28 January 2014 ã.
The basis for national security is a solid provision of population with domestic foodstuff produced within a preset volume. The grain is given the highest significance as it is socially most important strategic product. Therefore every kind of increases in grain production on the basis of using intensive technologies of cultivation is a first priority task of agro-industrial complex of Belarus.
Some 2,5–2,6 million ha of land are used to cultivate grain crops, leguminous plants and cereals in the Republic of Belarus. In recent years gross harvest of grain crops and leguminous plants (bunker weight) has grown by 1,5 times having reached more than 9,0 million tons in 2012. The task is set to yield no less than 12 million tons annually in the near-term outlook.
One of the most difficult technological operations in grain production is postharvest processing of the yield. It is also the most resources-consuming process in the whole technological chain which takes up 30–50 % of fuel, 90–98 % of electric power, 15–20 % of metal, 10–12% of labour costs and about 15–20 % of operating costs. The most urgent problem is the saving of energy resources (fuel, electric and heat power) during post-harvest processing by way of their most economical use. Certain well-directed measures are needed for these purposes: mass introduction of energy-saving machines and equipment, improvement of technological processes, rational organization of labour. Analysis of performance of machines and equipment for post-harvest processing of grain allows to determine factors which directly or indirectly influence the specific consumption of energy resources and to identify measures for decreasing energy consumption of these processes, especially drying of grain.
For reference: Each yearabout 80% (6,5–7,5 million tons) of harvestis dried. Agricultural enterprises of the Republic are equipped with some 3,2 thousand grain cleaning and grain drying complexes and around 1,3 thousand individually set grain-dryers, 1169 bunkers for active ventilation of the type BV-40. Most of machines and equipment is depreciated. Out of the whole amount of grain-dryers and grain drying complexes, only about 50 % have service life of 8 years, but some of them are used for more than 15 years and want change and renovation.
There are various technological ways to increase the effectiveness of removing moisture from grain. The main of them are:
- mixing of grain of different moisture and temperature
- temporary heating of raw grain (for its preliminary heating) or heating of a mix of raw and dried recirculating grain
- binning of a mix of grain or uniform (in terms of temperature and humidity) grain
- using different ways of advance of drying agent to grain
- utilization of heat energy when cooling the dried grain
Each of the mentioned ways renders different kind of influence on the improvement of the technology of grain drying and the decrease of power inputs.
Mixing of grain of different moisture. The nature of the process is the return (recirculation) of a part of dried grain into the dryer and mixing it with raw grain which is delivered on the dryer. The aim is to decrease the humidity and increase the temperature of raw grain at the expense of heat and mass exchange processes. This operation can be performed on modern dryers working in circulation mode (during dryer startup, when the first batch of grain is dried) or when drying grain with high humidity, e.g. maize grain, where mere pass of grain through the dryer is not enough for excess moisture to be removed. In some cases it can be quite effective but the efficiency of dryer is reduced in the process.
Preliminary heating of raw grain. Practically all shafts of modern dryers have three zones: preliminary heating, drying and cooling.
The main objective of preliminary heating is increasing of grain temperature up to the maximum permissible one (at given humidity level) and simultaneous vaporization of up to 30–40 % out of the total amount of moisture vaporized from grain during drying. In the process, diffusion of moisture from inner layers of grain seeds towards their surface intensifies which leads to the decrease of heat inputs for moisture vaporization during subsequent drying. As a result, it helps to save fuel.
Preliminary heating of grain can be performed within a slow-moving layer in special units with intrinsic source of heating of heat carrier. Such process of drying is implemented in grain cleaning and drying complex ZSK-30 of JSC “Amkodor” and ZSK-20–01 (illustration 1).
Illustration 1 – Technological scheme of grain cleaning and drying complex ZSK-20–01 of JSC “Amkodor”.
Raw grain goes from receiving-feeding unit 1 into ventilated accumulating bunker 2, where the pile of grain is heated and initial reduction of its humidity takes place as it is ventilated by the heated air from air-heater 10.
At this stage some 30–40 % of primary humidity is removed from grain (grain humidity is reduced by 3–5%). Slightly dried pile of grain is then directed for preliminary cleaning 3, and then to the dryer 5. After that it is delivered to the machine of primary cleaning 6. After primary cleaning it goes into ventilated accumulating bunker for dry grain 7, whereas waste is collected in bunker 4. In bunker 7 the grain is circulated and cooled by ambient air, at the same time the remaining excess moisture is removed from grain (grain humidity is reduced by 1–2 % and reaches standard condition). Dried cooled grain is discharged into a vehicle or into a mechanized warehouse.
As a result of using preliminary heating the subsequent drying progresses more effectively and in the long run inputs of heat energy are reduced by 15–20 % as compared with drying without preliminary heating.
Binning of grain. Binning of grain ( multi-component in terms of temperature and humidity) is based on partial redistribution of moisture between raw and dry components of grain mix and is performed simultaneously with equalization of their temperature. Effectiveness of inter-grain heat and mass exchange processes depends on the duration of binning.
Numerous researches of different authors have determined that when grain of different temperature is being mixed, its equalization takes place over short stretch of time (up to 10–15 minutes).
Another grain drying practice is binning of uniform grain. This is typical, for instance, for drying high-humidity grain which is successively passed through two parallel shafts of the same dryer or through the shafts of two parallel dryers.
In foreign countries this method is widely used for separate drying of grain when the main mass of moisture is removed in the dryer, and finish drying (after binning) is realized within active ventilation units.
In the first case the duration of binning of uniform grain is limited by the holding capacity of drying bunkers and depends on initial parameters of grain and on the modes of drying (speed of grain outlet from the dryer). In the second case the duration is determined by the holding capacity of units of active ventilation and can reach up to 6–8 hours.
During such binning the moisture from the inner layers of partially dried grain seeds diffuses towards their surface previously dehydrated in grain dryers, i.e. the grain is kind of sweated. As a result, the subsequent drying of such grain becomes more intensive which promotes the decrease of fuel energy expenses.
Different ways of advancing drying agent to grain. Modern practices of grain dehydrating involve convective advance of heat to grain. The process runs under constant speed of drying and is accompanied by gradual increase of grain temperature. For shaft and column grain dryers the mode of drying, together with the temperature of agent, is usually characterized by maximum permissible temperature of grain heating, the value of which for specific types of dryers is set with regard to the impact on non-uniformity of heating of such factors as non-uniform distribution of the drying agent along shaft section; along the length of drying duct and exhaust duct; non-uniform speed of movement of separate sheets of grain along the section of a shaft or column.
The grain reaches its maximum temperature by the time it goes out of the drying zone, therefore the process of drying is not effective enough. Another reason of low effectiveness is that in attempts to secure grain from overheating, engineers try to prevent it from being maximally heated. The temperature of heating is rated on the basis of heat stability of grain.
An important feature of continuous-flow grain dryers is the way of moving of the agent of drying against the grain. According to this criterion, drying units are grouped as follows: with crosscut advance of heat carrier, with parallel (along the movement of grain) and counterflow advance (against grain movement).
Crosscut advance of the drying agent is most widely used; heat carrier is advanced here towards grain mass perpendicular to the direction of its movement. All domestic and foreign column and shaft dryers work in this way.
Though, heat inputs for drying processes are most influenced by the way of supplying air to the dry chamber: charging or drawing. Most of domestic and foreign grain dryers utilize drawing of drying agent — in this way heat saving reaches up to 20 % as compared with charging thanks to underpressure in the dry chamber (vacuum effect) which accelerates moisture evaporation from grain seeds.
To ultimately cool the dried grain, atmospheric air is advanced to the grain. It is used to make grain uniform in terms of humidity and to align its temperature with that of the environment for its further long-duration storage without grain quality deterioration. Together with cooling, grain can be additionally dehydrated. Dehydration intensifies with the rise of temperature of grain directed for cooling, as well as with the increase of duration of binning (before cooling) in cooling columns or in collection bins. During final stage of grain cooling the most strongly linked part of moisture evaporating from grain is removed, therefore final cooling is especially important. In accordance with applicable standards, the grain after cooling should have temperature not exceeding the temperature of the outside air more than by 10 C°.
Engineers abroad pay special attention to the way of air advancing to the grain with the aim to reduce fuel and energy inputs. Air may be advanced to grain before or after drying. It was determined that during cooling of grain in silos or bunkers the way air is delivered to grain sheets is of great importance. One of the ways allowing to maximally save fuel and energy resources is drying based on drying and aeration. This method was borrowed from American engineers and has been widely used in France. In the heart of this process is slow separate cooling of grain after drying. Common drying dehydrates grain up to 15–16% of humidity, and the heat accumulated in grain is then removed by intense cooling with atmospheric air in the cooling chamber.
During drying and aeration, grain is cooled in chambers equipped with ventilation system. The process of grain drying consists of four stages:
1. Enhanced dehydration in the dryer up to 18–19 % of humidity with the temperature of drying agent being 110–120 C° and temperature of grain heating – 50–60 C°.
2. Grain with t 50–60 C° is directed to aeration chambers, where it is kept for 8–12 hours (including loading time) for binning and subsequent dehydrating so that the inside moisture of grain transfers into a drier peripheral surface zone.
3. Slow cooling of grain by atmospheric air for 12–15 hours with specific consumption of air — 40–60 m³ per 1m³ of grain. The grain here is not only cooled before being placed for storage — the remaining heat is used as vaporation energy, which allows to reduce humidity by 1,5–3,0 %.
4. Unloading of grain from aeration chambers and its delivery to storage.
The whole cycle of aeration is intended to last for 32 hours. Four-chamber unit with two ventilators secures normal operation of the dryer. Consecutive loading and unloading of grain secures full turn of chambers’ blocks in the course of 72 hours; the capacity of one chamber is designed for 8 h of dryer’s operation. Dry aeration method allows to increase the efficiency of drying up to 40 % and reduce the use of fuel and energy resources by 20–22 %, but at the same time it requires additional ventilation chambers (bunkers) with powerful ventilators. The process of continuous aeration developed by a French company “Law” eliminates these problems. A conveyor delivers non-cooled grain from the dryer to the upper part of isolated aeration unit, then the grain passes on to unventilated zone of binning, where it stays for 8 h on average. After that the grain moves to ventilated zone of cooling. The two zones are efficiently divided into unventilated and ventilated sections with the help of air ducts which direct exhaust air into ventilation pipe with the air not passing through grain in binning zone. The grain is being passed through ventilation zone during 8 hours, after which an unloading mechanism takes the cooled grain out from the silo. As a result, the capacity of aeration unit should be 16 times more than maximum hourly output of the dryer (not 32 time more as characteristic of the periodic dry aeration described above).
Such method is used at grain cleaning and drying complexes ZSK-60 Sh (80 Sh) and ZSK-100 produced by JSC “Amkodor”. Today these complexes are the most powerful ones in Belarus. Principle of their work is developed by the direction of chief constructor of “Amkodor” jointly with scientists of RUE “SPC of the NAS of Belarus for Agriculture Mechanization”. The grain is cooled and finally dried in the bunker-accumulator-cooler, whereas the dryer fully operates in heating mode. This creates a possibility for grain to be discharged from the dryer when it reaches humidity of 1–2% higher than the required value. As compared with fast cooling, this method allows to reduce fuel consumption by 14–18 % and to enhance the effectiveness of the dryer. ZSK-60 Sh dryer can work in full capacity – with three shafts (capacity – 138 tons, wheat), not in full capacity — with two shafts (92 tons) or one shaft (46 tons) (illustration 2).
Illustration 2 — Grain cleaning and drying complexes ZSK-60/80 JSK “Amkodor”
The quality of the drying is controlled by continuous moisture meters. The temperature of air and grain heating is regulated by sensors of automatic control system. The specified parameters are outputted on the display of control desk or on computer monitor.
Grain drying and cleaning complex with the capacity of 100 t/h (ZSK-100Sh) with shaft dryer of 150 t is equipped with linear gas burner with 7MW of power capacity (heat exchanger is not used for drying, instead of it a mix of fuel gases and air is utilized) and a self-contained cooling bunker with holding capacity of 100 t. Use of liner gas burner allows to reduce specific consumption of fuel by 20–22 % as compared with air-heater with heat exchangers. But in this case additional safety measures are needed to prevent the risk of inflammation of grain mass in the dryer — removing of grain dust from the drying agent, incorporating of spark suppressor in the construction of dryer. Therefore during design of ZSK complexes great deal of attention is paid to dust-removal from the outgoing air.
Recuperation (reusing) of heat of spent agent of drying.
Foreign and Belarusian engineers have certain experience in using the heat of the spent agent of drying. Heat losses, according to a number of studies, may reach 30–40% of all overhead heat in grain dryers.
Up to the recent time there have not been data in Belarus on the limits of possible use of heat of spent agent of drying with various moisture loads. It was generally accepted that only an agent with low humidity levels can be reused.
Foreign countries, France and USA in particular, employ different ways of technical solutions to this problem. Scientists of a French company “Law” have developed recuperation grain dryer in which the heat of the spent agent of drying passes through heat exchanger before being discharged into the atmosphere. When in heat exchanger, the agent gives part of its heat to the liquid (glycolic water). Warm water goes to another heat exchanger which is situated along the passing route of outdoor air arriving to the dryer. Thus the air is partially heated and fuel inputs for drying are reduced. Most French dryers reuse the spent agent (with low-saturated steam) which goes out of the lower part of the drying zone having temperature 50–60 °C and returns into the upper part of the dryer or to the burner (where it is mixed with drying agent). Depending on the way of using of secondary heat and on the dryer design, it is possible to save from 10 to 30 % of heat per each kg of moisture.
For reference: In recent years engineers from the USA have been using dryer with recirculation of the spent agent of drying and with cooling air which passed through a cooling chamber. According to companies producing such dryers, some 50% of fuel is saved when compared with common drying unit. In particular, dryers with parallel flow of drying agent are now applied (air moves from top to bottom in one direction with the grain) and cooling air (air moves against direction of the grain). The drying process is performed in three stages: in the first stage a part of the drying agent is used for preliminary heating of raw grain. At the second stage the grain is moving downward in one direction with the drying agent. At the final stage the cooled air passes through the dry warm grain picking a part of its heat. The whole spent cooling air and around 60% of spent drying agent both circulate between a ventilator and combustion chamber. According to data of the manufacturing company, such recirculation system allows to reduce fuel consumption by 35%.
Reduction of heat losses into the environment. As practice shows, losses of heat into the atmosphere through the elements of the dryer’s frame and air channels can amount to 20 % out of the quantity of heat spent for grain drying. Besides, heat losses at air channels which discharge the spent agent from the dryer, provoke condensation of steams on the walls, clog them with dust and small impurities. As a result, hydraulic resistance grows and energy consumption increases.
Big losses of heat occur at a section of air channel which connects air-heater with the dryer. Decrease in temperature of heat carrier in the context of reduced temperatures of outdoor air (e.g. in autumn during corn grains drying) can reach 40–60 C° which is quite important, given significant discharge of the drying agent. The main reason of such losses of heat is insufficient heat insulation of dryers’ surfaces or lack of insulation, as well as comparatively big length of air channel. It is also important to reduce the area through which heat losses take place, for which it is necessary to shorten connective air channel by approaching the burner closely to operating dry zone.
The Republic of Belarus produces shaft grain dryer SZShM -40–11 jointly with a German company “Riela”. This dryer features recuperation of a part of the spent drying agent and heat-insulated drying module. Grain dryer consists of heat-insulated shaft for grain, delivery and discharge air channels, two linear burners for direct heating, exhausters and unloading mechanism. It is completed with aspiration system (centrifugal heat arrester).
Illustration 3 — Grain dryer SZShM-40–11 and its technological scheme
During drying, outdoor air arrives to burning area through the shutters of the shaft where it is mixed with fuel combustion products (thus drying agent is prepared). Then drying agent goes to the shaft through supply air channel where it is evenly distributed along conical guiding channels (ducts). When passing through grain sheets, the drying agent is saturated with moisture and at the same time cools, after which it moves to discharge ducts for spent air from which the air full of water vapour is taken outside with the help of exhausters. Dried grain is cooled by outside atmospheric air in the cooling zone in the lower section of the shaft. The spent warm air of the cooling zone is directed into the delivery channel for secondary handling by ventilators where it is mixed with heat carrier. Use of heat recuperation from the cooling zone and shaft insulation allow to save no less than 17–22% of heat energy and accordingly, fuel for drying.
Advantages of grain dryer SZShM-40 as compared with analogs. All-purpose, is applicable for drying bread grain, forage grain and seeds, as well as corn grain. Gentle modes of drying. Low need in fuel thanks to heat insulation of the shaft and air channels, heat recuperation from the cooling zone. Rectangular aluminum frame of the dryer. Completely automated handling of the drying process.
Improvement of design of working elements of grain dryers. As practice shows, any design upgrading which intensify the drying process, reduces energy inputs for drying. For example, more even distribution of the drying agent along shaft’s section and along individual ducts is secured by reducing flow rate of the drying agent when it enters delivery ducts of the drying shaft. For this purpose it is necessary to make the section of the delivery duffuser bigger and to set it, if possible, along the whole height of the shaft. Such design solutions are applied in grain dryers of the company “Amkodor”, as well as in grain dryer SZShM-40–11 (modern project of RUE “SPC of the NAS of Belarus for Agriculture Mechanization” and “Riela” company (Germany).
Correct maintenance of grain dryers. Maintenance of grain dryers in accordance with instructions and technological regulations can not always fully guarantee quality drying with minimum energy inputs. A lot depends on the qualification of service personnel, on their knowledge about “bottlenecks” of the dryers, about scope of influence of various factors on the quality of grain, fuel and energy consumption for drying. Thus, it is possible to guarantee fuel savings thanks to the following approaches:
1. Automation of the drying process – arrangement of regulation of moisture and pre-set value of moisture of the dried grain. This guarantees maintenance of optimal flow rate of drying agent and air (overdrying is prevented).
2. Upgrading of grain dryer’s design:
- minimizing the unevenness of grain heating and drying (even distribution of drying agent along shaft’s section and along length of ducts), continuous discharge of the dried grain;
- heat recuperation of the spent drying agent with the use of special systems for dehydrating the spent drying agent delivered for reuse;
- drying under conditions of underpressure (by means of drawing the drying agent through grain sheet with the help of exhausters);
- grain cooling on free-standing cooling units under such conditions which allow to use inside thermal energy of grain to the maximum for moisture vaporization.
Search for additional ways for more rational use of fuel and energy resources for drying is possible on the basis of further research associated with the development of new technological means of dehydrating and absolutely new ways of drying and delivery of heat towards grain, including that with the use of solar power and substitution of traditional kinds of fuel with renewable sources.
Marinich Leonid – first deputy minister of Agriculture and Food
Samosyuk Vladimir – candidate of economic sciences, director general of the RUE “SPC of the NAS of Belarus for Agriculture Mechanization”.
Chebotarev Valery – candidate of technical sciences, first deputy director general of the RUE “SPC of the NAS of Belarus for Agriculture Mechanization”.
Baranovski Ivan — candidate of technical sciences, head of laboratory for grain and seeds harvest and post-harvest processing
Poleshchuk Leonid – head of Energy and Transport Department of the Ministry of Food and Agriculture of Belarus
Krutalevich Boris – chief constructor of agricultural machines of JSC “Amkodor”