 to 240 m.

The Transnis­tri­an coal-bear­ing area is locat­ed with­in the Ternopil and Lviv regions, in the form of a sub­lat­i­tu­di­nal strip with a width of 7 to 30 km, a length of 180 km from the city of Rava-Rus­ka to the city of Shum­sk. The coal-bear­ing stra­tum is con­fined to the Low­er Tor­ton­ian deposits of the Miocene, with a thick­ness of up to 30–50 m. Coal seams lie at a depth of 50 to 60 m. The seams are com­plex in struc­ture, the thick­ness is not sus­tained, often have the form of small lens­es with an area of from 0.1 to 2.5 km². The thick­ness of the seams is from 0.5 to 1.5 m, some­times reach­es 3 m. Coal has a high ash con­tent (10–50%) and sul­fur con­tent (2–10%), the yield of volatile sub­stances is 30–60%, and the car­bon con­tent is 53–70%.

The Pre­carpathi­an coal-bear­ing area is locat­ed with­in the Ivano-Frankivsk and Cher­nivt­si regions. It stretch­es from north­west to south­east in a strip 15–30 km wide and 130 km long from the city of Kalush to the city of Storozhynets.
Pro­duc­tive lay­ers are con­fined to the Upper Tor­ton­ian deposits of the out­er zone of the Pre­carpathi­an depres­sion. The Upper Tor­ton­ian deposits are rep­re­sent­ed by the Pistyn­ian for­ma­tion, which is divid­ed into three hori­zons: Low­er Pokuttyan, Pistyn­ian, and Upper Pokuttyan. The coal-bear­ing one is medi­um, it is rep­re­sent­ed by alter­nat­ing clays with sands and sand­stones. The thick­ness of the hori­zon is 290–320 m. There are three coal seams in total, with a thick­ness of 0.5 m or more. The coal is tran­si­tion­al from brown to long-flame type. Dense, strong, black in col­or, ash con­tent 10–20%, volatile mat­ter yield — 43–61%, sul­fur con­tent — 2.0–6.5%. Spe­cif­ic heat of com­bus­tion 20.5–33.5 MJ/kg.

The Tran­scarpathi­an coal-bear­ing area is locat­ed with­in the Tran­scarpathi­an region in the form of a strip 18–34 km wide. Coal seams lie among deposits of the Sar­ma­t­ian, Pan­non­ian and Pon­tic stages of the Neo­gene. The most coal-bear­ing is the Ilnyt­s­ka Suite, which is rep­re­sent­ed by alter­nat­ing clays, sands, silt­stones, sand­stones, tuffs and tuff brec­cias. The for­ma­tion con­tains up to 10 lay­ers and seams of brown coal with a thick­ness of sev­er­al cen­time­ters to 1–2 m, some­times up to 5 m.
The coal is con­t­a­m­i­nat­ed with tuffa­ceous mate­r­i­al, the mois­ture con­tent is 15–35%, the ash con­tent is 10–40%, the sul­fur con­tent is 1.5–5.8%, the volatile mat­ter yield is 38–54%, the calorif­ic val­ue is 16.6–25.1 MJ/kg.
Until 1971, the Uzh­gorod, Berezyn, Kryv and Gorb deposits were devel­oped in a closed way. Cur­rent­ly, the Ilnytske deposit is being devel­oped in an open way.

The Nizh­ny Dni­ester coal-bear­ing area is locat­ed with­in the Odessa region. A coal-bear­ing strip 10–15 km wide is traced from the south­west to the north­east for 35 km. The coal-bear­ing rocks here are deposits of the Sar­ma­t­ian and Pon­tic stages. The thick­ness of the lay­ers of the Sar­ma­t­ian stage is main­ly 0.05–0.5 m, only in the south­ern part, in the area of lakes Yalpukh and Cahul does the thick­ness of the lay­ers reach work­ing capac­i­ties. The thick­ness of the Pon­tic coal seam is from 0.1 to 1.2 m. Coal of the Sar­ma­t­ian and Pon­tic age has an ash con­tent of 25–30%, humid­i­ty – 25–27%, volatile mat­ter yield – 52–54%, sul­fur con­tent – 8.3%, car­bon con­tent – 58–60%, calorif­ic val­ue – 24.2–27.8 MJ/kg.

The coal occur­rences of the Crimean Penin­su­la are asso­ci­at­ed with Tri­as­sic, Juras­sic and Cre­ta­ceous deposits, locat­ed in the moun­tain­ous part, where 57 occur­rences have been dis­cov­ered and par­tial­ly explored. The Beshui deposit is com­posed of two coal seams 0.6–0.8 m thick. The coal here is high-ash (40–50%), of low meta­mor­phism, and tran­si­tion­al from brown to long-flame.

Distribution in the world

The world’s reserves of brown coal are esti­mat­ed at 1.3 tril­lion tons. The main part of the reserves is con­cen­trat­ed in the USA, Ger­many, Ukraine, Poland, the Czech Repub­lic, and Aus­tralia. The largest basins and deposits of brown coal are con­fined to the com­plex of Meso-Ceno­zoic deposits. In Europe, brown coal deposits are asso­ci­at­ed with Pale­o­gene-Neo­gene deposits, in Asia — main­ly Juras­sic, less often Cre­ta­ceous and Pale­o­gene-Neo­gene. On oth­er con­ti­nents with Cre­ta­ceous and Pale­o­gene-Neo­gene deposits.

Brown coal in the USA is mined in the west­ern and south­ern parts of the coun­try (the states of North and South Dako­ta, Wyoming, Mon­tana, Utah, Col­orado, Ari­zona, New Mex­i­co, Texas, Arkansas, Mis­sis­sip­pi, Louisiana, Alaba­ma). There are also lit­tle-devel­oped brown coal basins in Alas­ka (the Lis­burn-Colville basin). Coal deposits in the USA are char­ac­ter­ized by a gen­tle occur­rence of seams at shal­low depths, an aver­age thick­ness of about 1 m, a sig­nif­i­cant spread over the area, and a mod­er­ate con­tent of water and gas.

Brown coal in Ger­many is con­cen­trat­ed in the Low­er Rhine, Low­er Lusa­t­ian, Mid­dle Ger­man, Magde­burg and oth­er basins. Brown coal seams are con­fined to Eocene-Miocene deposits with a thick­ness rang­ing from 3–4 to 8–12 m.

In Poland, brown coal is dis­trib­uted almost through­out its ter­ri­to­ry. Coal-bear­ing com­plex­es are Juras­sic-Neo­gene deposits. The main deposits are main­ly Miocene and are rep­re­sent­ed by soft brown coal. The largest deposits are in the cen­tral and west­ern parts of the coun­try. Brown coal is mined in the open pit method in the Tur­o­she­vo, Belcha­tow, and Konin dis­tricts. The reserves of brown coal, which are mined in the cen­tral (Malinec, Adamów) and south­west­ern (Tur­ośów, Żary) regions of Poland, are esti­mat­ed at 14 bil­lion tons.

Brown coal min­ing in the Czech Repub­lic is car­ried out main­ly by open pit min­ing. The largest brown coal basin is the North Bohemi­an Basin, locat­ed in the Ore Moun­tains. Its length is 65 km, with a width of 1 to 26 km. The thick­ness of the coal seams varies from 1 to 65 m.

More than half of Indi­a’s coal reserves are con­cen­trat­ed in the states of Bihar, West Ben­gal and Mad­hya Pradesh. The largest brown coal deposit in India is the Neyveli deposit, locat­ed in the state of Tamil Nadu. The total brown coal reserves in this deposit are esti­mat­ed at 3.3 bil­lion tons. One seam with a capac­i­ty of 18 m is being devel­oped here. Low-ash coal (3%), humid­i­ty 50%, volatile mat­ter yield 25%, calorif­ic val­ue 10.2–15.0 MJ/kg.

In Aus­tralia, brown coal is mined in the Latrobe Val­ley basin, locat­ed in the state of Vic­to­ria. Total coal reserves are esti­mat­ed at 113 bil­lion tons. pro­duc­tive lay­ers are asso­ci­at­ed with Oligocene-Miocene deposits. The largest deposits are Yal­lorn and Mor­well, locat­ed in the Latrobe Riv­er val­ley.

Brown coal is an impor­tant nat­ur­al resource with large reserves both in Ukraine and in the world. In Ukraine, the main deposits are con­cen­trat­ed in the Dnipro brown coal basin, as well as in the Dnipro-Donet­sk depres­sion, the Transnis­tri­an, Pre­carpathi­an and Tran­scarpathi­an coal areas. These reserves have sig­nif­i­cant poten­tial, but mod­ern min­ing and man­age­ment tech­nolo­gies are required for their effec­tive use. The effi­cient use of lig­nite is key to the ener­gy secu­ri­ty and eco­nom­ic devel­op­ment of many coun­tries.