Opportunities and challenges in sustainable treatment and resource reuse of sewage sludge: A review

Tar is unavoidable by-product during biomass gasification process. Catalytic steam reforming of tar to syngas is a promising way for the removal of tar from the gas products. However, the key issue for this way is catalyst development. To date, the developed catalysts always have advantages and disadvantages: nickel-based catalysts have high activity, but they are easily deactivated by coking; noble metal based catalysts have high catalytic activity, long-term stability and high carbon deposition resistance, but they are expensive; other transition metal catalysts such as Fe, Co and Cu exhibit a good performance, but they are also deactivated easily by carbon deposition in the case of high heavy-tar content in the tar; alkali metal catalysts also have high catalytic activity for tar reforming, but they are easy to be evaporated with the generated gases; natural catalysts have been widely applied for the steam reforming of tar due to its inexpensive, abundant and disposable, but their catalytic activities are lower than those man-made ones, and especially have low mechanical strength, making them not suitable to be used in fluidized bed reactor; zeolite is suggested to be a good catalyst support due to its high thermal/hydrothermal stability, high resistance to sulfur compounds, and easy to be regenerated; biomass char has been used as the catalyst or catalyst support in the steam reforming of tar due to its low cost and its natural production inside the biomass gasifier; even biomass ash now is considered to be a good catalyst for tar removal. In this review, to get better understanding of the mechanism of catalytic steam reforming of tar derived from biomass, tar formation, tar properties and catalytic reaction mechanism are also introduced, and prospects and challenges are summarized.

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Catalytic steam reforming of biomass tar: Prospects and challenges

Sewage sludge, the inevitable by-product of municipal wastewater treatment plant operation, is a key issue in many countries due to its increasing volume and the impacts associated with its disposal. Thermochemical processing offers a new way of managing sewage sludge, not only by providing effective volume reduction, but also enabling transformation of carbon-rich organic fraction into valuable energy and fuel. Owing to some unique properties, sewage sludge differs from other solid fuels such as lignocellulosic biomass and coal, making its thermochemical conversion application somewhat complicated and challenging. This paper reviews the options of converting sewage sludge to energy and fuel via three main thermochemical conversion processes namely pyrolysis, gasification and combustion. The fundamental aspects of sewage sludge and its behaviour in each of thermochemical process are summarised. The challenges in adopting thermochemical conversion technology in sewage sludge management are addressed, and various alternative approaches deserving further consideration, such as the incorporation of pre-processing and co-utilisation, are discussed.

Thermochemical processing of sewage sludge to energy and fuel: Fundamentals, challenges and considerations

Thermochemical conversion of sewage sludge: A critical review

Gasification is a promising technology for reducing the volume of biowaste feedstock. Further, with the incorporation of steam this thermochemical treatment technology also concomitantly produces H2, a high value energy. This paper aims to summarize the status of biowaste gasification technology and detail the benefits and limitations of different gasification processes, especially for biowaste. First, we compare steam with other gasification agents (oxygen and air) to understand the specific effects of gasification agents on the resulting gas quality and quantity. Second, influencing process factors (reactor configurations, temperature, steam to biomass ratio, and catalyst incorporation) are evaluated in terms of their impact on the resulting H2/CO ratio, gas heating value, gas yield, tar yield, and energy recovery. Third, commercial biowaste gasification applications are detailed and the economics and societal impacts are elucidated. Finally, the current challenges facing the field of gasification and the future outlooks of this technology for reducing biowaste are presented.

Gasification of biowaste: A critical review and outlooks

Air and air-steam gasification of sewage sludge. The influence of dolomite and throughput in tar production and composition

Characterisation of tar from sewage sludge gasification. Influence of gasifying conditions: Temperature, throughput, steam and use of primary catalysts

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.

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Elena Roche

Papers

Air and air-steam gasification of sewage sludge. The influence of dolomite and throughput in tar production and composition

Characterisation of tar from sewage sludge gasification. Influence of gasifying conditions: Temperature, throughput, steam and use of primary catalysts

Sewage sludge gasification. dolomite performance under different operating conditions