No abstract

Time-weighted average (TWA) passive sampling using solid-phase microextraction (SPME) and gas chromatography was investigated as a new method of collecting, identifying and quantifying contaminants in process gas streams. Unlike previous TWA-SPME techniques using the retracted fiber configuration (fiber within needle) to monitor ambient conditions or relatively stagnant gases, this method was developed for fastmoving process gas streams at temperatures approaching 300 °C. The goal was to develop a consistent and reliable method of analyzing low concentrations of contaminants in hot gas streams without performing timeconsuming exhaustive extraction with a slipstream. This work in particular aims to quantify trace tar compounds found in a syngas stream generated from biomass gasification. This paper evaluates the concept of retracted SPME at high temperatures by testing the three essential requirements for TWA passive sampling: (1) zero-sink assumption, (2) consistent and reliable response by the sampling device to changing concentrations, and (3) equal concentrations in the bulk gas stream relative to the face of the fiber syringe opening. Results indicated the method can accurately predict gas stream concentrations at elevated temperatures. Evidence was also discovered to validate the existence of a second boundary layer within the fiber during the adsorption/ absorption process. This limits the technique to operating within reasonable mass loadings and loading rates, established by appropriate sampling depths and times for concentrations of interest. A limit of quantification for the benzene model tar system was estimated at 0.02 g m −3 (8 ppm) with a limit of detection of 0.5 mg m −3 (200 ppb). Using the appropriate conditions, the technique was applied to a pilot-scale fluidized-bed gasifier to verify its feasibility. Results from this test were in good agreement with literature and prior pilot plant operation, indicating the new method can measure low concentrations of tar in gasification streams. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Page 1 of 33A c c e p t e d M a n u s c r i p t 1 Highlights 1 ("Analysis of trace contaminants in hot gas streams using time-weighted average solid-phase 2 microextraction: proof of concept" manuscript by Woolcock et al.) 3  TWA-SPME method (retracted fiber) was developed for high temperature process gases. Ames, IA 50011, USA; koziel@iastate.edu, 19 ABSTRACT.Time-weighted average (TWA) passive sampling using solid-phase 20 microextraction (SPME) and gas chromatography was investigated as a new method of 21 collecting, identifying and quantifyi...

A pilot-scale 25 kg/h fluidized bed, oxygen/steam blown gasifier and syngas cleaning system was developed to convert switchgrass into clean syngas. The system is rated for operation at gage pressures up to 1 bar. The reactor vessel incorporated a novel guard heating system to simulate near-adiabatic operation of large commercial-scale gasifiers, and was effective for gasification temperatures up to 900°C. After removing particulate from the gas stream via cyclones, a warm-gas cleaning operation based on oil scrubbing was used to remove tars. Sulfur compounds were removed via solid-phase adsorption. Ammonia was removed by water scrubbing. Baseline gasification tests with steam and oxygen were conducted at equivalence ratios (ER) between 0.21 and 0.38 using switchgrass as fuel. Measurements on the raw and cleaned syngas included permanent gas composition, C 2 hydrocarbons, water, heavy and light tars, gasification residues (char and ash), hydrogen sulfide (H 2 S), carbonyl sulfide (COS), carbon disulfide (CS 2 ), ammonia (NH 3 ), and the first reported measurements of hydrogen cyanide (HCN) for oxygen/steam blown gasification. Heavy tars were removed with high efficiency by the method employed, although more difficult to remove light tars reduced overall tar removal efficiency to less than 80%. The sulfur scrubbing system demonstrated 99.9% removal efficiency, resulting in less than 200 ppb of H 2 S in the cleaned gas. The NH 3 scrubbing system also accomplished greater than 99.9% removal efficiency, resulting in final NH 3 concentrations of less than 1 ppm.

A new method was developed for collecting, identifying and quantifying contaminants in hot process gas streams using time-weighted average (TWA) passive sampling with retracted solid-phase microextraction (SPME) and gas chromatography. The previous lab scale proof-of-concept with benzene was expanded to include the remaining major tar compounds of interest in syngas: toluene, styrene, indene, and naphthalene. The new method was tested on high T (⩾100 °C) process gas from a pilot-scale fluidized bed gasifier feeding switchgrass and compared side-by-side with conventional impingers-based method. Fourteen additional compounds were identified, representing 40-60% improvement over the conventional method's detection capacity. Differences between the two methods were 1-20% and as much as 40-100% depending on the sampling location. Compared to the inconsistent conventional method, the SPME-TWA offered a simplified, solvent-free approach capable of drastically reducing sampling and sample preparation time and improving analytical reliability. The improved sensitivity of the new method enabled identification and quantification of VOCs beyond the capability of the conventional approaches, reaching concentrations in the ppb range (low mg/m3). RSDs associated with the TWA-SPME were <10%, with most lab-based trials yielding <2%. Calibrations were performed down to the lowest expected values of tar concentrations in ppb ranges (low mg/N m3, with successful measurement of tar concentrations at times >4000 ppm (up to 10 g/N m3). The new method can be a valid alternative to the conventional method for light tar quantification under certain conditions. The opportunity also exists to exploit TWA-SPME for process gas streams analysis e.g., pyrolysis vapors and combustion exhaust.