Effect of torrefaction on the physicochemical properties of pigeon pea stalk (Cajanus cajan) and estimation of kinetic parameters
Graphical abstract
Introduction
Most of the developing and under developed nations in South Asia, Africa and Latin America depend on agriculture due to their growing economy. Thus, agriculture being the economic backbone of those countries, its residues can play very important role in improving the living standards and simultaneously meeting the partial energy needs. Pigeon pea stalk as a biomass source derived from agricultural residue has a great potential, however, very limited work has been carried out on torrefaction and pyrolysis of pigeon pea stalk. For example, currently, India produces 611 million tons per year of agricultural residue [1], which can be utilized for setting up potentially bio-based industries. Biomass presently accounts for nearly 10% of the total primary energy consumption worldwide [2]. Among various alternate fuels available, biomass has the potential to meet the requirement as a substitute for fossil fuels and decreasing greenhouse gas (GHG) emissions. India being a developing country, along with second largest population in the world has around 2/3 population living in rural areas. Most of the population living in rural areas of a developing country like India depend on agriculture for their livelihood, which makes very important for those countries to increase the utility of the agricultural residue to generate energy efficiently. Also, shifting towards renewable energy sources like biomass, solar energy, and wind energy will help in reducing the negative impact on the environment caused due to excessive use of fossil fuels to meet the daily energy needs. Hydrogen generation via reforming process from biomass derived bio-oil is also an alternative source of energy [3]. Alternate fuels biodiesel, syngas, bio-oil, CNG (compressed natural gas), biogas and hydrogen will play major role in near future to meet the global energy demand and reducing environmental impact caused due to conventional fossil fuels like crude oil and coal [4].
Although potentially biomass has greater scope to meet the energy needs, still its application to generate energy is limited due to higher cost and technological constrains. One of the methods to improve the physical and chemical properties of biomass is torrefaction. Torrefaction is a pre-treatment process, also known as mild pyrolysis, has shown improvements in the properties of the raw biomass making it suitable for the energy generation. The torrefied biomass forms a charcoal-like carbonaceous material with better properties like low water content, low O/C ratio, and high energy density [[5], [6], [7]]. Torrefaction is a thermochemical process with operating temperature between 200 and 300 °C in an inert atmosphere [8,9]. Torrefied biomass exhibits better physical characteristics such as improved grindability and storage stability due to increase in hydrophobicity and less transportation cost due to enhanced energy density. Torrefied biomass has lesser ash content compared to fossil fuels, which makes it more promising addition in renewable source of energy and subsequently lowering net greenhouse gas (GHG) emission [10,11]. Operating parameters like reaction temperature, residence time and heating rate have influence on the final product obtained. However, Buratti et al. [12], based on statistical analysis, showed that impact of operating parameters on fuel properties like higher heating value (HHV) was in the sequence of temperature > residence time > heating rate. Similar trend was also observed by Mundike et al. [13] where heating rate had minimal influence on solid fuel properties of biomass during torrefaction as compared to temperature and residence time. Hence, in this study the influence of two operating parameters (reaction temperature and residence time) on physicochemical properties of pigeon pea stalk have been considered.
Even though India is rich in coal reserve, the major drawback is that most of the available coal are of inferior quality with very high ash content (∼38.39 wt%), lower gross heating value (16.15 MJ/kg), high nitrogen content (∼2.49 wt%) and high sulfur content (0.4 wt%) [14]. Also, most of the energy generation units in South Asia and African countries are coal-based and hence, have to witness strong environmental issues. On the other hand, agricultural residue co-fired with coal reduces the problems like ash handling, high SOX and NOX emission, as agricultural residue has less ash content, lower nitrogen content and negligible sulfur. However, the existing power plants are designed in accordance with coal properties. Hence, study and improvement of solid fuel properties obtained from agricultural residue like pigeon pea stalk becomes necessary in order to not compromise in the performance of combustion units. In this regard the combustion indices like fuel ratio (FR), combustibility index (CI) and volatile ignitability (VI) play an important role. Generally the industrial grade coals have FR value ranging in 0.5–2 [15,16].
Pyrolysis is a proven technique in utilization of agricultural residue in energy generation producing bio-oil, bio-char and gaseous product. Bio-oil is the major product obtained from pyrolysis, however, high oxygen content, lower higher heating value (HHV), high water content and poor fuel qualities makes plenty room for improvement [17]. These major drawbacks are due to the low quality biomass obtained from agricultural residue, hence it becomes necessary to pretreat the biomass to improve its fuel properties. Pyrolysis of torrefied biomass is one of the emerging technique to reduce the oxygen content and upgrade the quality of bio-oil. In our best of knowledge limited study is present on properties like flowability, compactibility and combustion indices of raw and torrefied biomass. The study of these properties for raw and torrefied biomass will help its utilization in other applications like gasification and combustion.
In recent years most of the studies are confined to popular agricultural residues like wheat straw, rice husk and sugarcane bagasse, leaving scope for the other agricultural residues like pigeon pea stalk, chickpea stalk and maze cobs. Pigeon pea stalk is a crop residue which is mostly cultivated in tropical and subtropical regions of India, Botswana, South Africa, China, Ethiopia, Zambia, etc. [18]. India being the largest producer and consumer of the pigeon pea in the world, which makes it the most suitable contender to efficiently utilize the crop residue obtained from pigeon pea. The agricultural residue obtained from pigeon pea stands around 2.9 tons/ha making it relatively high residue producing crop [1]. Presently, a major portion of pigeon pea stalk is utilized as a source to domestic ruminants including as a cooking fuel making it highly underutilized biomass fetching meager economic values to the producers. Agricultural residue can be utilized for energy generation by applying an efficient combination of torrefaction with other thermochemical processes such as pyrolysis and gasification. However, very limited literature was available for detailed study of torrefaction and pyrolysis of pigeon pea stalk.
In this article, effects of torrefaction on physicochemical properties of pigeon pea stalk (agricultural residue) has been investigated. TGA and DTG have been used to predict the pyrolysis behavior of raw and torrefied biomass, along with estimation of kinetic parameters using Arrhenius direct method with an assumption of three pseudo-components. The results obtained have been used to predict the best suited torrefaction condition.
Section snippets
Material selection
For the experimental studies pigeon pea stalk (Cajanus cajan) has been selected. The pigeon pea stalk has been obtained from a village located in Varanasi. Initially, the biomass has been washed with tap water and dried in sunlight for 3–4 days. Then, it has been cut into smaller lengths and then fed to a cutting mill (SM300, Retsch, Germany). After that, the milled material obtained has been sieved in sieve shaker, and then the fraction between 0.7 and 1.2 mm has been collected, sun-dried and
Effect of temperature and residence time on solid yield during torrefaction of biomass
In the present study, each torrefaction experiment has been repeated once and an average value has been reported. The products of torrefaction are solid, liquid (condensable vapors) and gas (non-condensable gases). The yield of each product depends on operating parameters like temperature, heating rate and residence time etc. Fig. 2 presents the yield of solid, liquid and gas with final temperature and residence time. As the temperature increased, decomposition of hydroxyl groups (as confirmed
Conclusions
Torrefaction as a pretreatment process has major influence on physicochemical properties of pigeon pea stalk. The solid yield for TPS-275-45 was 40.4% resulted in very low energy yield (0.50) making high temperature torrefaction non-beneficial for pigeon pea stalk. The van Krevelen diagram confirmed that torrefaction process improved the fuel properties of pigeon pea stalk, making it comparable to coal available in India. There was maximum decrease of 17.70% and 8.11% in the value of CCI and
Acknowledgement
The authors acknowledge the funding from SERB, New Delhi through fund no. SR/FTP/ETA-56/2012. We thank Dr. Debaprasad Shee (Associate Professor, Department of Chemical Engineering, Indian Institute of Technology Hyderabad) for providing TGA and DTG analysis.
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