Measuring Gases Produced by Waste Gasification to Improve Control and Efficiency

Edinburgh Sensors: Waste gasification is a type of waste to energy process where waste material is used as a fuel to create heat, electricity or other chemical products. Specifically, in waste gasification, the waste material is decomposed in such a way to produce gaseous products, often known as syngas, a combination of hydrogen, carbon monoxide and some carbon dioxide.¹

Conventional incineration uses heat and oxygen to combust waste products. While waste gasification still requires high temperatures, as it does not use oxygen, fewer pollutants are produced and no potentially toxic ashes. Syngas itself is also a valuable fuel either for steam and electricity product, or syngas can be used as chemical feedstock in the synthesis of more valuable products.²

Complex Processing

While waste gasification offers an attractive alternative to conventional incineration, the breakdown processes involved are complex. As the chemical composition and moisture content of the waste feedstock will constantly vary, so too will the optimum conditions for the most efficient conversion of waste to syngas.³

The whole gasification process can be broken down into several steps. First, the waste is dried by heating, then pyrolysis can begin in an oxygen-free environment. Some vapours and char are produced at this stage. At high temperatures, the waste feedstock will start to oxidise, forming carbon monoxide and water vapour as well as more heat, making the process self-sustainable. Then, there are the endothermic reduction reactions that produce the syngas of interest.

The oxidation stage requires the introduction of a ‘gasifier’ or a chemical species required for the necessary chemical reactions to occur. Gasifiers can be used at a range of pressures and temperatures, and a number of different gases are also suitable. Given the variation in the initial feedstocks and the number of variables in the processing, this means continuous monitoring of gas inputs and outputs is essential to maximise the efficiency of production.⁴

Gas Measures

One commonly used approach for ensuring optimal energy production values are maintained is using online gas monitors that continually track the levels of the gases produced in the syngas mixture or the input gas concentrations for oxidation.⁵ While gas collection for offline analysis can be useful, this can only be done periodically, is more labour intensive and does not provide as comprehensive monitoring as a series of online monitors.

Given the benefits of continuous monitoring and process adaptation,⁶ it is fortunate that Edinburgh Sensors offer a range of products that are ideal for gas monitoring, data logging, and integrated applications.

Monitoring Options

For detecting both oxidation gas concentrations and the chemical composition of syngas, Edinburgh Sensors offer a range of non-dispersive infra-red sensing instruments suitable for highly sensitive online monitoring of gases such as carbon monoxide and dioxide. All of these instruments come with pre- and post- sales and technical support to ensure their products match the customer’s needs exactly.

For CO or CO2 sensing, Edinburgh Sensors offer the Gascard NG⁷, the Guardian NG⁸, and the Chillcard NG⁹. While the Chillcard is targeted for the measurement of refrigerated gases, the other gas monitors are suitable for measurements in atmospheres of up to 1150 mbar of pressure and humidity conditions ranging between 0 – 95 %.  If just CO2 sensing is required, then there is the GasCheck, a low-cost, robust device capable of detecting CO2 concentrations between 0 – 3000 ppm.¹⁰ Alternatively, there is the compact IRgaskiT, weighing in at just 125g, that offers a range of advanced integration possibilities with monitoring and feedback systems for process control.¹¹

Simplicity and Sensitivity 

Two of the most popular gas monitors in the Edinburgh Sensor’s range offer impressive sensitivity, nearly the same as what can be achieved with offline analysers, for gas detection. The GasCard is now also available in a pre-built desktop unit, the Boxed Gascard¹², for even greater ease of use and installation.

The Guardian NG offers 0 – 3000 ppm detection ranges up to 0 – 100 % volume with an impressive response time of less than 30 seconds from the sample inlet. With an accuracy of ± 2 %, the Guardian NG can be relied upon to detect even the smallest changes in gas concentrations. The device comes with a display screen with a live readout of both pressure and true volume percentage as well as historical graphical data. Both the Guardian and GasCard can be connected to external devices though, either as part of alarm systems or for external logging and monitoring purposes, with the GasCard having true onboard RS232 connections along with the option of TCP/IP communications protocols.

For CO2 detection in harsher environmental conditions, the Gascheck is based on analogue electronics to give a zero-stability of ± 3 % over 12-months. As well as reducing the need for time-consuming collection and offline analysis of gases, the robust design of the Gascheck means that it can be relied upon to maintain accurate logging with minimal interference and recalibrations. 

These easy-to-install online gas monitors ensure that full control of the waste gasification process can be maintained, no matter what volume or type of starting material is used. By implementing such sensors into feedback and process control, optimum energy conversion can be achieved throughout continuous waste plant operation.

References

1. U. Arena, Waste Manag., 2012, 32, 625–639.
2. S. K. Gangwal and V. Subramani, Energy & Fuels, 2008, 22, 814–839.
3. M. Saghir, M. Rehan and A.-S. Nizami, in Gasification for Low-Grade Feedstock, 2018, vol. 6, pp. 97–113.
4. J. Gañan, A. A. K. Abdulla, A. B. Miranda, J. Turegano, S. Correia and E. M. Cuerda, Renew. Energy, 2005, 30, 1759–1769.
5. Gas Analysis in Gasification of Biomass, https://www.ieabioenergy.com/wp-content/uploads/2018/09/IEA-Bioenergy-Task-33-gas-analysis-report-Document-1-1.pdf, (accessed February 2019)
6. P. Kannan, A. Al and C. Srinivasak, in Gasification for Practical Applications, 2012, pp. 279–296.
7. Gascard NG, https://edinburghsensors.com/products/oem/gascard-ng/, (accessed February 2019)
8. Guardian NG, https://edinburghsensors.com/products/gas-monitors/guardian-ng/, (accessed February 2019)
9. Chillcard NG, https://edinburghsensors.com/products/oem/chillcard-ng/, (accessed February 2019)
10. Gascheck, https://edinburghsensors.com/products/oem/gascheck/, (accessed February 2019)
11. IRgaskiT, https://edinburghsensors.com/products/oem/irgaskit/, (accessed February 2019)
12. Boxed Gascard, https://edinburghsensors.com/products/oem/boxed-gascard/, (accessed February 2019)