2.2 MSW Valorization Technologies
2.2.3 Incineration and Combustion
Odor
As the organic components are composted, significant amount of odor is generated due to the release of hydrogen sulfides, volatile organic sulfides, ammonia, pyridine, alcohols, esters, ketones, and aldehydes [70]. Typically odor at composting sites are tackled by suf- ficient aeration and by the addition of bulking agents such as cornstalks, rice straws, wheat straws, wood chips, and sawdust [70]. Negative active aeration is an efficient method for tackling release of odor [53, 70].
Bioaerosol
Airborne microorganisms, called bioaerosol, are released when composting feedstock is handled or turned during composting. Bioaerosols can cause respiratory diseases, and other chronic lung diseases in composting facility workers [70]. It is recommended that the operating cab of heavy equipment being used at composting sites be equipped with pressurization systems and high efficiency particulate air (HEPA) filters [71].
Heavy Metals
Heavy metals in compost can reduce its quality. Sources of metal in compost are compost feedstocks. Source separation of the metal contaminants have been shown to be the most efficient in improving compost quality [72].
MSW incineration, and landfill gas (LFG) recovery system. After LFG is recovered, it is combusted to generate electricity via steam generation. Out of these two WtE methods, MSW incineration is the most economically favorable [76].
Moisture in MSW decreases its calorific value [77]. MSW of developed countries contain less moisture as compared to that of developing countries [77]. The calorific value of de- veloped country MSW is between 8-12 MJ/Kg [77]. Bangladesh is a developing country and the waste generated in Bangladesh contains more moisture and will not generate as much a heating value mentioned above.
The thermal energy within a MSW sample can be calculated by performing ultimate anal- ysis of the MSW. The Higher Heating Value (HHV) of MSW is the summation of the Lower Heating Value (LHV) of MSW and the latent head of vaporization of water [78].
HHV of MSW can be calculated using Dulong’s formula [79]:
HHV(BTU/lb) =14544C+62028(H−O/8) +4050S (2.2)
whereCis the carbon weight fraction ,His the hydrogen weight fraction,Ois the oxygen weight fraction, andSis the sulfur weight fraction.
The LHV can be calculated as follows [80]:
LHV =HHV−[W H2O+ (9×W H)]×1050 (2.3)
whereW H2O is the moisture weight fraction in MSW, andW H is the hydrogen weight fraction in MSW.
The general process for WtE facilities is to utilize the Rankine Cycle where water is the cycle media. Water is converted to superheated steam using the energy from waste. The superheated steam is then passed through a steam turbine to generate electricity; which in turn cools the steam and it is condensed back to liquid state. As corrosion is accelerated
at elevated temperatures, most WtE facilities do not operate at the most efficient config- uration due to corrosion problems within the boiler. WtE facilities are therefore limited to only about 25% efficiency [77]. For higher efficiency, other WtE technologies such as gasification, hydrothermal treatment, and pyrolysis are more desirable and they will be discussed in the later part of this chapter.
2.2.3.1 Combustion/Incineration Technologies
MSW combustion technologies are classified into Mass Burn Systems, Modular Combus- tion Systems, and Refuse Derived Fuel Systems.
Mass Burn Systems (MBS)are the most common type of combustion technology and 90%
of all WtE technologies in Europe are MBS [73, 81]. MBS are built as field-erect water- wall furnaces with a high MSW processing capacity of up to 400 tons a day [78]. MSW combustion can reduce methane emission from landfills [81]; however, MSW combustion releases more greenhouse gases than fossil fuel combustion [82]. MSW incinerators for electricity generation have an efficiency of about 13-24 %, which is lower than fossil fuel fired power plants [82].
Modular Combustion System (MCS)are cheaper to build than MBS. However, MCS can process only about 150 tons MSW per day, which is lower than the amount of MSW MBS can handle a day [78]. The only similarity between MBS and MCS is that they are both fed unsorted and unprocessed MSW. The boiler in MCS is different than that of MBS.
In MCS, the boilers are not water-wall boilers but waste heat boilers. In MCS, waste is fed in batches using front-end loaders in the combustion chamber instead of on grates or rollers like in MBS. In a MCS system, primary combustion takes place in a series of 3-7 refractory-lined hearths; the evolved flue gas is then combusted in a secondary combus- tion chamber. After combustion, the ash from the last hearth in the primary combustion chamber is quenched and collected for disposing off in a landfill [78].
Refuse Derived Fuel (RDF)combustion systems require presorting of MSW unlike MBS and MCS [81]. For this reason, some RDF facilities have an integrated waste sorting fa- cility. The sorting facility removes ferrous materials using a drum or belt magnet followed by shredding of MSW [78]. The desired size of combustable waste is between 2-6 inches [78]; waste is typically screened using trommels, discs, or other type of screening equip- ment to remove non-combustable or undesired waste size [78]. The combustion system in RDF is similar to that of MBS but different from MCS. In RDF, the waste feed travels through the combustor in a traveling grate. The waste is then discharged from the grate to a water quench trough. The boiler system is a water-wall system like that in a MBS.
Additives such as zinc oxide (ZnO), iron(III) oxide (Fe2O3), and aluminum oxide (Al2O3) can facilitate the combustion of MSW [83]. With sufficient supply of air and oxygen, the flue gas temperature increases [83, 84]. Since the fuel (sorted MSW) is more homo- geneous in RDF systems, less amount of excess air is needed for efficient combustion, making RDF systems more efficient. A fluidized-bed reactor can also be used to combust sorted waste; turbulent mixing takes place in the fluidized-bed, and once heated, the bed retains the heat for a longer time resulting in a stable combustion process [78]. The re- leased flue gas from a fluidized-bed reactor is passed through a boiler for heat recovery.
Expensive sulfur removing gas treatment processes, such as scrubbers, can be eliminated in fluidized-bed reactor systems. The addition of sulfur absorbing chemicals, such as limestone, in the bed will retain the sulfur within the sorbent.
2.2.3.2 Environmental Impacts of MSW Combustion/Incineration
MSW Combustion for producing energy are not clean processes. MSW Combustion fa- cilities produce many harmful gases as well as ash. Typical flue gas treatment involves removal of particulate matter. nitric oxides, acid gases, organic substances, and metals [77, 85]. However, treatment of the flue gas leaves behind harmful residue.
Particulate matter is removed using fabric fiber baghouses which are tightly woven fabric;
the flue gas is passed through the baghouse and particulate matter gets collected on the
fabric [73]. Particulate matter can also be removed using electrostatic precipitators and cyclone separators which uses electrostatic force on electric plates and centrifugal force, respectively, to separate particulate matter from the flue gas [85]. Toxic mercury and dioxin are removed from the flue gas by mixing the flue gas with powdered activated carbon just prior to the flue gas entering the baghouses [73]. Acid gases, such as sulfur dioxide (SO2), hydrochloric acid (HCl), and hydrofluoric acid (HF), can be removed using acid scrubbers. Acid scrubbers are very efficient in removing HCl; upto 95% HCl and approximately 33% HF can be removed using acid scrubbers [73]. Oxides of nitrogen (NOx) can be removed using either selective catalytic reduction (SCR) or selective non- catalytic reduction (SNCR). In SCR, ammonia is used to reduce NOxgases in the presence of vanadium oxide or titanium oxide catalyst; SCR can remove upto 90% of NOx from the flue gas. In SNCR, ammonia is sprayed in to the hot flue gas at high temperatures (870-1150◦C) to reduce the NOxto N2and H2O [73] without the use of any catalyst. The conversion efficiency of SNCR is around 30-75% [73]. All of the aforementioned flue gas treatment methods are tabulated below in Table 2.11.
Bottom ash from MSW combustion facilities are collected mechanically and screened for recyclable materials [73]. The residue is then disposed off at a landfill. Nonetheless, there is a potential risk of noxious components leaking out from ash depositories [86]. In contrast to gasification processes, combustion ash from moving grate incinerators (such as that in MBS and MCS) cannot be recycled immediately [87] and landfilling remains the only viable option. However, ash obtained from RDF systems differ significantly from MBS and MCS and have similar composition of that of cement [77].
TABLE2.11: Pollutants in flue gas of MSW incineration plants and their treat- ment
Pollutant Treatment / Method Comment
PM1
Fabric filter baghouses
Electrostatic Precipitators - - - Cyclone Separators
Mercury/Dioxins PAC2
SO2, 95% HCl removal
HCl, acid gas scrubbers 33% HF removal
HF NOx
SCR (NH3with V2O5or TiO2) 90% removal for SCR or SNCR ((NH3at 870-1150◦C) 30-75% removal for SNCR
1Particulate Matter
2Powdered Activated Carbon
2.2.3.3 MSW Combustion Potential in Bangladesh
There are no MSW combustion facilities in Bangladesh and majority of the generated waste ends up at landfills, untreated. There is huge potential for electricity generation from MSW in Bangladesh. The lower heating value (LHV) of MSW in Bangladesh was calculated to be 9.85 MJ/kg using modified Dulong’s equation [88]. Table 2.12 shows the projected MSW generation in 2030; the MSW generation growth was calculated using Compound Annual Growth Rate (CAGR) method to account for the amount of MSW generated in 2030.
TABLE2.12:ProjectedMSWgenerationrateandElectricitygenerationfor theyear2030.Adaptedfrom[88] DhakaChittagongKhulnaRajshahiBarisalSylhetRangpurBangladesh MSW1 GWh2 MSW1 GWh2 MSW1 GWh2 MSW1 GWh2 MSW1 GWh2 MSW1 GWh2 MSW1 GWh2 MSW1 GWh2 3.4021712.91.788900.20.446224.40.12261.30.10954.80.288144.90.17688.46.3313186.9 1 MunicipalSolidWasteinmilliontonsperyear 2 Gigawattshourofelectricityperyear