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.

Chlorine Corrosion, Ghost The Feared Powerplant

Superheater pipes corroded by chlorine

There are 3 groups of biomass used as powerplant fuel around the world based on its quality such as calorific value, ash content and ash chemistry.  First, woody biomass includes all woody trees such as eucalyptus, calliandra, gliricidia and so on. The second group is agro-industry wastes such as ricehusk, baggase, coconut shell, palm shell and so on. While in the third group is grasses.

Although in general the biomass has a low concentration of chlorine (except straw) compared with coal, heavy corrosion occurs in a powerplant that use biomass fuel and cause leaks in the pipes of heat exchanger with  less than 10,000 hours of operation. Corrosion of heat transfer apparatus is strongly associated with ash deposits in the pipes. The mechanism of the complex reactions of chlorine corrosion in boiler pipes like this chart:

The mechanism of corrosion chemical reaction of chlorine

Superheater steam pipes are generally not designed to accommodate the chlorine in biomass fuels. Corrosion mechanisms above are quite well understood by designers and users of biomass boilers in Europe, indications are superheater pipes installed in the low temperature gas zone. Another technology that is able to accommodate biomass fuels with high chlorine levels such as agro-industry wastes and grasses are Circulating Fluidised BedCombustion (CFBC). Temperature pulverized coal (PC) boilers which operate at temperatures up to 1500 C 1400 causing severe corrosion in the superheater pipes, while the CFBC operating at lower temperatures ranging from 850C to 900C.