If [gasification of garbage] is really commercially viable and uses old technology, then why hasn't it been done before?
It is viable, it is old technology and it has been done before (but perhaps not in exactly the way referred to).
There is existing technology for extracting energy from garbage. Sometimes the process is described as gasification.
Gasification was allegedly developed in the 1800s.
Existing gasification technology may not be the same technology as described in the NYT article - which may represent an improvement.
Gasification is the process of converting carbon based products such as coal, biomass, and petrol into carbon monoxide and hydrogen. This is done by causing a chemical reaction between the product and an amount of oxygen in a gasifier to produces a gas mixture known as synthetic gas, or syngas. Gasification is a highly efficient method for obtaining energy from organic materials, and can be used as a method for disposing of waste. Gasification was developed in the 1800s and was previously used to produce gas for lighting and cooking.
From Biomass Engineering
Oslo, Norway burns rubbish to power and heat homes, and they’ve run out. The city imports trash from Sweden, Ireland, England—and they wouldn’t mind taking on a few tons from the US.
And it’s not just Oslo. Northern Europe’s trash burning capacity outstrips its trash producing capacity by 550 million tons. Neighboring Sweden imports 800,000 tons of trash annually to make energy. Norway, Austria, and Germany are also getting in on the game. In total, 40 countries worldwide burn trash for energy.
From some random web site of unknown reliability
The first Energos facility is a 27MW gasification plant owned by Østfold Energi and is operating since 2002. It has been supplying 185Gwh a year of steam. The two facilities can process a total of 156,000t of waste.
From Power Technology - Gasification Plant, Norway
Energy-from-Waste (EfW), or Waste-to-Energy (WTE) is a process that takes post-recycled municipal solid waste – including plain old household rubbish – and burns it at a high temperature to reduce its weight and volume, and to produce heat and/or electricity.
From Covanta - Energy from waste
Waste and recycling company, AmeyCespa has received planning permission for its Milton Keynes Waste Recovery Park, which will utilise gasification technology from Manchester based Energos to recover energy from waste.
From Waste Management World
Unlike today's modern facilities, a number of plants that were developed during the late 1960s and 70s simply burnt or incinerated waste, using mains gas or other fossil fuels. However, modern facilities recover energy from the process and are therefore classed as ERF facilities, as opposed to being simply an incineration plant.ENERGOS ERF plants utilise Advanced Conversion Technology (ACT) and only use a small amount of natural gas to kick start the gasification process.
From Energos FAQ