Effect of Pressure on In-situ Catalytic Hydropyrolysis of Rice Straw

Research Article

Austin Chem Eng. 2020; 7(2): 1076.

Effect of Pressure on In-situ Catalytic Hydropyrolysis of Rice Straw

Naidu VS*, Kumar P*, Karri N, Singh PK, Gandham S and Rao BR

Fluid Catalytic Cracking Division, Hindustan Petroleum Green Research & Development Centre, India

*Corresponding author: V Satyam Naidu, Fluid Catalytic Cracking Division, Hindustan Petroleum Green Research & Development Centre, India; Pramod Kumar, India

Received: June 01, 2020; Accepted: July 02, 2020; Published: July 09, 2020


Pyrolysis of rice straw was investigated to study deoxygenation of biooil vapours with various parameters such as pressure, gas environment, and catalyst. Pyrolysis and hydropyrolysis experiments were performed at a temperature of 500oC and pressures of 1, 5 and 10 bar in a fluidized bed reactor under N2 and mixture of N2 and H2 environments. It was observed that the biooil yield increased from 28 to 42 wt.% and the bio char yield decreased from 43 to 33 wt.% with increase in pressure from 1 to 10 bar during hydropyrolysis in the presence of ZSM-5 catalyst. Gas analysis showed that carbonylation and decarboxylation were the major pathways for deoxygenation of pyrolysis vapours using ZSM-5and lighter hydrocarbons up to 13 wt.% were obtained under catalytic hydropyrolysis. The detailed analysis of carbon balance and oxygen balance was carried out to evaluate carbon efficiency and degree of deoxygenation of bio-oil vapours.

Keywords: In-situcatalytic pyrolysis; Hydropyrolysis; Deoxygenation; ZSM- 5; Pressure effect; Rice straw


The increase in demand for energy and huge dependence on fossil fuels create environment pollution such as greenhouse gas emissions, particulates formation etc. One of the main sources of alternate energy to replace fossil fuels is by thermo chemical conversion of biomass via gasification, pyrolysis, hydrothermal liquefaction routes to produce gaseous, liquid and solid char as fuels [1,2]. Pyrolysis of biomass is widely used technique to produce bio-oil which can replace petroleum products [3]. Lignocelluloses biomasses are widely used for the production of bio fuels, chemicals, etc.

As per the statistics of International Rice Research Institute (IRRI), rice is major crop in the Asian countries in which India being the second largest consumer with 97.35 million metric tons of rice consumption annually. Each kg of milled rice produces 0.7 to 1.4 kg of rice straw depending on the variety of rice crop, stubble cutting and moisture content during harvest. Rice straw is basically a by-product of rice when harvesting paddy and is abundantly available from the agro fields that can also be utilized to produce bio oils pyrolysis [4- 10]. In the Indian context, stubble burning by farmers of Haryana, Punjab, Uttar Pradesh and Delhi is a major contributor to the smogsoaked winters. Therefore, one of the objectives of the present study is to convert rice straw biomass to liquid fuels in order to reduce the particulates, pollutants and gas emissions into the atmosphere.

Various stages of pyrolysis kinetics involving drying, devolatilization and hydropyrolysis etc are not extensively studied in the literature [11,12]. It is widely known that higher bio-oil yield can be achieved from fluidized bed reactors [13-17]. Maximum biooil yield of 54 to 60 wt.% is obtained under fluidizing and spouted bed conditions [13,14]. Iisa et al. demonstrated that organic bio-oils with wide range of oxygen contents can be obtained by Catalytic Fast Pyrolysis (CFP), however, leaving more oxygen leads to better carbon efficiency and economics [18].

The pyrolysis techniques are classified as in-situ and ex-situ mode depending on the catalyst utilization in the pyrolysis reactor. These are defined as follows: Biomass pyrolysis and up gradation occur in the single reactor during fluidization is called in-situ catalytic pyrolysis whereas pyrolysis occurs in the first reactor and upgradation happens in the second reactor is called as ex-situ catalytic pyrolysis [18]. Recent studies on in-situ and ex-situ catalytic pyrolysis in the presence of zeolite catalysts demonstrated enhancement in the aromatic hydrocarbons [19-21]. Although ex-situ catalytic pyrolysis had inherent advantages in terms of low-oxygen content, high catalyst stability, in-situ catalytic pyrolysis is superior for high biooil yield, carbon retaining capacity with similar minimum fuel selling price in the range of $1.1 per litre [22,23]. Gamliel et al. compared in-situ and ex-situ Catalytic Fast Pyrolysis (CFP) bio-oil produced in PyGC analyser composition and reported that ex situ CFP more accurately predicts the molecular composition with spouted bed reactor [24]. Nolte et al. reported MoO3 was effective catalyst to produce hydrocarbons at higher yields in in-situ mode consisting of linear alkanes and aromatics in comparison to ex-situ HDO of bio-oil [25-26].

Zeolite catalysts have been extensively used for biomass catalytic pyrolysis with different silica-to-alumina ratio such as ZSM-5, H-beta, Y-Zeolite, USY, MCM-41 etc. to improve the organic bio-oil yield and quality [28-32]. However, the organic bio-oil yield has never exceeded the amount higher than organic bio-oil produced under thermal conditions and is the maximum yield produced from catalytic pyrolysis for fuel applications [33]. Two-stage zeolite catalysts such as ZSM 5 (micro pore catalyst) and MCM-41 (mesoporous catalyst) have also been used to produce bio-oil with approximately 77 % of favourable fractions, water content up to 42%, TAN of 43 mg-KOH/g and high gasoline range chemicals up to 98% were obtained [34,35].

Hydropyrolysis is evolved as an emerging technology to produce bio-oil in the presence of catalysts for the pressure range from 1 to 52 bar of H2 in an autoclave reactor [36]. Various noble metal catalysts on carbide and Al2O3 supports have been screened for hydropyrolysis and hydrodeoxygenation of bio-oil [37,38]. The rate of deoxygenation can be improved via hydrogenation with increase in H2 pressure over Ni/ZSM-5 [39]. Recent studies conducted on hydropyrolysis in the pilot-scale plants under fluidizing conditions to produce gasoline and diesel range fuels in two-stage processes yielded less than 1 wt.% oxygen at 22 to 25 bar [40-42]. Hydropyrolysis experiments are also performed in a single-stage process at 20.7 bar and produced lowoxygenate bio-oil with oxygen content up to 5 wt.% [43-45]. Although hydropyroysis experiments were performed atthe pilot scale level, the effect of pressure on bench scale and pilot scale experiments are very scarce.

The present work is focused on studying the effect of pressure on slow pyrolysis and hydropyrolysis for rice straw feedstock at 1, 5 and 10 bar at a reaction temperature of 500oC. Catalytic pyrolysis and hydropyrolysis experiments were performed using ZSM-5 catalyst to study the deoxygenation efficiency of bio-oil. The detailed analysis of bio-oils, biochar and Non Condensable Gases (NCG) is performed to determine the carbon efficiency and oxygen distribution in each of the products.

Experimental Section

Materials and characterization methods

Rice straw feedstock was procured from local agro fields in the Bangalore city in India. Rice straw was sun-dried before reducing its size to 2 to 5mm using a crusher. These particles were further reduced to 100 μm to 1000μm using a grinding mill. The crushed and ground biomass was sieved to obtain a size range of +300-700μ particles for the experiments. The reduced particles size was needed to avoid diffusion limitations and to improve the rate of heat transfer during pyrolysis under fluidizing conditions. Proximate and ultimate analysis were performed using sophisticated analytical instruments (CHNS analyzer and TGA) as per ASTM standards and the results are shown in Table 1. Commercial ZSM-5was used as catalyst in some experiments to study the deoxygenation mechanism of pyrolysis vapours under in-situ pyrolysis conditions. Catalyst was sieved to obtain a size range of +106-212μm particles in order to have proper mixing with biomass particles under fluidizing conditions.ZSM-5 catalyst was characterized for BET surface area, pore volume and acidity of the catalyst. The catalyst showed low surface area with 119m2/g and high micro pore surface area with more of weak acid sites as shown in Table 2 [46]. XRF analysis was performed to determine the metal composition of ZSM-5 catalyst as shown in Table 3 [46].