1. Brief introduction — coffee & coffee residues
Coffee is a prevalent beverage favored by people all over the world. It comes from coffee beans, namely the seeds of berries from coffee trees that are cultivated in many countries. Once the coffee fruit (i.e. coffee berries or cherries) are ripe, they would be picked, processed, and dried. Then the dried coffee seeds shall be roasted; the roasted coffee seeds then will be ground and brewed with hot water.
Coffee tastes good. However, coffee cherries must be processed to separate the beans from the husk. This post-harvest process generates considerable amounts of coffee residues. The residues comprise the husk and pulp, which are solid residues obtained after de-hulling the coffee cherries during dry or wet processing, respectively. It is roughly calculated that husk accounts for 50% of the coffee fruit. The large amount of coffee husks and pulp generated in coffee processing has caused environmental concerns. If not properly disposed of, they would do harm to the environment.
In addition, coffee ground, which refers to the remained dregs after brewing coffee, also needs to be properly treated. An appropriate solution for processing of coffee husk, pulp and ground is shown as below.
2. Steps for coffee husk/pulp pelleting
Coffee residues (i.e. coffee husk and coffee pulp) can be made into pellets fuel.
1) Residues collecting
Producers need to collect coffee husks or pulps when the ripe coffee cherries are harvested and hulled.
2) Grinding
In general, the standard-sized pellet mills require biomass that is ground into particles that are no more than 3mm in size. So the collected coffee residues need to be ground into small particles first. For the biomass of larger size and of higher density (like wood), the material should be chipped by a chipper first, and then be ground by a hammer mill to reduce the particles until they are of the required size. As for smaller and softer biomass (like straw), they can be fed directly into the hammer mill without being chipped. And for coffee residues, they just need to go through a hammer mill for grinding.
3) Drying
In order to make quality final pellets, keeping an appropriate moisture level in the material is very important. After grinding, the residues should be dried until they are of the desired moisture content, i.e. approx 12%. If the feedstock is too dry, moisture can be added by injecting steam or water into the feedstock.
4) Pelleting
The dried coffee husk or pulp shall go through the pelleting process. In this step, a pellet mill or press needs to be adopted.
Note: It would be better to blend these coffee wastes with other resources to make quality pellets, like blending corn cob or rice husks in appropriate proportion.
5) Cooling
As the pellets leave the die, they are of high temperature. They should be cooled by air or by cooling machine before storage and use.
6) Optional steps
The cooled pellets should be sieved to separate the pellets which do not form properly. If the pellets are for commercial use, a packing machine is recommended, so as to pack the pellets into uniform bags.
3. Property of coffee husk and coffee pulp
Coffee husk
The moisture content is roughly 10% on a wet basis; its ash content is about 12%; the volatile content is about 80%; the calorific value is nearly 18.3 MJ/kg.
Coffee pulp
The initial moisture content is roughly 90%; the calorific value of pellets made from coffee pulp is about 12,501kj/kg; the ash content of coffee pulp pellets is about 6.7%.
4. Why make coffee husk/pulp pellets
There are large amount of wastes from coffee industry. Use of this kind of source plays an important role in energy supply. Making and using of coffee husk or pulp pellets are recommended, because:
1) The raw materials are easily available.
Coffee trees are planted everywhere, including countries in America, Southeast Asia, and countries at the equator like India, Indonesia, etc. So the materials for pellets making from coffee residues are abundant.
2) The raw materials and the pellets are renewable.
The raw materials for coffee husk or pulp pelleting are renewable, unlike fossil fuels.
3) Pellets made from coffee husks and pulps burns efficiently.
While being burned, the pelleted coffee residues are more efficient than the original materials.
4) Coffee husk/pulp pellets burning is environmental friendly.
The application of coffee husk/pulp pellets as biofuel will lead to remarkable reduction of CO2 emissions, contributing to reduce global warming.
5. Application of pellets fuel from coffee wastes
Like wood pellets, the pellets made from coffee wastes can also be used as fuel. Usually they are applied for heating and electricity generation in/at
-Power plant
-Boiler
-Home
-Institutions (like school, hospitals, etc.)
6. Typical coffee producing countries
The beverage, coffee are popular among people from countless cities around the globe. As for coffee beans, the source of coffee drink, is quite relevant to the economy of its major producing countries such as Brazil, Vietnam, Colombia, Indonesia, Ethiopia, India, Honduras, Uganda, and Mexico. These coffee beans producing countries has generated a good deal of coffee residues, which are good sources for pellets production.
Brazil
Brazil is the largest coffee producer and exporter and the second largest consumer in the world. This country owns about 220,000 coffee farms, with an area of about 27,000 km2. Hence, there are abundant coffee residues generated from coffee processing industry.
Vietnam
Vietnam is the second largest coffee exporters in the world with an estimated annual production of one million tons. Coffee plantation accounts for a large share in its national economy. In addition, coffee plantation facilitates its employment. The majority of its coffee are exported. After coffee cherry harvesting, there are abundant residues left in coffee manors, which can be collected and hen pelleted.
Colombia
Colombian coffee is famous worldwide. Due to its climate, Colombia once was the second largest coffee producing country after Brazil. But since Vietnam’s production rapidly expanded, Colombia fell behind.
Other producers
Indonesia, Ethiopia, India, Honduras, Uganda, and Mexico produce large volumes of coffee beans every year as well due to their favorable climate and ideal geographical location.
The coffee residues generated there remain as a kind of valuable energy source, which is worthy of processing into pellet fuel.
