**1. Introduction**

The environmental issues associated with global warming have resulted in a strong tendency towards the reduction of greenhouse gas emissions in the field of energy production. This trend has thus drawn attention to clean-coal technologies, such as gasification, which, compared to conventional combustion, can reduce CO2 emissions by up to 90% [1,2]. The gasification process consists of partial oxidation of an organic feedstock that takes place at high temperatures (around 700–1200 ◦C) in the presence of a gasification agen<sup>t</sup> (air, steam, oxygen, or a combination of these). Gasification consists of a combination of chemical processes, such as drying, pyrolysis, combustion, and partial oxidation. The main product is syngas, which is a mixture of gases with a high calorific value, typically composed of H2, CO, CO2, and CH4. Some undesired products are also generated during the process, such as particulate, acid gases, and tars (condensable heavy hydrocarbons) [3], which have to be removed in order to make the gas usable for energy purposes or the production of base chemicals.

Gasification can be a promising option for exploiting abundant carbonaceous solid resources, such as lignite [4], for the production of a versatile fuel gas with a wide range of possible final uses. The product gas can in fact be used for the production of electricity in integrated gasification combined cycle systems (IGCC) or fuel cells [5], or it can be exploited for the synthesis of liquid fuels [6] and

Fischer-Tropsch synthesis [7,8], thus reducing imports of oil and natural gas from outside Europe and reducing emissions from the transport sector. Lignite, with its high heating value and low volatile content compared to biomass, could be a very suitable feedstock for the gasification process [9,10].

Olivine, a mineral mainly composed of magnesium, iron oxide, and silica, is used as an inventory of the fluidized-bed reactor, because of its advantageous qualities. As stated in the literature, olivine is recommended as bed material because of its reported activity in tar reduction, which is comparable to that of calcined dolomite. Furthermore, olivine has a stronger attrition resistance, compared to dolomite, which makes it more suitable as bed material [11,12].

Previous works have already reported lignite gasification in fluidized-bed reactors. Bayarsaikhan et al. studied lignite steam gasification (particles of 500–1000 μm) in a fluidized-bed reactor with a bed of silica and alumina in the range of temperatures of 850–950 ◦C [13]. Additionally, Kern et al. investigated lignite gasification (particles of 2–6 mm) in the dual fluidized-bed gasifier developed by Vienna University with olivine as bed material, at an operating temperature of 850 ◦C, with steam/fuel ranging from 0.9 to 1.4 [14]. Furthermore, Karimipour et al. performed a statistical analysis based on the experimental results of lignite gasification (particles of 70–500 μm) in a fluidized-bed reactor of silica sand particles with steam and oxygen as oxidizing agents [15].

The novelty of the present work is the assessment of the fluidized-bed technology for the steam gasification of small particle size lignite (around 50 μm) pre-treated by the WTA process (fluidized-bed drying process with internal waste heat utilization, in german: Wirbelschichttrocknung mit interner Abwа¨rmenutzung), with a bed of olivine particles in a temperature range of 750–850 ◦C and a steam/fuel ratio equal to 0.65. Furthermore, in this work, the operating temperature was changed in order to study its e ffect on the gas quality, and in some of the experimental tests, air injections were added in the freeboard, in order to reproduce, on a smaller scale, the oxygen added in the post gasification zone of the High Temperature Winkler (HTW) gasifier [2,16], and thus to assess its e ffectiveness in tar reduction.

This work was carried out within the European project LIG2LIQ [17], whose aim is to develop an economically e fficient concept for the production of liquid fuels, such as Fischer-Tropsch fuels or methanol, from lignite and solid recovered fuel from municipal waste by means of HTW gasification technology. Therefore, in the first step, the concept of HTW gasification, which was optimized with respect to the cold gas e fficiency and carbon conversion e fficiency, using lignite/waste mixtures as feedstocks for the production of syngas, was studied on a small scale. This paper describes the results of gasification experiments using only lignite as fuel in a laboratory fluidized bed. In the second step, experiments with a mixture of municipal waste and lignite were carried out.

Lignite gasification tests were carried out in a bench-scale fluidized-bed reactor, with the aim of investigating the best conditions to produce a high-quality syngas, with a low tar content and high H2 and CO fraction, foreseeing downstream processes for Fischer-Tropsch or methanol synthesis. Tests were carried out, as mentioned above, changing the operating temperature and carrying out small air injections in the freeboard of the gasifier, in order to study the e ffect of a temperature increase in the upper part of the reactor aimed at enhancing tar conversion. The results were evaluated in terms of the syngas composition, tar content, gas yield, and conversion rates. Additionally, pressure fluctuation signals were acquired in the reactor freeboard during experimental tests, in order to evaluate the fluidization quality at the explored process conditions and to detect possible sintering or particle agglomeration within the bed inventory; in fact, these phenomena can increase the particles' average diameter, and then negatively a ffect the fluidization properties [18].

Therefore, the aim of the work was to evaluate the quality of the product gas obtained from the gasification of fine lignite in a fluidized bed of olivine particles at di fferent temperatures, and to study the e ffect of air injections in the enhancement of tar reduction. In addition, the analysis of the materials before and after tests and the pressure fluctuation analysis helped to assess the eventuality of undesired phenomena, such as ash melting and the aggregation of bed particles, which could lead to defluidization of the bed.

#### **2. Materials and Methods**

#### *2.1. Experimental Test Rig*

The bench-scale gasification reactor represented in Figure 1 was used to carry out the experimental tests. The gasifier consisted of a cylindrical stainless steel reactor (internal diameter of 100 mm and height of 850 mm) externally heated with a 6 kW electric furnace. Steam was used as a gasification agen<sup>t</sup> and a flow of nitrogen was added in order to fluidize the bed; they were sent to a wind-box to be mixed and pre-heated, and then fed from the bottom of the gasifier through a porous ceramic distribution plate. In order to investigate the effect of an increase of temperature in the freeboard, in some tests, a small stream of air was injected in the freeboard through a steel tube of a 6 mm diameter. The bed material used was sintered, calcined olivine particles provided by Magnolithe GmbH [19] with a d3,2 diameter of 317 μm and density of 3000 kg/m3, with the following composition by weight: MgO, 48%–50%; SiO2, 39%–42%; and Fe2O3, 8%–10%. As reported in the literature, calcination at high temperatures (around 1100 ◦C or higher) allows the iron oxides contained in the olivine particles to emerge at the surface, and thus to be available for catalytic reactions [20–22]. The height of the bed in the reactor was approximately 200 mm. The feedstock consisted of WTA lignite, and Rhenish lignite from a process of fluidized-bed drying with internal waste heat utilization [23], supplied by RWE Power AG. Lignite was continuously fed to the bed of the reactor by means of a screw feeder and a feeding probe that delivered the material directly to the fluidized bed. The feeding probe was purged with a small N2 flow, in order to help the fall of the feedstock and to avoid the material from clogging the tube. The reactor was designed to host a ceramic filter candle in the freeboard above the bed, through which the product gas was forced to pass to exit the gasifier; in this way, the solid particulate remained on the external surface of the candle and the dust-free gas could exit the reactor.

The product gas downstream of the gasifier was sent to heat exchangers for gas cooling and condensation of the residual steam. Circulation of the gas was granted by a vacuum pump. The flow of the dry product gas was measured with a mass flow controller, and its composition was analysed by online gas analyzers; a slipstream of the gas produced, about 1 Nl/min, was sent to the tar sampling unit, carried out following the specification of the standard CEN/TS 15439. The gas passed through five impinger bottles containing 2-propanol and kept in a cold bath at −20◦C, to help the condensation of tars. The gas stream was moved by a vacuum pump and its flow rate was controlled by a mass flow controller. Finally, the main gas stream and the slipstream used for tar sampling were both sent to the vent.

Temperatures were measured by means of three K-type thermocouples: one was positioned in the reactor bed (T1); one was located in the freeboard (T2); and the other was situated at the exit of the candle, just at the outlet of the filter (T3). The operating temperature was considered the average of the values measured from T1, T2, and T3. Differential pressures were measured by means of pressure probes through the candle (ΔP1) and through the reactor (ΔP2). The pressure probes were connected to U-tube manometers, and were able to measure in the range of 0.5–90 mbar.

The fluidized-bed bench-scale reactor was equipped with a vertical probe in its freeboard, for acquisitions of pressure fluctuation signals. The probe was connected to a piezoelectric pressure transducer, in turn transmitting its signal to a charge amplifier KISTLER 5019A, and the operation parameters were tuned so as to obtain the highest amplification, without overloading. The resulting amplified voltage signal was then digitally converted and stored on a PC, provided with a tailored LABVIEW® routine. The data acquisition frequency was 100 Hz, which is much higher than the values typically observed in gas-fluidized beds under study (less than 10 Hz). The duration of each acquisition was 2–3 min, to ensure their repeatability and significance [24]. Stored signals were processed by means of a MATLAB® script, which calculated their standard deviations, as well as the power spectral density function (PSDF), by fast Fourier transform [25]. Standard deviations are directly related to the size of bubbles erupting at the upper bed surface (the higher the standard deviation, the bigger the

bubbles), while PSDF allows dominant frequencies of pressure fluctuations to be identified, related to the number of erupting bubbles [18].

**Figure 1.** Scheme of the bench-scale gasification test rig. 1—water pump; 2—steam generator; 3—air and N2 gas tanks; 4—air and N2 mass flow controllers; 5—bubbling fluidized-bed gasifier; 6—electric furnace; 7—ceramic filter candle (OD of 60 mm, length of 440 mm); 8—screw conveyor for feeding fuel; 9—heat exchangers for steam condensation; 10—tar sampling unit; 11—vacuum pumps; 12—syngas mass flow controllers; 13—gas analyzers.

The lignite gasification tests were carried out using olivine as bed material, with a constant feeding rate of lignite and steam. For each test, a new batch of calcined olivine was inserted as bed material in the reactor, in order to compare tests with equivalent initial conditions avoiding the accumulation of ash in the bed material, which could have beneficial effects, such as the enhancement of tar conversion [26,27]. The operating temperature was changed in the tests between 750 and 850 ◦C and air injections were added in three of the six tests. Tests #1, #2, and #3 were carried out at operating temperatures of approximately 750, 800, and 850 ◦C, respectively. Tests #4, #5, and #6 had the same input conditions adopted in the first three tests, but with an additional air stream of 8 Nl/min injected inthefreeboardofthereactor.

The operating conditions used are summarized in Table 1.

**Table 1.** Operating conditions of the test campaign.


#### *2.2. Analysis of Products*

Downstream of the steam condensers and the vacuum pump, a slipstream of the product gas was sent to online gas analyzers for an evaluation of the composition. Online analyzers allowed H2, CO, CO2, CH4, H2S, and NH3 to be detected (ABB URAS, LIMAS, CALDOS, and ULTRAMAT 6 Siemens).

The water content in the product gas was calculated from the quantity of water collected in the flasks connected to the steam condensers. The water conversion η*wc* (%) was thus calculated as

$$
\eta\_{\rm nuc} = \frac{\dot{m}\_{\rm water, in} - \dot{m}\_{\rm water, out}}{\dot{m}\_{\rm water, in}} \times 100,\tag{1}
$$

where .*mwater*, *in* and .*mwater*, *out* are the mass flows of the water input and output, respectively.

The quantity of dry product gas, measured by means of a mass flow controller, allowed the gas yield *Ygas Nm*3/kg*f eedstock* to be calculated:

$$Y\_{\text{gas}} = \frac{F\_{\text{gas, out}}}{F\_{f\text{cads stock, in}}},\tag{2}$$

where *F gas*, *out* is the total dry N2-free volume of gas flow produced, and *Ff eedstock*, *in* is the mass flow of the input feedstock.

From the analysis of the gas composition and the carbon content in the feedstock, it was possible to calculate the carbon conversion *XC* (%):

$$X\_{\text{C}} = \frac{n\_{\text{CO}} + n\_{\text{CO}\_2} + n\_{\text{CH}\_4}}{n\_{\text{C}\_{\text{in}}}} \times 100,\tag{3}$$

where *ni* represents the moles of the *i* carbonaceous species in the product gas (CO, CO2, and CH4), and *nCin* represents the total moles of C in the feedstock input.

Post-combustion was carried out after gasification: an air stream was fed to the reactor and the gaseous products (CO and CO2) were analysed and quantified, in order to evaluate the amount of the residual un-reacted char in the reactor. From this result, it was possible to calculate the char yield *Ychar* (%):

$$Y\_{char} = \frac{m\_{char}}{m\_{feadstock}} \times 100\,\text{.}\tag{4}$$

where *mchar* is the mass of the residual char estimated by the post-combustion in the gasifier, and *mf eedstock* is the mass of the total feedstock fed.

The cold gas efficiency η*CG*(%) was calculated as

$$\eta\_{\rm CG} = \frac{LHV\_{\rm gus}F\_{\rm gus, out}}{LHV\_{\rm light, i}F\_{\rm feedback, in}} \,\prime\,\tag{5}$$

where *LHVgas* and *LHVlignite* are the lower heating values of the gas produced and lignite, expressed in MJ/Nm<sup>3</sup> and MJ/kg, respectively.

The liquid tar samples collected in the impinger bottles were analysed offline by means of HPLC (Hitachi "Elite LaChrom" L-2130) for the detection and quantification of heavy hydrocarbons in the product gas. The tar compounds chosen as representative of a typical tar composition [28] were as follows: phenol, toluene, styrene, indene, naphthalene, acenaphthylene, fluorene, phenanthrene, anthracene, fluoranthene, and pyrene.

After each test, the bed material was extracted from the reactor and sieved, in order to separate the fly-ashes accumulated in the bed during the experimental run from the olivine particles; samples of ashes were thus collected for analysis after tests. Afterwards, the ashes were analysed by means of SEM/EDS and XRD analyses, in order to study their morphology and the present elements and compounds.

#### **3. Results and Discussion**

#### *3.1. Analysis of Materials Pre-Test*
