Present state of small-scale grain drying in the USA
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by Prof. F.W. Bakker-Arkema
Small-scale, on-farm drying of grain in the USA takes place in high-temperature, high-capacity dryers, or in low-temperature; low-capacity units. A new category, combination dryers, combines the two systems. High-temperature batch dryers are flexible, but are energy inefficient and often cause grain damage. Low temperature dryers require skill on the part of the operator and are limited to the drying of grains at moisture contents below 22% (w.b.); the units have the potential to produce excellent quality grain at minimum energy consumption.
Combination grain drying systems combine the advantages of the high-temperature and lowtemperature drying. All grain can be dried in combination drying systems regardless of the initial moisture content of the grain or the local weather conditions. It produces grain of superior quality, minimises fuel consumption, and has great flexibility. It appears suitable to Chinese conditions.
This paper parallels Chapter 6 and part of Chapter 7 in the book on grain drying by Brooker et al. (1974).
On-farm grain drying systems in the USA are considered to have a capacity of less than 10 tonnes per hour in removing five percentage points of moisture from wet grain. The systems include batch drying, natural air drying, supplemental-heat drying, solar drying, combination drying, and in-bin counterflow drying. Each system is able to dry any type of grain and legume, although some cannot dry material with initial moisture content over 21 - 22% (w.b.).
Cleaning of the grain before drying is strongly recommended regardless of the drying system employed because it enhances the air flow rate and the subsequent grain storability.
2. HIGH-TEMPERATURE SYSTEMS
Drying wet grain in batches in a bin from harvest to safe-storage moisture content is called batch-in-bin drying. Relatively large airflow rates (up to 20 m³/mintonne) and high air-temperatures (40 - 70°C) are used; the grain depth in the bin is limited to 0.6 -1.2 m to minimize overdrying of the bottom grain dryers. Dryingtime varies from 4 - 8 hours depending on the harvest moisture content, drying air-temperature, and the airflow rate. Automatic batch dryers are molecule units fulfilling the same purpose as batch-in-bin dryers.
The grain-quality and energy-efficiency of batchtype drying systems are marginal; their advantages are the high drying capacity and simplicity of operation.
3. LOW-TEMPERATURE SYSTEMS
Natural-air and supplemental-heat drying are similar processes. The grain is dried and stored in the same storage in on-floor flat warehouses or in metal bins. The distinction between the two drying methods is that no heat (except fan energy) is added in natural-air drying, while in the case of supplemental-heat drying, the air-temperature is increased 1 - 5°C (usually with electric heaters) to decrease the relative humidity of the drying air to below 55%. Natural-air and supplementalheat drying have for a long time been used in the USA to dry small grains harvested during the summer months at moisture contents below 18% (w.b.). Now both systems have been adapted to dry maize and paddy at moisture contents up to 24% (w.b.). The recommended minimum airflow rates and maximum grain depths depend on the environmental conditions. For instance, natural air drying requires 0.6 m³ mine, t(1) in the more southern state of Indiana. Thus, before these two in-store drying techniques can be transferred to the different provinces in China, the minimum airflow rates have to be established for those regions.
Solar grain drying (not to be confused with sundrying which does not require a mechanical air movement device) has been researched extensively in the USA during the past decade. The solar energy available at a particular location depends on the latitude and the local weather conditions. The price of solar collection depends on the labor costs and construction materials. These factors have contributed to the wide diversity in opinion about the value of solar grain drying systems. It is the opinion of the author that solar heat can replace significant amounts of energy in some grain drying systems, and is technically feasible; however, it will not become economically feasible in the USA until the fossil fuel prices have at least doubled.
Other recent developments in small-scale, lowtemperature grain drying include trickle-ammonia drying, heat pump drying, and stir-drying.
Low-temperature grain drying systems are lowcapacity systems but are able to produce higher quality grain at decreased energy consumption than hightemperature dryers. Considerable expertise is required of the dryer operators of low-temperature systems to ensure that the grain has dried before microbiological deterioration of the grain occurs.
4. COMBINATION DRYING
Combination drying is defined as a system in which high-temperature, high-capacity drying is followed by in-store low-temperature, low-capacity drying. The high-temperature phase of combination drying can be a batch-in-bin dryer or a mobile automatic-batch unit. The heat grain (usually maize) is harvested at 28 - 32% (w.b.), partially dried with high-temperature air to 18 22% (w.b.), and subsequently moved to a natural-air, or supplemental-heat in-store drying system for tempering and final drying/cooling. The low-temperature part of combination drying may require from 2 days to 2 months, depending on the moisture content of the grain placed in the low-temperature bin and on the airflow rate and temperature of the drying air. Combination drying systems produce grain of improved quality and require about half as much energy as high-temperature batch drying. Drieration is the best known of the combination drying system.
In-bin counterflow drying is a relative new combination drying system which has rapidly gained popularity because of its exceptional flexibility. This system also lends itself well to the use of biomass furnaces.
Brooker, D. B., Bakker-Arkema, F. W., Hall, G. W. 1974. Drying Cereal Grains. AVI Publishing Company, Westport, Connecticut, USA. Available in Chinese Translation.
SMALL-SCALE U.S. GRAIN DRYERS
1. IN-STORE DRYING
2. COLUMN BATCH AND CONTINUOUS FLOW DRYING
3. COMBINATION DRYING
IN-STORE (IN-BIN) DRYING OF MAIZE)
1. FULL-BIN DRYING (LOW TEMP. SYSTEM)
2. BATCH-IN-BIN DRYING (HIGH TEMP. SYSTEM)
3. IN-BIN COUNTERFLOW DRYING (COMBINATION SYSTEM)
SMALL-SCALE HIGH-TEMPERATURE MAIZE DRYERS
1. COMUN-BATCH DRYERS
2. CONTINUOUS-FLOW CROSSFLOW DRYERS
3. CONTINUOUS-FLOW CONCURRINT-FLOW DRYERS
LARGE-SCALE MAIZE DRYING IN THE USA
1. CROSSFLOW DRYERS
2. CONCURRENT-FLOW DRYERS
3. MIXED-FLOW DRYERS
POWER (WATTS) =
STATIC PRESSURE (P)
- TYPE OF GRAIN
- MOISTURE CONTENT OF THE GRAIN
- KERNEL SIZE OF THE HYBRID OR VARIETY
- PACKING OF THE GRAIN
- FINES IN THE GRAIN
- DEPTH OF THE GRAIN
- AIR FLOW RATE THROUGH THE GRAIN
- q = APB
- WHERE A AND B ARE PRODUCT CONSTANTS
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