Others

This chapter discusses some other technology concepts regarding waste treatment. These methods may not necessarily be oriented into disposing the waste, but to refining it or being a part of, for example, SRF production process.

Three separate technologies are discussed. These are bio drying, gasification, and com-posting. First chapter presents drying technologies by utilizing the heat generated by bac-teria in the waste itself. Second chapter, gasification, is further divided in bio- and thermal gasification and both of these are presented and discussed. Lastly, composting is dis-cussed as a method to treat and dispose bio waste.

4.5.1 Bio drying

Drying the fuel increases its calorific value, since there is less moisture to heat up to boiling point and then evaporate and further superheat the steam. This means, that one mass unit of the fuel produces more energy when combusted and thus increases the am-bient temperature more. This is because there is less amount of water and thus more fuel in the mass unit. On the other hand, since this moisture is not a component advancing the incineration, its heating and evaporation process only captures energy from the combus-tion of the actual fuel. This is not necessarily devastating for the boiler operacombus-tion, since the same energy is released in combustion, only part of it is stored as the thermal energy of steam in the flue gas and can still be retrieved in, for example, heat transfer pipes. On the other hand, this lowers the temperature of the flue gas, causing lower efficiency and heat transfer coefficient in the boiler.

Since the waste has usually quite high moisture content, drying it may be beneficial in order to capture the maximum amount of energy from the waste in combustion process.

On the other hand, drying itself requires also energy and time. The waste needs to be arranged properly for the drying and the conveying of the humid air out and dry air in has also to be arranged to enable the potential for drying. After the drying, the waste needs to be collected to make room for new wet waste. The drying also takes some time, depending on the drying conditions. For example, the drying time can be reduced by providing good drying conditions. In general, the lower the relative humidity, higher the temperature and greater the flow of the drying air, the faster the drying is.

One other method for drying is bio drying. In this method, the heat produced by the bac-teria as a side product of their metabolism, is used to heat the air and waste and thus to evaporate more moisture off of the waste. The waste is usually covered by membrane cover to let the air and steam pass, but keep the rodents and other animals out and to make

the system more sanitary. The organic material in the waste feeds the bacteria, enabling them to reproduce and to provide heat for the process. Air is blown into the waste so that the continuous airflow inside the waste would cause the relative humidity to remain low inside the waste and thus promoting the drying. Some of the organic content of the waste is lost in this operation, as the bacteria consume it and transfer it to carbon dioxide and heat. On the other hand, the drying is efficient, and the air does not need to be heated separately. The process can be also sustained in colder weathers. (Convaero 2015)

4.5.2 Bio- and thermal gasification

Gasification converts solid organic material into flammable process gas. This process gas can further be refined by cleaning and, if necessary, by liquefying, into high quality gas or liquid fuels. In this chapter, two gasification methods are presented. These are bio gas-ification and thermal gasgas-ification.

In bio gasification, biodegradable material is transformed by bacteria into methane, car-bon dioxide, and some impurities through various phases. This process happens in biore-actors in anaerobic state. For this reason, bio gasification is also referred to as anaerobic digestion. Other outputs for this process are solid digestate, which can be used as, for example, fertilizer, and waste water which requires further processing and purifying.

Thermal gasification converts material that contains elementary coal, into process gas, called syngas. The thermal gasification resembles combustion, since the process happens in relatively high temperature and produces heat, but it happens in sub stoichiometric state. This means, that there is less oxygen present in the reaction as would be needed for the material to combust in an ideal combustion by the chemical reaction equation. Thus, the combustion lacks oxygen and the combustion is imperfect. The syngas is then mostly carbon monoxide (CO) and hydrogen (H2), instead of carbon dioxide (CO2) and water (H2O). The syngas is then also flammable, since it can react with oxygen (O2). It can be then purified from other impurities and liquefied. High quality liquid fuels can be pro-duced from the syngas by further refining it. One example of this is producing liquid hydrocarbons in a process called Fischer–Tropsch process. (Spath & Dayton 2003) One advantage of thermal gasification is that the organic compounds containing coal are quite efficiently gasified, whereas the inert components remain in the bottom ash and can then be recycled properly. After purifying the process gas it can be incinerated in quite high temperatures, enabling high efficiencies for the heat transfer and steam processes.

On the other hand, the process where the extremely heterogeneous waste is present, hap-pens in relatively low temperature compared to, for example, direct combustion of waste.

The lower processing temperature of waste reduces the corrosive effects to the gasifier by the hazardous chemical components in the waste. On the other hand, investing in gas-ification process is an additional cost. Thus, the benefits gained must outweigh the addi-tional costs. (Spath & Dayton 2003)

4.5.3 Composting

Composting offers a quick and easy way to process easily degradable bio waste. This method works well on materials such as animal and plant tissue, but does not perform very effectively on materials such as wood, polymers or leather and does not work at all on materials such as glass, ceramics or metals (Diaz et al. 2002, p. 12.1). The main outputs of composting are carbon dioxide, water, and compost, which in essence is extremely fertile soil (Diaz et al. 2002, p. 12.1).

Composting is, like bio gasification, executed by microorganisms, such as bacteria, but also, for example, fungi, worms and larvae (Diaz et al. 2002, p. 12.3). On the other hand, while bio gasification is executed in absence of oxygen and thus is an anaerobic process, composting is aerobic process and consumes oxygen. In essence, carbon contained in compost is transformed into carbon dioxide by reactions with oxygen and the hydrogen of carbohydrates are transformed into water, also by reactions with oxygen. This is exo-thermic reaction and thus the composting produces also heat as the bio heat of the living organisms in the compost.

Due to the production of heat compost evaporates water quite rapidly when functioning correctly. For this reason, the compost might need to be irrigated, as a sufficient level of moisture is required for the microbial activity and thus for the composting to occur (Diaz et al. 2002, p. 12.10). The compost needs also sufficient aeration, since the process is aerobic due to the normal metabolism of the microorganisms. Sufficient aeration can be provided by, for example, turning the compost every now and then.