they compress a slurry of ground coal & water into a mold and then squeeze most of the water out.
I also found this...
Indoor air pollution in residences originates primarily from fuel combustion during cooking and migration of outdoor pollutants. Indoor air pollution characteristics in China are quite different from those in developed countries. Because of the general layout of the kitchen in the floor plan, the chief source of air pollution in rooms are cooking related. Coal and coal "cakes" or "balls" are still widely used for cooking. In Shanghai, about 50% of the households are using coal cake stoves (CCS) without chimneys and unvented, resulting in high concentrations of suspended particulars and gases. This is a health problem affecting about 1 million families in the city. Switching to electricity or gas for cooking is not feasible in the near future. Use of ventilators to exhaust the polluted air might be useful in ameliorating indoor air pollution. This project will reduce the respiratory disease burden of Shanghai residents using coal-cake stoves through better understanding of the health impacts of indoor air pollution and through the use of ventilation devices to improve the air quality indoors.
And then there's BINGO!
Caution to dial-uppers....this is a 1 meg .pdf, but it is pretty detailed!