Elsevier

Biomass and Bioenergy

Volume 95, December 2016, Pages 378-387
Biomass and Bioenergy

Research paper
Supercritical water gasification of timothy grass as an energy crop in the presence of alkali carbonate and hydroxide catalysts

https://doi.org/10.1016/j.biombioe.2016.05.023Get rights and content

Highlights

  • Timothy grass was gasified as an energy crop in supercritical water to produce H2.

  • Studies on temperature, feed concentration, reaction time and alkali catalysts.

  • High H2 yields at 650 °C, 1:4 biomass-to-water ratio and 45 min at 23–25 MPa.

  • Improvement in H2 yields by 3 % KOH with upheld water-gas shift reaction.

  • Raised temperature (650 °C) resulted in high gas yields and biochar aromatization.

Abstract

This study is focused on identifying the candidature of timothy grass as an energy crop for hydrogen-rich syngas production through supercritical water gasification. Timothy grass was gasified in supercritical water to investigate the impacts of temperature (450–650 °C), biomass-to-water ratio (1:4 and 1:8) and reaction time (15–45 min) in the pressure range of 23–25 MPa. The impacts of carbonate catalysts (e.g., Na2CO3 and K2CO3) and hydroxide catalysts (e.g., NaOH and KOH) at variable mass fractions (1–3%) were examined to maximize hydrogen yields. In the non-catalytic gasification of timothy grass, highest hydrogen (5.15 mol kg−1) and total gas yields (17.2 mol kg−1) with greater carbon gasification efficiency (33%) and lower heating value (2.21 MJ m−3) of the gas products were obtained at 650 °C with 1:8 biomass-to-water ratio for 45 min. However, KOH at 3% mass fraction maximized hydrogen and total gas yields up to 8.91 and 30.6 mol kg−1, respectively. Nevertheless, NaOH demonstrated highest carbon gasification efficiency (61.3%) and enhanced lower heating value of the gas products (4.68 MJ m−3). Timothy grass biochars were characterized through Fourier transform infrared spectroscopy, Raman spectroscopy and scanning electron microscopy to understand the behavior of the feedstock to rising temperature and reaction time. The overall findings suggest that timothy grass is a promising feedstock for hydrogen production via supercritical water gasification.

Introduction

The dedicated energy crops are the plant species that are explicitly cultivated as biofuel feedstocks. Some lignocellulosic plants identified as energy crops include alfalfa, bamboo, elephant grass, hybrid poplar, jatropha, reed canary grass, ryegrass, silvergrass, switchgrass and timothy grass. Conservative estimates suggest that the annual availability of energy crops in Canada can range up to 17.3 million tonnes, which has a potential to produce 4.7 billion litres of bioethanol per annum [1]. It should be noted that the availability of energy crop biomass largely depends upon the feedstock value and their propensity for biofuel production.

The salient features of an ideal energy crop are: (i) low cost and fast growth; (ii) short rotation harvesting; (iii) non-seasonal or perennial availability; (iv) high yield, i.e. maximum dry matter production per hectare; (v) lower requirement of intensive agricultural practices; (vi) reduced accumulation of environmental contaminants, e.g. chemical fertilizers, pesticides and heavy metals; (vii) no competition with food crops for nutrients or sunlight; (viii) ability to grow and regenerate in marginal or degraded lands; and (ix) resistance to extreme weather conditions. As the energy crops are non-food or non-cash crops cultivated on marginal soil, they usually do not pose any threat to the food supply and arable lands [2], [3].

Timothy grass is a perennial grass native to Europe and North America that grows well in heavy soil and is resistant to drought as well as humid, cold and hot weather conditions. Recently, a few studies have demonstrated the bioenergy potential of timothy grass. The physicochemical characterization of timothy grass is almost analogous to that of wheat straw [4], [5], which suggests that perennial grasses can potentially supplement the demand for agricultural biomass in biorefineries. Mohanty et al. [6] performed slow and fast pyrolysis of timothy grass at variable heating rates of 275 K min−1 and 723 K min−1, respectively. While fast pyrolysis led to the mass fractions of 42% bio-oil, 22% gas and 22% biochar, slow pyrolysis resulted in mass fractions of 18% bio-oil, 27% gas and 43% biochar from timothy grass. Nanda et al. [7] performed dilute acid pretreatment and enzymatic hydrolysis of timothy grass followed by bioconversion to ethanol (22.6 g L−1 in 36 h) and butanol (10.8 g L−1 in 60 h) using Saccharomyces cerevisiae and Clostridium beijerinckii, respectively.

Supercritical water gasification (SCWG) is an attractive hydrothermal technology for the conversion of lignocellulosic biomass to produce combustible syngas. SCWG employs supercritical water (SCW) as a homogeneous reaction medium and indigenous catalyst. The thermophysical properties of water transform beyond its critical temperature (≥374 °C) and critical pressure (≥22.1 MPa) that imparts enhanced mass transfer and solvation properties [8]. SCW has liquid-like viscosity and gas-like density along with high diffusivity, low dielectric constant and excellent heat transfer properties [9]. The efficiency of SCWG, as well as gas yields and composition, are determined by the applied temperature, pressure, feed concentration and reaction time [10]. H2-rich syngas obtained from SCWG can act as a direct fuel or be used to produce hydrocarbon fuels, green diesel and synthetic chemicals via Fischer-Tropsch process [11]. As a fuel, H2 enriched syngas can be used in high-efficiency power generation systems, combustion engines and fuel cells for both vehicular transportation and distributed electricity generation.

Currently, the primary routes for commercial H2 production from fossil fuels or hydrocarbons are via steam reforming, alkaline-enhanced reforming, partial oxidation and autothermal reforming. Nearly 59% of industrial H2 production is through steam methane reforming of natural gas, which contributes about 30 million tonnes of CO2 per annum [12]. The cost of H2 production through steam methane reforming is also sensitive to the price of natural gas. For instance, the cost of generating H2 by steam methane reforming of natural gas (10.30–13.5 $ GJ−1) is nearly three times higher than the price of natural gas (3.43–4.50 $ GJ−1) [12]. The cost of H2 obtained through pyrolysis and gasification is expected to be 1.47–2.57 $ kg−1 and 1.44–2.83 $ kg−1, respectively [13]. Balat and Balat [14] reported that the cost of H2 obtained from biomass pyrolysis ranges between 8.86 and 15.52 $ GJ−1. Hamelinck and Faaij [15] reported that the cost of biomass-derived H2 ranges from 10 to 14 $ GJ−1, with a net energy efficiency of 56–64%. However, H2 price could fluctuate depending on production capacity of the refinery, the cost of feedstock, co-product marketability and carbon trading.

Although the potential of dedicated energy crops for biorefining is well-known, yet their applied conversion is scarcely available in the literature. Hence, the current paper attempts to better understand the candidacy of timothy grass for H2 production during SCWG. Several parameters governing the SCWG of timothy grass such as temperature, feed concentration and reaction time along with homogeneous catalysts have been systematically investigated.

Section snippets

Energy crop biomass

Timothy grass (Phleum pratense subsp. pratense) was used as a representative energy crop biomass in this gasification study. Timothy grass bales weighing about 3–5 kg were procured from a local farm in Saskatchewan, Canada. The grass species was in the later flowering stage during harvest. The height of the cut was between 0.8 and 1.2 m in length that included leaves with a few flower heads. Any visible contaminants such as sand, soil or shell particles were manually removed by threshing the

Effects of temperature coupled with feed concentration

Timothy grass was gasified at 450–650 °C for 45 min to study the effect of temperature and feed concentration (1:4 and 1:8 BTW feed ratio) at 23–25 MPa. Fig. 1a shows the trend of gas yields from timothy grass at 1:4 BTW feed ratio with different temperatures. With the rise in temperature from 450 to 650 °C, H2 yields increased from 1.04 to 4.08 mol kg−1. Analogous to H2, the concentrations of CO2 (6.24 mol kg−1), CH4 (3.05 mol kg−1) and C2H6 (0.51 mol kg−1) also increased at 650 °C. The total

Conclusions

The SCWG of timothy grass resulted in a gradual increase in the yield of H2 and total gases with the rise in temperature from 450 to 650 °C. High temperatures (≥550 °C) caused extreme denaturation of timothy grass through dehydration, bond breakages and formation of transformational products which resulted is aromatization of biochars. Higher yields of H2 (5.15 mol kg−1) and total gases (17.2 mol kg−1) with a carbon gasification efficiency of 33% was found with 1:8 biomass-to-water feed ratio

Acknowledgements

The authors thank Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Research Chair (CRC) program for providing financial support to conduct this bioenergy research.

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