Hydrothermal Processing Of Municipal Solid Waste Environmental Sciences Essay

Municipal Solid waste are gabages, sludge or refuse gotten form a waste water treatment plant, air pollution control facility, or water supply treatment plant, EPA 2012. These also include solid, semi -solid, liquid or gaseous materials which result from industrial, mining, commercial or agricultural and household operations. The appropriate treatment of the increasing MSW and the need for energy is becoming a global environmental issue. This has prompted in different treatment methods to turn the waste to renewable energy as well as reduction of CO2 emissions.

Fig (1): Municipal waste before recycling (Onwudili, 2012)

Traditional means of waste disposal such as composting, landfill and open and closed incineration technologies have caused environmental concerns due to its adverse effects. (Onwudili and Wiliams). Chemicals can be released into the environment via erosion during composting. It also has the potential of releasing harmful gases. Future generations are at the risk of landfilling because of the release of greenhouse gases during the process and from potential toxic leachate consisting of organic pollutants and toxic metals that sip underground into drinking water. Recently built incinerators adhere to existing legislative emissions but there are still some public concerns owing to the fear of the adverse effects of the emitted levels.(PT Williams NOTE)

ADVANCED MSW TREATMENT

Due to the health and environmental hazards posed by the traditional methods of treating MSW, the EU has produced legislative directives guiding and driving better MSW handling for its member states in the European Union.

Fig (2): Some EU guidelines governing MSW treatment (Onwudili, 2012)

Different advanced treatment methods include Plasma Arc, Autoclaving, Microwave, Bio-refining, Hydrothermal Processes. Reasons for various MSW processing methods include, reduction in greenhouse gas emissions, achieving proper and maximum treatment of MSW, creation of job opportunities and commercial exploitation from the sale of the by-products.

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HYDROTHERMAL PROCESSES

Hydrothermal technology studies dates back as early as the 17th century and the first patent was issued to Robert Gardner in 1788 for his research in the area of gasification. However, the area of Hydrothermal technologies were dropped during the span from 1800-1970 due to the abundance of oil. Lawrence Berkley Laboratory researched on liquefaction in the early 1980’s and the initial experiments conducted showed that hydrothermal processes could convert different biomass sources to oil products. (Midgett, 2005)

Hydrothermal Processing is a method in thermochemical processing of waste. This is done in a very high temperature media so as to upgrade the waste in a short given time. This method of waste treatment is a progressive method in the conversion of biomass and MSW into clean energy resources because it can increase the drying and dehydration performances of biomass with high content of moisture and also increase the quality of the fuel produced from the MSW. (Kim, 2012)

Fig(3): Schematic of a Hydrothermal treatment process (Biller, 2012)

Principles of Hydrothermal processing

This process designates a system which is aqueous and at increased temperatures and pressures that are close to critical point of water (3740c, 22.1Mpa) or above it. It also requires an addition of a catalyst. This is to increase decomposition. It can also be added to speed up gasification rates. The strength and working duration of a catalyst can be influenced and reduced by substances such as chlorine, sulphur and metals.

Below are different Hydrothermal Processes for MSW

Hydrothermal Liquifaction (HTL), Hydrothermal Carbonisation (HTC), Hydrothermal Gasification (HTG), Wet Air Oxidation (WAO), Hydrothermal Refining (HTR), all these are under subcritical water treatment : T≤350oc, P ≤20MPa

Supercritical water gasification (SCWG), Supercritical water Oxidation (SCWO), Supercritical Refining (SCWR), all these are under supercritical water treatment: T≥374oc, P≥22.1MPa.

Figure(4): Different HTP methods and their by-products (Onwudili, 2012)

HYDROTHERMAL LIQUIFACTION

This method can also be referred to as ‘hydrous pyrolysis’. The process is synonymous to the production of fossil fuels due to geologic means. But the difference is that the technological HTL process works in a time frame which is measured in minutes unlike that of geologic time. HTL is a chemical process which reforms biomass in an oxygen starved enclosure which is heated and usually pressurised.

Researchers like Herbert R. Appell and his co-workers in the 1970’s were among the first to look into HTL processing of biomass. They used different feedstocks like urban refuse, cellulosic waste, sewage sludge in their research (REF)

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Fig(4): Schematic of a Hydrothermal Liquifaction treatment process

Liquid water is very important in HTL of lignocellulose feed stock and other types of biomass (algae and manure) because of its role as a vehicle and as a catalyst carrier for the feedstock. It also serves as both solvent and reactant. HTL when used to treat lignocellulose can convert to oil in quantity in the presence of a catalyst. However, the use of HTL commercially has its flaws due the fact that it is uneconomical relative to the cost of gasoline or diesel production. The transportation of huge quantities of biomass remains expensive and increases production cost.

HYDROTHERMAL CARBONIZATION (HTC)

Hydrothermal carbonization (HTC) is a method of hydrothermal conversion that used for treating or stabilizing waste streams with minimal production of greenhouse gas. (REF). The HTC process runs subcritically at a relatively wet low temperature of about 180-350oC and under autogenous pressures, can convert carbohydrates into hydrochar.

The Hydrothermal carbonization process (HTC) was first described by Friedrich Bergius in 1913, but was downplayed due to rise in the exploration and exploitation ofoil (REF)

Fig (5): HTC, its advantages and byproducts (Science Direct)

Water is a necessary and key ingredient of HTC as it acts as an organic solvent when its physical properties have changed due to increase in temperature. Due to the fact that HTL process shows a lower activation energy than other dry hydrothermal processes, lower temperature HTC reactions can continue with the same level of conversion efficiency as higher temperature processes. Presently, HTC has been mostly applied and studied on a limited number of feedstocks, ranging from pure substances to slightly more complex biomass such as wood (Berge 2011). The creation of low-cost carbon-based nanomaterials/nanostructures from carbohydrates has been a major drive instead of advancing in the utilisation of HTC as a sustainable waste management technique.

Previous studies show that a significant fraction of carbon remains within the hydrochar during the HTC process, suggesting carbonization of waste streams may mitigate greenhouse gas emissions.

Table(6): Ultimate and proximate analytical results of initial feedstocks and Produced hydrochar (Berge 2011)

HTC processing has the capability of degrading emerging compounds such as hormone destructive compounds and personal care products. The surface functionalization patterns which the HTC produced char contains makes it amendable for important and beneficial end use. It can be used as feedstock for carbon fuel cells, adsorbent for harmful pollutants, and similar to char from pyrolysis/gasification, it can be used for soil amendment. Since hydrochar is already sterilized, it may require less solids processing and handling.

HYDROTHERMAL GASIFICATION (HTG)

A broad range of various organic chemical functional types that contain carbon, oxygen, and hydrogen can be treated by the wet gasification process (Kyong 2006). The hydrothermal gasification method of treating waste also uses the hydrothermal processing conditions of 250-360oC and up to 22MPz) to process biomass, waste water and organics-in-water process residues. In the process, gases containing carbon dioxide and methane are converted from the organic contaminants. High levels of carbon-to-gas conversion at up to 350oC (relatively low temperature) can be converted through a metal catalyst gasification of wet biomass. The gas produced from this kind of process is called a syngas (synthesis gas) or a producer gas and is composed of CO, CO2, CH4, H2 and N2.

Applications of the by-products include:

1. Power generation for gas turbines, commercial internal combustion engine, fuel cells

2. Production of Hydrogen

3. Chemical feedstock like Methanol, Methane, Liquid transport fuels

Fig (7): Energy Contents of synthetic gas Jenny Jones (2011)

CONCLUSION

Traditional Methods of MSW treatment

Incineration: This is a method of MSW treatment which tends to be one of the most expensive solid waste management options which requires highly skilled personnel and careful maintenance. This makes incineration a good choice but only when other less expensive and simpler choices are not available. It is not advisable for underdeveloped countries to run incinerator plants because of the requirement of highly skilled operators and finance to run maintenance. It can be advantageous to site an incinerator when a landfill cannot be sited because of the lack of proper sites or long and expensive haulage distances. On the other hand, the disadvantages tend to be harsher than the advantages in the sense that its emissions primarily deals with human and environmental health. The plant involves high operating costs and requires the throughput of both foreign and local in the lifetime of the plant. Residues from cleaning of the flue gas can contaminate the environment if not handled properly. It should be properly disposed of in a controlled landfill to prevent surface and ground water pollution. There are other disadvantages associated with incineration. Emission of NOx, SO2, HCL, dioxins and fine particles, carbon dioxide from fossil-derived waste (eg plastics) and N2O which contributes to global warming.

Landfill: This is a Municipal Solid Waste treatment process with a scientfically engineered structure built into or on the ground that is designed to isolate waste from the environment. Landfills operate in accordance with EPA’s Subtitle D regulations. Waste is isolated by a liner that is placed on the bottom of the landfill to collect and remove any water that might pass through the waste and by a cover system over the waste to control water infiltration. In addition, gas collection systems are installed to collect any gas generated by the decomposition of the waste and groundwater-monitoring wells are installed around the landfill to monitor the performance of the liner systems.