Potassium and Chlorine, 2 Elements Need More Attention at Wood Pellet

Most of the current power plants use pulverized combustion technology which operates at temperatures greater than 1400 C. The high operating temperature makes the requirements for the fuel used quite tight, meaning that not all fuels can be received immediately. The standard power plant fuel is designed using coal, so that when using biomass fuels it might be necessary to modify it. In the ratio of small cofiring, for example, 5% of the power plant is likely to operate on a standard basis without modification. The big question is why the pulverized combustion technology power plant whose capacity can be hundreds or even thousands of MW can not directly use biomass up to 100% without obstacles? That's what we will try to review in the article below.

The main difference between biomass fuel and coal in terms of the power plant is the chemical ashes. The chemistry of coal ash is composed of inorganic materials which have very high melting points and tend not to be corrosive to metals at high temperatures. This makes technically coal fuel more friendly to pulverized combustion technology. Although reviewed environmentally, coal fuel is not friendly because there are many fly ash, ash waste is classified as hazard material / B3 and SOx emissions cause acid rain. While a review of climate change and global warming, it is clear that coal is a fossil fuel and is a carbon positive that increases the concentration of CO2 which is a greenhouse gas in the atmosphere. Many countries now reduce their use of coal in their policies.

While biomass fuels have a chemical content of ash consisting of inorganic materials that have low melting points and tend to be corrosive so that it becomes an obstacle to the pulverized combustion technology. Potassium is one of the chemical elements of ash in biomass which is in the spotlight, this is because potassium has a low melting point and in a large amount of biomass. The melted potassium ash will cover and be deposited on the heat exchanger pipes in the generator boiler. These deposits make heat transfer efficiency decrease so that fuel consumption will increase. This is indicated by the increased chimney temperature which means that there is a large heat loss.

Another element that is the main highlight besides potassium is chlorine. This chlorine is corrosive and is like a ghost for the pulverized combustion power plant. Corrosive properties will shorten the life span or operating life of the power plant, for example with high chlorine content in the fuel, making the operating life of the power plant to be half or a quarter of what it should be. Of course this is very detrimental, for more details can be read here. In addition to the two things above which are considered unfavorable from the use of biomass fuels, but along with the problem of environmental damage in the form of climate change and global warming, biomass fuels are the solution to the problem. This is because biomass fuels are renewable, sustainable, carbon neutral and various other environmental benefits.

The most popular biomass fuel today is wood pellets. When it turned out that wood pellet which is a biomass fuel containing potassium and high chlorine, it became less desirable and was even rejected by the pulverized combustion power plant. Wood pellet producers must pay attention to this issue if the market segment is electricity generation. Ensuring wood pellet products with chlorine and potassium content according to technical requirements are mandatory for these producers. When wood pellets have been produced but the specifications cannot meet the requirements, it is necessary to change the market or improve the quality of the wood pellets.

Basically to overcome the content of chlorine and potassium it can be in two ways, namely from the production side of wood pellets and from the user side. Wood pellet producers can choose raw materials that can meet the specifications requested or even do a number of treatments so that the specifications can be achieved. Whereas from the user side, that is by using electricity generation technology with lower operating temperatures so that the problem of potassium and chlorine can be reduced and even eliminated. The technology of electricity generation with fluidized beds and gasification can be a solution to this.

PKS For Coal Powerplant in Europe

Europe with its bio-economic program in the RED (Renewable Energy Directive) has targeted the use of renewable energy to reach a minimum of 20% by 2020 with biomass consumption reaching 70% of all renewable energy and by 2030 to be at least 27%. For biomass energy, Europe is also the largest producer of wood pellets, which is currently estimated at 13.5 million tons / year while its consumption is 18.8 million tons / year. The biggest wood pellet producing countries in Europe are Germany and Sweden. Even though the production of wood pellets of 13.5 million tons / year has not been able to meet the internal needs of the region, so it still needs supply from outside. America and Canada are the main suppliers of wood pellet needs for the country. Most of the use of wood pellets for electricity generation. In addition to wood pellets, PKS has also been imported from Indonesia. Along the size of the target to be achieved, the need for biomass fuel is predicted to increase.

Although most power plants currently use pulverized coal boiler technology which reaches around 50% of the world's electricity generation, the use of grate combustor boiler technology and fluidized bed boilers is also increasing. Pulverized coal boiler is mainly used for very large capacity plants (> 100 MW), while for ordinary medium capacity uses fluidized bed technology (between 20-100 MW) and for smaller capacity with combustor grate (<20 MW). The advantage of boiler boiler combustion and fluidized bed technology is fuel flexibility including tolerance to particle size. Various agricultural waste, municipal waste, used tires and so on can be used as fuel. When the pulverized coal boiler requires a small particle size (1-2 cm) like sawdust so that it can be atomized on the pulverizer nozzle, the combustor grate and fluidized bed the particle size of gravel (max. 8 cm) can be accepted. Based on these conditions agricultural waste, namely PKS has a great opportunity as fuel for these boilers.

49 MW Biomass Powerplant with Fluidized Bed Technology
in Japan that use PKS as fuel, has been operating since 2015 

To be able to fuel the boiler combustor grate and fluidized bed boiler PKS can be used directly, without additional pretreatment. More specifically for fluidized bed boilers, circulating fluized beds (CFB) boilers are more suitable for PKS compared to boiler fluidized bed (BFB) boilers, for more details read here. Then is PKS not suitable for pulverized coal boiler? There are several things that need to be considered for the use of PKS in pulverized coal boilers. The first thing that can be done is to reduce PKS particle size to a maximum of 2 cm so that it can be atomized in a pulverized system. The second thing to note is the percentage of PKS in coal, or the term cofiring. Unlike a grate and a fluidized bed combustor that can be flexible with various types of fuel, pulverized almost all use coal only. Of course it can also be pulverized to be replaced with biomass, especially PKS, but there are specific things that distinguish biomass and coal fuels, namely ash content and ash chemistry. Both of these things greatly influence the combustion characteristics in the pulverized system.

Coal ash content is generally greater than biomass, besides that coal ash chemistry is very different from biomass ash chemistry. Biomass ash has lower an inorganic than coal, but the alkali content in biomass can change the properties of coal ash, especially aluminosilicate ash. Practically, if you want to change the pulverized system from coal to biomass, especially PKS, it is necessary to modify the power plant and this is also not cheap, but if you want without modification or just a small modification of the power plant is needed, namely the cofiring method. Biomass cofiring with coal in small portions for example 3-5% does not need to modify the pulverized power plant. For example, Shinci in Japan with a capacity of 2 x 1,000 MW of supercritical pulverized fuel with 3% cofiring requires 16,000 tons / year of biomass and no modification, likewise with Korea Shoutheast Power (KOSEP) 5,000 MW with 5% cofiring requiring 600,000 tons / year of biomass and also without modification. Why is the cofiring in the pulverized system discussed a lot? In addition to this type of generator, the most number with a very large electricity production capacity so as to be an effective means of reducing CO2 levels in the atmosphere which also automatically reduces coal use, also the use of biomass in cofiring has an effect on plant operations and the price of electricity produced. The other main reason is because is the cheapest way to enter renewable sector especially for big coal powerplants.

Denmark 700 MW Studstrup power station conducts cofiring up to 20% with straw

Combustor grate technology, fluidized bed and pulverized are basically combustion technologies. Combustion technology is one of the 3 thermal biomass processes that are widely applied, with the other two are gasification and pyrolysis. Gasification as well as pyrolysis can also be used for electricity production, but its use is not as much as combustion technology and its electricity production capacity is generally also small. Almost the same as a combustor grate and fluidized bed, fuel for gasification and pyrolysis is also flexible, including coal and PKS. In gasification technology mainly to maximize gas products (syngas) while in pyrolysis to maximize its solid products. PKS can be pyrolyzed to produce charcoal while coal will produce coke if it is pyrolyzed. Charcoal from PKS can be used for fuel, briquette production and activated charcoal while coke for steel smelting. Syngas is a pyrolysis by-product that can be used for electricity production while in gasification, syngas is the main product that can also be used for electricity production.

Why use PKS for the powerplant fuel? This is because PKS has almost the same characteristics as wood pellets, many are available and are cheap. Indonesia and Malaysia are the two main producers of PKS. PKS is produced from palm oil processing and considered as waste. With an area of ​​Indonesian oil palm plantations reaching 12 million hectares in Indonesia and 5 million hectares in Malaysia, the number of PKS produced from both countries reached 15 million tons / year. The number of PKS in both countries exceeds the production of wood pellets from the United States and Canada, or the two largest producers of wood pellets today. And of course the United States and Canada cannot produce PKS, because they do not have oil palm plantations, but Indonesia and Malaysia can also produce wood pellets because they have large forests. The production of wood pellets in Indonesia and Malaysia is still small today, which is less than 1 million tons / year, but the production of PKS is quite large which can act as an initial driver of bioeconomy in the countries and supply the PKS biomass to Europe.

Optimization of Biomass-Coal Cofiring in Coal Fired Powerplant

Biomass-coal cofiring has been commonly performed by a number of coal power plants in Europe and America with a primary motivation for reducing the environmental impact of emissions. Currently the percentage of cofiring biomass with coal is still small on average below 10%. It can also occur due to a limited supply of wood pellets. But in terms of operational cofiring biomass-coal fly ash will reduce significantly. On the other hand, if the percentage of biomass-coal cofiring is added will cause deposits on boiler pipes that will disrupt the process of heat transfer in the furnace causing inefficient use of fuel with one indicated by the high temperature of the fluegas.

There are three techniques commonly used in biomass-coal cofiring:

1.       Mixing of biomass and coal in the fuel handling system (then fed to the boiler).

2.       Setting up a separate biomass with coal, and then inject into the boiler.

3.       Gasification of biomass to produce a gas which is then burned in the boiler directly or using the integrated gasification combined cycle (IGCC) system.Worldwide reported more than 200 coal power plant that has been tested with biomass (IEA 2010).

Several cofiring  options are available on in coal powerplant, among others:

-Cofire with a low percentage of biomass, with a slight modification of the equipment.

-Cofire with a high percentage of biomass, by upgrading equipment.

- Convert/repower individual coal burners to be fired with biomass

- Convert/repower entire coal plants to be fired with biomass

- Cofire with torrefiedwood

Ash content in coal and biomass are generally differ quite large and moreover  ash chemistry are also a lot of different. This is factor that causes a lot or at least a deposit in the boiler pipes. Percentage of cofiring biomass-coal up to 10% is generally acceptable. Optimal percentage that causes minimal boiler tube deposits and significant reduction in fly ash can be searched based on the variable characteristics of coal and biomass are used.