Evaluation of feasibility of pelletized wood co-firing with high ash Indian coals
Introduction
It has been widely acknowledged that CO2 emissions from the combustion of fossil fuels are major contributors to the enhanced global warming. The CO2 emissions from fossil fuel combustion and industrial processes contributed about 78% to the total GHG emission increase between 1970 and 2010 [1]. Therefore focus has been shifted to development of CO2 capture technologies such as pre-combustion capture, post-combustion capture, oxyfuel combustion, and chemical looping combustion to remove CO2 from the source itself. However, these technologies are highly energy intensive and require significant amount of additional capital cost than conventional power plants. At this juncture, utilization of carbon neutral fuels such as biomass for power generation is an economical option to reduce net CO2 emissions [2] and also the implementation cost would be lower [3]. Many countries have initiated incentives in recent years to encourage the use of biomass for power generation. Based on the IEA Bioenergy Task 32 database [4], 228 plants across the globe have already experienced with biomass co-firing with coal, at least on a trial basis. Most of these plants are located in Finland, USA, Germany, UK and Sweden. In India, there is a significant potential available for utilization of biomass in coal fired thermal power plants. It has been estimated that about 500 million metric tonnes per year of biomass is produced in India. Out of which the surplus biomass available for energy production is about 120–150 million metric tonnes per annum and that has potential energy generation capacity of about 18,000 MW [5]. However, the power generation by biomass and co-generation is only 2.48% of the total installed capacity of 315,426.32 MW [6].
Although the constituents of biomass and coal are same, their origin is very different from each other, therefore, the following parameters are to be considered while co-firing biomass with pulverized coal (PC) in power plants, such as type of boiler, type of milling system, particle size distribution (PSD), chemical composition of fuels to be co-fired, alkali content, sulphur, chlorine in the biomass, type of firing, percentage of co-firing, unburnt carbon, ash deposition in the boiler, etc. Therefore, in the present study, to evaluate the feasibility and optimal proportion of pelletized wood (PW) co-firing with Indian coal in a coal fired boiler with minimal modifications in the milling and fuel firing section, a series of tests have been conducted in pilot scale plant. These studies include composition analysis of fuels, particle size analysis after milling, pulverized fuel (PF) transportation, flame stability, emission of pollutants and analytical studies of slagging while PW co-firing with coal at different proportions. Based on these studies, optimal proportion of PW co-firing with coal has been suggested by considering all these parameters to retrofit in the existing PC fired boiler.
In general, bulk densities typically range from 1100 kg/m3 for low-rank coals to 2330 kg/m3 for high-density pyrolytic graphite [7] but the biomass fuel bulk densities ranges from 100 kg/m3 for straw to 500 kg/m3 for forest wood [8] therefore, pulverizing biomass along with coal in the same mill may be an issue. However, the dried and pelletized biomass were very useful for direct co-firing the biomass with coal [9], [10] in bulk quantities but milling and transportation issues were observed at higher biomass proportion mixing with coal [11], [12]. Apart from that when co-milling biomass with coal the fraction of particles of size less than 75 μm and fraction of particles of size greater than 300 μm are increasing with increase in biomass proportion [12], [13], [14]. Increases in PF mean particle size range was also reported [15], [16] while co-milling various biomass fuels with coal. A Biomass usually consists of 70–80% volatile matter whereas coal consists of 10–50% volatile matter [3], it causes more elongated flames with combustion. It was observed during biomass co-firing with coal and lignite fuels that the slower combustion reactivity and elongated flames have increased the unburnt carbon [17].
The optimum proportion of biomass co-firing with coal has been varying for each case of study, however, most of the studies were mentioned it around 20% by weight [16], [18], [19], [20]. In general most of the experimental work conducted in literature reported significant reduction of NOx [14], [15], [16], [20], [21] while co-firing the biomass with coal due to lower fuel-N in the biomass fuels. On the contrary, no significant reduction of NOx was also reported [11] while co-firing raw and torrefied wood with coal. Another experimental work [22] observed that NOx emission was increased when co-firing pelletized saw dust with coal. Similarly reduction in SO2 emission was observed while co-firing biomass with coal and reduction in SO2 depends on the sulphur content in coal and biomass fuels [14], [15], [22], [23], [24]. Since biomass consists of higher volatile content than coal, no flame stability issues were reported in the literature but increase in CO emissions were claimed based on 500 MWe down shot fired power plant [18] and an increase in unburnt carbon while co-firing biomass with coal [13], [23]. However, on the contrary, large reduction of unburnt carbon in the fly ash was observed [22]. This explains the sensitivity of the selection of optimum ratio for co-firing the biomass fuel with coal. While studying the ash deposition characteristics for biomass co-firing with coal in limited proportion many studies have reported that no significant deposits were observed [11], [20], [25].
From all these studies, it was observed that the percentage of biomass proportion with coal, emissions and unburnt carbon have been varying based on type and processing of biomass fuel, type of coal, configuration of test facility, etc. Therefore, the present paper discusses PW co-firing with coal in the laboratory and as well as in the experimental facility for implementing in large coal fired boilers.
Section snippets
Analysis of composition for PW and Indian coal
In the present study, laboratory analysis for Indian coal and PW was conducted on as received basis to determine the elemental composition and calorific value. The composition analysis was conducted using proximate analyser (Leco, TGA 701), ultimate analyser (Elementar, Vario Micro Cube) and for calorific value bomb calorimeter (Parr, Isoperibol 6300), these results are shown in Table 1. From this analysis, it can be seen that the volatile matter is significantly higher, sulphur and nitrogen
Description of test facility
The pilot scale solid fuel burning test facility (SFBTF) has been designed to fire solid fuels at a capacity of 1000 kg/h. The objective of this facility is to test various type of solid fuels, different burner designs, and optimizing the air staging to burner for minimizing the hazardous gaseous emissions at various blended fuels. The SFBTF furnace is placed horizontally on concrete floor and one end of the furnace is connected with wall burner and the other end is connected with vertical
100% coal firing experimental study
To compare the results of PW co-firing with coal, a base line test was conducted at PC flow rate of 1000 kg/h. Stable combustion has been established with wall burner and ignition of PC was observed at distance (x/L) of 0.07 and intense flame was observed after x/L of 0.2. Where ‘x’ is any distance within the furnace and ‘L’ is the total length of the furnace. The flame length has been extended most part of the horizontal furnace of SFBTF. During combustion, the flame parameters such as flame
Conclusions
Present study discusses about PW co-firing with high ash Indian coal at pilot scale test facility, from these experimental work it was observed good flame stability at all proportions of PW co-firing with coal. The higher volatile matter present in PW helps in stabilizing the flames better while co-firing with coal, especially for coals which has high ash and low volatile matter. However, pipeline clogging issues were observed during transportation of pulverized mixture of PW and coal,
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