Plasma Technology
Plasma technology has emerged as a possible solution to the problems created in the conventional gasification of solid wastes for synthesis gas production. Broadly speaking, the technology uses an applied voltage to convert molecules/atoms of a gas into a stream of high energy electrons, ions, free radicals etc, which put together is called plasma. These species are then imparted on the surfaces of substrates – typically gas or high porosity solids in gasification. This catalyzes a chain of reactions which for the case of gasification favours the creation of mostly H2 and CO. Low molecular weight hydrocarbons are produced but the preference is to convert these compounds into typical synthesis gas components – H2 and CO.
In plasma technology, the undesired tar, solids and particulate matter created in large amounts by conventional gasification become plasma substrates and are converted into more syngas increasing plasma carbon efficiency. Carbon efficiencies tend to increase with increases in applied voltage as this increases plasma species density and reaction rates. However, it comes with increased temperature, higher plasma work as more electrical energy is consumed and increased operations cost. Additionally, high temperature plasmas (operating in the temperature range of between 1000oC and 1200oC and above) results in a technology that’s more expensive. The need for a low temperature (non-thermal) plasma is warranted.
Nonthermal Plasma
Non-thermal plasma (NTP or low temperature plasma (LTP) is a non-equilibrium plasma that mainly excites the vibrational rather than the translational or rotational mode of substrates. This increases the concentrations of free radicals rather than raising the temperature of the species. Hence higher thermal and even carbon efficiencies are achievable with NTPs/LTPs. This means NTP-based plasmas are cheaper in both OPEX and CAPEX while increasing syngas yields.
Plasma Types, Configurations and Energy Consumption
Plasmas can be deployed as either single-stage or two-stage. In single-stage gasification, the generated plasma torch/arc is sent into the gasifier reactor to act on the surfaces of solid particles. This configuration is appropriate for high temperature plasmas, with the plasma work reaching about 800-1000 kwh/ton of MSW.
In single-stage plasma configurations, biomass is first gasified and the syngas and its by-products are directed to a plasma torch from a decoupled plasma source. Most suitable for low temperature plasma systems. In single-stage configuration energy consumption is in the range of 30-240 kwh/MT MSW. This is good for NTP plasma systems.
The paper written by Serang Kwon et al (2022) entitled Feasibility of Non-thermal Plasma for Waste-to-Energy Power Plant used NTP to generate synthesis at a temperature as low as 400 oC as part of an integrated plasma gasification combined cycle (IPGCC). The feedstock was MSW with only 10.7 wt % plastic. Using a two-stage configuration, a plasma power consumption in the range of 0-100 kwh/MT MSW was selected.
The authors studied different cold gas efficiencies (CGE) reaching a maximum of 95 % with CGE defined as:
Because feedstock heating value remains constant at the right moisture content (typically 5 % for MSW), the CGE increases with temperature since the heating value of syngas increases with temperature. For gasifiers prioritizing hydrogen production, the conversion of low-molecular weight hydrocarbons into H2 and CO (the carbon efficiency) is more important. Therefore creating more plasma free radicals that can cause the production of more H2 and CO from low molecular weight hydrocarbons is probably of more economic value. This might require operating at the higher end of the plasma energy consumption spectrum, ie 100 kwh/MT MSW – 340 kwh/MT MSW while still maintaining a low temperature compared to high-temperature plasma.
At 95 % efficiency, the following results were obtained
The authors don’t specifically mention the feed rate of the MSW used that would have enabled the mass flow rates of each of the components to be determined. However, on a mole basis, LDPE feeds from previous paper produced more hydrogen compared to MSW with just 10.7 % plastic that’s not necessarily LDPE. The focus should be on getting a technology that will increase the carbon efficiency by converting all H2 gases especially, C2H4, C3H6, C2H2 etc into CO and H2.
In terms of drying, plastic wastes especially LDPE tend to have very low moisture contents (MC), usually less than 1 % compared to the 5 % needed for moist other wastes. This means that CGE of LDPEs can reach 95 % or more, which will increase hydrogen yields. However, LDPE collected from an area with a high humidity could temporarily have a high surface moisture which can be removed by drying in a room that’s part of a building holding the shredder machine. This direct drying can be done using about 1 % of the syngas coming out of the gasifier.
Note: CO is toxic. Hence personnel should avoid getting into air-tight drying chamber during scheduled drying periods. Full ventilation after drying for about 30 minutes or so is required before access into room can be safe
Eliasu A Teiseh
Research and Development Manager, PhD