Biochar to Increase Biogas Production

Charcoal (biochar) is the raw material for the production of activated carbon. The production of activated charcoal itself goes through two main processes, namely carbonization (pyrolysis) and activation. The surface area of charcoal (biochar) is also smaller than that of activated carbon, but larger than the raw biomass. The carbonization process increases the surface area of the raw biomass. The ratio of surface area between raw biomass, charcoal and activated charcoal is approximately 25 m2 / gram, 200 m2 / gram, 2000 m2 / gram. The larger the surface area of the biomass material that is inserted into the biogas reactor, the greater the penetration of bacteria into the substrate so that the fermentation process that occurs is more perfect so that biogas production will increase. Biochar itself does not participate in fermentation because the main component is stable carbon, while hemicellulose, cellulose and lignin have been decomposed during the carbonization process.

Another example is the addition of biomass briquettes to the biogas reactor, briquetting processs with high pressure and heat also open pores or expand the surface of the biomass, so that biogas production also increases, for more details, read here. The addition of biomass briquettes to the biogas reactor will also increase the C / N ratio, even biochar and activated charcoal have high carbon (C) content.

Charcoal (biochar) has been widely used in the agricultural world to repair damaged soil / soil amendment and thus increase fertility. Good soil fertility will also increase agricultural production. The biochar becomes a home for soil microbes, so that organic materials or compost will break down more completely and be absorbed by plants more as plant nutrients. The charcoal (biochar) pores are the home for these microbes. The more pores, the more microbes will inhabit the biochar as their “house”. The same principle applies to the biogas unit. Another bonus of using biochar is that it absorbs CO2 from the atmosphere, thereby contributing to lowering the greenhouse gases that cause climate change and global warming.

Research in Germany shows that adding 5% biochar to a biogas reactor increases methane production by 5% - based on the dry matter of biochar to the substrate. But when the amount of biochar became 10%, it turned out that no more methane was added. This shows that the optimum condition for adding biochar is the amount of 5%. The microbes in the biochar increase the volume of microbes in the reactor so that the production of biogas or especially methane also increases up to 5%. Biochar itself is not decomposed in the fermentation process. 

Meanwhile, the addition of biomass briquettes per 1 tonne of briquettes will increase biogas production by 400 Nm3. This is because in the biomass briquettes, both cellulose, hemicellulose and lignin have not been decomposed, thus adding to the substrate in the biogas reactor. Whereas in biochar, both cellulose, hemicellulose and lignin have been decomposed during the thermal carbonization process, so there is practically no additional substrate, but only microbial addition occurs in the biochar pores.

The important thing about the addition of biochar is that the compost or digestate produced is of better quality with the addition of the biochar. Biochar will make the compost which is produced as a slow release organic fertilizer. This further encourages biochar production, especially for palm oil companies that care about environmental issues and even strive for zero waste conditions.

Palm oil mills have the potential to apply biogas and biochar units. Solid wastes such as empty bunches and mesocarp fiber can be used for biochar production. Palm oil mills can even replace the furnace in the boiler with a gasifier or pyrolyser. This becomes more profitable because in addition to heat energy being used for production of steam which is used for power generation and sterilization of fresh fruit, biochar will also be produced. The biochar produced is then used to increase biogas production and improve the quality of the compost, as well as a fertilizer mixture in palm oil plantations. And even the potential use of biochar to save fertilizer on large palm oil plantations, for more details can be read here.

PKS for FBC Powerplant Part 2

Between the two variants of fluidized bed combustion (FBC) technology, the type of CFBC (circulating fluidized bed combustion) is more widely used for power generation than BFBC (bubbling fluidized bed combustion). That is because the level of efficiency is higher, the use of large capacity and the amount of flue gas produced is less. Stoichiometric air requirements for CFBC (1.1 - 1.2) are less than BFBC (1.2 - 1.3), this also results in less flue gas produced at CFBC compared to BFBC. The main difference between CFBC and BFBC is that there is a circulation of the use of bed material in CFBC and CFBC using air velocity greater than BFBC.

The fuel used for CFBC has a smaller particle size compared to BFBC so it will require higher preparation / pretreatment costs. This is one of the drawbacks of CFBC. The usual preparation is foreign material control and size reduction. Fuels used like coal must be cleaned of a number of impurities such as metals. Magnetic separator is a equipment commonly used to separate the impurities of these metals. The metals that pollute the coal apart from damaging the screen on the crusher (if using a hammer mill type crusher) it is also possible to cover the perforated plate at the bottom of the fluidized bed furnace. The closure of these pores causes disruption of air circulation which will certainly affect the output of the unit.

PKS or palm kernel shell is a biomass fuel that are widely available in Indonesia and have characteristics close to wood pellets. The PKS can be used for CFBC fuel both for cofiring and fullfiring (100% PKS). Regarding the preparation or pre-treatment of fuel before it is fed or used in CFBC, it also needs to be done for PKS. PKS measuring 2-5 cm need to be cleaned from a number of impurities and size reduction before use. In cofiring, when equipments for coal preparation can also be used for PKS, it means no new line is needed for that. But if it can't, a new line needs to be made. Some power plants in Japan even use 100% PKS as fuel with the CFBC technology and especially in the framework of environmental and climate change solutions supported by the government.

CFBC biomass power plants in Japan with 49 MW capacity
which has been operating since 2015 with PKS

The use of fluidized bed combustion (FBC) technology has actually been carried out since the 1960s, which at that time was mainly used to treat municipal and industrial waste. At present it is estimated that more than 300 FBC units are installed and operating throughout the world. PKS as palm oil mill solid waste (CPO mill) is available in abundance in Indonesia which is estimated to reach more than 10 million tons / year and in Malaysia is estimated to be more than 5 million tons / year. This is so that the use of PKS on FBC is very possible because the volume and availability is sufficient. Economic factors are especially limiting the use of the PKS, although in terms of the environment and climate change, it is strongly supported. The location of the palm oil mills which is in the middle of the plantation commonly in very remote locations also makes logistical costs expensive so it affects the selling price of the PKS.