Raw biomass
Table 1 compiles the proximate analysis of the various biomass wastes — rice straw (RS), Lantana Camara (LC), tree leaves litter (TL), and common grass (GR) to understand their suitability as a fuel in thermal power plants. Data in Table 1 reveals that volatile matter (VM) in raw biomass is between 59% - 88%, whereas domestic coal has low VM, generally below 35%. Imported coal used in Indian power plants, on the other hand, has VM up to 40% on a wet basis. Power plants designed for handling coal face challenges while handling biomass, which has significantly higher VM content.
Based on our in-house studies done in the past and subsequent Indian government notifications, biomass, considering the constraints, can only be safely blended in coal below 10% [19] without significant plant modifications.
|
RS
|
LC
|
TL
|
GR
|
Imported Coal
|
Moisture%
|
7.56
|
9.01
|
5.35
|
4.85
|
14.56
|
Ash%
|
24.7
|
7.14
|
10.4
|
10.95
|
5.52
|
Volatile Matter %
|
54.78
|
80.52
|
69.35
|
67.64
|
40.36
|
Fixed Carbon %
|
12.96
|
3.33
|
14.9
|
16.56
|
39.56
|
Table 1: Proximate Analysis of RS, LC, TL, and GR (raw samples)
Torrefied Biomass
Torrefaction of biomass improves grindability and reduces volatile matter; hence can be a better choice for mixing higher ratios in the coal, provided the other constraints are managed. To examine the suitability of torrefied biomass for replacing a higher percentage of coal in PC-TPP, we proceeded with the torrefaction of the biomass samples and investigated them for their gross calorific values and proximate analysis.
We torrefied the biomass at temperatures ranging from 200 °C to 275 °C for various residence times to examine its impact on desirable fuel qualities. The graphs in Figure 1 depict the results of torrefaction at 200 °C, 225 °C, 250 °C and 275 °C for 30, 60 and 90 minutes. We found that the torrefaction temperature has a more significant impact than torrefaction time, as evidenced by the thin cross-section along the constant temperature lines, which confirms previous findings [20].
Volatile matter in biomass fuel should not exceed 40% (on a wet basis), as in the case of imported coal. To enable large-scale replacement of mineral coal, preferably, it should be below 30%, like typical domestic design coal of Indian PC-TPPs. Torrefaction at higher temperatures reduces volatile matter; however, torrefaction above 300 °C results in rapid weight and energy loss due to the thermal decay of cellulose [28]. We, therefore, explored the torrefaction temperatures below 300 °C, resulting in lower volatile matter safe for Indian power plant use. We, therefore, tried the torrefaction temperatures below 300 °C, resulting in lower volatile matter (VM) safe for Indian power plant use.
We carried out the proximate analysis (Table 2) of the products that revealed torrefaction temperature below 275°C is unsuitable for making thermal-grade fuel, primarily due to higher volatile matter. We, therefore, decided to do the torrefaction at 275 °C and evaluated the charred biomass for its suitability in thermal power plants. Torrefaction at higher temperatures reduces the VM content further and corresponding energy loss. We, therefore, conclude that 275° C is the ideal temperature for making charcoal for power plants using rice straw. However, we observe that VM is a little higher in the case of tree leaves. If mixed with rice straw coal, their proportion will be very insignificant due to their low availability, not affecting the suitability of the fuel.
|
RS (250)
|
RS
(275)
|
TL (250)
|
TL
(275)
|
GR
(250)
|
GR
(275)
|
Moisture
|
3.65
|
4.27
|
4.36
|
1.32
|
3.75
|
8.17
|
ASh
|
26.94
|
32.59
|
12.59
|
14.59
|
13.12
|
11.75
|
volatile matter
|
47.64
|
33.58
|
60.77
|
56.7
|
53.26
|
42.02
|
Fixed Carbon
|
21.77
|
29.56
|
23.39
|
27.39
|
29.87
|
38.06
|
GCV
|
4018
|
4282
|
4960
|
5199
|
5164
|
5545
|
Table 2: Proximate Analysis of Torrefied charcoal at 250 °C and 275 °C for 90 minutes.
We also examined torrefied samples for HGI and found them highly grindable with HGI values 210, 146 and 142 for RS, TL, and GR, respectively, whereas the design HGI Index for coal mills in PC-TPPs is generally 50 or more.
Washing Treatments
Slagging and fouling are the two leading problems associated with stand-alone or high ratios of biomass firing in PF power plants, even with torrefied fuel based on agricultural residue. Slagging is the molten ash or partially fused deposits on the surfaces exposed to radiant heat in the boiler or furnace. Molten or soft ash particles strike the cold walls of the furnace or radiant heat exchangers and solidify, forming thick deposits. It is, therefore, essential to have fuels with higher initial deformation temperatures (IDT) than the furnace temperature to avoid slagging. Fouling occurs on convective zones of the boiler, like the superheater or reheater, where ash and volatiles condense, encountering colder surfaces and forming a coating. Alkali metals affect ash fusion temperatures and cause slagging and fouling [6]
Conventional slagging and fouling indices calculated from the chemical analysis of mineral coal are not applicable for biomass ash [21] as coal ash predominantly contains alumino-silicate. On the other hand, biomass ash primarily has quarts and some inorganic salts, chiefly phosphates, sulphates and chlorides of potassium, sodium, magnesium, and calcium. Therefore, we resorted to the ash fusibility tests instead of the detailed elemental analysis. Similarly, treatment to improve ash fusion temperatures (AFTs) of resultant fuel is of prime importance for substituting it for coal in PC-TPPs, which the current literature does not target.
AFTs are a fair indicator of slagging and fouling propensities. According to tests, the initial deformation temperature (IDT) of torrefied RS was 980 °C. Values below the furnace exit temperatures (usually around 1100 °C) may cause heavy slagging, and the product is, therefore, unsuitable for high-concentration firing. Woody biomass wastes, however, have high AFTs. As revealed in our experiments, torrefied LC, a forest-origin biomass, already has high ash fusion temperature (IDT:1102, ST:1314, HT> 1500 and FT>1500) and may not require water washing.
Literature reported that water-washing of agricultural biomass improves AFTs [6], [22]. Jensen et al. 2001 [23] also documented that post-torrefaction washing of wheat straw with simple water and hot water removes potassium and chlorine effectively. Many researchers have worked on pre-washing the biomass; however, not enough published work is available on the post-torrefaction wash. We, therefore, decided to work on the comparative advantages of both pre-torrefaction and post-torrefaction washing of RS, TL, and GR to improve their fuel characteristics.
We water-washed the biomass before torrefaction (TR1) and examined water-washing after torrefaction (TR2). Both choices affected ash fusion temperatures, as depicted in Figure 2. However, we discovered that ash fusion temperatures increased much more in the biomass samples by water washing where they were too low (Figure 2), such as in RS. However, the biomass samples having moderate AFTs, for instance, TL, did not exhibit such high improvements with any of the two treatments.
It may be due to potassium in RS finding its way from fertilizers used in farming. Studies indicate that higher potassium, sodium, and phosphorous salts in biomass in farm residue biomass are responsible for lower ash fusion temperatures [12].
Water washing before or after torrefaction drives out these salts, especially potassium salts, and increases AFTs. We performed paired t-tests to analyze the impact of pre-torrefaction and post-torrefaction water wash on AFTs because the pairs have a significant positive correlation between them, and experimental units are heterogeneous. It indicates RS and GR with TR1 demonstrate definite improvement in AFTs with more than 95% confidence. However, TR2 does not exhibit statistically significant improvement in AFTs over TR1 for TL and GR.
Further, the paired t-test revealed that TL already has higher AFTs, and neither of the two water-washing treatments accomplishes improvement in AFTs with more than 95% confidence. It may be due to the low potassium and other water-soluble salts playing a crucial role in reducing AFTs.
AFTs of GR improve significantly with pre-torrefaction wash, but surprisingly, they exhibit a slight reduction in AFTs with post-torrefaction wash, though with a confidence level of about 93% (Table 3) and (Table 4). It may be due to the drain of Mg and Ca salts in post-torrefaction wash, which is associated with high melting point ash [21].
|
Two tailed P value ¥
|
Test statistics
|
DF £
|
interpretation
|
Rice Straw
|
0.0014
|
5.1130
|
7
|
Strong evidence of statistically significant improvement
|
Tree leaves
|
0.0594
|
2.2472
|
7
|
No evidence of statistically significant improvement
|
Grass
|
0.0001
|
10.0047
|
7
|
Very strong evidence of statistically significant improvement
|
¥ Note: (hypothesis = µ (tor) - µ (TR1) =0), where µ is the mean value of AFTs
£ DF: degree of freedom
Table 3: P values for statistical analysis of improvement of AFTs with pre-washing treatment.
|
Two tailed
P value Ψ
|
Test
statistics
|
DF
|
interpretation
|
Rice Straw
|
0.3573
|
0.9854
|
7
|
Strong evidence of statistically significant improvement
|
|
Tree leaves
|
0.6176
|
0.5222
|
7
|
No evidence of statistically significant improvement
|
|
Grass
|
0.0332
|
2.644
|
7
|
Weak evidence of statistically significant difference
|
|
|
|
|
|
|
|
|
|
|
|
Ψ Note: For treatment causing significant improvement of AFTs with 95% confidence level (i.e. two tailed p value=0.025) test statistics should be > 2.84. (hypothesis = µ (TR1)-µ (TR2) =0); where µ is the mean value of AFTs.
Table 4: P values for statistical analysis for improvement of AFTs with post- torrefaction washing treatment with reference to pre-torrefaction washing treatment.
Testing the ash samples of torrefied RS and water-washed torrefied RS for elemental analysis confirmed that washing reduces potassium considerably (Figure 3). Further, comparing the ash samples of biomass RS with coal revealed that coal ash is primarily composed of alumina silicate, while RS biomass ash is chiefly silicates.
Post-torrefaction wash would be more cost-effective because of the need to treat a small amount of material, reduced due to the torrefaction process. Post-torrefaction washing treatments also improved gross calorific value (Figure 4) and decreased volatile matter (Figure 5) and will be more economical due to lower mass for reforming fuel for use in PC-TPPs.
Suitability of the studied samples as fuel for thermal power plants
The study and experiments reveal that post-torrefaction washing (TR-II) for RS results in a calorific value like G-10 and G-11 grade Indian coal (Figure 4). Its ash content, volatile matter (VM) and fixed carbon (FC) are also in the same band as coal, as depicted in Figure 5. Results also demonstrate that slagging and fouling tendencies of these fuels should also be within the acceptable band, as evident from the ash fusion temperature (Figure 2). Biomass fuels analyzed did not contain a detectable amount of Sulphur and, therefore, need no FGDs to eliminate SOX emissions during combustion in power plants.
|
GCV
|
N
|
C
|
H
|
RS(Tor)-TR2
|
4282
|
1.35
|
44.80
|
4.12
|
TL(Tor)-TR2
|
5199
|
1.8
|
45.95
|
4.53
|
GR(Tor)-TR2
|
5404
|
2.65
|
52.55
|
4.46
|
Lantana-TR2
|
5904
|
3.35
|
55.49
|
4.68
|
Table 5: Ultimate Analysis and gross calorific values of various biomass derived charcoals torrefied at 275° C.
While the properties of the torrefied and washed RS resemble domestic coal, torrefied LC, TL, and GR with required treatment can become a valuable resource, having most properties like imported coal. From (Table 5) it is seen that the GCV of washed torrefied fuel derived from TL, GR, and LC are in the band of G-8 to G-5. Previous researchers have reported work on the torrefaction of Lanata focusing on optimizing energy yield [24], whereas we have targeted replacing coal in thermal power plants with matching properties. According to the findings of our study, a suitable torrefaction process and treatment can transform biomass derived from LC, TL, and GR into a fuel that can replace imported coal in Indian PC-TPPs for blending with domestic coal or for imported coal-based plants. Moreover, experiments with TL reveal clear statistical evidence that washing can improve AFTs. They also have lower ash content and, with a little increase in torrefaction temperatures, are suitable for replacing imported coal in Indian power plants.
The main results can be summarized as below,
- Torrefaction below 275°C does not reduce the volatile matter suitable for firing in Indian PC-TPPs.
- Rice straw torrefied at 275° C with TR2 makes it like domestic coal in most properties, making it suitable as a potential replacement for domestic coal in Indian PC-TPPs.
- TR1 significantly improves the ash fusion temperatures of biomass fuels having lower ash melting points. However, those having higher ash fusion temperatures (AFTs) show no significant improvement.
- TR2 also significantly improves the AFTs, as in the case of TR1. However, the fuels already having high ash fusion temperatures (AFTs) show no significant improvement. Washing treatments (TR1 or TR2) for such biomass fuels may be redundant.
- Comparing TR1 and TR2, we found that TR2 significantly improves the initial deformation temperature (IDT) for oxidizing and reducing environments for RS over TR1. However, TR2 and TR1 result in statistically similar improvements in AFTs. Nevertheless, weak statistical evidence indicates that TR2 for grass reduces its AFTs over TR1.
The results indicate biomass wastes can produce fuel suitable for 100% firing in Indian thermal power plants. However, an important question remains whether we have sufficient biomass waste in the country to replace fossil coal in power generation. The authors investigated this question and referred to several research papers and literature to understand the quantum of biomass waste annually available in India and summarized in the following section.
Availability of biomass in India
Biomass available in India includes 230 million tons of agricultural residue [25], forest residue consisting of pine needles and many invasive alien species (IAS). Pine needle litter is predominantly available in hill states such as Himachal, Uttarakhand, Sikkim, Jammu & Kashmir. They cause forest fires and destroy forest diversity. Pine forests cover approximately 869,000 hectares [26] in India. The area under the pine forests in Uttarakhand is about 394000 hectares, with a gross pine needle yield of 1.9 MMTPA [27]. With the same proportion, pine needle yield for the entire country with 869000 ha of pine forest would be about 4.19 MMTPA. Assuming about 30% of it is unavailable due to collection, storage, transportation losses and alternate uses, 2.93 MMTPA of pine needle biomass is available for capturing energy. LC is an invasive alien species (IAS) that has infested about 15.4 million hectares of forest land [28] in India. LC contains 70-80% of woody biomass, and its production rate is 40-60 tons per hectare per year [29]. Considering a conservative estimate of 40 tons/hectare/year, the annual availability of LC is 616 MMTPA. Further, considering 80 % of it is available for energy use, India has around 492 MMTPA of LC available for energy needs.
The 2019 census estimates the total cattle population in India as 302 million [30], and each animal daily produces 09 kg-15 kg of cow dung [31]. Annual wet cow dung production in India is 1322 MMTPA, considering an average of 12 kg of cow dung daily. Further, considering 85% of moisture and 85% of unused cattle dung [32], the dry mass of cattle dung available for energy recovery is 168 MMTPA.
Additionally, India generates miscellaneous biomass wastes not precisely quantified here. They include sawdust, waste wood, invasive species like P. Juliflora and Mexican Devil, Napiergrass and horticultural wastes and others. Conservatively considering about 200 MMTPA of miscellaneous biomass aggregate availability in India is about 950-1100 million tons, as summed up in Table 6.
Biomass
|
Gross Availability
(million tons per year)
|
For energy use
(million tons per year)
|
Gross calorific Value
(kCal/kg)
|
Agro- residue
|
750
|
230
|
3000-4000
|
MSW
|
58
|
40
|
2200-2800
|
Lantana
|
616
|
492
|
4300-4600
|
Pine Needles
|
4.19
|
2.93
|
4100-4500
|
Cattle Dung
|
1322
|
168
|
3800-4000
|
Others
|
300
|
200
|
3000-4000
|
Total
|
|
1132
|
|
Table 6: Annual gross and net biomass availability for energy use in India.
Greenhouse gas emission reduction
The life cycle analysis (LCA) from the reviewed literature indicates that the emission factor of Biomass power in co-firing mode is around 230 gm/kWh[33]. The CO2 absorbed from the atmosphere during the growth phase is almost balanced with the emission during combustion. The emission factor of coal-based power plants is 970 gm/kWh [34] in India, and coal-fired plants generated about 1138 BU in 2020-21 [35] Accordingly, entire fuel switching from coal to biomass for electricity generation can save 740 gm/kWh, resulting in a CO2 emission reduction of 842 million metric tons per annum (MMTPA) from the electricity sector.
Cautions:
The findings pave the way to use a substantial portion of about one billion tons of annual biomass waste for green energy generation in PC-TPPs without extensive modifications of the plant facilities, having the potential to reduce 840 million tons of carbon emissions. Nevertheless, entirely replacing coal with biomass fuels is possible but requires some caution. It is a new fuel with a vastly different ash composition and no large-scale use history in PC-TPPs. Its performance in swirl and high-velocity jet burners regarding combustion behavior, unburnt fuel, and NOx generation needs careful observation during the initial trials. Burner original equipment manufacturers (OEMs) have the capabilities and set-up to evaluate such performances and may be an area of future research. If the performance demands, the existing coal burners may need replacement with specially designed biomass burners.
The ash fusion temperatures (AFTs) indicate no enhanced risk of slagging and fouling; however, actual field trials may divulge more details about the impact on heat exchanger surfaces with prolonged biomass firing, if any. In case of any issues, the experience of the industry with waste-to-energy plants, firing Municipal Solid Waste (MSW), a more difficult fuel, can help replace or modify selective heat exchangers with appropriate coating. We recommend initial field trials on old subcritical plants using traditional metallurgy to reduce risks.
There is not much experience available about the behavior of biomass ash in electrostatic precipitators (EP), which is critical as biomass ash has vastly different types of ash particles. The literature points out (reference) that biomass ash may contain particles of sub-1-micron size whose charging mechanism may not be adequate to precipitate them in the EP [36]. The efficiency of the EP in terms of the number of particles may decrease, while it may not be affected in terms of mass. Fortunately, however, the ash content in most of the biomass is less than that of domestic coal, and the issue, if observed during field tests, can be tackled with bag filters or a combination of EP and bag filters.
From the above, we can sum up that with torrefaction and the recommended treatment, entire coal replacement with biomass is possible. However, during initial field trials, we should closely watch its effects on burning profile and EP performance and use the appropriate solutions. Ash meting profile indicates that slagging fouling should not occur; however, it may also require cautious observations.
Discussion
Past approaches to torrefaction focus on maximizing energy yield. However, as the objective of the present work is to make a fuel most suitable for power plant use, we have optimized the torrefaction process to bring VM into permissible limits and subsequently apply the energy yield criteria. According to the study, for rice straw, the optimum temperature is 275 oC.
We further focused on treatments for improving ash fusion temperature to address critical issues of slagging and fouling with agricultural residue-based fuels. We analyzed treatments TR-1 and TR-2 and statistically compared their effectiveness for various biomass wastes. The comparison revealed that washing treatment is effective for RS, and the efficacy of both washing treatments is not uniform for all the biomass wastes studied and thus requires careful application depending upon the biomass variety.
Conclusions
The work investigates the prospects of replacing coal entirely in Indian pulverized coal thermal power plants with torrefied and treated biomass. It requires matching the prepared fuel with domestic and imported coal used in Indian pulverized coal thermal power plants. The approaches followed previously in Europe used hardwood biomass and suitable plant modifications, and their experience with agricultural biomass firing was discouraging due to excessive slagging and fouling (ref). However, the present work reveals that agricultural residue, like rice straw, can also be converted to domestic Indian coal-like fuel using torrefaction and washing treatment. We have demonstrated that with the suggested treatment and processing, the main properties of the torrefied and treated biomass, such as GCV, VM, and ash fusion temperatures, match with domestic thermal coal and can suitably replace it. The other tested biomass, viz lantana camara, grass and tree-leaf litter, can suitably replace high volatile and high GCV imported coal. Secondary research also reveals that we have more than 1100 billion metric tons of biomass waste available in India, which is enough to replace entire fossil coal for power generation in the country. With this radical breakthrough, switching to biomass-derived fuels from fossil-origin coal can foster a rapid energy transition with associated environmental and food cycle benefits. The authors, additionally, identify the areas of cautious observation during initial trials to take corrective actions if required.