Drying Technologies of Lignite Coals


Drying Technologies of Lignite Coals

Coals are generally ranked as anthracite, bituminous, sub-bituminous, and lignite, with anthracite being the oldest and lignite the youngest in the age. As coal ages, its moisture content decreases and heating value increases. The lignite coal is often being referred to as brown coal. It is considered to have the lowest rank, lowest carbon (C) content and highest moisture content. Moisture content in lignite coals can be even 60 % or more.

Lignite coals are usually shallow buried facilitating its easy open mining. These coals besides high moisture content also have high volatile content and low calorific value (CV) with easy spontaneous ignition. High moisture content is the main restraint for the application of lignite coals.

Moisture content of coal causes many difficulties during processing, storage, transport, grinding, and combustion. The high moisture content considerably reduces the CV and combustion efficiency of the coal. It also results into higher heat loss in the exhaust gas.

In the combustion of lignite coals, the important part of the energy is consumed to evaporate the moisture inside the coal. The combustion of the high moisture content coal creates several problems such as the additional energy consumption for the moisture evaporation, the insufficient combustion and the additional exhaust discharge etc.

Moisture content of the lignite coals can be classified into the following three types.

  • Surface moisture – It is also known as external moisture. The moisture adheres to the surface of coal particulates or in the bigger capillary cavities. It is the moisture, which can be removed by the coal drying in air at ambient temperature (around 25 deg C). It depends on water conditions in deposit.
  • Inherent moisture – It is a naturally combined part of the coal deposit. It is also called hygroscopic moisture which is the moisture in the air-dry state. It is chemically bound water, so called constitutional and intermolecular water. The hygroscopic water content decreases with the increase of the rank.
  • Crystallized moisture – It is the chemically combined with the mineral matters in coal. It is also called decomposition moisture and is the water which is formed during thermal decomposition of the coal.

The drying or dewatering of the lignite coals decreases the problems caused by the high moisture content. This reduces the burden on the coal handling system, conveyers and crushers. Also, since dried coal is easier to convey, this reduces maintenance costs and increases availability of the coal handling system.

Removal of moisture from the lignite coals improves the CV which results into reduction of pollutants. Reduction of the moisture content also reduces energy consumption in the grinding mills, lowers the loss of heat with flue gases, reduces transportation costs while increasing the combustion efficiency, safety, and reducing the amount of exhaust gases.

A number of drying processes for lignite coals are being developed in different counties. Many of these drying processes depend on high grade heat to reduce coal moisture content, or are employing complex equipment arrangements using expensive materials to recover latent heat of vaporization. These approaches significantly increase the cost of thermal drying.

Drying curve for the lignite coals is given in Fig 1.

Fig 1 Drying curve for lignite coals

Drying of lignite coal is usually the first and essential step in most processes and technologies which are based on the use of such coal. However, there is no single universal method of drying of the lignite coal. There are very large number of patents of coal dryers and drying processes all over the world. Never the less, only a few of these patented technologies are truly viable. Some of the ideas suggested are not even practical.

Methods of the drying of the lignite coals can be broadly divided into two groups. In the first group, the method of drying is known as evaporative drying. In the evaporative drying of the coal, the heat is provided to remove the water from the coal particle. The drying medium can be air, flue gas, or superheated steam. In the drying process, both the heat and the mass transfer mechanisms are active. The heating of the lignite coals can be either by direct contact or by indirect contact. The drying process utilizes either fixed bed, fluidized bed, or entrained bed. In the second group, the method consists of non-evaporative drying. The processes generally employed for the non-evaporative drying are based on thermal dewatering, thermal mechanical dewatering, or solvent extraction dewatering. Some of the processes of the drying of the lignite coals are described below.

Hot gas drying

Drying of the lignite coals by hot flue gas has been carried out many years back. It is a mature process with simple equipments. In this process, there is a direct contact between lignite and hot flue gas. Moisture absorbs heat in flue gas and evaporates. The drying medium which is hot flue gas is easy to obtain in power plants, from furnace or rear flue gas pass. Low oxygen content in flue gas can prevent the possibility of ignition and explosion during lignite drying process. Drying in coal pulverizer belongs to the drying methods by hot flue gas and is one of the most applied methods in power plants at present. The disadvantages of the process include high energy consumption and possibility of ignition and explosion.

Fleissner process

This is a very old process for drying low-rank coals, first developed in Austria by Professor Hans Fleissner in 1927. This process is based on the principle that uneven shrinking of the coal and consequent disintegration can be prevented by controlled removal of the water. The saturated steam atmosphere prevents evaporation until the lump is heated, and then loss of water can be controlled by gradual reduction of the steam pressure. It is a thermal drying process, in which the action of high pressure steam on a lump of lignite produces these effects. As the temperature rises and the pressure increases part of the colloidal water is expelled from the lump as a liquid. The lump shrinks as water leaves and the cells collapse, and when the pressure is lowered, more water leaves by evaporation caused by the sensible heat stored in the lump. When the pressure is lowered further by vacuum, additional moisture is evaporated, which cools the lump. Many methods of drying are based on Fleissner process.

Rotary tube drying

The drying process is carried out in a rotary tube heat exchanger and employs non?direct contact between steam and lignite. If no air infiltrates during the process then at the process end there is only water vapour. Hence, it is possible to recover the latent heat of vaporization. The process uses bulky device with low capacity of drying.

Superheated steam drying

Drying of lignite coal with steam is a recent process. In this process there is direct contact between lignite and superheated steam. The possibility of ignition and explosion during lignite drying process can be avoided due to the inertia of superheated steam. During the process there is no mass transfer resistance between moisture in lignite and superheated steam even though there is a high drying rate. In case of power plants using lignite coal as fuel in boiler, steam from turbine can be used as drying medium. If the latent heat of vaporization in off?gas can be entirely recovered, the energy consumption of drying by superheated steam is only around 20 % of that dried by hot air. Hence, drying by superheated steam has energy saving potential.

Mixed?bed drying

Mixed bed drying is a process of evaporative drying. The drying of the lignite coal is carried out in circulating fluidized bed where the hot bed material supplies heat for drying. Drying off?gas is water vapour which is easy to be recovered and utilized. The drying off?gas is cyclic utilization with heat transfer taking place in drying chamber where lignite coal gets dried.

Coldry process

Coldry technology has been developed in Australia in the early 1980s as a result of investigations in the Department of Organic Chemistry, University of Melbourne, in collaboration with CRA Advanced Technical Development, and patented by Environmental Clean Technologies Limited. It is a coal upgrading technology for lignite and sub-bituminous coals (brown coals) by removing natural high moisture content and certain pollutants.

The drying process is based on the release of moisture in the coal, by initiating an exothermic reaction, due to abrasion of the C particles together. The result is a concentrated product in the form of densified pellets which are durable, easy to store and transport, and which have similar energy value normally associated with many of the black coals, whilst significantly reducing CO2 emissions compared to its original brown coal form. The process consists of six steps namely (i) screening and feed control, (ii) attritioning and extruding, (iii) conditioning, (iv) pack bed drying, (v) water recovery, (vi) production of Coldry pellets.

During the first step, the feed of lignite is crushed and sieved to a size which is less than 8 mm diameter. Then the crushed particles consisting of C grains and water mixture is fed into an ‘Attritioner’ which rubs the coalfaces together. This initiates an exothermic chemical reaction which triggers a natural process of expelling water from the coal. The reaction accelerates when the now plasticized mixture is extruded under low pressure and sent to the conditioning unit. Here the extruded pieces of coal are heated for about an hour at a temperature just 40 deg C. The hardened and dried product is separated in the form of pellets and directed into the dryer. The final moisture content is in the range of 10 % and 14 % depending on the as-mined moisture, the characteristics of the feedstock and parameters of the process especially the temperature provided by the heat exchange unit, and the drying time allowed.

The produced dry Coldry pellets are typically of 16 mm diameter and 45 mm in length. They have bulk density of around 700 kg/cum to 750 kg/cum with a moisture content of around 12 % and have a high heat value of around 5520 kcal/kg.

The main benefits of the Coldry process are (i) increase of CV of lignite coal in the range of 200 % to 250 %, (ii) liberation of large volumes of water which can be recovered from the coal for immediate industrial use without expensive treatment, (iii) possibility of feeding the recovered water to the power stations’ cooling circuits, (iv) decrease in ash content, (v) reduction of CO2 emissions, (vii) reduction of ash accumulation, (viii) low temperature process since it needs waste heat at around 40 deg C, (ix) this low heat is sourced via heat exchange from a co-located power station, (x) low pressure process which requires less energy, and (xi) possibility of using the existing power boilers.

Thermal dewatering

Thermal dewatering of the lignite coal simulates the coal forming process under high temperature and high pressure to reduce the moisture content. It upgrades the lignite coal to a coal which is similar to bituminous coal. Process parameters are temperature in the range of 280 deg C to 350 deg C, pressure in the range of 10 atmospheres to 130 atmospheres. It is a non?evaporative drying method where the moisture in lignite extracted in liquid form. In addition to the drying, thermal dewatering also reduces the hydroscopic nature of the coal while increasing its CV. Some inorganic and organic matter is also removed during this process. The technological requirements are high and are difficult to realize in a large scale plant.

Thermal mechanical dewatering process

Thermal mechanical dewatering process of the lignite coal consists of the combined action of temperature and mechanical force. Moisture of the coal is extracted in liquid form. The investigation work on the thermal mechanical dewatering is being carried out in Germany, Australia, and China.

The process results into good drying with the removal rate of moisture higher than 60 %. The tendency of spontaneous ignition and the hygroscopic nature of the coal are reduced. Technological requirements consisting of a temperature lower than 200 deg C and a pressure of less than 2 atmospheres can be easily realized. Some inorganic matter is removed together with the moisture of the coal.

Mechanical thermal expression process

Mechanical thermal expression (MTE) process is the combination of mechanical expression and thermal dewatering process. It is a method which uses mild heat and mechanical compression. For getting substantial benefit from MTE process, it is necessary to heat the lignite coal above the normal boiling temperature of water. However, the processing temperature is to be low enough to prevent significant release of organics into the product water. Around 10 % to 60 % of the initial water is removed during the stage of mechanical compression. The compressive pressure is the major factor influencing the quantity of water removed.

Mechanical dewatering process is held in back-pressure to prevent evaporation, ensuring that the water is removed only by mechanical forces. Further moisture reduction is achieved by flash evaporation in the processed lignite coal by exposing it to atmospheric conditions.

The MTE process results into the removal of water which is around 75 % maximum of the original moisture content. The MTE process has certain drawbacks such as (i) the need for prior grinding of coal,(ii) the need for clean water produced, (iii) time consuming, and (iv) high investment and operating costs.

Electromagnetic mill

The drying of the lignite coal in the electromagnetic mill comes under the thermal-mechanical method of drying of the brown coals. In this method, coal is heated by steam at elevated temperature of 150 deg C to 200 deg C and at the pressure ranging from 5 atmospheres to 16 atmospheres It is then compressed in a hydraulic press to squeeze out the water.

The electromagnetic mill uses ferromagnetic grinding mediums with a very low weight. Hence, the power consumption is very low. The grinding mediums follow changes of the magnetic field reaching high kinetic energy. To increase the productivity of the mill, or get a finer grain size of the product, the multi-section structure (parallel or serial) can be introduced.

Advantages of this method include (i) a short drying time which is around 30 seconds, (ii) low energy consumption, and (iii) removal of water to the extent of around 75 % of the original water content.

Drying in fluidized bed

In the drying process for the lignite coal in fluidized bed, there is direct contact between the coal and the drying medium, with lignite coal particulates remaining in suspension condition. Fluidizing medium which can be used generally consists of hot air, hot flue gas, and superheated steam. It is feasible to have built?in heat exchanger which can supply more heat for the drying.

Characteristics of drying of the lignite coal in fluidized bed include (i) high drying rate, (ii) compact structure, and (iii) easy to achieve large scale operations. Built?in heat exchanger can supply most heat, decrease fluidized medium flow, reduce the size of the dryer, and decrease energy consumption of the fan. If water steam is used as drying medium, spontaneous ignition of lignite coal can be avoided, with high mass transfer efficiency achieved. Fluidized medium and hot fluid in built?in heat exchanger can be extracted from boiler or turbine, which is easy to integrate with power generation system.

The WTA (Wirbelschicht Trocknung Anlage) technology

WTA technology was developed by German company RWE Power AG. It is a technology of drying in a fluidized bed with internal waste heat utilization. Fig 2 shows a schematic overview of the process.

                        Fig 2 Schematic overview of the WTA lignite fine drying process

The raw coal is ground down to a size less than 2 mm in two hammer crushers directly connected in series. After the grinding, the coal is fed into the fluidized bed, in which the fluidizing medium is the vapour arising from the drying process. Evaporation of water occurs at 110 deg C under slight over-pressure by heat exchangers integrated into the fluidized dryer and heated with steam. The residence time of lignite coal in the drying chamber is in the range of 60 minutes to 90 minutes.

The dried coal leaving the stationary bed is separated from the accompanying vapour first in a cyclone and then in an electrostatic precipitator. The vapour at the outlet of the cyclone is the vapour used for fluidization of the bed and the vapour at the outlet of the electrostatic precipitator is discharged into the atmosphere. In addition, there is a coarse extraction for the coal at the bottom of the bed, which is mixed with the coal separated at the cyclone and the electrostatic precipitator after having passed an intermediate cooler.

The heat needed for the drying of the coal is supplied by external steam, which is normally taken from the turbine with the heat transfer taking place in tube bundles located inside the bed. The drying in the fluidized bed further reduces the grain size, so that the dry coal leaving the drier typically has a grain size of less than 1 mm with around 9 % more than 1 mm. The dried coal has a moisture content of around 12 %. By controlling the fluidized bed temperature, the moisture content can be adjusted and kept constant at the desired value. WTA technology is an important element to reduce CO2 emissions in lignite coal electricity generation.

Presently this technology is working at the 1000 MW capacity Niederaussem power station. The system, which can process 210 tons of raw coal an hour, has an evaporation capacity of 100 tons of water per hour and is the biggest lignite drying plant in the world. It can generate 110 tons of dry lignite an hour.

The major advantages of the WTA technology are (i) high energy efficiency because of drying at low temperature, and energetic use of the evaporated coal water (through vapour condensation or mechanical vapour compression), (ii) very safe because of drying of coal in an inert atmosphere thus avoiding explosive coal dust-air mixtures, (iii) compact design due to integrated raw lignite fine milling system and where needed secondary dried lignite milling as well, and (iv) utilization of the energetic vapour avoiding significant steam and dust emissions. The vapour condensate is a water source which can be used.

Combined grinding and drying process

Lignite coal is normally ground prior to its utilization. The heat produced during grinding can reduce the moisture content considerably while reducing the particle size. One of the commercial devices combining these two functions in the application for coal drying is the KDS (kinetic disintegration system) Micronex grinder/dryer. The equipment consists of a high volume grinder, which grinds and dries the coal in a single step process, without needing any heat input. The mechanism of drying is partly thermal and partly mechanical dewatering. KDS technology uses significantly less total energy (70 %) than needed for the conventional drying and grinding combined.

Solvent extraction dewatering process

The solvent extraction dewatering process is based on the principle of variation of water solubility in nonpolar solvent. The common solvents used are dimethyl ether (DME), supercritical CO2, toluene, and anisole etc. The process reduces the tendency of spontaneous ignition. For some solvents like DME, technological requirements and energy consumption are low. The organic solvent increases the moisture extraction cost. The drying rate of the lignite coal by this method is low. The process is difficult to realize for large scale installation.

Pristine-M process

Pristine-M process is being developed by Clean Coal Technologies, Inc. (CCTI. The process is for converting raw coal into a cleaner, more efficient fuel source. It has been developed to dewater coals whose moisture content are high (30 % to 60 %). It combines a unique concept known as ‘Vapour Phase Deposition’.

CCTI’s process addresses three fundamental challenges. These challenges are (i) to produce a product which does not re-absorb moisture, (ii) to produce a product of low friability which can be safely transported with minimal risk of spontaneous combustion, and (iii) the process to be inexpensive and economically viable. The process does not need pulverization of the feed coal. The raw coal suffers almost no degradation and, consequently, briquetting or pelletizing is not a part of the process.

Pristine-M is a continuous process and is comprised of three separate components. The process utilizes a devolatizer to produce gases which are used for the process heat as well as to stabilize the dry coal. Only a small portion of the feed coal (typically less than 7 %) is devolatized. Process parameters are optimized so as to produce only enough volatile gases for the mentioned purposes. Liquid byproducts are not desirable in this process. Excess devolatized coal is blended back with the dry and stabilized coal at the end of the process and, thus, is not lost.

The second component of the process comprised of Carrier-designed dryers. Drying takes place at around 120 deg C, a temperature that is adequate to drive off inherent moisture with the degree of removal, e.g., down to 15 % or 10 % or 5 %, being a function of residence time, bed depth and temperature. Certain coal types have a tendency to degrade into fines as a function of the degree to which the raw coal is dried. In such cases, the removal of moisture is reduced otherwise briquetting of dried coal is required. The process is designed to remove fines at various stages and making them available for combustion (process heat), if required. The small amount of fines which enters the third phase of the process tends to agglomerate and hardens on the surface of the dry and stabilized coal.

In the third stage of the process which is the stabilization/ Vapour Phase Deposition phase, the volatile matter is made to be absorbed into the pores of coal from where the moisture has been removed.  For achieving the desired result, stabilization parameters are established based on the chemical profile of the feed coal.

The Pristine-M process makes the coal impermeable. Also the structural integrity of the coal is maintained and its heat value can be enhanced beyond the value achieved with only the removal of moisture. The Hardgrove Grindability Index (HGI) of the product coal remains the same as that of the feed coal.

The Pristine-M process is modular. A commercial module which is designed to feed throughput of 30 tons per hour and handling lignite coal with 50 % moisture has a capacity to produce around 160,000 tons of dry coal per year. A one million ton per year plant based on Pristine-M process is comprised of 6 such modules. The process is continuous with resident times estimated to be around 15 minutes, depending on the degree of moisture removal and the inherent moisture in the coal.  The plant operates at the pressure of 1 atmosphere.