2.2 MSW Valorization Technologies
2.2.5 Pyrolysis
HTL processes are not yet commercially viable and more work is needed to understand the feasibility of HTL plants before commercialization becomes a reality. It is difficult to understand if HTL plants will be feasible to produce bio-oil from MSW in the future.
2.2.4.3 Hydrothermal Gasification (HTG)
HTG uses supercritical water at 400◦C and 250 bar to convert the organic materials into gaseous products such as hydrogen (H2), CO2, CH4, CO, etc. [108]. The composition of gaseous product in HTG is similar to that of the syngas from high temperature steam gasification process. HTG can completely convert the organic fraction within MSW to produce H2 or CH4 rich gas depending on the catalyst used. Although MSW is highly inhomogeneous, HTG can convert the organic fraction to gaseous products completely while destroying the harmful pathogens in MSW [109, 110]. Molinoet al. [111] carried out lab-scale HTG experiments on MSW leachate to produce syngas and synthetic natural gas. A syngas with a ratio of H2/(CO + CO2) between 0.45 to 1.12 was obtained using nickel (Ni) based catalyst. However, the use of alkali catalyst is more logical as solid catalysts can be easily poisoned by the alkali and sulfur present in the system [109, 112].
Moreover, alkali catalyst is naturally sourced and can be in dissolved form during gasi- fication process [113]. At this point, only a limited number of study on HTG of MSW exists in literature and it is expected that more study will be carried out in the future to enhance the understanding of the process.
The products of pyrolysis depends on the temperature at which pyrolysis is carried out.
Above 760◦C, predominantly H2, CH4, CO, and CO2 are formed; between 450-730◦C, predominantly tar, char, and liquids such as oils, methanol, and acetone are formed [114].
Char is formed in two steps; primary and secondary char are formed directly from feed- stock and from volatiles interacting with the surface of the primary char formed, respec- tively [115]. Char production is favored by low moisture content, low temperature, high pressure, long vapor residence time, low heating rate, larger feedstock particle size, and efficient heat transfer [115].
2.2.5.1 Pyrolysis Types and Heating Methods
Pyrolysis processes are either oxic or anoxic. Heat for pyrolysis process can be provided from the exothermic reactions taking place in pyrolysis; initially a portion of the feedstock is combusted to produce hot gases which can sustain pyrolysis reactions by transferring heat via a heat transfer surface. The other way is to directly expose the feedstock to the hot combustion gases from the combustor into the pyrolysis reactor; in this method, the feedstock is in direct contact with the hot combustion gases. The latter method of heat transfer is more efficient and suitable for larger reactors [115].
Pyrolysis processes are broadly classified as - slow, fast, and flash. In slow pyrolysis, the heating rate is significantly lower than the reaction time [116]. Whereas, in fast and flash pyrolysis, the heating rate is significantly higher than the reaction time [116]. All pyrolysis processes produce solids, liquids, and gases in proportions dependent on the type of pyrolysis executed [115]. Table 2.15 below shows the conditions required for desired yield of product type. Slow pyrolysis yields the highest amount of char; char properties are dependent on the pyrolysis treatment temperature [115, 116]. The gases produced are condensible and incondensible. Among condensible gases are methanol, acetic acid, tar;
non-condensible gases include carbon dioxide, carbon monoxide, hydrogen, and methane [117].
TABLE2.15: Different types of pyrolysis processes. Adapted from [115]
Pyrolysis Type Temperature Heating Rate Product Yield Solid Liquid Gas
Slow 400 - 600◦C 0.1 - 1 K/s 35 30 35
Fast 500◦C 200 - 1000 K/s 12 75 13
Flash >800◦C >1000 K/s 10 5 85
Raveendranet al. reported that solids or char products of pyrolysis have higher heating value than liquids or gaseous products. The heating value of char obtained from pyrol- ysis is 33 MJ/kg; the heating value of pyrolysis liquids vary between 22-25 MJ/kg; and the heating value of pyrolysis gases vary between 5-16 MJ/kg [118]. Therefore, slow pyrolysis is a more practical approach as the product, char, is more energy dense.
2.2.5.2 Pyrolysis of MSW
Unlike MSW combustion plants, pyrolysis plants can be established in small and medium urban centers [119]. Japan has been carrying out pyrolysis on MSW since 2002 and opened up five additional plants subsequently [119]. Hwanget al. surveyed and studied four MSW carbonization facilities in Japan; Japanese MSW carbonization facilities have an added advantage as waste is source-separated into combustible, non-combustible, recy- clable, and bulky wastes [119]. In the study the four facilities were not named, however, they were labelled as A, B, C, and D. Facilities A, C, and D carbonizes combustible and bulky wastes whereas facility B carbonizes combustible waste only. Facilities A, B, and D do not carbonize plastic and packaging materials due to theContainer and Packaging Recycling Law in Japan. However, facility C treats plastics and packaging material ex- cept polyethylene terephthalate (PET) bottles [119]. Out of the four facilities mentioned, facility D is privately owned; facility D operates 24 hours/day, this exposes the business opportunity to invest in pyrolysis plants.
Type of furnace greatly influences the char properties [119]. Fluidized-bed furnace is used by facility D; the LHV obtained from the volatile matter and char is lower than that of the char obtained from rotary kiln furnaces [119]. To increase the LHV of char, it needs to be
dried [119]. These four Japanese facilites are able to sell their char as an alternative to fuel to cement kilns, power generation plants, and steel manufacturing plants [119]. Facility B and facility C were able to sell their char at US $4 per ton and $10 per ton, respectively [119]. Table 2.16 shows the amount of waste treated, char and ash produced by the four facilities in Japan.
TABLE2.16: Char obtained from four Japanese facilities. Adapted from [119]
A B C D
Waste (ton/year) 13,856 4,183 9,027 19,341 Products (ton/year)
Fly Ash 108 83 237 967
Char 3,247 313 1,379 967
Char Yield (weight %)
Wet Yield 23 7.5 15 5
Dry Yield 35 15 42 11
Char produced from pyrolysis of MSW is superior in term of thermal energy recycling compared to RDF or incineration. Unlike MSW, char is resistant to seasonal variation and char is easier to store and transport. Additionally, char can be used in conventional boilers without any modification, retrofitting or pretreatment. However, pyrolysis processes for MSW treatment are inefficient and they draw more energy in producing the char than the energy that can be drawn from the produced char [119].
2.2.5.3 MSW Pyrolysis in Bangladesh
There are no government established pyrolysis plant in Bangladesh. There are reports of private individuals in Bangladesh practicing pyrolysis for producing fuel oil from plastics [120, 121]. A Chinese company reported that they made sales of four 10 ton/day capacity pyrolysis plants to a private customer in Bangladesh [122, 123]. Accordingly, there are at least four 10 tons/day pyrolysis plants in Bangladesh with an output of 30% and 45% fuel oil from waste tyres [122, 123]. As mentioned above in section 2.2.5.1, slow pyrolysis
which predominantly produces char is more efficient. Therefore, there is room for im- provement in the pyrolysis processes in Bangladesh. Private organizations, as well as the Government of Bangladesh, should explore options of utilizing MSW to generate energy and fuel.