Co-gasification of woody biomass with organic and waste matrices in a down-draft gasifier: An experimental and modeling approach

https://doi.org/10.1016/j.enconman.2021.114566Get rights and content

Highlights

  • Full scale co-gasification of woody biomass with waste matrices has been verified.

  • A proper air distribution system assures tar reduction and high gasifier efficiency.

  • Cold gas efficiency of 72–76% is attained in co-gasification tests.

  • The pseudo-kinetic approach reliably simulates the gasification process.

  • Preheating of the gasifying air is effective for enhancing gasification efficiency.

Abstract

In this paper an extensive experimental campaign about the co-gasification of virgin wood biomass with organic and waste matrices in a full-scale downdraft air gasifier is presented. In particular, wastes from cherry processing, plastic wastes, sewage sludges and hazelnut shells were co-gasified with biomass adopting different mixtures. The syngas composition was continuously measured in order to assess the gasifier behaviour and performances using these mixtures. Results obtained mixing biomass with other feedstocks up to 60% (by mass) showed that the downdraft gasifier adopted, characterized by a patented internal air distribution system, was able to maintain a good gasification performance in terms of syngas lower heating value (LHV) (>5.18 MJ/Nm3), syngas flow rate (>400 Nm3/h) and cold gas efficiency (>71.9 %).

A pseudo-kinetic model of the gasifier using Aspen Plus® code was developed as well. The model, calibrated with the experimental data, proved to fit the syngas composition and physical properties with satisfying accuracy. The numerical model confirmed its usefulness in predicting the performance of the gasifier as the operating parameters vary and clearly highlighted that secondary air flow and preheating of the gasifying air are effective ways for the enhancement of the cold gasification efficiency, which can reach values around 80%.

Introduction

It is well known that biomass gasification is a thermochemical process in which the biomass is converted into a mixture of gases (called producer gas or syngas) which consists mainly of CO, H2, CO2, CH4 and N2 (this last is present if air is utilized as gasification agent) as well as other hydrocarbon species. The gasification process operates at temperatures from 700 to 1200 °C. In addition to air, other gasification agents can be used, such as O2, steam and CO2 or a combination of them in the biomass gasification process [1], [2], [3], [4]. Depending on its composition, the producer gas can be conveniently transformed into chemicals, fuels or energy (both heat and power), this last thanks to the utilisation of an appropriate thermal machine, such as a reciprocating internal combustion engine (RICE) or a gas turbine [5], [6], [7].

Increasing attention has been recently given to a new gasification strategy which sees the addition of organic and waste matrices to biomass (co-gasification). This approach is applicable for many feedstocks such as organic waste from agricultural processing or food industry, sewage sludge, plastic, municipal solid waste, etc. [8], [9], [10]. These matrices, despite having good thermo-physical characteristics, are difficult to dispose of using conventional techniques, mainly owing to the high heterogeneity and moisture content and low volumetric energy density. Thus, co-gasification opens up new perspectives for their treatment, even if some critical aspects have still to be solved.

The accurate description of gasification and co-gasification process characteristics is not the scope of the present work since it can be found in many scientific papers. However, it is important to recall that different gasification technologies are available which include fixed bed gasifiers (updraft and downdraft), fluidised bed gasifiers (bubbling and circulating) and entrained flow gasifiers. In particular, fixed bed downdraft gasifiers, as the one applied in the present experimental activity, are very attractive for small scale application (typically, downdraft gasifiers have a thermal input of 10 kW–1 MW, but some applications can reach 2 MW), especially when applied in CHP plants which see the presence of a RICE, due to their low fabrication costs, easy operation and low tar content in producer gas [11], [12]. Nevertheless, some reliability drawbacks are still challenging to solve such as grate blocking, reaction bed channeling and accurate syngas cleaning, in particular the long chain hydrocarbons (tar) reduction. Another disadvantage is that downdraft gasifiers require feedstock with low moisture content.

While biomass-coal co-gasification is a well-established technology [13], [14], the utilization of organic and waste matrices with biomass is still at the experimental stage, especially in fixed-bed downdraft gasifiers, as the recent limited scientific literature confirms, so the present experimental–numerical activity gives further insight into this promising waste disposal approach.

E. R. Widjaya et al. [15] analysed recent progress in non-woody biomass gasification, evidencing that the potential utilization of this type of biomass is promising, given that it is abundant and widely available. Energy conversion through gasification processes can achieve higher conversion efficiencies and lower pollution when compared to standard combustion. However, it has been found that the low density and the high silicate content, characteristics of typical non-woody biomass, can make this type of material relatively difficult to handle in the gasification process. Efforts in the selection and improvement of gasifier designs are necessary to match the nature of low density non-woody properties. Authors also highlight that upgrading the non-woody material into good quality solid fuel by adding lignocellulosic biomass may be the best alternative option for feeding the gasifier. This approach also fits well with the simplest gasifier designs such as the fixed bed type.

P.R. Bhoi et al. [16] co-gasified municipal solid waste (MSW) with switchgrass using a commercial-scale 100 kg/h downdraft gasifier. MSW as a feedstock was studied at 0%, 20%, and 40% co-gasification ratios (CGR, by wt.). The experiments results indicated that co-gasification is acceptable utilising up to 40% wt. of MSW, while at CGR of 60% ash agglomeration was observed during gasification. The heating values of syngas were 6.2, 6.5 and 6.7 MJ/Nm3 for CGR of 0%, 20% and 40%, respectively. The cold gas efficiencies reached a value of about 60% for co-gasification ratio of 40%. Authors also evidenced that as CGR increased from 0 to 40%, tar content of syngas decreased significantly from 26.1 to 9.9 g/Nm3 because the cellulosic constituents of feedstock, which are responsible for tar generation, are decreased as CGR increased.

Z. Ong et al. [17] studied, experimentally and numerically, the co-gasification of woody biomass and sewage sludge. During the experimental activity, co-gasification of sewage sludge and woody biomass was successfully performed in a small fixed-bed downdraft gasifier to produce syngas. A maximum of 20 wt% dried sewage sludge in the feedstock was effectively gasified to generate producer gas with an average LHV of 4.5 MJ/Nm3. Further increasing the sludge content up to 33 wt% led to the blockage of gasifier mainly due to the formation of agglomerated ash. A simulation model was developed as well by adapting several kinetic models developed in the literature. Authors evidenced how the simulation model was useful to study the effects of equivalence ratio, biomass composition and moisture content on the co-gasification process avoiding gasification experiments which are usually quite expensive and time-consuming.

S. Szwaja et al. [18] experimentally and numerically analysed the co-gasification process utilizing dried granulated sewage sludge and Virginia Mallow. Tests were carried out on an industrial scale downdraft gasification system with a maximum feedstock consumption of 300 kg/h, utilising three different mixtures in a weight percentage of 0/100, 50/50 and 100/0. Authors found that at 50/50% of sewage sludge and Virginia Mallow the gasification process provided satisfactory conditions to obtain maximal temperature at gasification zone of 850 °C, which led to generate syngas with calorific value LHV of 5.3 MJ/Nm3. Authors also concluded that the numerical model created could be applied as a preliminary tool for predicting syngas calorific value as well as process temperature and for evaluating economic justification for gasification of poor fuels as sewage sludge is.

Other than the positive thermo-chemical characteristics of the co-gasification technology, another important aspect is that it can lead to the installation of small plants widespread on the territory, minimizing the environmental impact and the cost of transportation. S. Ramachandran et al. [19] studied a decentralised sewage sludge and woody biomass co-gasification system for Singapore. The blend utilized was composed by 20 wt% of sewage sludge and 80 wt% of woody biomass. Downdraft gasification technology was chosen among the others since it proved to be a standout choice for small to medium scales, while the decentralize strategy allowed to maintain the average transport distance, per kg of sewage sludge, around 35 km. Authors evidenced how this approach could reduce concretely the CO2 emission, compared to the existing system, thanks to an increased process efficiency, no requirement of supplementary fuel for sludge disposal and CO2 sequestration in biochar. Moreover, the decentralised system reduced the kg-km driven by 42%. The proposed system was able to increase the net electricity production from sewage sludge and woody biomass by 3–24%.

However, as anticipated, all the aforementioned works highlight how, currently, information on operative behaviour and syngas quality through the co-gasification of biomass and wastes are still lacking and additional investigation on its conversion process is still necessary, in an attempt to consolidate co-gasification as a promising technique for the exploitation of energy residues. Therefore, the aim of the present work is to give additional information on the co-gasification process of woody biomass with organic and waste matrices, showing the results obtained during an experimental activity performed on a full scale down-draft gasifier. Tests were conducted mixing biomass with different percentages (by wt.) of cherry processing waste, plastic, sewage sludge and hazelnut shells. The results obtained confirm some already presented in the scientific literature, while others, such as those obtained by co-gasifying hazelnut shells, are shown for the first time.

In addition to the experimental activity, another novelty of the present work is the definition of a pseudo-kinetic model of the co-gasification process implemented in Aspen Plus® environment. It is well known that the creation of a numerical code, which also takes into consideration the geometry of the gasifier, is a valuable tool for identifying which parameters are appropriate to act on to improve the performance of the gasifier, avoiding time consuming and expensive experimental activities [20], [21], [22]. The utilization of the Aspen Plus® platform, besides facilitating the modeling of the thermochemical processes involved in the gasification process (drying, pyrolysis, oxidation, etc.), can also give the possibility to simulate the entire gasification plant (pipes, pumps, heat exchangers, etc.), thus offering a satisfactory evaluation of the performance of the system as a whole.

Section snippets

Description of the gasification plant

The experimental activity was conducted on a gasification plant equipped with a downdraft gasifier (Imbert type) and a syngas cleaning system. The gasifier has a gross thermal power of about 800 kWth at maximum load, corresponding to a woodchips consumption of roughly 300 kg/h with a moisture content of 10 wt%. Air was used as gasification agent and dried woodchips (with moisture content < 15 wt%) as woody biomass. In the present experimental activity, the syngas cleaning system was not

Feedstock characterization

Dried woodchips (conifer) were used as woody biomass in gasification and co-gasification tests. Woodchips were blended with different waste matrices in co-gasification experiments. Three waste matrices of different origin and features were tested:

  • a “simulated” Organic Fraction of Municipal Solid Waste (OFMSW), prepared by mixing fruit wastes from cherry production process (88.5 wt%) and plastic wastes (11.5 wt%);

  • Sewage Sludge;

  • Hazelnut Shells, a typical organic waste from food industry.

Fruit

Experimental results

The co-gasification of the different mixtures was experimentally investigated in the full-scale downdraft gasifier described above. In order to compare gasification performance, gasification tests with pure woodchips were also carried out. A biomass feeding rate ranging from 207 to 225 kg/h was employed during woodchips gasification. The value of the equivalence ratio ER (i.e., the ratio between the actual air supplied to the stoichiometric value required for complete fuel combustion) ranged

Modelling activities

The large amount of experimental data reported above made it possible to reliably validate a simulation model of the gasifier with the aim to obtain an effective tool for the prediction of its performances with many different operative conditions. This approach is well known in the scientific literature and many studies deal with the creation and setting of numerical models dedicated to the gasification process, as reported in several review papers [20], [21], [22]. Moreover, the possibility to

Conclusions and future remarks

Recently, co-gasification of biomass and wastes has been drawing attention around the world as a promising technology to produce energy from waste, overcoming the problem of residues disposal and processing, simultaneously contributing to the reduction of greenhouse gas emissions. However, information on the use of biomass and wastes in co-gasification process is still lacking and investigation on their characterization and conversion method is still necessary.

The present work gives further

CRediT authorship contribution statement

Federica Barontini: Conceptualization, Methodology, Investigation, Validation, Visualization, Writing - original draft, Writing - review & editing. Stefano Frigo: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing, Funding acquisition. Roberto Gabbrielli: Conceptualization, Methodology, Software, Investigation, Visualization, Writing - original draft, Writing - review & editing, Funding acquisition. Pietro Sica: Resources, Investigation, Writing

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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