Active feedback stabilization of pressure-driven modes in tokamaks is studied computationally in toroidal geometry. The stability problem is formulated in terms of open-loop transfer functions for fluxes in sensor coils resulting from currents in feedback coils. The transfer functions are computed by an extended version of the MARS stability code [A. Bondeson et al., Phys. Fluids B 4, 1889 (1992)] and can be accurately modeled by low order rational functions. In the present paper stability is analyzed for a system with an ideal amplifier (current control). It is shown that feedback with modest gain, and a single coil array poloidally, gives substantial stabilization for a range of coil shapes. Optimum design uses sensors for the poloidal field, located inside the resistive wall, in combination with rather wide feedback coils outside the wall. Typically, the feedback does not strongly modify the plasma-generated magnetic field perturbation. A future companion paper [C. M. Fransson et al., Phys. Plasmas (accepted for publication)] will apply control theory to study the limitations arising for finite time-constant of the amplifier-feedback coil circuit.

According to the Intergovernmental Panel on Climate Change (IPCC), scenarios that have a good chance of restricting global warming to less than 2°C involve substantial cuts in anthropogenic greenhouse gas (GHG) emissions, implemented through large-scale changes in energy systems. The use of renewable energy sources and fossil fuels, in combination with carbon capture and storage (CCS), could help to reduce GHG emissions in the AbstractThis paper presents the main experiences gained and conclusions drawn from the demonstration of a first-of-its-kind wood-based biomethane production plant (20-MW capacity, 150 dry tonnes of biomass/day) and 10 years of operation of the 2-4-MW (10-20 dry tonnes of biomass/day) research gasifier at Chalmers University of Technology in Sweden. Based on the experience gained, an elaborated outline for commercialization of the technology for a wide spectrum of applications and end products is defined. The main findings are related to the use of biomass ash constituents as a catalyst for the process and the application of coated heat exchangers, such that regular fluidized bed boilers can be retrofitted to become biomass gasifiers. Among the recirculation of the ash streams within the process, presence of the alkali salt in the system is identified as highly important for control of the tar species. Combined with new insights on fuel feeding and reactor design, these two major findings form the basis for a comprehensive process layout that can support a gradual transformation of existing boilers in district heating networks and in pulp, paper and saw mills, and it facilitates the exploitation of existing oil refineries and petrochemical plants for large-scale production of renewable fuels, chemicals, and materials from biomass and wastes. The potential for electrification of those process layouts are also discussed. The commercialization route represents an example of how biomass conversion develops and integrates with existing industrial and energy infrastructures to form highly effective systems that deliver a wide range of end products. Illustrating the potential, the existing fluidized bed boilers in Sweden alone represent a jet fuel production capacity that corresponds to 10% of current global consumption. 7

SUMMARYThe Gothenburg Biomass Gasification plant (2015) is currently the largest plant in the world producing biomethane (20 MW biomethane ) from woody biomass. We present the experimental data from the first measurement campaign and evaluate the mass and energy balances of the gasification sections at the plant. Measures improving the efficiency including the use of additives (potassium and sulfur), high-temperature pre-heating of the inlet streams, improved insulation of the reactors, drying of the biomass and introduction of electricity as a heat source (power-to-gas) are investigated with simulations. The cold gas efficiency was calculated in 71.7%LHV daf using dried biomass (8% moist). The gasifier reaches high fuel conversion, with char gasification of 54%, and the fraction of the volatiles is converted to methane of 34% mass . Because of the design, the heat losses are significant (5.2%LHV daf ), which affect the efficiency. The combination of potential improvements can increase the cold gas efficiency to 83.5%LHV daf , which is technically feasible in a commercial plant. The experience gained from the Gothenburg Biomass Gasification plant reveals the strong potential biomass gasification at large scale.

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Active feedback of nonaxisymmetric resistive wall modes in tokamaks is investigated using control theory. Control systems are designed to stabilize the resistive wall mode of toroidal mode number n=1 and meet certain performance specifications for a set of test equilibria. The response of the plasma and resistive wall is described by low order rational functions from an electromagnetic model [Y. Q. Liu et al., Phys. Plasmas 7, 3681 (2000)]. Simple coil arrangements are assumed, both for the sensor and feedback coil arrays, and the sensors detect the perturbed poloidal field. The active coils are modeled both as broad strips and as thin wires, and several different controllers: P (proportional), PD (proportional plus derivative) and H∞ are investigated. An important parameter is the ratio, τ, of control system response time to the resistive wall time, and the analysis shows the restrictions on this ratio for acceptable performance. For an equilibrium that exceeds the no-wall beta limit by 63%, good control with broad strips and a PD controller is possible for τ≲5.4, while thin-wire coils require τ≲2.1. H∞ controllers give some improvement for thin wires and about a factor of 2 increase in τ for broad strips. The upper limit in τ decreases with increasing pressure. A control system designed to stabilize a certain pressure generally works well at lower pressures.