14.3 Characteristics of Hydrothermal Treatment Product
14.3.3 Characteristics of Product as Solid Fuel
Observing the heating values of hydrothermally treated products compared to raw MSWs and conventional solid fuel as shown in Fig.14.15, it can be shown that the heating values of the products were relatively similar regardless of their hydro- thermal treatment conditions. The hydrothermally treated products showed the heating value of about 18 MJ/kg to 22 MJ/kg, competitively similar to that of high quality sub-bituminous coal (class A) with the heating value of about 25 MJ/kg [44], and typical RDF with the heating value of 16 MJ/kg [45].
0 20 40 60 80 100
0 24 48 72
Moisture Content (%)
Drying Time (hour)
Raw MSW (rice) Raw MSW (vegetable) Product B22 (225 ºC) Product C11 (235 ºC)
Fig. 14.14 Natural drying characteristics of MSWs and treated products
0 5 10 15 20 25 30
Coal RDF MSW-1 225 C,
90min
225 C, 90min Naturally
Dried
MSW-2 235 C, 90min
235 C, 90min Naturally
Dried
Heating Value (MJ/kg)
dry basis (db) as received basis (ar)
Fig. 14.15 Heating value of hydrothermally treated products compared to conventional solid fuels
In the case of kimchi and paper, the calorific values for both materials increased with the increase of the reaction temperature due to the increase of the fixed carbon content as shown in Fig.14.16. Due to higher carbon content of paper, the calorific value increase of paper is more significant than in the case of kimchi.
Since the hydrothermally treated product will be used in the pulverized coal burner, its grindability, known as HGI (Hardgrove Grindability Index), was also observed for two samples processed in 2 MPa, 200C and holding time of 30 min and 60 min, respectively. It is shown from Table14.7that the HGI of Sample-60 (60 min holding time) was higher than that of Sample-30 (30 min holding time);
therefore, longer holding time is considered to produce easy-to-pulverize solid fuel. As for 60 min holding time, its HGI was higher than 60, which is higher compared to that of coal (normally within 40–60). This means that the product is easier to be pulverized compared to coal.
As a result of good grindability of the product, complete burn out, easy mixing, no blockage or bridging to the feeding system, and no sedimentation can be expected [46]. However, the adhesion of the sample was observed to be high compared to coal, as many of the samples were seen adhered to the ball mill.
Combustion tests have been conducted to confirm the usage of treated MSW as solid fuel. The burning characteristics of the Indian and Australian coals, treated MSW and their various blends have been studied using thermogravimetry analysis (TGA) as shown in Fig.14.17.
0 10 20 30 40 50 60
0 5 10 15 20 25
Raw material Experiment at 180
Experiment at 200
Experiment at 220
Calorific value (MJ/kg)
Paper Kimchi
Carbon content of Paper Carbon content of Kimchi
Carbon content(%)
Fig. 14.16 Effect of the hydrothermal treatment on the calorific value and carbon content of paper and kimchi
Table 14.7 Result of HGI
measurements Sample-30 Sample-60
HGI 30.1 60.3
With the rise of the temperature, after a release of moisture, combustion of samples took place with associated weight losses. For the blends, quicker weight losses were observed, mainly due to early emission of volatile matter, which differentiates burning behavior of treated MSW compared with coal. The com- bustion of coal is mainly due to the combustion of the fixed carbon whereas in the case of treated MSW, this was dominated by combustion of the volatile matters, obviously due to their high volatile content. As for Indian coal, the addition of treated MSW in the blend reduce the ash component in the mixed fuel, resulting in improved combustion performance of Indian coal.
In larger scale experiment, a drop tube furnace (DTF) was used to observe the burnout efficiency of high ash content Indian coal with its blend with treated MSW as shown in Table14.8.
This shows that the burnout efficiency increased 3.5 % when the Indian coal is blended with treated MSW. This increase in the burnout efficiency can signifi- cantly reduce the unburnt loss in a power plant and thus increase in the combustion efficiency. This improvement is attributed to the enhancement of the low reactive char component of the coal due to the addition of treated MSW. It has been
0
150 250 350 450 550 650
Temperature [OC]
Temperature [OC]
Indian Coal 10% blend 20% blend 30% blend 50% blend MSW
(a)
0 20 40 60 80 100
0 150 250 350 450 550 650
Sample Weight [wt %]
20 40 60 80 100
Sample Weight [wt %]
Australian Coal 10% blend 20% blend 30% blend 50% blend
(b)
Fig. 14.17 aTG profile for the Indian coal blend.bTG profile for the Australian coal blend
demonstrated that MSW blending with Indian coal enhances the reactivity of the low reactive char component [47].
Figure14.18 shows the mass loss profile for treated MSW blends of 20 % studied by TGA analysis. When comparing their DTG (differential thermogravi- metry) profiles, the profile for the MSW blended fuel differs with coal profile in three ways. A clear difference can be found in the low temperature region i.e., around 250C, due to the early release of volatile from the MSW fuel which will support the ignition of coal. Second, there is a difference in the height of the DTG profile between coal and treated MSW blend, which was lower. Hence, the weight loss rate for the MSW blended fuel is slightly lower than coal combustion in the char burning stage. It can be concluded that the blending of MSW with coal increases the reactivity of volatile component and reduces the reactivity of char component. The improvement in the reactivity of the volatile component enhances the ignition characteristics and hence the reduction in the ignition temperature [48].
The third and important difference lies in the higher temperature region at around 450C. In general char has two regions of reactivity, low reactive com- ponent and high reactive component [49]. The low reactive component is responsible for a slow weight loss rate creating a shoulder in the right hand side of the DTG curve, act as the main contributor for the unburnt carbon present in the fly ash of a thermal power plant.
Treated MSW blending with coal can significantly enhance the reactivity of the low reactive component of the coal char; hence the shoulder portion of the DTG Table 14.8 Burnout
efficiency Burnout efficiency (%)
Indian coal 89.5
Indian coal+20 % MSW 93.0
0 20 40 60 80 100
150 250 350 450 550 650
Temperature [oC]
Sample Weight [wt %]
0 0.25 0.5 0.75 1 1.25 1.5
Weight loss rate [mg/min]
TG IC TG MSW blend DTG IC DTG MSW blend 20% Blend
Fig. 14.18 TG and DTG profile for Indian coal (IC) and MSW 20 % blend
profile is completely absent for the MSW blended fuel, and this improvement in the combustibility reduces the unburned char of the coal.
A coal-based bubbling fluidized bed (BFB) reactor was adopted for the co-combustion behavior investigation to verify the feasibility of the replacement of coal with treated MSW without major furnace modification. The BFB was made of a 77 mm i.d. corundum ceramic tube, with a total height of 1100 mm. The operating temperatures were 700, 800, and 900C and the excess air of 1.3 with residence time was kept in the range of 1.5–2.0 s. Contents of the flue gas were analyzed by a GASMETTMDX-4010 FT-IR Gas Analyzer.
As shown in Fig.14.19, when it comes to the co-combustion practice at low blending ratios (10, 20 %), dramatically lower CO concentrations than that of coal were observed, while at high blending ratios (30–50 %) higher CO concentrations were occurred. For coal monocombustion, the higher CO emission could be ascribed to diffusion controlled reactions due to its high ash content (12.01 %) compared to the treated MSW (1.09 %). It is reported that the samples with high ash content is more likely to have the trend to follow a shrinking sphere model which results in an ash layer surrounding the FC making oxygen diffusion difficult [50]. This kind of incombustible ash layer led to the accumulation of unburnt char particles which eventually created a fuel-rich zone in the bed thus resulting in higher CO emission.
When the treated MSW was mixed as a co-combustion fuel, first the ash content was reduced so that the oxygen diffusion could be promoted. Second, burning of the treated MSW during co-combustion was expected to occur at a rapid rate compared to coal and therefore the heat released helped to provide a higher temperature zone which elevated the speed of coal combustion as well as the CO burnout. Third, the high percentage of oxygen content in the treated MSW might increase the amount of O, OH radicals near the char particles which promoted the heterogeneous char oxidation.
Figure14.20shows the changes of SO2emission as a function of the temper- ature at different combustion conditions. It is obvious that the SO2emitted from the combustion of low sulfur content treated MSW was almost negligible and the
0 300 600 900 1200 1500
700 800 900
Temperature/[˚C]
CO/[ppm] (at 11% O 2)
Coal 10% MSW 20% MSW
30% MSW 50% MSW MSW
Fig. 14.19 CO emissions as a function of the temperature
SO2concentrations decreased with the increase of the treated MSW share. Fur- thermore, there was a monotonic increase trend for SO2with temperature rising for all the samples, though the levels were rather small due to the low sulfur content in coal (0.44 %).
In the BFB combustion system, fuel NOx is believed to be predominant, which primarily formed via two different pathways: one is through gas-phase oxidation of the nitrogenous group in the volatiles, and the other is through the heterogeneously catalyzed oxidation of the char-bound nitrogen species [51–54]. Moreover, the distribution of the nitrogen between the volatiles and the chars is roughly pro- portional to the volatile matter in the fuel. Especially, during the devolatilization of treated MSW, as the char content is relatively small, most of the nitrogen is believed to be emitted during the devolatilization phase (66–75 %) [55].
As shown in Fig.14.21, in the case of co-combustion at low blending ratios (10, 20 %), the blending of the treated MSW contributed to the abatement of NO emitted from coal by providing more available fixed carbon surfaces. In addition to that, the burning of the treated MSW around the coal particles also offered more available NH3effective for NO reduction. For the co-combustion with a higher
0 20 40 60 80
700 800 900
Temperature/[ C]
SO 2/[ppm] (at 11% O 2)
Coal 10% MSW 20% MSW
30% MSW 50% MSW MSW
o
Fig. 14.20 SO2emissions as a function of the temperature
0 200 400 600 800 1000
700 800 900
Temperature/[ C]
NO/[ppm] (at 11% O 2)
Coal 10% MSW 20% MSW
30% MSW MSW
o
Fig. 14.21 NO emissions as a function of the temperature
MSW blending ratio (30 %), it is noteworthy that the NO emission was even lower than the case of individual MSW. Besides the reasons abovementioned, the rapid heat release from the burning of VM of the HT MSW along with the more available oxygen emitted from high oxygen content HT MSW enhanced the thermal devolatilization of coal thus creating more pore structure of coal char which could be useful for NO reduction in both of the cases of coal and the HT MSW. Additionally, the VM released from the HT MSW also created temporarily the reducing environment with high concentrations of CHiand HCCO groups near the coal particles, which assisted the reduction of NO strongly as well.