Sewage sludge gasification. Dolomite performance under different operating conditions

Gasification is a technology that can replace traditional management alternatives used up to date to deal with this waste (landfilling, composting and incineration) and which fulfils the social, environmental and legislative requirements. The main products of sewage sludge gasification are permanent gases (useful to generate energy or to be used as raw material in chemical synthesis processes), liquids (tars) and char. One of the main problems to be solved in gasification is tar production. Tars are organic impurities which can condense at relatively high temperatures making impossible to use the produced gases for most applications. This work deals with the effect of some primary tar removal processes (performed inside the gasifier) on sewage sludge gasification products. For this purpose, analysis of the gas composition, tar production, cold gas efficiency and carbon conversion were carried out. The tests were performed with air in a laboratory scale plant consisting mainly of a bubbling bed gasifier. No catalyzed and catalyzed (10% wt of dolomite in the bed and in the feeding) tests were carried out at different temperatures (750ºC, 800ºC and 850ºC) in order to know the effect of these parameters in the gasification products. As far as tars were concerned, qualitative and quantitative tar composition was determined. In all tests the Equivalence Ratio (ER) was kept at 0.3. Temperature is one of the most influential variables in sewage sludge gasification. Higher temperatures favoured hydrogen and CO production while CO2 content decreased, which might be partially explained by the effect of the cracking, Boudouard and CO2 reforming reactions. At 850ºC, cold gas efficiency and carbon conversion reached 49% and 76%, respectively. The presence of dolomite as catalyst increased the production of H2 reaching contents of 15.5% by volume at 850 °C. Similar behaviour was found for CO whereas CO2 and CnHm (light hydrocarbons) production decreased. In the presence of dolomite, a tar reduction of up to 51% was reached in comparison with no catalyzed tests, as well as improvements on cold gas efficiency and carbon conversion. Several assays were developed in order to test catalyst performance under more rough gasification conditions. For this purpose, the throughput value (TR), defined as kg sludge “as received” fed to the gasifier per hour and per m2 of cross sectional area of the gasifier, was modified. Specifically, the TR values used were 110 (reference value), 215 and 322 kg/h·m2. When TR increased, the H2, CO and CH4 production decreased while the CO2 and the CnHm production increased. Tar production increased drastically with TR during no catalysed tests what is related to the lower residence time of the gas inside the reactor. Nevertheless, even at TR=322 kg/h·m2, tar production decreased by nearly 50% with in-bed use of dolomite in comparison with no catalyzed assays under the same operating conditions. Regarding relative tar composition, there was an increase in benzene and naphthalene content when temperature increased while the content of the rest of compounds decreased. The dolomite seemed to be effective all over the range of molecular weight studied showing tar removal efficiencies between 35-55% in most cases. High values of the TR caused a significant increase in tar production but a slight effect on tar composition.

Behaviour of dolomite, olivine and alumina as primary catalysts in air-steam gasification of sewage sludge

Sewage sludge gasification assays were performed in an atmospheric fluidised bed reactor using air and air–steam mixtures as the gasifying agents. Dolomite, olivine and alumina are three well known tar removal catalysts used in biomass gasification processing. However, little information is available regarding their performance in sewage sludge gasification. The aim of the current study was to learn about the influence of these three catalysts in the product distribution and tar production during sewage sludge gasification. To this end, a set of assays was performed in which the temperature (750–850 °C), the in-bed catalyst content (0, 10 and 15 wt.%) and the steam–biomass ratio (SB) in the range of 0–1 were varied with a constant equivalence ratio (ER) of 0.3. The results were compared to the results from gasification without a catalyst. We show that dolomite has the highest activity in tar elimination, followed by alumina and olivine. In addition to improving tar removal, the presence of water vapour and the catalysts increased the content of H2 in the gases by nearly 60%.

Air-steam gasification of sewage sludge in a bubbling bed reactor: Effect of alumina as a primary catalyst

Numerous references can be found in scientific literature regarding biomass gasification. However, there are few works related to sludge gasification. A study of sewage sludge gasification process in a bubbling fluidised bed gasifier on a laboratory scale is here reported. The aim was to find the optimum conditions for reducing the production of tars and gain more information on the influx of different operating variables in the products resulting from the gasification of this waste. The variables studied were the equivalence ratio (ER), the steam-biomass ratio (SB) and temperature. Specifically, the ER was varied from 0.2 to 0.4, the SB from 0 to 1 and the temperature from 750 °C (1023 K) to 850 °C (1123 K). Although it was observed that tar production could be considerably reduced (up to 72%) by optimising the gasification conditions, the effect of using alumina (aluminium oxide, of proven efficacy in destroying the tar produced in biomass gasification) as primary catalyst in air and air-steam mixture tests was also verified. The results show that by adding small quantities of alumina to the bed (10% by weight of fed sludge) considerable reductions in tar production can be obtained (up to 42%) improving, at the same time, the lower heating value (LHV) of the gas and carbon conversion.

Gasification technology assessment of sterile coal to clean electrical power generation.

Sterile coal is a low-value residue associated to the coal extraction and mining activity. According to the type and origin of the coal bed configuration, sterile coal production can mainly vary on quantity, calorific value and presence of sulphur compounds. In addition, the potential availability of sterile coal within Spain is apparently high and its contribution to the local power generation would be of interest playing a significant role. The proposed study evaluates the availability and deployment of gasification technologies to drive clean electricity generation from waste coal and sterile rock coal, incorporating greenhouse gas emission mitigation systems, like CO2, H2S and NOx removal systems. It establishes the target facility and its conceptual basic design proposal. The syngas obtained after the gasification of sterile coal is processed through specific conditioning units before entering into the combustion chamber of a gas turbine. Flue gas leaving the gas turbine is ducted to a heat recovery steam generation boiler; the steam produced within the boilerdrives a steam turbine. The target facility resembles a singular Integrated Gasification in Combined Cycle (IGCC) power station. The evaluation of the conceptual basic design according to the power output set for a maximum sterile contribution, established that rates over 95% H2S and 90% CO2 removal can be achieved. Noticeable decrease of NOx compounds can be also achieved by the use of commercial technology. A techno-economic approach of the conceptual basic design is made evaluating the integration of potential unitsand their implementation within the target facility aiming toachieve clean power generation. The criterion to be compliant with the most restrictive regulation regarding environmental emissions is setting to carry out this analysis.

Simulation of an Integrated Gasification Combined Cycle with Chemical-Looping Combustion and CO2 sequestration

Chemical-looping combustion allows an integration of CO2 capture in a thermal power plant without energy penalty; secondly, a less exergy destruction in the combustion chemical transformation is achieved, leading to a greater overall thermal efficiency. This paper focus on the study of the energetic performance of this concept of combustion in an integrated gasification combined cycle power plant when synthesis gas is used as fuel for the gas turbines. After thermodynamic modelling and optimization of some cycle parameters, the power plant performance is evaluated under diverse working conditions and compared to a conventional integrated gasification combined cycle with precombustion capture. Energy savings in CO2 capture and storage has been quantified. The overall efficiency increase is found to be significant and even notable, reaching values of around 7%. In order to analyze the influence of syngas composition on the results, different H2-content fuels are considered.

Biomass Gasification in a Fluidized Bed Reactor: Effect of Temperature, Stoichiometric Ratio and Biomass Type

This paper investigates the gasification of two biomass types (pine wood and olive stones) in a laboratory scale bubbling fluidized bed reactor, in order to evaluate comparatively their potential in the production of syngas.