Article pubs.acs.org/EF
Gasification of Wood and Torrefied Wood with Air, Oxygen, and Steam in a Fixed-Bed Pilot Plant N. Cerone,†,‡ F. Zimbardi,*,† A. Villone,† N. Strjiugas,† and E. G. Kiyikci† †
Research Center of Trisaia, Energy Technologies Department, Italian National Agency on New Technologies, Energy and Sustainable Economic Development (ENEA), 75026 Rotondella, Italy ‡ Department of Chemical Engineering, Materials and Industrial Production, University of Naples Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy S Supporting Information *
ABSTRACT: In this work, the efficiency of the updraft gasification of different biomass was analyzed using a 20 kg/h pilot facility. Eucalyptus wood chips, torrefied Eucalyptus wood chips, and torrefied Spruce chips were investigated. Absolute air, mixes of air with steam, and mixes of pure oxygen with steam were used as gasification media. The direct comparison between the parental and torrefied biomass emphasizes the positive effect of this pretreatment on syngas properties and plant performances. Typically, the use of torrefied feedstock resulted in a reduction to about 1/5 of the tar load in the syngas and in 44% increment of the thermal power of the plant when compared to the performances obtained with the parental wood. The introduction of steam as co-gasifying stream was effective to avoid hot spots inside the reactive bed and to stabilize the process. Moreover, the use of steam positively affected the molar ratio of H2/CO that reached the value of 1.17 with the H2 concentration in the syngas of 39 vol %. The cold gas efficiency was 0.85 with torrefied biomass and pure oxygen with steam (0.82 when taking into account the energy for steam production), whereas the use of parental wood in similar conditions gave a value of 0.67 (net of 0.65). The treatment capacity of the plant was directly proportional to the reactivity of the feedstock as assessed by thermogravimetric analysis when the feed was reported as volumetric flow.
1. INTRODUCTION Biomass accounts for 10% of the energy supply worldwide, and its use has a lot of potential given the new environmental policies.1 Unlike most other energy sources, e.g., solar and wind energy, bioenergy can be stored in liquid and solid products and commercialized. On the basis of this, it is expected that biofuels will represent a significant proportion of the energy sources also in the future. It has been estimated that, in the European Union (EU), the production of electricity, heating, cooling, and fuels obtained from biomass will increase from 8.5% in 2012 to 11.7% in 2020, with an absolute growth of 47% (3967 PJ in 2012 to 5941 PJ in 2020).2 The trade of biofuels is creating some issues regarding their standardization and stability, especially for seasonal feedstock or imports from countries where prices are lower and availability is ensured all year.3 The achievement of sustainability targets in the EU implies that imports may reach 236 PJ by 2020, i.e., 400% of their 2010 levels.4 Biomass torrefaction has been investigated for the last several decades, and it is a well-assessed process that can provide significant advantages in both trading and thermochemical processing, such as combustion and gasification.5,6 This pretreatment is a pyrolysis carried out at a low temperature and low heating rate. The main chemical effect is the degradation of the hemicellulose that is the most thermolabile component. The molar ratios of O/C and H/C in the solid decrease because highly oxygenated molecules, including water, are lost as volatiles. The van Kreevelen diagram with these ratios shows that, after torrefaction, the biomass approaches the fossil fuel region.7 The resulting solid product is more stable and has higher energetic yield per unit © 2016 American Chemical Society
mass. The slight carbonization produces a sterilized feedstock that is resistant to biological degradation because of the lower hydrophilicity and the lack of hemicellulose, which is the first macrocomponent attacked by microbes. From the energetic point of view, the upgraded biomass retains about 90% of its initial energy, while the produced gas, containing CH4, H2, and CxHy, can be burnt to sustain the process. Moreover, the torrefied biomass can be ground with less power and lower energy requirement than biomass,8 which gives a big advantage when producing particles with a size of hundreds of micrometers suitable for co-firing second-generation biofuels, because the grinding energy can be reduced by a factor of 2 or 3.9 An important characteristic of torrefied biomass is the lower production of smoke and volatiles than other thermal conversions.10 These improvements make torrefied biomass suitable for co-firing with fossil fuels in power plants, the most popular technology used to meet the targets of reduction of greenhouse gas (GHG) emissions.11 Although the advantages of using this kind of fuel instead of raw biomass were predicted in several process configurations in terms of overall efficiency and exergy balance,7 very few studies are available in the literature on the gasification of torrefied biomass in direct comparison to the parental feedstock, especially at a pilot scale. Higher performances were obtained in a fluidized gasifier using the torrefied biomass compared to the parental biomass,5 but the loss of carbon as elutriated solid negatively affected the Received: January 19, 2016 Revised: March 8, 2016 Published: April 15, 2016 4034
DOI: 10.1021/acs.energyfuels.6b00126 Energy Fuels 2016, 30, 4034−4043
Article
Energy & Fuels
and syringaldehyde contents using calibration curves and external standards. Other organic molecules were determined using a GC− mass spectrometry (MS) apparatus (Agilent model 5975 equipped with a quadrupole electron impact ionization and auto sampling device) and an Agilent column DB5MS using helium as a carrier 6.0 at 1 mL/min at 45 °C for 10 min, followed by heating at a rate of 8 °C/ min up to 325 °C. To identify the chemical species, the mass spectra were matched with the National Institute of Standards and Technology (NIST) library using the peak area to quantify them and benzene, phenol, cresol, and 1-methyl-naphthalene as standards.17 The thermal behavior of the fuels was studied with a micro thermogravimetric analysis (TGA) apparatus (PerkinElmer model TGA7) by loading 2−5 mg of milled sample (particle size of
