Limitations of 24/7 Operation of a Capacity Briquette Plant For Binderless Fibrous Biomass
Press-to-log technology in North America and binderless briquetting technology in Europe were essentially designed to work with granular wood waste such as sawdust, wood chips and bark. Wood waste generated by wood processing plants is mostly dry (15 to 20 percent moisture) so briquetting plants are used primarily for waste disposal. And in developing countries, most of the briquetting plants installed between 1985 and 2000, utilized residue from the agricultural processing industry such as rice husks, ground nut shells and husks from various other plants.
Preliminary testing on almost 200 of these 90-millimeter (mm)-diameter presses with 15-cubic-meters per-hour displacement indicates that they are under-designed for new-generation crop briquetting of the fibrous biomass found in Europe and North America.
The challenge is to improve briquetting operations to efficiently utilize fibrous agricultural residues from crops, horticulture operations and forests. The limitations of briquetting fibrous biomass utilizing briquette presses and processes designed for granular biomass are addressed herein.
Preparing Fibrous Biomassfor Briquetting
Most types of fibrous biomass have low bulk density and normal- to medium-moisture content, so processing, including cutting/chopping and shredding at the harvest site, natural or mechanized drying, and grinding for increased bulk density are required prior to briquetting
Also, the availability of these materials is seasonal so they may have to be stored for considerable periods prior to briquetting.
Binderless Briquetting 24/7
Even after grinding fibrous material through a 6 mm screen, the bulk density is only 286 to 310 pounds per cubic meter (lb/m3) as compared to 352 to 396 lb/m3 for wood waste. Energy consumption is high and the briquette press is de-rated. Also, wear and tear on parts such as the ram, die, split die, wear ring, etc. is excessive.
In developing countries, most of the briquette plants utilized to process fibrous material operate 10 to 12 hours per day with low productivity, low output per press and low output per person. Power consumption is also high.
Limitations of 24/7 Operation
• Overheating: For briquetting, the temperature required to press the fibrous material is more than 300 degrees Celsius (572 degrees Fahrenheit). Cooling is required to maintain the briquetting head temperature below 100 degrees C. Most of the European presses in operation today do not have head cooling and will overheat after eight to 10 hours, at which time the operation must stop to allow for cooling and lubricating.
• Low Output: Standard 90 to 200 diameter briquette presses are rated for outputs of 2 tons per hour on granular biomass with bulk densities of 352 lb/m3. However, processing unground fibrous biomass can de-rate the output to as low as 1.2 tons per hour. Another cause of low output is the alignment accuracy between the ram and pressing parts. For optimum output, the desired accuracy is 0.2 mm, however, the highest accuracy of presses used in developing countries is only about 0.5 mm. This disparity increases wear and, as a result, replacement of wear parts is required within 100 hours of operation to maintain minimum production levels.
• Low Productivity: A standard 90 to 200 diameter briquette press requires a minimum of three people to operate. To increase productivity to one person, the following improvements must be made to avoid jamming the die holder, which frequently occurs when briquetting material has not been preprocessed: re-engineering the die holder fixing and extraction arrangement, mechanized handling of 300-pound die holder; and automatic die holder pressure control.
Limitations of Design and Construction
Briquette press outputs are size-specific. In most of the installations where manual firing is popular, the preferred briquette size is about 50 to 65 mm, thus the output of an extrusion briquette press is only about 1.5 to 1.1 tons per hour. Because this is typically adequate for developing countries some manufacturers tend to under-design high-output rated presses.
Today, 90 mm diameter presses with 15-cubic-meters per-hour displacement are being widely marketed. Preliminary testing of almost 200 of these presses indicates that they are under-designed in the areas of power, centering accuracy, oil contamination, die holder fixing and heating.
• Powering the Press: A standard 90 mm diameter press with 200 strokes per minute is powered by 95 horsepower (75 kilowatt) motor and can deliver up to 2.2 tons per hour using granular biomass with a bulk density of 352 lb/m3. These presses have been de-rated to 1.1 to 1.3 tons per hour with 75 horsepower motors.
• Centering Accuracy: The presses must be designed with centering accuracy of 0.1 mm (in 90 mm diameter) for optimal throughput and the support system should be “slide” rather than “bush” for high centering accuracy. Most of the presses beingbuilt today have a much wider gap than 0.1 mm and they have a bush support system, which results in less output and decreased service life on wear parts.
• Oil Contamination: Vital moving parts are exposed and not shrouded, resulting in oil contamination, increased oil consumption and increased wear on the pump.
• Die Holder Fixing: For maintenance and service, die holder fixing and removal should be quick and easy. This should be a 15-minute task yet, in many presses this requires more than one hour to complete.
• Heating the Briquette Press: The die holder is fixed with the briquetting head for support with close tolerance, and the die holder temperature is maintained around 300 degrees C for effective briquetting of fibrous biomass. To avoid production interruption due to overheating and to guard against burns at human touch points, the head temperature should be sustained at under 1,000 degrees C.
Preprocessing of Biomass Material
Most of the biomass residues resulting from crop harvest or otherwise have the following characteristics.
• Low bulk density of 88 to 132 lb/m3 with odd sizes to handle.
• High moisture upon harvest is 25 to 30 percent as compared to 12 percent moisture required for briquetting.
To facilitate effective briquetting of these materials the following activities are required:
• Mechanized Collection and Transportation: A high percentage of crop residue is left in the field because there is not an effective mechanism to collect it. Because of the variety of residue types and sizes, there are more than a dozen different mechanisms required to collect it. Following collection, the residue must be transported to a processing site, which is generally four to seven miles away.
• Drying: Granular material at a size of 3 mm is the only residue that can be effectively dried in a low-cost/low-temperature flash dryer. A rotary dryer must be used to dry larger fibrous materials, however, a rotary dryer is not economical to operate with these quantities. Solar drying is an alternative for drying, however, the extended drying time and large amount of space required are cost prohibitive.
• Grinding of Materials: Grinding of material is optimal for increasing the bulk density and reducing sharp edges, which can cause wear and tear on the press, and for preventing heavy material such as rocks and bricks from entering the press.
A critical problem with the process is that residue size should be relatively small for the drying process and the residue moisture should be low for the grinding process. Simultaneous drying and grinding is possible in a few cases.
Biomass residues are uneven in size and bulky in nature, so manual handling in larger operations is labor intensive and cost prohibitive. And, where there are long transport distances and higher elevations, mechanization can even be difficult. In large, integrated biomass processing plants, mechanization is critical and must be incorporated at all stages of the process from harvesting to primary storage, from primary storage to main storage and from main storage to the processing plant.
An electrically powered tipper/extractor and a pneumatic conveyor system are more cost effective than diesel-powered equipment.
Mechanization reduces processing time and labor costs as well as optimizing space utilization.
For 24/7 operation and high productivity, automation should be incorporated in these key areas:
• Drying: To maintain output moisture of around 15 percent, the temperature of the discharge flue gas should be maintained at 85 degrees C. This can be done by controlling the cold air injection into the hot air from the hot air generator.
• Die Holder Temperature: For smooth production of briquettes, the die holder temperature should be maintained around 300 degrees C by regulating the water flow into the die holder.
• Cooling Oil Temperature: The lubricating oil temperature should be controlled by an automated radiator fan.
• Die Holder Pressure: The die holder pressure is similarly maintained at the desired level.
• Grinding: It is difficult to optimize loading of the grinding system, however, traveling bottom bins are effective. The bottom bin speed is adjusted and controlled in proportion to the desired input into the grinder and increases until the maximum grinding load is achieved. On/off controls start functioning at optimum loading.
• Densification Equipment: Controlling the material input flow from the storage bin into the briquette press is relatively easy. The input flow increases to maximum load per the load setting on the press.
Future of Biomass Briquetting
Economizing on Power Consumption: The current practice for briquetters is to install multiple presses for bulk production. Few twin, die-holder briquette presses were installed in India but the power connection for twin presses was 1.5 times compared to double output. While no real-time recording was done, the following was concluded:
• For a single 90 mm diameter press the connected load is 67.5 kilowatts (kW) for the main motor, 7.5 kW for the vertical feeder, 1.5 kW for the pump and 3.5 kW for the feeding screw. It appears that besides the main motor, all remaining load is for 2.2 tons per hour production.
• A no-load trial run indicated that 90-mm diameter presses consume about 15 kilowatt hour (kWh) units of power in one hour. While producing 1.1 to 1.3 tons per hour, power consumption is about 55 kWh units. This is comparable to the international norm for briquetting 40 to 45 units per ton of production. Higher per metric ton production is due to under-utilization of the press. If the briquette press is fed preprocessed material, it produces about 2.2 tons per hour consuming about 85 units of power.
• The 90-mm diameter twin press will have 141 kW (main motor 125 kW+ vertical screw 2 x 5=10 kW+, feeding screw 2 x 2.2=4.4 kW to oil pump 1.5 kW). If briquette presses consume about 90 percent of connected power while producing 4 tons per hour of power; electricity consumption per ton will be just above 30 kWh units.
In view of the above, it is expected that a well-engineered, twin briquette press can reduce power consumption from 45 to 50 kWh units per ton to 30 to 35 kWh units per ton.
Economizing in Packaging: Briquettes are commonly used in Europe and North America as household fuel for winter heating, while commercial applications use compressed natural gas based on convenience and economics. Briquette packaging in 17- to 22-pound bags may be in demand in the future, therefore it’s difficult to estimate the packaging and shipping costs for European and North American markets.
Briquettes are most commonly used in industrial boilers and shipped in bulk or in 110-pound gunny sacks. The gunny sacks are used only once for packaging and are thrown into the boilers along with the briquettes for convenience in charging.
Today, small 8- to 10-mm diameter pellets in 11-pound bags are being marketed for residential heating. This fuel is inexpensive at about $5.50 for a 10-pound bag.
An automatic robot system is necessary to pack briquettes for commercial production.
Higher Productivity: With better press design, it should be possible to operate a briquetting plant 24/7, and with mechanized coding of the grinder it is possible to operate a twin 90-mm diameter (4-ton-per-hour) briquette press with three operators for mechanized feeding into grinder, briquette press and plant supervisor.
Other requirements for higher productivity are:
• Wear parts with a minimum life of 200 hours.
• Reversible hammer mill for sustained operation for 150 hours.
• Intermediate storage of the post-grind material to optimize the feed rate to the briquette press.
• Automated controls to maintain constant die holder temperature, briquette head temperature and moisture in ground material, which when optimized delivers nonstop briquettes.
• An inventory of spare parts and replacement wear parts must be maintained to minimize downtimes.
Raw Material: Chances are wood waste/biomass prices are still not established in your region and in many cases it is still a disposal problem. This is always a matter, however, of supply and demand. Check for availability of low-cost raw material in your area.
Authors: Priya Jain
Business Development Manager, Hi-Tech Agro
Founder, Hi-Tech Agro