Ash From Palm Oil Waste Combustion As A Biomaterial In Concrete Construction

Palm kernel shells (PKS) and mesocarp fibres are commonly used as fuel for palm oil mills for the production of electricity and steam for sterilizers. The byproduct of combustion in the form of ash a lot produced by the process. A palm oil mill can produce 5 tons / day or more of the ash depending on the amount burned because the average ash content of the palm oil waste is around 5%. Most of the palm oil mills do not make use of the residual ash from the combustion but just throw it away. Though the ash can actually be used for various purposes, one of which is as a biomaterial substitute for sand in various building construction projects. The locations of palm oil mills or surrounding communities usually in the remote areas difficult to obtain a supply of sand for the construction of various buildings, so that the presence of ash as a substitute for sand will be helped.

Typical boiler in palm oil mill

At present, a number of domestic industries have started to use biomass fuel, as an environmentally friendly fuel. Palm kernel shell or PKS is one of the favorite biomass fuels at this time. One of the interesting things about palm kernel shells is that they are  almost similar properties to wood pellets, but at a lower price. Ash from the palm kernel shells as well as ash from the palm oil mills can also be used for sand substitution for various building construction projects. The location of industries in urban areas so that the use of ash can also be in line with various city construction constructions such as the construction of roads, high rise buildings and aircraft runways. Although the ash cannot replace 100% sand but it will help save costs by around 25% substituted with the ash.

Main chemical compounds content in sand

Main chemical compounds content in boiler bottom ash

The palm kernel shell ash itself contains high silica which is around 60%, while the ash mixture from the shell and palm fiber in the boiler furnace contains about 30% SiO2. SiO2 functions as a filler in the manufacture of concrete which will affect the strength of the concrete. While the composition of the main compounds in the sand can be seen that SiO2 has a relatively large composition. Mud in sand is impurity so that the mud content in sand cannot exceed 5%. Mud levels of more than 5% result in the hydrogen bonding of cement paste and sand being reduced due to the influence of sludge as impurities. Boilers ash from palm oil mills can be used as a substitute for sand in making concrete because it has compounds that play a role in making concrete. If it is to be used as a cement substitution material, a material must contain a relatively large amount of lime compounds because cement basically functions as a binder and mainly functions as lime. The content of SiO2, Al2O3, and CaO is contained in boiler scale ash (bottom ash) which is needed in making concrete.

The yield of ash density is 2.11 g / cm3, lower than the density of sand. According to ASTM C128-93, the good density for making concrete is above 2.50% so it includes rough aggregate. The absorption value of water produced by boiler scale ash fulfills the requirements of preventing or reducing empty voids in concrete. The maximum limit of SiO2 content contained in sand for making concrete is around 30%, so that the boiler bottom ash is included in a good aggregate and meets the standard for partial sand replacement. Based on the composition of the chemical content, boiler bottom ash is superior to sand because CaO in ash plays a role in helping cement as a binding material. Similarly Al2O3 is very influential in accelerating hardening of concrete.

Comprehensive concrete strength tester

If calculated the potential ash from the waste of palm oil is also quite large potential. Say with 1,000 palm oil mills now operating in Indonesia with each mill producing 5 tons / day of ash then 5,000 tons of ash a day or 150,000 tons of ash a month. While the excess of palm kernel shells from palm oil mills that can be utilized by other industries in Indonesia is estimated to reach 11 million tons, which means that the ash potential that can be produced is 550 thousand tons. In addition, the difference between ash from palm oil waste classified as biomass ash and coal ash is palm kernel shell ash excluding B3 (hazardous) waste so handling is easier while coal ash including B3 (hazardous)waste whose handling is more difficult. Palm kernel shell ash also comes from biomass which is a renewable resource while coal is from non-renewable resource group.

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.

Ash Problem Up to Modification of Power Plant in the Framework of Coal Substitution to Wood Pellet on Pulverized System

The ash of various biomass in combustion causes a number of problems in the pulverized system. That's because the pulverized system is designed to burn coal. Does not the coal also produce ash after burning even more quantity? Yeah right, but the chemical content of coal ash is different from that of biomass. The difference is that coal ash contains a lot of heavy metals such as arsenic, cadmium, mercury, selenium, lead, and nickel which have high melting point so it does not cause problems to the pulverized, whereas biomass ash mainly contains alkali metals such as potassium and alkaline earths are calcium, which has a low melting point so it creates a problem on the pulverized. The content of coal ash is also much larger compare with biomass ash content.

Ash Slagging: Melt ash deposits on boilers or generally on radiant exposed surfaces in a furnace at temperatures above
1,000 C

High-temperature fouling (around 1000 ° C) while low-temperature fouling (300-600 ° C) usually occurs in boiler pipes

What is the problem with the biomass ash chemistry? There are several problems caused by the ash chemistry on the pulverized system, namely slagging and fouling. How to avoid it? There are several ways to avoid it, namely first, determine the exact percentage of biomass fuel in the pulverized system or co-firing. At the appropriate percentage of biomass fuel can be used simultaneously (co-firing) with coal. The type of biomass fuel also determines its percentage, eg wood pellets will have a larger portion than agro-waste pellets. Secondly, by modifying the coal-fired power plant so that it can even be 100% with biomass fuel such as wood pellets.

Burning biomass fuel in addition to environmentally friendly or carbon neutral, fewer CO2 emissions, SO2 emissions are also very small, very little fly ash even does not occur, and the ash is rich in potassium, and phospur so it becomes a good fertilizer for plants. While coal ash is otherwise and even categorized as dangerous waste due to the heavy metal content. Based on these things, it is natural that the use of biomass fuel continues to be improved even has become government policy in some countries.

The entry point for massive biomass fuel usage is co-firing with coal. The modification of the coal-fired power plant has become an opportunity, so a number of companies have emerged to capture such opportunities as Ramboll and Doosan Babcock. While in Indonesia it seems still long enough to make biomass fuel has a large portion as a source of energy, especially in power plants, because there is no policy that supports it.

Focus on Biomass Ash

Utilization of biomass with thermal route which is combustion and gasification will be generated ash residue. While the pyrolysis due to work at low temperatures (400-600 C) and without oxygen / air then there is no ash. Minerals contained in the biomass will remain behind in the form of charcoal products in the pyrolysis process. Ash content and ash chemistry greatly affects the utilization of biomass in addition to heating value, particle size and moisture content. In general characterization of ash from biomass is described by Bryers as follows:

1.     High  silica (Si) and potassium (K) ash content  while the calcium (Ca) is low, with low fusion temperature derived from the group of agricultural biomass wastes.

2.     Low silica (Si) and potassium (K) ash content while calcium (Ca) is high, with a high fusion temperature derived from a group of nearly all the woody biomass. Specifications are best for combustion and gasification.

3.     High potassium (K) and phosphorus ash content, with low fusion temperature derived from faecal matter such as poultry and cattle dung.

Biomass Ash Content

The ash content of various types of biomass indicate slagging behavior.  In general, the higher ash content, the greater the tendency of its slagging behavior. But this does not mean that low ash slagging not show the phenomenon.  Operating temperature, ash chemistry and ash content are variables the occurrence of slagging. If conditions favor the slagging will be even greater. Minerals such as SiO2, Na2O and K2O more tendency toward the occurrence of slagging. Usually slagging occurs in biomass with ash content of more than 4% and non-slagging fuel with ash content less than 4%. According to the composition of the melt, fuel-biomass fuels are grouped into severe or moderate slagging.

Biomass Ash Chemistry 

Issues arising from these ashes is clogging the air intake holes or slagging that will reduce the thermal efficiency of the gasification and combustion process related air supply and biomass distribution in the process unit.

Ash Slagging

So before processing the biomass waste, consider and note the ash content and its chemistry so that it can deliver optimum results.

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.